ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉At the Antamina deposit, Peru, accurate classification of exoskarns and endoskarns can be problematic when textures are mottled. In this study, we use whole-rock geochemical compositions (62 elements) of 221 samples to differentiate texturally similar endoskarns and exoskarns by comparing their compositions to least altered precursors (wall rocks and intrusive rocks). We present a simple method for discriminating these skarn types using immobile element bivariate plots. The most effective discriminators partition endoskarn and exoskarn into distinct domains defined by the composition of each precursor; these include Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉 versus heavy rare earth elements and some high field strength elements. Using these geochemical parameters, undifferentiated skarn samples can be more reliably classified as endoskarn or exoskarn. The effectiveness of these element pairs is attributed to their significantly different initial concentrations in wall rocks versus igneous precursors and their immobility during skarn formation. While immobile elements can differentiate the skarns, mobile element gains and losses (quantified using isocon analysis) provide insight on the bulk mineralogical and mass changes that take place during skarn formation.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Tongkuangyu copper deposit in the Zhongtiaoshan region, southern Trans-North China orogen, is hosted by a poorly constrained sequence of Paleoproterozoic volcano-sedimentary (quartz-sericite schist and biotite schist) and granitic rocks that have been metamorphosed to lower greenschist facies and variably deformed. The deposit has previously been proposed to be either a porphyry-type or a sediment-hosted stratiform Cu deposit, and its age of formation has been debated.The quartz-sericite schist is interpreted to be a felsic crystal tuff and consists of angular quartz crystals in a fine-grained sericite-altered matrix. Two quartz-sericite schist samples yielded zircon U-Pb upper concordia intercept ages of 2512 ± 12 (2〈span〉σ〈/span〉, mean square of weighted deviates [MSWD] = 0.19) and 2335 ± 16 Ma (2〈span〉σ〈/span〉, MSWD = 0.80). Biotite schist, which is interleaved locally with the quartz-sericite schist and is interpreted to be a basaltic-andesitic sill, yielded a younger zircon U-Pb upper concordia intercept age of 2191 ± 10 Ma (2〈span〉σ〈/span〉, MSWD = 1.7). Five samples of granodiorite and granodiorite porphyry that intruded the schist sequence yielded similar zircon U-Pb ages, with a weighted mean upper concordia intercept age of 2182 ± 7 Ma (2〈span〉σ〈/span〉, MSWD = 1.3). These results suggest that the volcanic sequence was deposited between ∼2.5 and 2.3 Ga and was intruded by basaltic-andesitic sills and a suite of granodiorite and granodiorite porphyry intrusions at ∼2.19 to 2.18 Ga.Two stages of copper mineralization are interpreted to have formed after pervasive sericite alteration of the felsic volcanic rocks. Stage 1 mineralization includes disseminated and deformed quartz veinlets containing chalcopyrite ± pyrite ± magnetite ± molybdenite associated with biotite ± K-feldspar alteration in granodiorite porphyry and schist. Stage 2 comprises undeformed quartz-chlorite-carbonate veins with bornite ± chalcopyrite ± magnetite associated with local chlorite and silicic alteration. Allanite crystals intergrown with chalcopyrite in the granodiorite porphyry yielded an approximate concordia U-Pb age of 2115 ± 31 Ma (2〈span〉σ〈/span〉, MSWD = 2.3). Two molybdenite samples in a deformed quartz-chalcopyrite-molybdenite vein yielded Re-Os model ages of 2106 ± 9 and 2089 ± 9 Ma (2〈span〉σ〈/span〉), consistent with previously published results.Hydrothermal monazite grains with Cu-Fe sulfide inclusions in the granodiorite porphyry, quartz-sericite schist, and undeformed chlorite-bornite-quartz veins yielded much younger U-Pb upper concordia intercept ages of 1832 ± 16 (2〈span〉σ〈/span〉, MSWD = 0.48), 1810 ± 14 (2〈span〉σ〈/span〉, MSWD = 0.92), and 1809 ± 12 Ma (2〈span〉σ〈/span〉, MSWD = 0.38), respectively. The results are in agreement with four Re-Os model ages for pyrite mineral separates from undeformed quartz-sulfide veins, which yielded a weighted mean age of 1807 ± 4 Ma (2〈span〉σ〈/span〉, 〈span〉n〈/span〉 = 4, MSWD = 0.42). In contrast, hydrothermal rutile crystals in the quartz-sericite schist and biotite schist yielded a range of roughly concordant ages between 2.1 and 1.8 Ga, reflecting isotopic disturbance.We interpret these results to indicate original copper mineralization at ∼2.1 Ga that is significantly later than the granodiorite (∼2.18 Ga) and schists (∼2.5–2.2 Ga), followed by hydrothermal remobilization and metamorphism at ∼1.8 Ga. The metavolcanic and granodiorite porphyry host rocks, alteration styles, and disseminated and veinlet form of the earlier mineralization are strongly reminiscent of porphyry Cu deposits, and ages of ∼2.1 Ga have been reported for one intrusion and three volcanic rock samples from the district. The Tongkuangyu, therefore, represents one of the oldest known porphyry copper deposits. Remobilization of copper occurred at ∼1.8 Ga during the Zhongtiao orogeny.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉This study integrates new geologic mapping and structural analysis with previous work near Walnut Canyon and Telegraph Canyon to address the style and magnitude of shortening and the relationship between contractional structures and porphyry preservation and localization between the Ray and Resolution porphyry copper deposits. Cenozoic extensional structures were superimposed on earlier contractional structures formed during the Laramide orogeny, which dates from ~80 to 50 Ma. This superposition requires that Cenozoic normal faults be restored prior to analysis of Laramide contractional structures and their relationship to nearby porphyry copper deposits. Five distinct sets of normal faults within the study area progressively tilted the region 65° east. The amount of extension was 10.3 km or 276%. Using key constraints such as offset strata, cutoff angles between faults and various units, and Laramide fault geometries, the study area was structurally reconstructed and verified using 2-D kinematic modeling of reverse fault offset and related folding. Total shortening is 7.2 km or 98%. Laramide reverse faults are interpreted as thick-skinned basement-cored uplifts, because they restore to moderate angles, have related fault-propagation folds, and involve significant crystalline basement rock. The Telegraph Canyon reverse fault has at least 5.3 km of offset, and the Walnut Canyon reverse fault has 3.2 km. The preferred estimate of the total vertical uplift for the fault system is 5.2 km but could be several kilometers greater. The restored strike direction of these faults combined with mid-Cenozoic erosion surfaces throughout the region suggests that this fault system may be responsible for the Laramide uplift of the Tortilla Mountains and Black Hills. In addition, most major porphyry centers appear to have been intruded into the footwall of this large uplift, with local examples including Ray and Resolution, suggesting that topography generated from this uplift may have been critical to preservation of these ore systems. Though definitive crosscutting relationships do not exist in the immediate map area, geologic relationships in a broader area suggest that shortening here began after 74 Ma and, in the Ray area, had ended by ~69 Ma and that porphyry formation postdated reverse faulting by as much as 5 m.y. to as little as 〈1 m.y.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Mt. Carlton Au-Ag-Cu deposit, northern Bowen basin, northeastern Australia, is an uncommon example of a sublacustrine hydrothermal system containing economic high-sulfidation epithermal mineralization. The deposit formed in the early Permian and comprises vein- and hydrothermal breccia-hosted Au-Cu mineralization within a massive rhyodacite porphyry (V2 open pit) and stratabound Ag-barite mineralization within volcano-lacustrine sedimentary rocks (A39 open pit). These orebodies are all associated with extensive advanced argillic alteration of the volcanic host rocks. Stable isotope data for disseminated alunite (〈span〉δ〈/span〉〈sup〉34〈/sup〉S = 6.3–29.2‰; 〈span〉δ〈/span〉〈sup〉18〈/sup〉OSO〈sub〉4〈/sub〉 = –0.1 to 9.8‰; 〈span〉δ〈/span〉〈sup〉18〈/sup〉O〈sub〉OH〈/sub〉 = –15.3 to –3.4‰; 〈span〉δ〈/span〉D = –102 to –79‰) and pyrite (〈span〉δ〈/span〉〈sup〉34〈/sup〉S = –8.8 to –2.7‰), and void-filling anhydrite (〈span〉δ〈/span〉〈sup〉34〈/sup〉S = 17.2–19.2‰; 〈span〉δ〈/span〉〈sup〉18〈/sup〉O〈sub〉SO〈sub〉4〈/sub〉〈/sub〉 = 1.8–5.7‰), suggest that early advanced argillic alteration formed within a magmatic-hydrothermal system. The ascending magmatic vapor (〈span〉δ〈/span〉〈sup〉34〈/sup〉S〈sub〉ΣS〈/sub〉 ≈ –1.3‰) was absorbed by meteoric water (~50–60% meteoric component), producing an acidic (pH ≈ 1) condensate that formed a silicic → quartz-alunite → quartz-dickite-kaolinite zoned alteration halo with increasing distance from feeder structures. The oxygen and hydrogen isotope compositions of alunite-forming fluids at Mt. Carlton are lighter than those documented at similar deposits elsewhere, probably due to the high paleolatitude (~S60°) of northeastern Australia in the early Permian. Veins of coarse-grained, banded plumose alunite (〈span〉δ〈/span〉〈sup〉34〈/sup〉S = 0.4– 7.0‰; 〈span〉δ〈/span〉〈sup〉18〈/sup〉O〈sub〉SO〈sub〉4〈/sub〉〈/sub〉 = 2.3–6.0‰; 〈span〉δ〈/span〉〈sup〉18〈/sup〉O〈sub〉OH〈/sub〉 = –10.3 to –2.9‰; 〈span〉δ〈/span〉D = –106 to –93‰) formed within feeder structures during the final stages of advanced argillic alteration. Epithermal mineralization was deposited subsequently, initially as fracture- and fissure-filling, Au-Cu–rich assemblages within feeder structures at depth. As the mineralizing fluids discharged into lakes, they produced syngenetic Ag-barite ore. Isotope data for ore-related sulfides and sulfosalts (〈span〉δ〈/span〉〈sup〉34〈/sup〉S = –15.0 to –3.0‰) and barite (〈span〉δ〈/span〉〈sup〉34〈/sup〉S = 22.3–23.8‰; 〈span〉δ〈/span〉〈sup〉18〈/sup〉O〈sub〉SO〈sub〉4〈/sub〉〈/sub〉 = –0.2 to 1.3‰), and microthermometric data for primary fluid inclusions in barite (Th = 116°– 233°C; 0.0–1.7 wt % NaCl), are consistent with metal deposition at temperatures of ~200 ± 40°C (for Au-Cu mineralization in V2 pit) and ~150 ± 30°C (Ag mineralization in A39 pit) from a low-salinity, sulfur- and metal-rich magmatic-hydrothermal liquid that mixed with vapor-heated meteoric water. The mineralizing fluids initially had a high-sulfidation state, producing enargite-dominated ore with associated silicification of the early-altered wall rock. With time, the fluids evolved to an intermediate-sulfidation state, depositing sphalerite- and tennantite-dominated ore mineral assemblages. Void-filling massive dickite (〈span〉δ〈/span〉〈sup〉18〈/sup〉O = –1.1 to 2.1‰; 〈span〉δ〈/span〉D = –121 to –103‰) with pyrite was deposited from an increasingly diluted magmatic-hydrothermal liquid (≥70% meteoric component) exsolved from a progressively degassed magma. Gypsum (〈span〉δ〈/span〉〈sup〉34〈/sup〉S = 11.4–19.2‰; 〈span〉δ〈/span〉〈sup〉18〈/sup〉O〈sub〉SO〈sub〉4〈/sub〉〈/sub〉 = 0.5–3.4‰) occurs in veins within postmineralization faults and fracture networks, likely derived from early anhydrite that was dissolved by circulating meteoric water during extensional deformation. This process may explain the apparent scarcity of hypogene anhydrite in lithocaps elsewhere. While the Mt. Carlton system is similar to those that form subaerial high-sulfidation epithermal deposits, it also shares several key characteristics with magmatic-hydrothermal systems that form base and precious metal mineralization in shallow-submarine volcanic arc and back-arc settings. The lacustrine paleosurface features documented at Mt. Carlton may be useful as exploration indicators for concealed epithermal mineralization in similar extensional terranes elsewhere.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉New SHRIMP U-Pb data from dioritic to granodioritic synmineral intrusions associated with the Jebel Ohier porphyry copper deposit (mineral inventory, including NI43-101-compliant total inferred and indicated resources, of 593 million tonnes [Mt] at 0.33% Cu and 0.05 ppm Au, for 1.953 Mt of contained Cu and 933,600 oz of Au at 0.15% Cu cutoff) in the Red Sea Hills of northeastern Sudan have bracketed the age of mineralization to ca. 816 to 812 Ma. This age range, as well as constraints from new and existing lithogeochemical data, is consistent with the deposit’s formation from a productive parental magma source during the early stages in the evolution of an intra-Mozambique Ocean island arc. The Jebel Ohier porphyry copper deposit bears many similarities to well-documented Phanerozoic analogues elsewhere in terms of (1) the mapped style and zonation of hydrothermal alteration (i.e., proximal K-silicate–dominated, to sericitic, to distal propylitic alteration), (2) the occurrence of intense Cu-bearing A- and B-type vein stockwork, as well as sulfide-only C-type veins, anhydrite veins, and younger, peripheral D-type veins, and (3) the geochemical fingerprint of the associated porphyry, which is akin to those of ore-related Tertiary porphyries in the Escondida area in northern Chile. The multiphase intrusion hosting the Jebel Ohier porphyry copper deposit has been intruded by several generations of mafic to felsic postmineralization dikes and voluminous plutons, with a peak in magmatic activity coinciding with the suturing of the Gebeit terrane at ca. 724 Ma. In spite of, or perhaps because of, the occurrence of extensive postmineralization magmatism, and regardless of subsequent deformation, regional metamorphism, uplift, and erosion, the deposit has remained remarkably intact. The discovery of a relatively ancient, yet well-preserved porphyry copper deposit in the Neoproterozoic Arabian-Nubian Shield has major implications for the exploration potential of this resource-rich geologic terrain.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The newly discovered Shuangjianzishan Ag-Pb-Zn deposit, with 145 Mt of ore grading 128.5 g/t Ag (locally up to 32,000 g/t) and 2.2 wt % Pb + Zn, is located in the Great Hinggan Range metallogenic belt, northeastern China, and is currently the largest Ag deposit in Asia. The Ag-Pb-Zn orebodies occur as veins and are hosted primarily by a Permian slate. Recent drilling and core logging have identified a partially Mo mineralized granite porphyry intrusion adjacent to the Ag-Pb-Zn mineralized veins. This well-preserved magmatic-hydrothermal system therefore offers an excellent opportunity to evaluate the possible temporal and genetic relationship between Mo-mineralized porphyry intrusions and Ag-Pb-Zn veins. Three primary paragenetic stages of veining have been recognized: (I) early pyrite + quartz ± K-feldspar, (II) main ore sulfide + sulfosalt + quartz + calcite + sericite + chlorite ± epidote, and (III) post-ore quartz. The silver mineralization occurs mainly in the late paragenetic part of Stage II, in which canfieldite (Ag〈sub〉8〈/sub〉SnS〈sub〉6〈/sub〉), argentite (Ag〈sub〉2〈/sub〉S) and freibergite [(Ag, Cu)〈sub〉12〈/sub〉Sb〈sub〉4〈/sub〉S〈sub〉13〈/sub〉] are the dominant Ag-bearing ore minerals. A combination of ore mineral chemical and sulfur isotope geothermometers and physicochemical calculations suggest that the Ag-Pb-Zn mineralization took place at a temperature of 250° to 200°C, a pH of 6.7 to 5.6, and a Δlog〈span〉f〈/span〉o〈sub〉2〈/sub〉 (HM) of –2.4 to –8.7.A conspicuous enrichment of Sn and Se in the ore, which is represented by minerals containing the metal suite Ag-Pb-Zn-(Cu-Sn-Se-Sb), likely reflects a close genetic association between the base metal mineralization and a magma. In situ analyses show that the 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values of the sulfides and Ag-bearing sulfosalts from the Ag-Pb-Zn mineralized veins vary from –4.67 to +2.44‰; the mean value is –2.11 ± 1.49‰ (〈span〉n〈/span〉 = 77). The calculated mean 〈span〉δ〈/span〉〈sup〉34〈/sup〉S〈sub〉H2S〈/sub〉 value of the ore-forming fluid is –1.65 ± 0.83‰, which is indicative of a magmatic sulfur source. In situ Pb isotope analyses of the ore minerals yielded a narrow range of values (〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb of 18.243–18.310, 〈sup〉207〈/sup〉Pb/〈sup〉204〈/sup〉Pb of 15.503–15.563 and 〈sup〉208〈/sup〉Pb/〈sup〉204〈/sup〉Pb of 38.053–38.203, 〈span〉n〈/span〉 = 59). Comparisons to corresponding isotopic data for the various rock units in the area and sulfides from nearby ore deposits indicate that there were substantial contributions of Pb and other metals (e.g., Ag and Zn) to the Shuangjianzishan deposit from a Mesozoic granitic source.Diorite-granodiorite dikes and dacite are crosscut by the Ag-Pb-Zn veins, and therefore, predate ore formation. These rock units have zircon U-Pb ages of 250.2 ± 2.0 and 133.9 ± 1.4 Ma, respectively. A concealed, weakly Mo mineralized granite porphyry intrusion proximal to the Ag-Pb-Zn mineralized vein system yielded zircon U-Pb ages of 134.4 ± 1.0 (MSWD = 0.1) and 134.4 ± 1.0 Ma (MSWD = 0.2), for coarse- and fine-grained facies, respectively. These ages are indistinguishable within the uncertainty from the zircon ages for the dacite and a granite intrusion ~2 km north of the mineralized veins, which has a weighted mean zircon U-Pb age of 135.2 ± 1.4 Ma (MSWD = 0.78). Molybdenite from three quartz vein/veinlet samples hosted by slate immediately above the porphyry intrusion yielded Re-Os model ages from 136.3 ± 0.9 to 133.7 ± 1.2 Ma and a weighted mean Re-Os age of 134.9 ± 3.4 Ma. Finally, three pyrite samples separated from the Ag-Pb-Zn mineralized veins have a weighted mean Re-Os model age of 135.0 ± 0.6 Ma. The very similar zircon U-Pb ages for the Mo-mineralized granite porphyry and dacite, and Re-Os ages for molybdenite and pyrite in the Shuangjianzishan ore district indicate that the Mesozoic magmatic-hydrothermal activity was restricted to a relatively short time interval (~136–133 Ma). They also suggest that the weakly Mo mineralized granite porphyry was likely the source of the fluids and metals that produced the Ag-Pb-Zn mineralization.Based on our geological observations and an extensive analytical database, a model is proposed for the genesis of the giant Shuangjianzishan Ag-Pb-Zn deposit in which the ore-forming fluid and its metals (i.e., Ag, Pb, and Zn) were exsolved during crystallization of the final phase of a composite granite porphyry intrusion. This fluid transported metals to the distal parts of the system, where they were deposited in preexisting faults or fractures created by the withdrawal of magma during the waning stages of the magmatic-hydrothermal event. The present study of the Shuangjianzishan Ag-Pb-Zn deposit and those of other magmatic-hydrothermal ore deposits in the region provide compelling evidence that the widespread Mesozoic felsic magmatism and Ag-Pb-Zn mineralization in the southern Great Hinggan Range took place in an intracontinental extensional tectonic setting, which was synchronous with, and spatially associated to, Paleo-Pacific slab rollback and lithospheric delamination and thinning.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Orogenic Au deposits have contributed the majority of Au recovered globally throughout history. However, the mechanism that concentrates Au to extremely high bonanza grades in small domains within these deposits remains enigmatic. The volume of fluid required to provide extreme Au endowments in localized occurrences is not reflected in field observations (e.g., in the extent of quartz veining or hydrothermal alteration). Detailed optical, scanning and transmission electron microscopy, nanoscale secondary ion mass spectrometry, and 3-D neutron tomography have been used to investigate the processes responsible for development of anomalously high grade ore (upward of 3% Au) found in quartz veins at Fosterville gold mine (Victoria, Australia). Distinct textural settings of visible Au include (1) Au concentrated along pressure solution seams associated with wall-rock selvages, (2) as nano- to microscale dusty Au seams parallel to pressure solution seams, and (3) in microscale tension fractures perpendicular to stylolitic seams. The distribution of Au in arsenopyrite and pyrite hosted within pressure solution seams changes as a function of the extent of deformation. Sulfides in highly deformed pressure solution seams exclusively host Au as nano- to micrometer-sized clusters within features associated with corrosion and brittle failure, whereas sulfides in mildly deformed pressure solution seams have Au bound in the crystal structure. It is proposed that Au supersaturation in fluids introduced during seismic periods led to the deposition of abundant Au nanoparticles in quartz-carbonate veins. Subsequent pressure dissolution of vein quartz and carbonate during interseismic intervals allowed for episodic increase in the Au/quartz ratio and permitted liberation and migration of Au nanoparticles, promoting Au grain growth in favorable textural settings. Galvanic corrosion and brittle fracturing of auriferous sulfides during the interseismic period allowed additional remobilization and/or enrichment of sulfide-hosted Au. Repetition of this mechanism over the time scale of deposit formation acted to concentrate Au within the lodes. This Au ore upgrading model, referred to as “aseismic refinement,” provides a new insight for the genesis of ultrarich Au mineralization and, based on textures reported from many Au deposits, may be a globally significant component in the formation of orogenic Au deposits.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉To better understand processes leading to porphyry Mo deposit formation, the metal content, volatile content, and crystallization conditions of melt inclusions from pre- and synmineralization intrusions in six porphyry(-skarn) Mo deposits of northeastern China (Aolunhua, Hashitu, Lanjiagou, Songbei, Wanbaoyuan, and Yangjiazhangzi) were investigated by means of laser ablation-inductively coupled plasma-mass spectrometry and electron microprobe analysis. The ore-forming silicate melts were one to four times more evolved than average granite with 1 to 7 ppm Mo. The ore-related intrusions crystallized predominantly at 760° to 690°C and 3.7 to 1.0 kbar, except for the one at Hashitu, which crystallized at 770° to 740°C and lower pressures (2.0–1.0 kbar). Fertile silicate melts at Hashitu contain up to 0.4 wt % F, 0.03 to 0.09 wt % Cl, 5.0 to 7.0 wt % H〈sub〉2〈/sub〉O, 10 to 24 ppm Cs, and 200 to 500 ppm Rb, whereas those at Yangjiazhangzi and Wanbaoyuan contain less Cs (3–6 ppm and 5–7 ppm, respectively), less Rb (180–220 ppm and 200–240 ppm, respectively), and negligible F (〈0.15 wt %) but have similar Cl (0.03–0.05 wt %) and H〈sub〉2〈/sub〉O (5.3–6.5 wt % and 4.0–5.2 wt %, respectively) contents. Calculated melt viscosities in fertile magmas (log 〈span〉η〈/span〉 = 4.3–6.1 Pa s) are at the lower end of the values reported for felsic melts at the same temperature.Comparison between syn- and premineralization intrusions in individual deposits reveals that the ore-related intrusions were similarly evolved and had similar Mo contents and crystallization conditions as the nonmineralizing intrusions. The only difference is that the premineralization intrusions tend to occur as batholiths. The key to porphyry Mo mineralization lies in the focusing of fluid into and through a small rock volume on the top of the intrusion.For the studied porphyry Mo deposits, the mineralizing magmas are all Mo poor, indicating Mo enrichment is not required to form porphyry Mo deposits. Metal endowments in porphyry Mo deposits have no direct relationship with the composition and crystallization condition of mineralizing melts but are linked with the fluid flux released from the underlying magma chamber through a cupola.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉High-grade hematite mineralization is widely developed in banded iron formations (BIFs) worldwide. However, in the North China craton where Neoarchean-Paleoproterozoic BIFs are abundant, economic high-grade hematite ores are scarce. High-grade hematite ores hosted in the Paleoproterozoic Yuanjiacun BIFs represent the largest occurrence of this type of ore in the North China craton. The orebodies are fault controlled and show sharp contacts with lower greenschist facies metamorphic BIFs. In situ U-Pb geochronology of monazite and xenotime intergrown with microplaty hematite and martite in high-grade ore established two episodes of metamorphic-hydrothermal monazite/xenotime growth after deposition of the BIFs. The earlier episode at ca. 1.94 Ga is interpreted as the timing of lower greenschist-facies metamorphism, and the later episode at 1.41 to 1.34 Ga represents the timing of high-grade hematite mineralization. Petrography and microthermometry of primary fluid inclusion assemblages indicate that the high-grade hematite ore formed from hot (313°–370°C), CO〈sub〉2〈/sub〉-rich, and highly saline (~20 wt % NaCl equiv) hydrothermal fluids. These fluids channeled along faults, which concentrated iron through interaction with the BIFs—a process similar to typical hematite mineralization elsewhere. The deposition of hematite was probably related to tectonic extension in the North China craton related to the breakup of the Columbia/Nuna supercontinent. Our results challenge a previously proposed model ascribing the scarcity of high-grade hematite ores in the North China craton to the lack of prolonged weathering conditions. Rather, we argue that the high-grade ore formed in lower metamorphic-grade BIFs at shallower depths than magnetite mineralization and was largely eroded during later exhumation and uplift of the craton.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Ore assemblages in uranium roll-front deposits are highly variable and heavily dependent on Eh/pH conditions. Sulfur isotopes in pyrite traditionally have been employed to distinguish between biogenic or abiogenic redox pathways as drivers of roll-front propagation. However, the extent of and constraints on bacterial productivity have never been quantified, nor have the chemical conditions imposed by either primary formation mechanism. Moreover, this approach implicitly assumes that deposits form via one process or the other and disregards the possibility that both processes participate simultaneously in generating some orebodies. In this study, we analyzed sulfur isotopes from pyrite coprecipitated with uranium in two Wyoming roll-front deposits: Lost Creek and Willow Creek Mine Unit 10. The results document contrasting isotopic fractionation that correlates with pyrite morphology. Both deposits evolved with both abiogenic and biogenic redox mechanisms as active contributors to ore formation. In the past, bimodal fractionation behavior with pyrite morphology has been attributed to distinct temporal episodes of pyrite formation, driven by either a change in redox mechanism or multiple independent fluid events with unique isotopic signatures. However, neither explanation is appropriate for the isotopic trends identified in this study, where the two pyrite morphologies appear coeval in both deposits. Moreover, the contemporaneous formation of both pyrite morphologies cannot occur under the same conditions by the same precipitation mechanism because of the difference in their free energies of formation. The data suggest a third alternative in which pyrite morphology correlates to its biogenic or abiogenic mode of formation. Given the isotopic composition of pre-ore pyrite, sulfur isotope fractionation trends within the ore zone can be applied to establish prolificacy of bacteria and chemical conditions of the ore-forming solution.In both study sites, framboidal pyrite occurred as the primary by-product of sulfur-reducing bacteria, and the corresponding fractionation pattern constrains the sulfur availability and bacterial productivity. Euhedral to anhedral pyrite precipitated from abiogenic redox, the sulfur fractionation recording Eh/pH gradients during ore evolution. At Lost Creek, framboidal pyrite produced 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values from –50.8 ± 0.5‰ to +142.8 ± 0.3‰, while subhedral pyrite ranged from –68.1 ± 0.4‰ to +33.8 ± 0.3‰ with 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values increasing toward the barren, unaltered contact. Pre-ore pyrite at Lost Creek ranged from –0.8 ± 0.5‰ to +70.6 ± 0.3‰. 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values from biogenically derived pyrite at Lost Creek indicate a closed system with limited sulfate availability and a slow rate of bacterial reduction, implying restricted bacterial activity. Abiogenic fractionation behavior indicates a system driven by an Eh drop under neutral or basic pH conditions, and pyrite distribution across the roll identifies abiogenic pyrite recycling as the dominant redox mechanism at Lost Creek. At Willow Creek Mine Unit 10, framboidal pyrite ranged from –32.5 ± 0.4‰ to +68.2 ± 0.4‰, and subhedral pyrite ranged from –45.1 ± 0.4‰ to +5.4 ± 0.4‰. The subhedral pyrite 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values initially increased into the center of the roll and subsequently decreased again approaching the barren, unaltered contact. Pre-ore pyrite ranged from –48.1 ± 0.4‰ to +15.6 ± 0.5‰. Willow Creek Mine Unit 10 biogenically produced 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values show minimal fractionation from pre-ore pyrite, indicating an open system with abundant sulfate and rapid reduction from prolific bacterial activity. The abiogenic trends indicate an Eh drop and low pH at the barren, altered contact progressively neutralized across the orebody. This correlates to the anticipated Eh/pH gradients in a system dominated by biogenic redox.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Karangahake deposit is the third largest gold producer in the Hauraki goldfield, Coromandel, New Zealand. Production was mainly from the Maria and Welcome/Crown veins that were mined over a strike length of 〉1,300 m and vertical interval of 700 m. The veins are hosted in andesite that is strongly altered over an area of approximately 4.2 × 2.7 km. By using petrography, qualitative and quantitative X-ray diffraction (XRD), and automated mineralogy from a scanning electron microscope (SEM), hydrothermal alteration has been characterized in detail for surface rocks exposed along the Ohinemuri and Waitawheta rivers and along the underground Keillors crosscut, which links the Maria and Welcome/Crown veins. The chemistry of hydrothermally altered rocks was analyzed using portable X-ray fluorescence (pXRF). The automated mineralogy reveals alteration mineral occurrences, abundances, and some textural relationships that are not as apparent from conventional studies.Hydrothermal alteration of the andesite is typically strong to intense, with 100% conversion of primary to secondary minerals, but decreases to moderate to weak, comprising 〈25% secondary minerals toward the west and east, where the margins of alteration are exposed. The andesite is altered by combinations of quartz, chlorite, adularia, albite, illite, pyrite, calcite, mixed-layered illite-smectite, smectite, and rare epidote. Quartz and chlorite are common and present in both strongly and weakly altered rocks. Adularia is widespread and extends laterally up to 500 m from the Maria and Welcome/Crown veins. Illite is broadly coextensive with and locally occurs beyond the adularia halo. The zone of illite alteration is bordered to the east by patchy mixed-layered illite-smectite and outlying smectite alteration. Along the Keillors crosscut, the rocks between the Maria and Welcome/Crown veins are altered to adularia, albite, illite, chlorite, and rare calcite and epidote.Automated mineralogy and quantitative XRD mineral estimates indicate many rocks have been altered by 〉30% adularia (max. 74%) and that the amount of adularia can exceed that of quartz. Adularia and albite occur throughout the Keillors crosscut, with adularia most abundant within 20 to 50 m of the Maria, Mystery, and Welcome/Crown veins, whereas albite is more abundant between these veins.Hydrothermal alteration has resulted in the variable gains in K and Rb forming anomalous halos that surround veins, which are attributed to adularia and illite. By contrast, Ca, Sr, and to a lesser extent Fe are variably lost around veins in broadly overlapping zones of depletion.The key hydrothermal alteration minerals are illite, illite-smectite, smectite, adularia, and albite. The systemscale zonation of illite, illite-smectite, and smectite outlines the broad thermal structure, with illite stability indicating 〉230°C, illite-smectite indicating 130° to 230°C, and smectite indicating 〈130°C. Adularia indicates zones of inferred high permeability and the central upflow of boiling hydrothermal fluids, whereas albite indicates lower inferred permeability and peripheral-style alteration. Adularia represents a broad-scale vector toward veins, and its occurrence and abundance can be determined using field portable XRD or chemically by proxy from the K/Al value.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Chengchao Fe skarn deposit (280 Mt @ 45% Fe; nearly half of the total Fe reserve has an ore grade higher than 53 wt %) is the largest high-grade magnetite skarn deposit in the Middle-Lower Yangtze River metallogenic belt. The magnetite orebodies and skarns occur mainly along the contact zones between granitic (129 ± 2 Ma) rocks and diorite (133 ± 1 Ma) and Triassic strata. The paragenesis of the Chengchao deposit includes the following five stages: Na-K alteration stage (albite + K-feldspar), prograde skarn stage (garnet + pyroxene), retrograde skarn stage (magnetite + phlogopite + amphibole), sulfate-sulfide stage (anhydrite + pyrite ± garnet ± magnetite), and carbonate stage (calcite).Four generations of garnet and magnetite with distinct textures are recognized. The elemental compositions and fluid inclusion data of the three generations of garnet from the prograde skarn stage indicate fluctuating changes in the oxygen fugacity during their precipitation. Some magnetite grains from the retrograde skarn stage are characterized by well-developed zonation with respect to Al, K, Ca, and Si, with concentrations of up to hundreds of parts per million. The higher contents of (V + Ti) and (Al + Mn) in Mt2 relative to Mt1 (0.8 and 2,321 ppm for Mt1, and 182 and 10,315 ppm for Mt2, respectively) may be indicative of increasing fluid temperatures from episodic magmatic events. Fluid inclusion data show that the fluids responsible for the prograde skarn stage have high temperature (〉750°C), high salinity (〉50 wt % NaCl equiv) and high Fe concentration (e.g., magnetite and pyrite daughter crystals within garnet and pyroxene). Oxygen and hydrogen isotope data suggest that magmatic water was dominant during the prograde skarn stage and that increasing percentages of meteoric water were involved during later stages. Multiple episodic incursions of Fe-rich fluids, which were probably related to the emplacement of granitic rocks and diorite, were crucial for controlling the formation of the high-grade Chengchao Fe skarn deposit.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉High- and low-temperature platinum nuggets from two historical localities, Chocó in Colombia and Córrego Bom Sucesso in Brazil, are compared with respect to trace elements. Supergene platinum nuggets from Córrego Bom Sucesso are enriched in Se and Hg, but depleted in siderophile and chalcophile elements, and have fractionated platinum group element patterns, in comparison with magmatic platinum nuggets from Chocó. In particular, Se concentrations over ~100 〈span〉μ〈/span〉g/g Se and S/Se ratios above unity indicate Se recycling in a supergene environment with abundant organic matter.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The transport and deposition of gold from colloidal suspensions in hydrothermal fluids has been a persistent theme in ore deposits research. Studies of active geothermal systems show that a complete model of gold transport must include both dissolved and particulate forms. However, samples of the hydrothermal fluids are commonly spiked with aqua regia after collection in order to put any solids back into solution, thus preventing a quantitative assessment of the particle load. Although attempts have been made to filter the solids, gold nanoparticles (Au NPs) will mostly pass the 0.2-〈span〉μ〈/span〉m filters that are in common use, and a simple technique for analyzing suspended particles in the liquids has been lacking. In this study, we demonstrate how time-resolved acquisition of mass 197 in a conventional inductively coupled plasma-mass spectrometer (ICP-MS) can be used to detect and measure Au NPs in the filtered liquids, with an example of well-characterized fluids from the Reykjanes geothermal field on Iceland. The technique allows for precise monitoring of the solution as it is introduced into the plasma with the capability of identifying individual particles carried in suspension. Results show that Au particles passing the 0.2-〈span〉μ〈/span〉m filters are abundant in the studied samples, and measurements of the individual particles can be used to determine their size. The experiment highlights the potential of emerging ICP-MS techniques, including very fast data acquisition and multielement analysis of single particles in time-of-flight mode, for characterization of NPs in hydrothermal fluids.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We report the first Ag isotope data for the Paleozoic orogenic Au deposits in the Victorian Goldfields, southeast Australia, a world-class province with a historic production of 2,400 tonnes of Au. Despite their relatively uniform geology—similar host-rock types, age, mineralization style—deposits in Victoria show a wide range in 〈sup〉107〈/sup〉Ag/〈sup〉109〈/sup〉Ag ratios in native Au (〈span〉ε〈/span〉〈sup〉107〈/sup〉Ag −6.6 to +8.3, relative to the NIST SRM 978a Ag standard), comparable to the entire previously known terrestrial range (−9.4 to +5.3). The data show no correlation with mineralization age or host-rock composition, and there is no obvious isotopic link to established “mantle” or “crustal” Ag isotope values, implying that source rock signatures are unlikely to be the main control on Ag isotope variations. Instead, it is suggested that the Ag isotope variation is primarily related to physicochemical processes, particularly Ag isotope fractionation during redox reactions such as conversion of Ag〈sup〉0〈/sup〉 in native Au to Ag〈sup〉+〈/sup〉 in dissolved Ag(HS)〈sub〉2〈/sub〉〈sup〉−〈/sup〉 or sulfide-borne Ag. Repeated Ag〈sup〉0〈/sup〉〈strong〉←→〈/strong〉 Ag〈sup〉+〈/sup〉 reactions along transport pathways and at sites of ore accumulation could generate a wide range in 〈span〉ε〈/span〉〈sup〉107〈/sup〉Ag, and evidence of this range is presented in the data here. Silver isotope fractionation via numerous deposition-dissolution cycles provides a different perspective into large-scale ore genesis that has not previously been recognized for orogenic gold systems; multistaged metal remobilization along fluid transport pathways is standard during their formation. Detailed Ag isotope studies have considerable potential for understanding the history of episodic metal addition and within-deposit redistribution.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Four porphyry Cu-Mo systems were investigated by Re-Os molybdenite geochronology to constrain their timing with respect to the geodynamic and magmatic evolution of the eastern Pontides, Turkey. Molybdenite from the Ispir-Ulutaş deposit yielded an Re-Os age of 131.0 ± 0.7 Ma, which is consistent with Early Cretaceous U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon ages of local calc-alkaline intrusions. It demonstrates that porphyry deposits were already formed during Early Cretaceous subduction of the Neotethys along the eastern Pontides, and that they can be correlated with porphyry Cu events in the adjacent Lesser Caucasus. Molybdenite Re-Os ages of 76.0 ± 0.4 and 75.7 ± 0.4 Ma at the Elbeyli prospect and 77.2 ± 1.0 Ma at the Emeksen prospect overlap with U-Pb LA-ICP-MS zircon ages of shoshonitic to high-K calc-alkaline intrusions in the region, which were emplaced during Late Cretaceous Neotethys subduction. A 50.7 ± 0.3 Ma molybdenite Re-Os age at the Güzelyayla deposit confirms porphyry Cu-Mo emplacement coeval with Eocene postcollisional, calc-alkaline adakitic magmatism of the eastern Pontides.An electron microprobe study of molybdenite samples, supplemented by data obtained during Re-Os dating, shows that the Eocene Güzelyayla deposit and the Late Cretaceous Emeksen prospect have the highest Re enrichment. Postcollisional melting of a thickened mafic lower continental crust and melting of a metasomatized lithospheric mantle with little to no interaction with upper crustal rocks may explain the Re enrichment at Güzelyayla and Emeksen, respectively.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The trace element composition of zircon, especially in tandem with U-Pb geochronology, has become a powerful tool for tracing magmatic processes associated with the formation of porphyry copper deposits. However, the use of the redox-sensitive Eu and Ce anomalies as a potential mineral exploration proxy is controversial. This study presents a comprehensive, temporally constrained data set of zircon trace element compositions (n = 645) for three compositionally distinct magmatic series identified in the Meghri-Ordubad pluton, southernmost Lesser Caucasus. The 30 million years of Cenozoic magmatism in the Meghri-Ordubad pluton are associated with several ore-forming pulses leading to the formation of porphyry copper deposits and epithermal-style mineralization. Our zircon geochemical data constrain the thermal and chemical evolution of this complex intrusive suite and allow an evaluation of the usefulness of zircon as a mineral exploration proxy for porphyry copper deposits. Our results combined with Rayleigh fractionation modeling indicate that the trace element composition of zircon (Th/U, Hf, Ti, Yb〈sub〉N〈/sub〉/Dy〈sub〉N〈/sub〉, Eu anomalies) is influenced by the composition and the water concentration of the parental magma, as well as by co-crystallizing titanite and apatite. In contrast, the variations of Ce anomalies remain difficult to explain by magmatic processes and could rather be ascribed to relative fluctuations of the redox conditions. In the Meghri-Ordubad pluton, we do not observe any systematic patterns between the trace element composition in zircons and the different ore-forming pulses. This questions the reliability of using the trace element composition in zircon as an exploration mineral proxy, and it rather emphasizes that a good knowledge of the entire magmatic evolution of a metallogenic province is required.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Muratdere Cu-Mo (Au) porphyry deposit in western Turkey contains elevated levels of rhenium and is hosted within granodioritic intrusions into an ophiolitic mélange sequence in the Anatolian belt. The deposit contains several stages of mineralization: early microfracture-hosted molybdenite and chalcopyrite, followed by a quartz-pyrite-chalcopyrite vein set associated with Cu-Au grade, a quartz-chalcopyrite-pyrite-molybdenite vein set associated with Cu-Mo-Re grade, and a later polymetallic quartz-barite-sphalerite-galena-pyrite vein set. The rhenium in Muratdere is hosted within two generations of molybdenite: early microfracture-hosted molybdenite and later vein-hosted molybdenite. In situ laser ablation-inductively coupled plasma-mass spectrometry analysis of sulfides shows that the later molybdenite has significantly higher concentrations of Re (average 1,124 ppm, 〈span〉σ〈/span〉 = 730 ppm, n = 43) than the early microfracture-hosted molybdenite (average 566 ppm, 〈span〉σ〈/span〉 = 423 ppm, n = 28). Pyrite crystals associated with the Re-rich molybdenite have higher Co and As concentrations than those in other vein sets, with Au associated with As. The microfracture-hosted sulfides have 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values between −2.2‰ and +4.6‰, consistent with a magmatic source. The vein-hosted sulfides associated with the high-Re molybdenite have a 〈span〉δ〈/span〉〈sup〉34〈/sup〉S signature of 5.6‰ to 8.8‰, similar to values found in peridotite lenses in the Anatolian belt. The later enrichment in Re and 〈span〉δ〈/span〉〈sup〉34〈/sup〉S-enriched S may be sourced from the surrounding ophiolitic country rock or may be the result of changing redox conditions during deposit formation.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Kapan mining district in the southernmost Lesser Caucasus is one of the few locations along the central Tethyan metallogenic belt where ore-forming processes were associated with magmatic arc growth during Jurassic Tethys subduction along the Eurasian margin. Three ore deposits of the Kapan district were investigated in this study: Centralni West, Centralni East, and Shahumyan. The ore deposits are hosted by Middle Jurassic andesitic to dacitic volcanic and volcaniclastic rocks of tholeiitic to transitional affinities below a late Oxfordian unconformity, which is covered by calc-alkaline to transitional Late Jurassic-Early Cretaceous volcanic rocks interlayered with sedimentary rocks.The mineralization consists of veins, subsidiary stockwork, and partial matrix replacement of breccia host rocks, with chalcopyrite, pyrite, tennantite-tetrahedrite, sphalerite, and galena as the main ore minerals. Centralni West is a dominantly Cu deposit, and its host rocks are altered to chlorite, carbonate, epidote, and sericite. At Centralni East, Au is associated with Cu, and the Shahumyan deposit is enriched in Pb and Zn as well as precious metals. Both deposits contain high-sulfidation mineral assemblages with enargite and luzonite. Dickite, sericite, and diaspore prevail in altered host rocks in the Centralni East deposit. At the Shahumyan deposit, phyllic to argillic alteration with sericite, quartz, pyrite, and dickite is dominant with polymetallic veins, and advanced argillic alteration with quartz-alunite ± kaolinite and dickite is locally developed.The lead isotope composition of sulfides and alunite (〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb = 18.17–18.32, 〈sup〉207〈/sup〉Pb/〈sup〉204〈/sup〉Pb = 15.57–15.61, 〈sup〉208〈/sup〉Pb/〈sup〉204〈/sup〉Pb = 38.17–38.41) indicates a common metal source for the three deposits and suggests that metals were derived from magmatic fluids that were exsolved upon crystallization of Middle Jurassic intrusive rocks or leached from Middle Jurassic country rocks. The 〈span〉δ〈/span〉〈sup〉18〈/sup〉O values of hydrothermal quartz (8.3–16.4‰) and the 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values of sulfides (2.0–6.5‰) reveal a dominantly magmatic source at all three deposits. Combined oxygen, carbon, and strontium isotope compositions of hydrothermal calcite (〈span〉δ〈/span〉〈sup〉18〈/sup〉O = 7.7–15.4‰, 〈span〉δ〈/span〉〈sup〉13〈/sup〉C = −3.4−+0.7‰, 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr = 0.70537–0.70586) support mixing of magmatic-derived fluids with seawater during the last stages of ore formation at Shahumyan and Centralni West.〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar dating of hydrothermal muscovite at Centralni West and of magmatic-hydrothermal alunite at Shahumyan yield, respectively, a robust plateau age of 161.78 ± 0.79 Ma and a disturbed plateau age of 156.14 ± 0.79 Ma. Re-Os dating of pyrite from the Centralni East deposit yields an isochron age of 144.7 ± 4.2 Ma and a weighted average age of the model dates of 146.2 ± 3.4 Ma, which are younger than the age of the immediate host rocks. Two different models are offered, depending on the reliability attributed to the disturbed 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar alunite age and the young Re-Os age. The preferred interpretation is that the Centralni West Cu deposit is a volcanogenic massive sulfide deposit and the Shahumyan and Centralni East deposits are parts of porphyryepithermal systems, with the three deposits being broadly coeval or formed within a short time interval in a nascent magmatic arc setting, before the late Oxfordian. Alternatively, but less likely, the three deposits could represent different mineralization styles successively emplaced during evolution and growth of a magmatic arc during a longer time frame between the Middle and Late Jurassic.〈/span〉

Description: 〈span〉〈span〉Economic Geology〈/span〉 thanks the following individuals for book reviews that they prepared for the 2018 volume of the journal. Their reviews appear on the page indicated by the number that follows each name.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Igarapé Cinzento (GT-46) iron oxide copper-gold deposit is located in the northwestern part of the Cinzento shear zone in Carajás province, Brazil. Copper mineralization in the deposit is hosted by a hydrothermally altered metavolcanosedimentary sequence that consists of interlayered amphibolites, (actinolite)-biotite schists, almandine-biotite schists, and metamorphosed banded iron formations, intruded by granitoids (tonalite to granite) and mafic dikes.The complex evolution of the deposit involved (1) deposition of a volcanosedimentary sequence (2774 ± 19 Ma U-Pb zircon), (2) a tectonometamorphic event (D〈sub〉n〈/sub〉 = upper amphibolite facies), restricted in time between 2.77 and 2.72 Ga, (3) development of the Cinzento shear zone (D〈sub〉n + 1〈/sub〉) at 2.72 Ga, with associated hydrothermal activity and deposit formation, (4) superimposed hydrothermal events (ca. 2.6 and 2.5 Ga), and (5) granitoid emplacement at ca 2.5 Ga in the region.The GT-46 deposit is the result of overprinting hydrothermal stages with at least two distinct mineralization events. Variations in alteration styles are a consequence of changes in the mechanisms of fluid control, first regulated by ductile deformation and later by brittle fracturing. Early hydrothermal alteration is characterized by zones of sodic-calcic and potassic alteration as well as iron metasomatism with associated disseminated mineralization I, controlled by ductile structures (chalcopyrite-bornite-magnetite, 2718 ± 56 Ma, Re-Os molybdenite). Veins and breccias with chalcopyrite-magnetite(-chlorite-calcite-quartz) represent an overprinting episode of mineralization (II) at ca. 2.6 Ga. Late alteration stages (ca. 2.5 Ga) represent fluid remobilization with chloritization zones and crystal growth spatially related to pegmatite intrusions. Re-Os molybdenite ages of 2503 ± 51 and 2449 ± 44 Ma provide evidence for younger episodes of hydrothermal fluid circulation in the deposit.Fluid inclusion study reveals aqueous-saturated, highly saline (32−〉50 wt % NaCl equiv, total homogenization temperature [T〈sub〉ht〈/sub〉] = 203°−〉450°C) and carbonic fluids (CO〈sub〉2〈/sub〉 melting temperature [T〈sub〉mco〈sub〉2〈/sub〉〈/sub〉] = –56.3° to –54.5°C) for early alteration zones and saline (2.1–27.5 wt % NaCl equiv, T〈sub〉ht〈/sub〉 = 153.3°–215.5°C) solutions for late alteration zones. These data provide evidence for several alternative fluids involved in the evolution of the deposit: (1) hypersaline fluids derived from hydrothermal-magmatic brines or evaporative brines/evaporite-derived fluids, (2) saline solutions generated by progressive dilution of primary hypersaline fluids with shallower, colder, and less saline fluids (e.g., meteoric, seawater), and (3) carbonic components sourced from magmatic fluids, devolatilization of carbonate units and, partially, metamorphic fluids.Sulfur is interpreted to have been leached from country rocks in the earlier hydrothermal stages (mineralization I, 〈span〉δ〈/span〉〈sup〉34〈/sup〉S = 0.01–1.99‰), and to have come from mixed granitic and external oxidized sources for later overprinting events (mineralization II, 〈span〉δ〈/span〉〈sup〉34〈/sup〉S = 9.75–11.25‰). These data provide evidence for the recurrence of hydrothermal activity, indicating the existence of different potential sources of metals and sulfur during the evolution of the deposit.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The world-renowned Pliocene El Laco iron deposit in northern Chile is the youngest and the best-preserved Kiruna-type deposit in the world. The genesis of the El Laco Kiruna-type iron deposit—i.e., whether it is magmatic or hydrothermal in origin—is a long-standing controversy. The interstitial Fe-P phase lined by early formed magnetite crystals observed in massive ores at El Laco is morphologically and geochemically consistent with that produced by recent immiscible experiments, confirming that the massive magnetite ores were products of complete solidification of an iron-rich mush comprising early crystallized magnetite and an interstitial immiscible Fe-P melt. The hydrothermal features in geochemistry of massive magnetite are similar to the features of magnetite in altered andesites, implying a superimposed hydrothermal process. The occurrence of melt inclusions with high homogenization temperatures (〉700°C) hosted by the apatite in Cristales Grandes ores indicates that the veined ores formed shortly after the massive ores genetically related to the magmatic system. Some vesicles in the massive ores and the magnetite scoriae, previously interpreted as compelling evidence of volcanic structures, are demonstrated to be the residual pore spaces formed after interstitial phases between magnetite grains were removed by postmagmatic hydrothermal fluids and the accumulated debris eroded from massive ores with minor altered andesites, respectively. Thus, we propose a two-step genetic model in which massive ores resulted from magmatic processes and then were modified by subsequent magmatic-hydrothermal alteration. This model could be applied to the metallogenesis of most Kiruna-type iron deposits.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Reconstructing the thermal evolution of hydrothermal ore deposits mainly relies on their fluid inclusion record, which is limited by favorable trapping conditions, calling for alternative temperature constraints. Muscovite and tourmaline coexist in many hydrothermal ore deposits and in the granites or pegmatites related with them. Whereas in situ analyses of boron isotopes in tourmaline are widely applied to constrain fluid sources and evolution, muscovite has seldom been used in this way, and the potential for isotope exchange thermometry with this mineral pair is unexplored. The different boron coordination in muscovite and tourmaline causes a significant, temperature-dependent isotopic fractionation between them, which has been determined experimentally. We used this relationship to study mineralization conditions and fluid evolution at the Panasqueira W-Sn-Cu deposit in Portugal, where the source and evolution of the mineralizing fluids are still debated. The difference in 〈sup〉11〈/sup〉B/〈sup〉10〈/sup〉B ratios of coexisting muscovite and tourmaline, expressed as Δ〈sup〉11〈/sup〉B〈sub〉mica-tourmaline〈/sub〉 = 〈span〉δ〈/span〉〈sup〉11〈/sup〉B〈sub〉mica –〈/sub〉〈span〉δ〈/span〉〈sup〉11〈/sup〉B〈sub〉tourmaline〈/sub〉, yields median temperatures for vein selvages from 400° to 460°C within a total range of 350° to 600°C, which agrees with published Ti-in-quartz temperatures. Mineral pairs from a late fault zone yield a lower median temperature of about 250°C (range 220°–320°C), which fits with published homogenization temperatures of quartz-hosted fluid inclusions from the veins. Taking these temperatures into account, the calculated fluid composition of the early and late muscovite generations is about 〈span〉δ〈/span〉〈sup〉11〈/sup〉B〈sub〉fluid〈/sub〉 = −6 ± 2‰, which indicates that the recurrent fluid pulses had a uniform composition and a magmatic-hydrothermal origin.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Faults are important structures in the formation of many mineral deposits, often acting as conduits for ore-forming fluids and sometimes providing, or generating, the bounding structures to associated mineralizing sites. Using 3-D analysis and modeling of the Lisheen and Silvermines deposits within the Irish ore field, we investigate the geometry of normal fault systems and their implications on the origin and nature of associated deposits. These Irish-type deposits are carbonate hosted and developed within the hanging walls of normal faults arising from an Early Carboniferous episode of north-south rifting, with relatively limited amounts of later deformation. Structural analysis of high-quality mine datasets indicates that fault segmentation is ubiquitous with left-stepping segments arising from north-south stretching developed above generally ENE-NE-trending fault arrays, which are subparallel to older Caledonian penetrative fabrics and structure within underlying Silurian and Ordovician rocks. Fault segments occur on different scales and have a profound impact on structural evolution, with larger scale segments and intervening relay ramps defining distinct orebodies within deposits and smaller scale segments and relays potentially providing paths for upfault fluid flow. The difference in behavior is attributed to the integrity of associated relay ramps where intact ramps represent orebody-bounding structures, and smaller breached ramps provide enhanced associated hydraulic properties and act as vertical conduits. Hanging-wall deformation along the rheological boundary between host-rock limestones and underlying shales has an important control on the localization of earlier dolomitization and/or brecciation and later mineralization adjacent to this contact, and on the migration pathways for basinal brines and mineralizing fluids.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Active sea floor hydrothermal systems are modern analogues of ancient volcanogenic massive sulfide deposits. Hydrothermal sulfide chimneys are one of the important components in these systems and are formed by rapid mixing between seawater and metal-rich hydrothermal fluids venting onto the sea floor. Previous models of chimney growth have been built up mainly based on studies including optical petrographic observations and scanning electron microscopy (SEM). The present study, conducted on a sample from the PACMANUS hydrothermal field (Manus basin, Papua New Guinea), reports for the first time the electron backscattered diffraction (EBSD) observations on a modern sea floor chimney, coupled with synchrotron X-ray fluorescence (SXRF) and scanning electron microscopy-backscattered electron (SEM-BSE) imagery to reveal fine-scale and primary microstructures of sphalerite, allowing the reconstruction of crystal growth history. The results show that sphalerite clusters are formed via the coalescence of multiple smaller sphalerite globules. Electron backscattered diffraction images also highlight that each globule includes an inner zone with fine-grained particles (〈1 〈span〉μ〈/span〉m) and an outer zone with elongate blade-shaped crystals (length up to 40 〈span〉μ〈/span〉m), in some cases showing branching dendritic habit. Both zones are dominated by sphalerite with minor other sulfides, such as chalcopyrite, pyrite, and wurtzite. The individual globules are interpreted as forming under conditions of supersaturation within high-temperature gradients, for example, during the mixing between high-temperature (e.g., 300°C) hydrothermal fluids and ambient cold seawater. The occurrence of these contrasting inner and outer zones reflects fluctuation between two regimes: bursts of rapid nucleation from supersaturated fluids that occurred almost instantaneously during the initial mixing and the skeletal crystal growth of particular crystal faces from limited nucleation sites where diffusion-limited boundary layers developed around the growing crystals. Those growing globules then coalesced into columnar aggregates. These observations could potentially have important implications for identifying fossil chimneys in ancient ore deposits. Moreover, this study emphasizes the benefits of advanced techniques, such as EBSD and SXRF, in order to characterize sulfides in various hydrothermal chimneys to reveal crystal growth and fluid mixing history and gain further insight into chimney growth processes.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Combined whole-rock lead isotope and high-precision trace element data are reported for shales from the Wolverine deposit and surrounding prospects (e.g., Puck, Fisher, Sable) to elucidate the source of Pb in the shales and their potential utility as an exploration vector. Shales exhibit distinct variations in time-integrated U/Th/Pb evolution with proximity to mineralization. Distal samples resemble the local crustal Pb isotope composition of the northern Cordilleran continental crust (i.e., shale curve). More proximal samples exhibit evidence for hydrothermal activity, but were not significantly mineralized. They have very high U/Th and U/Pb and anomalous present-day 〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb (〉20) coincident with elevated Y/Ho, Zr/Hf, and carbonate alteration. These signatures were derived from oxygenated seawater and imprinted on the shales by oxygenated, CO〈sub〉2〈/sub〉-rich hydrothermal fluids that contained uranyl carbonate ions (e.g., UO2(CO3)34−). It is envisioned that this was due to shallow-level (within 50 m of the seafloor), near-vent ingress of oxygenated seawater associated with hydrothermal venting. A third population of samples is most proximal to mineralization. These, too, have trace element signatures (i.e., high Y/Ho, Zr/Hf, CO〈sub〉2〈/sub〉-U enrichment) inherited from seawater, but they do not exhibit anomalous present-day 〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb, being more juvenile in 〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb, 〈sup〉207〈/sup〉Pb/〈sup〉204〈/sup〉Pb, and 〈sup〉208〈/sup〉Pb/〈sup〉204〈/sup〉Pb. This isotope composition coincides with enrichments in Pb, Zn, Cu, Tl, Sb, W, and Sn (±Cu, Mo). The Pb enrichment associated with mineralization effectively buffered these samples from appreciable Pb isotope evolution, despite overprinting by uranyl carbonate. The juvenile Pb associated with the shales proximal to mineralization argues for deep-penetrative circulation of hydrothermal fluids and leaching of Pb from mafic igneous rocks at depth and/or influence from juvenile magmas at depth (i.e., magmatic fluids). Both of these processes were likely critical for the genesis of mineralization at Wolverine.This study illustrates the utility of quadrupole inductively coupled plasma-mass spectrometry (ICP-MS) for providing sufficiently precise and accurate Pb isotope determinations for district-scale metallogenic evaluations. Furthermore, the coupling of solution ICP-MS Pb isotopes with high-precision trace element geochemistry from the same digest provides a powerful tool for regional exploration studies for hydrothermal mineralization in shale basins.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Carbonate-hosted Au deposits (12 million tonnes at 5.0 g/t Au) are located less than 1.5 km away from Cu-Au skarn deposits in the Fengshan area, Middle-Lower Yangtze River metallogenic belt, eastern China. The skarn deposits occur in the contact zones of late Mesozoic granodiorite porphyry intrusions. Bismuth tellurides and coloradoite (HgTe) occur in Cu-Au skarn and peripheral Au-As ores, respectively, whereas tellurides, including coloradoite, and lorandite (TlAsS〈sub〉2〈/sub〉) are present in carbonate-hosted Au ores outside the marble front. New zircon U-Pb ages indicate that the granodiorite porphyry intrusions spatially related to both Cu-Au skarn and carbonate-hosted Au deposits were emplaced between 149.9 ± 1.3 and 145.9 ± 0.7 Ma, which overlaps with molybdenite Re-Os ages of 149.1 ± 2.1 to 145.4 ± 2.9 Ma from the Cu-Au skarn deposits. Sulfur isotope compositions of sulfide minerals from the Cu-Au skarn deposits (〈span〉δ〈/span〉〈sup〉34〈/sup〉S = –2.5 to 6.4〈span〉‰〈/span〉) are similar to those from carbonate-hosted Au deposits (〈span〉δ〈/span〉〈sup〉34〈/sup〉S = –4.8 to 2.6〈span〉‰〈/span〉), suggesting a similar magmatic source of sulfur. Geologic, mineralogical, and sulfur isotope evidence collectively indicate that the carbonate-hosted Au deposits are the distal product of Cu-Au skarn mineralization rather than Carlin-type Au deposits, as has been previously suggested. This study proposes a genetic model to illustrate possible linkages between oxidized Cu-Au skarn deposits (associated with oxidized magmas) and carbonate-hosted Au deposits, which may have important implications for Au deposits in the more distal parts of Cu-Au skarn systems in the Middle-Lower Yangtze River metallogenic belt and elsewhere.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉High-resolution trace element mapping (2 〈span〉µ〈/span〉m beam) was performed by synchrotron radiation X-ray fluorescence microscopy (SXFM) on Cu- and Zn-rich chimneys from Brothers volcano, Kermadec arc. The maps cover 84- to 136-mm〈sup〉2〈/sup〉 cross sections of the inner chimney walls and document the distribution of Fe, Cu, Zn, As, Se, Sr, Pb ± Ga, Au, Bi, and U. Comparative element maps were generated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at a lower resolution (47 〈span〉µ〈/span〉m beam), which additionally measured Co, Ni, Mo, Ag, Cd, In, Sn, Sb, Ba, Au, Tl ± V, Te, and Bi. In two Zn-rich chimneys, Cu distribution varies from Cu-bearing sphalerite with chalcopyrite disease to distinct chalcopyrite-lined channels, implying a progression of chalcopyrite replacement of sphalerite. Conversely, the two Cu-rich chimneys have different styles of massive chalcopyrite lining their conduits. The first displays elongate chalcopyrite grains that radiate into and infill the conduit; these radial grains may be related to decreased fluid flow during sealing of the chimney at both its top and bottom. The second style involves multiple, concentric chalcopyrite laminations (0.25–1 mm) inside the conduit, which progressively narrowed the orifice as they were deposited. Fine (15–40 〈span〉µ〈/span〉m) trace element rings are revealed within and between laminations that exhibit variable contents and distributions of Co, Ni, Zn, As, Se, Mo, Ag, Cd, Sn, Te, Au, Tl, Pb, Bi, and U. The presence of U in the rings specifically indicates seawater ingress into the chimney interior despite it vigorously discharging 274°C fluids at the time of sampling. During these periodic seawater incursions, rapidly changing chemical gradients within the chimney wall induce the instantaneous precipitation of metals from the vent fluid. Thus, the trace element rings are a proxy for the secular evolution of vent fluid compositions. We compared enrichment factors of trace element rings to those of fumarole condensates studied at subaerial arc and rift volcanoes and molten S pooled atop a submarine back-arc volcano. Our enrichment factors show remarkable consistency with the other volcanoes and indicate Au, Te, Bi, Cu, Ag, and Se in Brothers’ chimneys were magmatically derived.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Although magmatic-hydrothermal ore deposits may contain as much as 1 billion tonnes (Gt) of sulfur as hydrothermal sulfides and sulfates, whether the genetically related magmas are particularly sulfur rich remains an open question. In oxidized melts, the presence of magmatic anhydrite would provide some evidence of this, but its preservation potential in the rock record is low, owing to the high solubility of sulfate in low-temperature aqueous fluids. In this study we have examined granitic and porphyry samples from porphyry copper ore deposits using quantitative evaluation of materials by scanning electron microscopy (QEMSCAN) and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) to identify small and sparse inclusions of anhydrite preserved in a range of igneous minerals including apatite, amphibole, plagioclase, quartz, and titanite. In total, anhydrite inclusions were present in 11 different samples from six sulfur-rich magmatic-hydrothermal ore deposits. Electron microprobe analysis (EMPA) of anhydrite inclusions suggests that primary magmatic anhydrite typically has elevated Sr (〉2,500 ppm) and P (〉500 ppm) compared to hydrothermal anhydrite; however, in many cases this is obscured by postentrapment exchange between the inclusion and host mineral or hydrothermal alteration of host minerals. Where the composition of inclusions is inconclusive, we have used cathodoluminescence imagery of zoning patterns in host minerals to help distinguish between igneous and hydrothermal origins. This study provides evidence that anhydrite saturation may be a common feature of arc magmas, particularly those associated with ore deposits, even though little evidence of magmatic anhydrite is preserved. Widespread anhydrite saturation has significant implications for any consideration of magmatic sulfur budgets, particularly in low-temperature, evolved magmas, which have little capacity to carry sulfur species in the melt.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Irish-type mineralization is commonly attributed to fault-controlled mixing of a seawater-derived, sulfur-rich fluid and basement-derived, metal-rich fluid. However, maar-diatreme volcanoes discovered in close spatial and temporal association with Zn-Pb mineralization at Stonepark in the Limerick basin (southwest Ireland) bring a new dimension to established geologic models and may increase the deposit-scale prospectivity in one of the world’s greatest Zn-Pb districts. Stonepark exhibits many incidences of dolomitic black matrix breccias with associated Zn-Pb mineralization, the latter typically occurring within 150 m of the diatremes. Highly negative 〈span〉δ〈/span〉〈sup〉34〈/sup〉S pyrite values within country rock-dominated black matrix breccias (–12 to –34〈span〉‰〈/span〉) are consistent with sulfide precipitation from bacteriogenic sulfur reduction in seawater-derived brines. However, 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values of Zn-Pb sulfides replacing black matrix breccias (–10 to 1〈span〉‰〈/span〉) reflect multiple sulfur sources. Diatreme emplacement both greatly enhanced country rock fracture permeability and produced conduits that are filled with porous volcaniclastic material and extend down to basement rock types. Our 〈span〉δ〈/span〉〈sup〉34〈/sup〉S data suggest that diatremes provide more efficient fluid pathways for basement-derived fluids. The diatremes introduce another potential sulfur source and facilitate a greater input of metal-rich basement-derived hydrothermal fluid into the system compared to other Irish-type deposits such as Navan and Lisheen, evidenced by Stonepark’s more positive modal 〈span〉δ〈/span〉〈sup〉34〈/sup〉S value of –4〈span〉‰〈/span〉. Irish-type deposits are traditionally thought to form in association with extensional basement faults and are considered unrelated to extensive Carboniferous magmatism. Our results indicate that a direct link exists between diatreme volcanism and Zn-Pb mineralization at Limerick, prompting a reevaluation of the traditional Irish-type ore formation model, in regions where mineralization is spatially associated with volcanic pipes.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Textural and compositional data of magnetite from Igarapé Bahia, Alemao, Sossego, Salobo, and Candelaria iron oxide copper-gold (IOCG) and El Romeral Kiruna-type iron oxide-apatite (IOA) deposits show that some magnetite grains display oscillatory zoning or have been reequilibrated by oxy-exsolution, coupled dissolution and reprecipitation (CDR) reactions, and/or recrystallization. Textures formed via CDR are most widespread in the studied samples. The original oscillatory zoning was likely derived from the crystal growth during fluctuating fluid compositions rather than from variation in temperature and oxygen fugacity. The oxy-exsolution of ilmenite in magnetite is attributed to increasing oxygen fugacity and decreasing temperature with alteration and mineralization, resulting in product magnetite with lower Ti and higher V contents. Recrystallization of some magnetite grains is commonly due to high-temperature annealing that retained primary compositions. Two different types of CDR processes are defined according to textures and chemical compositions of different generations of magnetite. The first generation of magnetite (Mag-1) is an inclusion-rich and trace element-rich core, which was replaced by an inclusion-poor and trace element-poor rim (Mag-2). The third generation of magnetite (Mag-3), inclusion poor but trace element rich, occurs as veins replacing Mag-2 along fractures or grain margins. Type 1 CDR process transforming Mag-1 to Mag-2 is more extensive and is similar to processes reported in skarn deposits, whereas type 2 CDR process is local, transforming Mag-2 to Mag-3. During type 1 CDR process, minor and trace elements Si, K, Ca, Mg, Al, and Mn in magnetite are excluded, and Fe contents increase to various extents, in contrast to type 2 CDR process, which is characterized by increased contents of Si, K, Ca, Mg, Al, and Mn. Type 1 CDR process is possibly induced by the changing fluid composition and/or decreasing temperature during progressive alteration and ore formation, whereas type 2 CDR process can be interpreted as post-ore replacement due to a new pulse of magmatic-hydrothermal fluids.The identification of magnetite core (Mag-1) with igneous origin and rim (Mag-2) with magmatic-hydrothermal origin in the Sossego IOCG and El Romeral IOA deposits supports a fluid changing from magmatic to magmatic-hydrothermal during IOCG and IOA formation and indicates a genetic link between these two deposit types. The large data set here further demonstrates that magnetite is susceptible to textural and compositional reequilibration during high-temperature magmatic and magmatic-hydrothermal processes. Reequilibrated magnetite, particularly that formed by CDR processes, has a chemical composition that can be different from that of primary magnetite. Modified magnetite, therefore, cannot be used to discriminate its primary origin or to interpret its provenance in overburden sediments. Therefore, in situ chemical analysis of magnetite combined with textural characterization is necessary to understand the origin of magnetite in IOCG and IOA deposits.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Tinakula is the first seafloor massive sulfide deposit described in the Jean Charcot troughs and is the first such deposit described in the Solomon Islands—on land or the seabed. The deposit is hosted by mafic (basaltic-andesitic) volcaniclastic rocks within a series of cinder cones along a single eruptive fissure. Extensive mapping and sampling by remotely operated vehicle, together with shallow drilling, provide insights into deposit geology and especially hydrothermal processes operating in the shallow subsurface. On the seafloor, mostly inactive chimneys and mounds cover an area of ~77,000 m〈sup〉2〈/sup〉 and are partially buried by volcaniclastic sand. Mineralization is characterized by abundant barite- and sulfide-rich chimneys that formed by low-temperature (〈250°C) venting over ~5,600 years. Barite-rich samples have high SiO〈sub〉2〈/sub〉, Pb, and Hg contents; the sulfide chimneys are dominated by low-Fe sphalerite and are high in Cd, Ge, Sb, and Ag. Few high-temperature chimneys, including zoned chalcopyrite-sphalerite samples and rare massive chalcopyrite, are rich in As, Mo, In, and Au (up to 9.26 ppm), locally as visible gold. Below the seafloor, the mineralization includes buried intervals of sulfide-rich talus with disseminated sulfides in volcaniclastic rocks consisting mainly of lapillistone with minor tuffaceous beds and autobreccias. The volcaniclastic rocks are intensely altered and variably cemented by anhydrite with crosscutting sulfate (± minor sulfide) veins. Fluid inclusions in anhydrite and sphalerite from the footwall (to 19.3 m below seafloor; m b.s.f.) have trapping temperatures of up to 298°C with salinities close to, but slightly higher than, that of seawater (2.8–4.5 wt % NaCl equiv). These temperatures are 10° to 20°C lower than the minimum temperature of boiling at this depth (1,070–1,204 m below sea level; m b.s.l.), suggesting that the highest-temperature fluids boiled below the seafloor. The alteration is distributed in broadly conformable zones, expressed in order of increasing depth and temperature as (1) montmorillonite/nontronite, (2) nontronite + corrensite, (3) illite/smectite + pyrite, (4) illite/smectite + chamosite, and (5) chamosite + corrensite. Zones of argillic alteration are distinguished from chloritic alteration by large positive mass changes in K〈sub〉2〈/sub〉O (enriched in illite/smectite), MgO (enriched in chamosite and corrensite), and Fe〈sub〉2〈/sub〉O〈sub〉3〈/sub〉 (enriched in pyrite associated with illite/smectite alteration). The 〈span〉δ〈/span〉〈sup〉18〈/sup〉O and 〈span〉δ〈/span〉D values of clay minerals confirm increasing temperature with depth, from 124° to 256°C, and interaction with seawater-dominated hydrothermal fluids at high water/rock ratios. Leaching of the volcanic host rocks and thermochemical reduction of seawater sulfate are the primary sources of sulfur, with 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values of sulfides, from –0.8 to 3.4‰, and those of sulfate minerals close to seawater sulfate, from 19.3 to 22.5‰.The mineralization and alteration at Tinakula are typical of a class of ancient massive sulfide deposits hosted mainly by permeable volcaniclastic rocks with broad, semiconformable alteration zones. Processes by which these deposits form have never been documented in modern seafloor massive sulfide systems, because they mostly develop below the seafloor. Our study shows how hydrothermal fluids can become focused within permeable rocks by progressive, low-temperature fluid circulation, leading to a large area (〉150,000 m〈sup〉2〈/sup〉) of alteration with reduced permeability close to the seafloor. In our model, overpressuring and fracturing of the sulfate- and clay-cemented volcaniclastic rocks produced the pathways for higher-temperature fluids to reach the seafloor, present now as sulfate-sulfide veins within the footwall. In the geologic record, the sulfate (anhydrite) is not preserved, leaving a broad zone of intense alteration with disseminated and stringer sulfides typical of this class of deposits.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The trace element composition of igneous and hydrothermal magnetite from 19 well-studied porphyry Cu ± Au ± Mo, Mo, and W-Mo deposits was measured by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and then classified by partial least squares-discriminant analysis (PLS-DA) to constrain the factors explaining the relationships between the chemical composition of magnetite and the magmatic affinity and porphyry deposit subtypes. Igneous magnetite can be discriminated by relatively high P, Ti, V, Mn, Zr, Nb, Hf, and Ta contents but low Mg, Si, Co, Ni, Ge, Sb, W, and Pb contents, in contrast to hydrothermal magnetite. Compositional differences between igneous and hydrothermal magnetite are mainly controlled by the temperature, oxygen fugacity, cocrystallized sulfides, and element solubility/mobility that significantly affect the partition coefficients between magnetite and melt/fluids. Binary diagrams based on Ti, V, and Cr contents are not enough to discriminate igneous and hydrothermal magnetite in porphyry deposits.Relatively high Si and Al contents discriminate porphyry W-Mo hydrothermal magnetite, probably reflecting the control by high-Si, highly differentiated, granitic intrusions for this deposit type. Relatively high Mg, Mn, Zr, Nb, Sn, and Hf but low Ti and V contents discriminate porphyry Au-Cu hydrothermal magnetite, most likely resulting from a combination of mafic to intermediate intrusion composition, high chlorine in fluids, relatively high oxygen fugacity, and low-temperature conditions. Igneous or hydrothermal magnetite from Cu-Mo, Cu-Au, and Cu-Mo-Au deposits cannot be discriminated from each other, probably due to similar intermediate to felsic intrusion composition, melt/fluid composition, and conditions such as temperature and oxygen fugacity for the formation of these deposits.The magmatic affinity of porphyritic intrusions exerts some control on the chemical composition of igneous and hydrothermal magnetite in porphyry systems. Igneous and hydrothermal magnetite related to alkaline magma is relatively rich in Mg, Mn, Co, Mo, Sn, and high field strength elements (HFSEs), perhaps due to high concentrations of chlorine and fluorine in magma and exsolved fluids, whereas those related to calc-alkaline magma are relatively rich in Ca but depleted in HFSEs, consistent with the high Ca but low HFSE magma composition. Igneous and hydrothermal magnetite related to high-K calc-alkaline magma is relatively rich in Al, Ti, Sc, and Ta, due to a higher temperature of formation or enrichment of these elements in melt/fluids.Partial least squares-discriminant analysis on hydrothermal magnetite compositions from porphyry Cu, iron oxide copper-gold (IOCG), Kiruna-type iron oxide-apatite (IOA), and skarn deposits around the world identify important discriminant elements for these deposit types. Magnetite from porphyry Cu deposits is characterized by relatively high Ti, V, Zn, and Al contents, whereas that from IOCG deposits can be discriminated from other types of magnetite by its relatively high V, Ni, Ti, and Al contents. IOA magnetite is discriminated by higher V, Ti, and Mg but lower Al contents, whereas skarn magnetite can be separated from magnetite from other deposit types by higher Mn, Mg, Ca, and Zn contents. Decreased Ti and V contents in hydrothermal magnetite from porphyry Cu and IOA, to IOCG, and to skarn deposits may be related to decreasing temperature and increasing oxygen fugacity. The relative depletion of Al in IOA magnetite is due to its low magnetite-silicate melt partition coefficient, immobility of Al in fluids, and earlier, higher-temperature magmatic or magmatic-hydrothermal formation of IOA deposits. The relative enrichment of Ni in IOCG magnetite reflects more mafic magmatic composition and less competition with sulfide, whereas elevated Mn, Mg, Ca, and Zn in skarn magnetite results from enrichment of these elements in fluids via more intensive fluid-carbonate rock interaction.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Haigou lode gold deposit (〉40 tons [t] at 3.4 g/t), which is located near the eastern boundary of the Central Asian orogenic belt and the North China craton, is one of the largest gold deposits in northeastern China. Native gold is intergrown with molybdenite and pyrite in auriferous quartz veins hosted by a monzogranite-monzonite stock and locally by Proterozoic gneiss, thereby offering an excellent opportunity to directly date the mineralizing event. Uranium-Pb age determinations for zircon yielded ages for the monzogranite and monzonite of 327.1 ± 1.1 and 329.5 ± 1.0 Ma, respectively. Numerous mafic to felsic dikes, which are crosscut by ore veins (pre-ore), parallel to these veins (possibly synore), or crosscut by them (post-ore), were carefully examined and dated. Their zircon 〈sup〉206〈/sup〉Pb/〈sup〉238〈/sup〉U ages are 318.3 ± 1.0, 310.9 ± 1.1, and 134.9 ± 0.4 Ma, respectively, thereby placing the timing of gold mineralization within the relatively large interval of 318.3 ± 1.0 to 134.9 ± 0.4 Ma. The age of mineralization was determined directly using the Re-Os method applied to molybdenite. A total of 19 molybdenite samples separated from auriferous quartz veins yielded widely differing Re-Os model ages of 467 to 155 Ma, and replicate analyses of individual samples also yielded widely differing ages. Significantly, the wide range is attributable entirely to the results obtained for some coarse-grained molybdenite samples and is interpreted to be due to Re and Os isotope decoupling, the considerable spatial Re heterogeneity, the analytical procedure (e.g., use of small sample aliquots), and the post-ore deformation. Nine of the samples, which are all fine grained, yielded a robust weighted mean model age of 310 ± 3 Ma and an isochron age of 309 ± 8 Ma. Thus, the molybdenite Re-Os ages are identical, within uncertainty, to those of the dikes that are parallel to the ore veins, indicating that these dikes were emplaced contemporaneously with the ore and that they and the Haigou gold mineralization are of late Paleozoic age (ca. 310 Ma). Finally, a sericite sample obtained from an auriferous vein returned a 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar plateau age of 165.3 ± 1.2 Ma, which is much younger than the age of the mineralization constrained by Re-Os age determinations of molybdenite. This indicates that the 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar isotope system was reset by post-ore thermal events.Our new geochronological data provide evidence for late Paleozoic gold mineralization in Haigou, which makes it the oldest known lode gold deposit in the easternmost Central Asian orogenic belt, a finding that has important implications for precious metal mineral exploration in the eastern part of the Solonker-Xar Moron-Changchun-Yanji suture zone between the Central Asian orogenic belt and the North China craton. This study also indicates that accurate and reproducible molybdenite Re-Os ages representing the true timing of ore deposition need an integrated combination of careful petrography, proper sampling procedures, sufficiently large analyzed aliquots, multiple analyses of individual samples, and multiple dating methods.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Lone Tree deposit is located in the northern Battle Mountain mining district, Nevada. Prior to mine closure in 2006, Santa Fe Pacific Gold and Newmont produced 4.2 Moz of gold at an average grade of 2.06 g/t at Lone Tree, primarily from the N-S– to NNW-SSE–striking Wayne zone. The ore is located between the Roberts Mountain and Golconda thrusts in siliciclastic rocks of the Ordovician Valmy Formation and in the Pennsylvanian-Permian Battle Mountain and Edna Mountain Formations, and above the Golconda thrust in siliciclastic and carbonate rocks of the Mississippian to Permian Havallah sequence. Ore is also hosted by rhyolitic dikes that were emplaced at 40.95 ± 0.06 Ma based on zircon U-Pb chemical abrasion-thermal ionization mass spectrometry.The gold is associated with sericitic and argillic alteration of the siliciclastic rocks and dikes and with decarbonatization and Fe carbonate alteration of the carbonate-bearing units, as well as in Fe-As sulfide and finegrained quartz alteration of all rock types. Oxidation affects 30 to 45% of the deposit, penetrating into the stratigraphy along numerous steeply dipping north-south, east-west, and north-northeast–south-southwest structures. Gold is positively correlated with Ag, As, Hg, and Sb. The highest Au grades occur in quartz-sulfide ore hosted in siliciclastic and carbonate sedimentary rocks and rhyolitic intrusions. In this ore style, fine-grained quartz and sericite are intergrown with disseminated sulfide minerals (quartz-sericite-pyrite alteration), constituting cores of weakly mineralized pyrite or marcasite, which are surrounded by fuzzy arsenopyrite rims that contain up to ~2,000 ppm Au. Low gold grades occur in late-stage banded pyrite breccias consisting of a finely zoned Au-poor pyrite matrix surrounding jigsaw-fit clasts of quartz-, illite-, barite-, and adularia-altered siliciclastic rock. The timing of main-stage mineralization is bracketed between the emplacement of the dikes and an adularia 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar age of 40.14 ± 0.74 Ma.Sericite intergrown with arsenopyrite-rimmed pyrite in phenocrysts of the rhyolite dikes gave 〈span〉δ〈/span〉〈sup〉18〈/sup〉O values of 1.6 to 9.5‰ and 〈span〉δ〈/span〉D values of –105 to –145‰. For temperatures of 300 ± 100°C, the calculated fluid isotopic compositions are consistent with felsic magmatic water and minor modifications by mixing with meteoric water and exchange with wall rocks. In the silica-sulfide ore, in situ isotopic laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) analyses of pyrite cores yielded 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values ranging from 3.4 to 7.7‰, with average values of 5.6‰ in the felsic dikes, 4.5‰ in the siliciclastic rocks, and 5.3‰ in the carbonate rocks. These values match conventional pyrite 〈span〉δ〈/span〉〈sup〉34〈/sup〉S data reported for Eocene porphyry systems elsewhere in the district. Nanoscale secondary ion mass spectrometry analyses show that gold and associated trace elements occur in submicron-scale zones within arsenopyrite rims on pyrite. The average 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values of the arsenopyrite rims are 5.3 to 6.5‰ heavier than the pyrite cores, indicating cooling and an increasing H〈sub〉2〈/sub〉S/SO〈sub〉2〈/sub〉 ratio. The highest grades resulted from episodic pulses of a gold-rich fluid that was partly derived from, or exchanged with, the sedimentary host rocks. In situ LA-MC-ICP-MS 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values for the late-stage banded pyrite breccia become progressively lighter from veinlet margin to center, reaching a low of –32‰. These veinlets indicate a shift from main-stage quartz-sericite-pyrite and intermediate argillic alteration to more neutral pH and oxidizing conditions during late-stage mineralization, indicating either increasing interaction between the fluid and sedimentary sulfur sources in the host-rock package or bacterial sulfate reduction and supergene sulfide precipitation.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Nanling region of South China hosts the largest W-Sn metallogenic province in the world, accounting for more than 54% of global tungsten resources as well as important resources of tin and rare metals. An important feature of this province, which is shared by a number of other W-Sn metallogenic provinces, is that W deposits occur separately from Sn and Sn-W deposits, with the latter concentrated in the western part of the region (especially along the deep, NE-trending Chenzhou-Linwu fault) and the W deposits to the east of them. All the deposits are associated with ilmenite series, peraluminous granites. However, the granites associated with the Sn and Sn-W deposits can be distinguished from the W granites by their higher bulk-rock 〈span〉ε〈/span〉Nd values and their higher zircon 〈span〉ε〈/span〉Hf values. Most importantly, the Sn and Sn-W granites are characterized by higher zircon saturation temperatures (800 ± 20°C) than the W granites (650°–750°C). The Sn and Sn-W granites also contain abundant mantle-derived mafic microgranular enclaves, whereas such enclaves are rare in the W granites. A model is proposed in which the protolith to the W granites released W to the melt as a result of the breakdown of muscovite. The temperature of melting, however, was too low for biotite to melt. In the west, particularly along the Chenzhou-Linwu fault (the location of the Sn and Sn-W deposits), higher temperatures enabled the breakdown of both muscovite and biotite and the consequent release of both Sn and W to form Sn and Sn-W granites. This model, which is based on differences in the protolith melting temperature and thus mobilization temperatures for Sn and W, is potentially applicable to any Sn-W metallogenic province in which the Sn and Sn-W deposits are spatially separated from the W deposits.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The use of indium in modern technologies has grown in recent decades, creating a growth in indium demand; thus, there is a need to constrain the spatial and temporal distribution of indium-bearing, granite-related deposits. Toward this end, a conceptual model and exploration vectors for the formation of granite-related indium deposits have been developed. The magmatic-hydrothermal system is modeled by consideration of crystal-melt and vapor-melt equilibria. The model calculates the efficiency of removal of indium from a melt into a volatile phase, which may serve as a component of an ore-forming fluid. The results of the model suggest that as the proportion of ferromagnesian minerals increases in the associated granites, the probability of indium ore formation decreases. Further, for a given modal proportion of ferromagnesian minerals, as the modal proportion of amphibole increases, the probability of indium ore formation decreases. Lastly, for a given modal proportion of biotite, as the magnesium content of the biotite increases (as would result from increasing oxidation of the magmatic system), the probability of indium ore formation decreases. Granites with the highest probability of being associated with indium ore formation will typically be part of A- or S-type igneous systems and will likely be highly fractionated (e.g., A-type topaz granites). I-type granites will generally have a lower potential of being associated with indium-bearing deposits. However, some I-type granites may be associated with indium-bearing deposits if the deposits contain granites (sensu stricto) or other related rocks (e.g., alaskites) that lack amphibole or other ferromagnesian phases.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Molybdenite-bearing porphyry deposits are the predominant supplier of molybdenum to industrialized society and one of the main hosts of Mo in the upper continental crust. The Mo isotope compositions (δ〈sup〉98/95〈/sup〉Mo, normalized to NIST3134 equals 0‰) of molybdenite show considerable variation (–1.62 to +2.27‰), but the factors controlling this variability remain poorly constrained. This information is critical for underpinning genetic models of porphyry deposits, understanding elemental cycling, and utilizing the δ〈sup〉98/95〈/sup〉Mo of marine sediments as a paleoredox proxy. Using the well-characterized Qulong porphyry Cu-Mo deposit (Tibet) as an example, here we discuss how rapid cooling, facilitated by mixing hot magmatic fluid with cold meteoric water, can be a controlling factor on efficient mineralization, and then tackle how fluid evolution regulates molybdenum isotope fractionation. Molybdenites, which preferentially partition isotopically light Mo (Rayleigh fractionation), precipitated from a single fluid will develop a heavier δ〈sup〉98/95〈/sup〉Mo composition over time, and this also creates heterogeneous δ〈sup〉98/95〈/sup〉Mo between molybdenite grains. Whereas a fluid undergoing multiple episodes of intensive boiling will gradually lose its isotopically heavy Mo to the vapor phase, molybdenites crystallizing successively from the residual liquid will then have lighter δ〈sup〉98/95〈/sup〉Mo over time. However, when mineralization efficiency becomes too low, a negligible variation in δ〈sup〉98/95〈/sup〉Mo of molybdenite is observed. Given that the mineralization efficiency (i.e., the amount of Mo crystallized as molybdenite from the fluid) rarely reaches 100% and molybdenite favors isotopically light Mo, the presence of a residual fluid with isotopically heavy Mo is inevitable. This residual fluid may then become trapped in alteration halos; hence, δ〈sup〉98/95〈/sup〉Mo has the potential to aid in locating the mineralization center (e.g., lighter δ〈sup〉98/95〈/sup〉Mo toward the orebody). The residual fluid may also feed surface hydrological systems and eventually impact Mo cycling. Our study highlights that understanding the controls of isotope fractionation is critical to bridge the gap between ore formation and elemental cycling, and that other transition metals (e.g., Cu, Fe, and Zn) may follow similar trajectories.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Zangezur-Ordubad mining district of the southernmost Lesser Caucasus is located in the central segment of the Tethyan metallogenic belt and consists of porphyry Cu-Mo and epithermal Au and base metal systems hosted by the composite Cenozoic Meghri-Ordubad pluton. Ore-hosting structures and magmatic intrusions are predominantly confined to a central N-S–oriented corridor 40 km long and 10 to 12 km wide, located between two regional NNW-oriented right-lateral faults, the Khustup-Giratagh and Salvard-Ordubad faults. The anatomy and kinematics of the main fault network are consistent with dextral strike-slip tectonics controlled by the NNW-oriented Khustup-Giratagh and Salvard-Ordubad faults.Dextral strike-slip tectonics was initiated during the Eocene, concomitantly with final subduction of the Neotethys, and controlled the emplacement of the Agarak, Hanqasar, Aygedzor, and Dastakert porphyry Cu-Mo and Tey-Lichkvaz and Terterasar epithermal Au and base metal deposits. The Eocene structures were repeatedly reactivated during subsequent Neogene evolution in transition to a postsubduction geodynamic setting. Ore-bearing structures at the Oligocene world-class Kadjaran porphyry Cu-Mo deposit were also controlled by dextral strike-slip tectonics, as well as porphyry mineralization and its epithermal overprint hosted by an early Miocene intrusion at Lichk.Eocene to early Miocene dextral strike-slip tectonics took place during NE- to NNE-oriented compression related to Paleogene Eurasia-Arabia convergence and subsequent Neogene postcollision evolution. Paleostress reconstruction indicates major reorganization of tectonic plate kinematics since the early Miocene, resulting in N-S– to NW-oriented compression. Early Miocene epithermal overprint at the Kadjaran porphyry deposit and left-lateral reactivation of faults and mineralized structures are linked to this late Neogene tectonic plate reorganization.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Au-rich polymetallic massive sulfide orebodies of the Kassandra mining district belong to the intrusion-related carbonate-hosted replacement deposit class. Marble lenses contained within the Stratoni fault zone host the Madem Lakkos and Mavres Petres deposits at the eastern end of the fault system, where paragenetically early skarn and massive sulfide are spatially associated with late Oligocene aplitic and porphyritic dikes. Skarn transitions into predominant massive and banded replacement sulfide bodies, which are overprinted by a younger assemblage of boulangerite-bearing, quartz-rich sulfide and late quartz-rhodochrosite vein breccias. The latter style of mineralization is most abundant at the Piavitsa prospect at the western end of the exposed fault system. The sulfide orebodies at the Olympias deposit are hosted by marble in association with the Kassandra fault, where textural and mineralogical similarities to the sulfide bodies within the Stratoni fault zone suggest a genetic relationship. Estimated trapping temperatures and pressures based on fluid inclusion data indicate that carbonate replacement mineralization took place at depths less than about 5.9 km.Carbon and oxygen isotope patterns in carbonate from the Stratoni fault zone support isotopic exchange principally through fluid–wall-rock interaction, whereas decarbonation and fluid-rock exchange reactions were important at the Olympias deposit. Carbonate minerals associated with skarn and replacement sulfide throughout the district have isotopic compositions that are consistent with formation from a hydrothermal fluid of magmatic origin. Lower homogenization temperatures and salinities in the younger quartz-rich sulfide assemblage and quartz-rhodochrosite vein breccias, together with low 〈span〉δ〈/span〉〈sup〉18〈/sup〉O values of gangue carbonate, suggest dilution of a primary magmatic fluid with meteoric water late in the evolution of the hydrothermal system in both the Olympias area and the Stratoni fault zone. The replacement sulfide orebodies in the district likely inherited their uniform Pb isotope composition from a late Oligocene igneous source and the isotopically heterogeneous metamorphic basement units.Metal distribution patterns at the scale of the Stratoni fault zone show diminishing Cu concentration with decreasing Pb/Zn and Ag/Au ratios from Madem Lakkos to Mavres Petres and the Piavitsa prospect in the west. The sulfide orebodies at the Olympias deposit exhibit elevated Cu values in the east with increasing Pb/Zn and Ag/Au ratios down-plunge to the south-southwest. Metal concentration and ratios support zoning related to temperature and solubility changes with increasing distance from a probable magmatic source. Structural and igneous relationships, together with fluid inclusion microthermometric and carbon-oxygen isotope data and metal distribution patterns, are supportive of a zoned hydrothermal system that exceeded 12 km along the Stratoni fault zone, sourced by an igneous intrusion to the southeast of the Madem Lakkos deposit. The Olympias replacement sulfide orebodies, associated with the Kassandra fault, resulted from a local hydrothermal system that was likely derived from a concealed igneous intrusion to the east of the deposit.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Major Au and Cu deposits in the Western Tethyan magmatic belt formed during two main periods of Cretaceous and Cenozoic magmatism. The Cretaceous deposits are dominantly Cu-Au porphyry, high-sulfidation epithermal, and volcanic massive sulfide deposits, whereas in the Cenozoic Cu is significant only in porphyry systems. However, the Cenozoic contains approximately three times greater total Au endowment (for Au deposits 〉0.5 million ounces), and also has a greater deposit diversity, including porphyry Au-Cu and Au-only deposits, high-, intermediate-, and low-sulfidation epithermal Au systems, and Au-rich carbonate replacement and sediment-hosted styles. The differences in endowment and deposit styles likely reflect regional-scale tectono-magmatic processes as well as local preservation and emplacement levels. The Cu ± Au endowment of the Cretaceous is consistent with typical subduction-related arc environments and generation of calc-alkaline porphyry to high-sulfidation epithermal systems, whereas Au enrichment related to Cenozoic magmatism appears to be related to high-K calc-alkalic to shoshonitic compositions. In many of the Au-rich Cenozoic magmatic belts, there is geochemical evidence for sourcing subcontinental lithospheric mantle that was previously enriched by Cretaceous subduction-related metasomatism. Additional differences in Au endowment may reflect the preservation of shallow-level systems in the Cenozoic, particularly for the Au-rich Miocene porphyry deposits such as Kışladağ and Bierly Vrch and the Apuseni porphyry Au-Cu deposits. However, in both the Cretaceous and Cenozoic, crustal exposure levels vary across the belt and cannot explain all the differences in Cu and Au endowment.A compilation of exploration discovery methods highlights the importance of historic workings in addition to geochemistry and geology as an initial vector, whereas geophysics has had limited involvement in direct discovery, primarily due to its limited application historically. Geologic models for well-understood systems such as porphyry and proximal epithermal systems provide excellent guides for explorers; however, more distal deposits such as Au-rich carbonate replacement deposits and deposits with poorly constrained models such as sedimentary rock-hosted and intermediate-sulfidation deposits are more challenging for exploration.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The newly defined, 1,500-km-long, late Cenozoic Anatolian metallogenic trend of Turkey is the central segment of the western Tethyan metallogenic belt and formed after the closure of the southern Neotethys Ocean. Mineral deposit discoveries along this trend show that the Oligocene to Miocene igneous units are highly prospective for gold-rich porphyry- and epithermal-style mineralization (~27 Moz) but that copper endowment is poor. However, the temporal and spatial constraints on late Cenozoic gold districts and isolated prospects and their tectonic affinity are poorly known, despite recent efforts. We herein provide new U-Pb and Re-Os age data and field observations from Miocene gold prospects and deposits throughout the Anatolian trend, which we interpret together with previously published age data in the region. We define nine new porphyry and epithermal districts: Simav, İzmir, Uşak, Bodrum, Konya, Aksaray, Kayseri, Tunceli, and Ağri. Gold-rich porphyry and epithermal systems peaked at (1) 25 to 17 Ma in eastern Anatolia, (2) 21 to 9 Ma in western Anatolia, and (3) 10 to 3 Ma in central Anatolia. The westward migration of porphyry and epithermal mineralization from eastern to central Anatolia is interpreted to reflect slab break-off propagation and gap opening after the onset of the Arabian continental collision. Conversely, the southwestward migration of the magmatic front and associated mineralization in western Anatolia resulted from the acceleration of the Aegean slab rollback and subsequent lateral tearing (15–8 Ma). Thus, the bulk of gold mineralization formed in response to the slab segmentation and thermal events at 25 and 15 Ma.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The formation of ore deposits in the Lavrion Pb-Zn-Ag district was associated with Miocene detachment that accommodated orogenic collapse and exhumation of high-grade nappes across the ductile-brittle transition. This district consists of (1) low-grade porphyry Mo style, (2) Cu-Fe skarn, (3) high-temperature carbonate replacement Pb-Zn-Ag, and (4) vein and breccia Pb-Zn-Ag mineralization. The vein and breccia mineralization locally contains high-grade silver in base metal sulfides that are cemented by fluorite and carbonate gangue. The rare earth element contents of these gangue minerals, chondrite-normalized patterns, and fluid inclusion studies suggest that they precipitated from a low-temperature hydrothermal fluid. Primary and pseudosecondary fluid inclusions in fluorite and calcite are characterized by a wide range of homogenization temperatures (92°–207°C) and salinities of up to 17.1 wt % NaCl equiv. Secondary fluid inclusions only represent 〈5 vol % of the total fluid trapped. Fluids extracted from inclusions in fluorite have values of 〈span〉δ〈/span〉D = –82.1 to –47.7‰ (Vienna-standard mean ocean water [V-SMOW]) and 〈span〉δ〈/span〉〈sup〉18〈/sup〉O = –10.4 to –5.1‰ (V-SMOW). These data and low ratios of Cl/Br measured by crush-leach analyses for fluids in fluorite (102–315) and calcite (162–188) are compatible with the ore fluid being the result of mixing of meteoric water with evaporated seawater. These data suggest that fluids leading to the deposition of late Pb-Zn-Ag–rich vein- and breccia-style mineralization in Lavrion were related to circulation of mixed evaporated seawater and meteoric fluids that was enhanced by brittle deformation. This contrasts with the fluids of magmatic origin related to the formation of low-grade porphyry Mo, Cu-Fe skarn, and high-temperature carbonate replacement deposits spatially related to the Plaka granodiorite.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The calc-alkalic Mitchell Au-Cu-Ag-Mo porphyry deposit, hosted in intrusive rocks of the Stikine volcanic arc terrane of northwestern British Columbia, is the largest undeveloped gold resource in Canada, with 40.72 Moz of contained gold. It lies within the KSM trend, a 12-km-long linear porphyry array in the Sulphurets district. It is genetically related to Early Jurassic Sulphurets stocks: phase 1 diorite to monzodiorite hosts Cu-Au mineralization in potassic assemblages (stage 1), a phase 2 granodiorite plug cores a molybdenum halo (190.3 ± 0.8 Ma, 191.3 ± 0.7 Ma; Re-Os, molybdenite) that is accompanied by phyllic alteration (stage 2), and a poorly mineralized phase 3 diorite plug temporally overlaps with quartz-pyrophyllite alteration at shallow levels (stage 3). Two deformation events (D〈sub〉1〈/sub〉 and D〈sub〉2〈/sub〉), attributed to orogen-scale mid-Cretaceous transpression, structurally modified Mitchell. D〈sub〉1〈/sub〉 deformation, expressed as steep E-striking pressure solution cleavage (S〈sub〉1〈/sub〉) and related W-plunging folded veins (F〈sub〉1〈/sub〉), is heterogeneously developed as a function of alteration type. D〈sub〉2〈/sub〉 is divided into two progressive events: D〈sub〉2a〈/sub〉, defined by N-plunging, folded veins (F〈sub〉2a〈/sub〉), and D〈sub〉2b〈/sub〉, defined by E-vergent thrust faults, including the Mitchell thrust fault, which offsets the updip continuation of Mitchell (the Snowfield deposit, 192.0 ± 1.0 Ma, 191.1 ± 0.8 Ma; Re-Os, molybdenite) ~1,600 m to the east-southeast. Host structures for the KSM trend may have been long-lived, N-striking basement lineaments that provided transcrustal magma and fluid pathways. East-trending intrusions, hydrothermal veins, alteration and metal distribution at Mitchell are attributed to subsidiary E-striking cross faults. These original anisotropies in turn influenced the geometry of Cretaceous faults and flattening domains within the deposit.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Regional exploration for rare and precious metal enrichment traditionally uses multielement geochemical analysis of the fines (〈150 〈span〉μ〈/span〉m) fraction of stream sediments; however, these data can be misleading in glaciated regions with complex geology. Here, we compare the spatial distributions of data from sediment fines with distributions of heavy mineral concentrates, including detrital gold, in the same region. Gold grains are characterized according to abundance and morphology plus a microchemical signature from the combination of alloy analysis and systematic identification of mineral inclusions revealed in polished sections. These inclusions survive indefinitely and are indicative of hypogene mineralogy, whereas the metal loadings of fines may be affected by weathering or anthropogenic activity. All three approaches are mutually supportive: sediment fines analyses provide a basis for more labor-intensive targeted gold-grain studies, which in turn highlight specific gold-element associations useful for interpretation of geochemical data sets. Spatial distributions of resistate heavy mineral suites constrain the directions and extents of glacial transport, which facilitates more confident interpretations of placer-lode relationships from gold-grain studies.Characterization of 2,160 gold grains from 40 localities in the auriferous region of southeast Ireland provided a clear indication of proximity of gold to source and identified gold derived from different episodes of mineralization. A distinction is apparent between gold in the south of the region (Wexford), likely derived from widespread stratabound Au-As-Fe-S mineralization, and that in the north (Wicklow), where the historical placer mining district of the Goldmines River yielded gold with inclusions exhibiting a distinctive Pb-Bi-As association. The Goldmines River placers formed by the efficient accumulation and preservation of detrital gold derived from several discrete intravalley sources. We recommend that a combination of classic stream-sediment geochemistry, heavy mineral analysis, and gold-grain studies is used more widely to provide additional insights on the location and nature of gold mineralization and regional metallogeny in regions of poor exposure and complex geology.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The giant Pulang porphyry Cu-Au district (446.8 million tonnes at 0.52% Cu and 0.18 g/tonne Au) is in the southern segment of the Yidun arc (Zhongdian arc), part of the Sanjiang Tethyan orogenic belt in southwest China. The district consists of three deposits: South Pulang (~96% of the total ore reserves) and the smaller East and North Pulang deposits. Four intrusive phases host the three Pulang deposits, which are, in order of emplacement, premineralization fine-grained quartz diorite and coarse-grained quartz diorite, intermineralization quartz monzonite, and late-mineralization diorite porphyry. The complex intruded carbonaceous rocks of the Late Triassic Tumugou slates. Zircon U-Pb laser ablation-inductively coupled plasma-mass spectrometry dating shows that intrusive activity occurred at about 216 ± 2 Ma.Hydrothermal alteration of the intrusions at Pulang includes five main types: K-silicate, epidote-chlorite, chlorite-illite, quartz-illite, and clay alteration. K-silicate alteration, subdivided into early K-feldspar alteration and late biotite alteration (dominant), mainly affected the central quartz monzonite and adjacent coarse-grained quartz diorite and fine-grained quartz diorite. Epidote-chlorite alteration, the most widespread alteration in the district, extends from the deposit core outward and has overprinted K-silicate alteration at South and North Pulang. Late chlorite-illite, quartz-illite, and clay alteration have overprinted preexisting K-silicate and epidotechlorite alteration assemblages and are locally developed in all four intrusive phases. Copper and gold are positively correlated and are mainly (90%) associated with epidote-chlorite alteration and, to a lesser degree, with K-silicate and chlorite-illite alteration. Hypogene pyrrhotite is intergrown with chalcopyrite and mainly occurs in chlorite-illite– and quartz-illite–altered, coarse-grained quartz diorite at East Pulang. Molybdenite Re-Os dating shows that mineralization in the district occurred at 216.54 ± 0.87 to 216.13 ± 0.86 Ma.The sequence of intrusion emplacement, alteration and veining, and sulfide associations at the three deposits suggests that South and North Pulang are two separate porphyry Cu-Au deposits, whereas East Pulang is probably a distal part of South Pulang. The dominance of primary magnetite over ilmenite and the assemblage titanite + magnetite + quartz in the causative quartz monzonite, and the abundant hydrothermal anhydrite veins associated with early K-silicate and main-mineralization epidote-chlorite alteration indicate the oxidized nature of the felsic intrusion and resultant early hydrothermal fluids. The pyrrhotite related to late chlorite-illite and quartz-illite alteration suggests local reduction due to interaction with the carbonaceous Tumugou slates. The atypical association of epidote-chlorite alteration and Cu mineralization at Pulang either is due to fluids from another porphyry deposit nearby overprinting epidote-chlorite alteration onto preexisting copper mineralization and K-silicate alteration at Pulang or is the result of collapse of epidote-chlorite–stable fluids into the K-silicate-altered core during waning hydrothermal activity.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Carbonate-hosted hydrothermal deposits typically show narrow visible mineralogical and textural alteration halos, which inhibit exploration targeting. In contrast, hydrothermal modification of the country rock’s stable isotope composition usually extends far beyond the limited visible alteration. Hence, stable isotope studies should be an effective tool to aid exploration for carbonate-hosted deposits. Here we present new insight into the development of a large stable isotope alteration halo based on 910 O and C isotope analyses of carbonate veins and hydrothermally altered limestone hosting the Cinco de Mayo Pb-Zn-Ag (Au, Cu) carbonate replacement deposit (CRD), in Chihuahua, Mexico. Our results demonstrate that stable isotope alteration is consistent with reactive, magmatic fluid flow into unaltered limestone and represents a powerful tool for the characterization of these hydrothermal ore systems. Synmineralization veins are texturally and isotopically distinct from those formed during pre- and postmineralization diagenesis and fluid flow and show distinct gradients along the direction of mineralizing fluid flow: this appears to be a promising exploration vectoring tool. Downhole variations in wall-rock isotope values reveal aquifers and aquicludes and outline the principal hydrothermal flow paths. Furthermore, wall-rock 〈span〉δ〈/span〉〈sup〉18〈/sup〉O〈sub〉VSMOW〈/sub〉 systematically decreases toward mineralization from ~23‰ to 〈17‰ over a distance of ~10 km, providing another vectoring tool. The extent of the stable isotope alteration halo likely reflects the overall fluid volume and areal extent of a fossil hydrothermal system, which may be expected to scale with the mineral endowment. This suggests that constraining the size, shape, and degree of isotopic alteration has direct application to mineral exploration by outlining the system and indicating the potential size of a deposit.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Stollberg ore field occurs in the Bergslagen region of south-central Sweden, a polydeformed ca. 1.9 Ga igneous province dominated by bimodal felsic and mafic rocks. Sulfide mineralization is hosted by metavolcanic rocks, marble, and skarn and consists of massive to semimassive polymetallic sulfides and iron oxide in a semiregional F〈sub〉2〈/sub〉 syncline termed the Stollberg syncline. The dominant country rocks are rhyolitic pumice breccia and rhyolitic ash-siltstone with minor mafic sills metamorphosed to the amphibolite facies. On the eastern limb of the Stollberg syncline, sulfide mineralization occurs as stratabound premetamorphic replacement of volcaniclastic rocks and limestone that grades into iron formation. The development of skarn assemblages is the result of low-temperature replacement of limestone and volcaniclastic rocks rather than formation by high-temperature metasomatism or synmetamorphic or late hydrothermal replacement of marble. Metamorphosed, hydrothermally altered rocks on the eastern limb are dominated by the assemblages garnet-biotite and gedrite-albite. Silica-altered rocks are generally subordinate in the Stollberg ore field; however, sulfides at Gränsgruvan, on the western limb of the syncline, are located in a silicified zone along with metamorphosed, altered rocks dominated by sericite and the assemblage quartz-garnet-pyroxene. Although the Tvistbo and Norrgruvan prospects along the northern end of the syncline are small, they show geologic characteristics that are transitional to deposits found on the western and eastern limbs of the syncline. Ore at Tvistbo is hosted by skarn and is spatially associated with quartz-garnet-pyroxene rocks, whereas sulfides at Norrgruvan are hosted by quartz-fluorite rocks that are similar to those hosting the Brusgruvan deposit on the eastern limb of the syncline.Whole-rock analyses of variably altered host rocks in the Stollberg ore field suggest that most components were sourced from felsic volcaniclastic rocks and that Zr, Ti, Al, Hf, Nb, Sc, Th, Ga, U, and rare-earth elements (REEs) were immobile during alteration. These rocks are enriched in light REEs, depleted in heavy REEs, and have negative Eu anomalies, whereas sulfide-bearing rocks (Fe- and base metal-rich) and altered rocks in the ore zone show the same REE pattern but with positive Eu anomalies. Indicators of proximity to sulfides in altered rocks in the Stollberg ore field include positive Eu anomalies, an increase in the concentration of Pb, Sb, As, Tl, Ba, Ba/Sr, and K〈sub〉2〈/sub〉O, as well as an increase in a modified version of the Ishikawa alteration index, which accounts for the presence of primary Ca in an original limestone component. Garnet and pyroxene enriched in either Ca or Mn are also considered to be pathfinders to ore. Cooling of an acidic, reduced hydrothermal fluid that carried sulfur and metals, which became neutralized as it reacted with limestone, is likely responsible for the formation of sulfides in the Stollberg ore field. The nature of the host rock types, the style of the alteration spatially associated with sulfide mineralization, and the spatial association with iron formation bear some resemblance to volcanogenic massive sulfide and Broken Hill-type deposits. However, the stratabound replacement of limestone by sulfides distinguishes it from these deposit types and is a so-called SVALS-type ore system, which is a class of stratabound, volcanic-hosted, limestone-skarn deposits restricted to the Bergslagen district.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉With longshore processes enabling sediment littoral migration along coastal shelves for tens to hundreds of kilometers, beach placers form important locations of heavy mineral accumulation and mining. We report a method for rapid composition and morphology characterization of placer sediment mineralogy using scanning electron microscope element mapping and image analysis, and we apply this technique to investigate garnet from 13 strandline samples from beaches along 427 km of the Westland coast of New Zealand’s South Island with the purpose of establishing patterns of mineral composition, morphology, and distribution in modern and associated raised beaches. Mineral modes show garnet to be least abundant on the South Westland beaches, intermediate in abundance along the Paparoa coast, and most abundant in Central Westland and North Westland. The garnets are typically almandine rich and are mostly derived from the Alpine Schist, even when this unit is distal such as along the Paparoa coastline. Garnet average grain size, which is an important parameter for a garnet resource, is coarsest in Central and North Westland (〉200 〈span〉μ〈/span〉m) and finest along the Paparoa coast. Once it is abraded to ≤40 〈span〉μ〈/span〉m, it is largely removed from the sediment load. Inclusions in garnet, which reduce the viability of a resource, decrease northward. As extensive historical prospecting has been undertaken on ilmenite on narrow but elongate stranded beach sand terraces formed during higher sea levels in Westland, the calculated garnet-ilmenite area ratio for the beach placer samples coupled with the historical ilmenite bulk values indicate that several raised beach terraces likely have 〉5 million tonnes (Mt) of detrital garnet. Our study therefore shows that (1) garnet and other heavy minerals (including ilmenite and gold) are moved northward along the Westland coastline except where submarine canyons intercept the transport direction, (2) garnet composition can be used to fingerprint the heavy mineral sources, and there are systematic morphology variations with distance from source, (3) many beach placers and stranded terraces that formed during higher sea levels in Westland are extremely rich in garnet, and (4) the Central Westland beaches contain garnet of suitable composition, morphology, and lack of inclusions to be worthy of further mineral exploration. The scanning electron microscope and image analysis techniques employed could be readily applied to other sedimentary mineral deposits.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Zudong heavy rare earth element (HREE) deposit in South China is the largest regolith-hosted HREE deposit in the world, with a resource of ~17,600 tonnes (t) of rare earth oxides (REOs) at an average grade of ~0.1 wt % REOs. Despite more than 40 years of exploration and exploitation, the genesis of this deposit is poorly understood. Subtropical weathering of the parent A-type granite formed orebodies hosted mainly within the lower B to upper C horizons of the resulting soil, which is developed on the hillsides of a moderately incised landscape (relief ~150 m). The thickness of the orebodies varies from a few meters to up to 10 m. REE concentrations increase from ~300 ppm in the A horizon to ~1,500 ppm in the lower B to upper C horizons and decrease with further depth in the profile. The entire soil profile is enriched in HREEs, with (La/Yb)〈sub〉N〈/sub〉 ratios 〈1, but the REE-rich lower B horizon is less enriched in HREEs, with (La/Yb)〈sub〉N〈/sub〉 values up to 0.9, than the underlying upper C horizon. Exchangeable REEs, representing REEs that are adsorbed in the deposit, constitute ~65% of the bulk REE content; the light REEs (LREEs) are preferentially adsorbed. The main minerals adsorbing the REEs are kaolinite and halloysite; the proportions of REEs adsorbed by Fe-Mn oxyhydroxides and organic matter are negligible. The remaining REE content is hosted by supergene chernovite (YAsO〈sub〉4〈/sub〉) and residual aeschynite/euxenite-group and fergusonite-group minerals, xenotime-(Y), zircon, and thorite.The Zudong deposit formed from weathering of a HREE-rich A-type granite, containing between ~200 and ~450 ppm REEs dominantly as synchysite-(Y), yttrian fluorite, gadolinite-(Y), hingganite-(Y), and yttrialite-(Y), which are susceptible to chemical weathering. Decomposition of these minerals released REEs to the weathering fluid, and the interaction between alkali feldspar, muscovite, and the weathering fluid produced halloysite and subordinate illite and smectite. With progressive weathering, these minerals accumulated together with quartz, refractory zircon, xenotime-(Y), and aeschynite/euxenite-group and fergusonite-group minerals. In the upper parts of the soil profile, where conditions were acidic, dissolution of REE minerals is promoted, and the REEs tend to remain in solution. Deeper in the profile (lower part of the B horizon and upper part of the C horizon), conditions become less acidic, leading to adsorption of the REEs and As (released by weathering from an adjacent coal unit) on the surface of halloysite. With time, halloysite, a metastable phase in respect to kaolinite, gradually transforms to the latter, releasing As and variable proportions of the adsorbed REEs. The released REEs and As are transported downward, partially readsorbed by clay minerals, and the remaining REEs combined with As to precipitate supergene chernovite-(Y). Ongoing operation of this eluviation-illuviation process at Zudong has gradually enriched the soils in HREEs to form a resource that now supplies much of the world’s HREEs.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We describe a unique occurrence of a world-class Proterozoic ore deposit, Century, affected by an Ordovician meteorite impact, the Lawn Hill impact structure. The meteorite excavated a complex crater with a Proterozoic core surrounded by an annulus filled with Cambrian carbonate rocks. The Century deposit is located at the southwestern edge of the crater and is bounded by postore faults that indicate an originally larger orebody. It is overlain by breccias, which contain evidence of impact-related textures that we interpret as fallback suevite. (Suevite is a rock consisting partly of melted material, typically forming breccia containing glass and crystal or lithic fragments, formed during an impact event.) Above the suevite are slumped Cambrian carbonate and Proterozoic shale, including a million-tonne block of Century ore detached from the main orebody and contained in the carbonate breccias. The Cambrian rocks were partially consolidated prior to impact, became fluidized on impact, and then resurged into the crater from surroundings areas. The resurge process resulted in a fivefold thickening of carbonates in the annulus, preserved to 600-m depth. Beneath the suevite, in the more competent orebody, fracturing and slab formation were the dominant responses to the impact. A restoration of the displacements on the impact-related faults suggests that the orebody was moved toward the crater. The inference is that, prior to impact, the deposit continued for several hundred meters beyond the current faulted northern boundary of the orebody and that this missing segment was displaced and may be buried within the annulus.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Late Ordovician-early Silurian (~455–435 Ma) Northparkes system is a group of silica-saturated, alkalic porphyry deposits and prospects that developed within the Macquarie island arc. The system is host to a spectacular and diverse range of rocks and alteration-mineralization textures that facilitate a detailed understanding of its evolution, in particular the nature and controls of porphyry-related propylitic alteration.The first intrusive phase at Northparkes is a pre- to early-mineralization pluton that underlies all the deposits and varies in composition from a biotite quartz monzonite to alkali feldspar granite. Prior to total crystallization, this pluton was intruded by a more primitive quartz monzonite that marks the onset of a fertile fractionation series. Toward its upper levels, the quartz monzonite is porphyritic and locally rich in Cu sulfides. Subsequently, a complex series of synmineralization quartz monzonite porphyries was emplaced. The quartz monzonite porphyry intrusions have a distinct pipe-like morphology and are ubiquitously K-feldspar–altered with a crystal-crowded porphyritic texture. The textures of the quartz monzonite porphyries and common occurrence of porphyry-cemented contact breccias indicate they were forcibly emplaced and of relatively low viscosity. The quartz monzonite porphyries are therefore interpreted as crystal-bearing, silicate melt-aqueous fluid slurries that represent the conduits through which deep-seated magmatic-derived ore fluid was discharged into the shallow crust (1–2 km depth).Each deposit is centered on a multiphase cluster of quartz monzonite porphyry intrusions that drove discrete hydrothermal systems. Initial fluid evolution was similar in all the deposits, with three major alteration facies developed as largely concentric zones around the quartz monzonite porphyry complexes. The innermost zone is host to Cu sulfide ore and dominated by K-feldspar alteration. This transitions outward through a shell of magnetite ± biotite alteration, with pyrite and minor chalcopyrite, to an outer halo of propylitic alteration. Generally, epidote, chlorite, and pyrite are abundant in the most deposit-proximal propylitic zone, with a decrease in the abundance of pyrite, and then epidote, with increasing distance away from deposit centers.Propylitic alteration, particularly within relatively low permeability rocks, is fracture-controlled and a hierarchy of veins is observed. Veins of chlorite-quartz-pyrite ± calcite ± hematite ± epidote ± chalcopyrite (P1) appear to represent the principal fluid conduits. They are surrounded by pervasive and intense alteration halos with a distinct mineralogical zonation from vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite, through hematite-sericite-chlorite ± epidote, ultimately to a vein-distal hematite-albite ± chlorite ± epidote assemblage. These P1 veins are surrounded by regions in which smaller epidote-chlorite ± calcite ± quartz ± pyrite veins (P2) are abundant, again with zoned alteration envelopes: vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite grades out into an epidote-rich zone, which in turn transitions into vein-distal albite-hematite ± chlorite ± epidote. Areas of weakest propylitic alteration, distant from both P1 and P2 veins, are characterized by small epidote-only veinlets (P3) with albite-hematite halos. Mineralogical transitions across the propylitic zone are therefore repeated in the evolution from P1 to P3 veins, as well as in the halos around these veins. It is the overall vein abundance and overlap of associated alteration halos that controls the intensity and appearance of propylitic alteration in most rocks. Such scale invariance and spatial relationships strongly suggest the transition from P1 to P3 veins reflects a broadly decreasing outward flux of (magmatic-derived?) fluid that passed through the fracture network and progressively reacted with country rocks. Further support for this hypothesis comes from crosscutting relationships and Rb-Sr dating of epidote (returning an age of 450 ± 11 Ma), which demonstrate the bulk of propylitic alteration was coeval with mineralization and potassic alteration.Late-stage fluid evolution at each deposit was unique. Much of the E48 orebody, and locally the GRP314 deposit, was overprinted by texturally destructive, white sericite-albite-quartz-alunite ± chlorite alteration. In the E26 deposit and in regions of the GRP314 deposit a series of quartz-anhydrite ± pyrite ± Cu sulfide veins with distinctive, vein-proximal, sericite-dominant alteration halos cuts the primary, deposit-concentric alteration facies. The vein-distal mineralogy of these alteration halos is controlled by their distance from deposit centers, changing from K-feldspar ± biotite in deposit-proximal veins to chlorite ± epidote-albite in depositdistal veins. Late-mineralization quartz monzonite porphyries at E26 and GRP314 also appear to be related to the generation of anhydrite-quartz ± sphalerite veins and a set of quartz-calcite-pyrite-sphalerite ± chalcopyrite ± galena veins. Postmineralization magmatic activity produced relatively primitive and barren monzonite porphyries and younger alkali basalt dikes.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Josemaría porphyry copper-gold deposit is located in the Frontal Cordillera of San Juan Province, Argentina, near the present-day northern limit of the Chilean-Pampean flat-slab segment of the central Andes, and midway between the Maricunga and El Indio metallogenic belts. The deposit is centered on small, multiphase dacite porphyry intrusions that were emplaced at the contact between rhyolitic volcanic and tonalitic plutonic rocks of Late Permian to Triassic age. The earlier, more intensely quartz ± magnetite-veined porphyry phases and contiguous wall rocks display a telescoped sequence of alteration-mineralization zones, from shallow advanced argillic (mainly quartz-pyrophyllite) and underlying sericitic to deeper chlorite-sericite and minor remnant potassic. All the alteration types are mineralized, but the highest copper and gold grades are present as a low-arsenic, high-sulfidation assemblage in the quartz-pyrophyllite and sericitic zones. The outermost parts of the copper-gold zone are overlapped by a pronounced molybdenum-bearing annulus.New U-Pb zircon ages show that the deposit was formed at ~25 to 24.5 Ma, partially unroofed during continued NNE-striking, high-angle reverse faulting, and then unconformably overlain by red-bed conglomerate and sandstone capped by andesitic and dacitic tuff and lava. The andesite reported an age of ~22.35 Ma. A second, discrete pulse of currently undated, advanced argillic alteration, accompanied by minor high-sulfidation enargite mineralization, locally affected the southern periphery of the deposit, including its postmineral cover. Following erosional removal of the volcano-sedimentary strata from the northern and central parts of the deposit, the NNE-trending fault zone underwent minor normal displacement and localized economically significant supergene chalcocite enrichment. However, probably because of the rapidity of deposit unroofing, supergene processes were barely able to keep pace with erosion, resulting in a thin supergene profile over much of the exposed deposit. The southern part of the deposit remains beneath the postmineral cover and, hence, escaped the enrichment.Josemaría is unusual among the many central Andean porphyry copper deposits formed during rapid uplift because it preserves evidence for not only alteration-mineralization telescoping but also exceptionally rapid postmineral exhumation and subsequent burial beneath thick volcano-sedimentary cover. Unroofing of porphyry copper deposits in 1 to 2 m.y. is more typical of the high erosion rates that characterize pluvial tropical climates than the semiarid conditions that prevailed during and since the formation of Josemaría.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Small-scale variations in mineral chemistry, textures, and platinum group element (PGE) mineralization were investigated in the Lower and Middle Group chromitite layers LG6, LG6a, MG1, MG2, and MG2 II from vertical drill core profiles at the Thaba mine in the northwestern limb of the Bushveld Complex. We present detailed geochemical profiles of chromite composition and chromite crystal size distribution curves to shed light on the processes of chromite accumulation and textural modification as well as mineralization. Multiple samples within each layer were assayed for PGE concentrations, and the respective platinum group mineral association was determined by mineral liberation analysis (MLA).There is strong evidence for postcumulus changes in the chromitites. The crystal size distribution curves suggest that the primary chromite texture was coarsened by a combination of adcumulus growth and textural equilibration, while compaction of the crystal mush played only a minor role. Mineral compositions were also modified by postcumulus processes, but because of the very high modal amount of chromite and its local preservation in orthopyroxene oikocrysts, that phase retained much primary information. Vertical variations of chromite composition within chromitite layers and from one layer to another do not support the idea of chromite accumulation from crystal-rich slurries or crystal settling from a large magma chamber. Instead, we favor a successive buildup of chromitite layers by repeated injections of relatively thin layers of chromite-saturated magmas, with in situ crystallization occurring at the crystal mush-magma interface. The adcumulus growth of chromite grains to form massive chromitite required addition of Cr to the layers, which we attribute to downward percolation from the overlying magma.The PGE concentrations are elevated in all chromitite layers compared to adjacent silicate rocks and show a systematic increase upward from LG6 (avg 807 ppb Ir + Ru + Rh + Pt + Pd + Au) to MG2 II (avg 2,062 ppb). There are also significant internal variations in all layers, with enrichments at hanging and/or footwalls. The enriched nature of chromitites in PGEs compared to host pyroxenites is a general feature, independent of the layer thickness. The MLA results distinguish two principal groups of PGE mineral associations: the LG6, LG6, and MG1 are dominated by the malanite series, laurite, and PGE sulfarsenides, while the MG2 and MG2 II layers are characterized by laurite and PGE sulfides as well as Pt-Fe-Sn and PGE-Sb-Bi-Pb alloys. Differences in the PGE associations are attributed to postcumulus alteration of the MG2 and MG2 II layer, while the chromitites below, particularly LG6 and LG6a, contain a more pristine association.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉A fundamental question in the study of magmatic-hydrothermal ore deposits is whether the mineralization potential of intrusions was already predetermined by the metal content of the exsolving fluids. The present study aims at addressing this question by reviewing the large number of microanalytical data (mostly laser-ablation ICP-MS data) obtained on fluid inclusions from this type of ore deposits over the last 20 years. Published data sets were screened for analyses of high-temperature fluid inclusions that are representative of premineralization fluids. A set of criteria was developed to distinguish such fluids from later, lower temperature fluids. In order to compensate differences in absolute metal concentrations caused by fluid immiscibility, all element concentrations were normalized to Na. A numerical model was developed to explore at which stage different metals are most efficiently extracted from a cooling pluton. The results suggest that the timing of most efficient metal extraction varies from metal to metal and strongly depends on pressure, the fluid/melt partition coefficient and the bulk mineral-melt partition coefficient. As a consequence, fluid compositions were chosen over the entire range of Cs/Na ratios recorded from a given pluton, as this ratio gives an indication of the fractionation degree of the silicate melts from which a fluid exsolved. In order to avoid bias toward occurrences from which a large amount of data are available, maximum four intermediate-density (ID)-type fluid inclusion assemblages plus four brines assemblages were chosen from each occurrence.Using the above-mentioned criteria, 169 fluid compositions from 12 Cu (Mo, Au) mineralized intrusions, 10 Sn/W mineralized intrusions, two Mo mineralized intrusions, and one U-Th-REE mineralized intrusion were finally chosen and plotted in graphs of X/Na versus Cs/Na. The results reveal that Sn- and Cu-mineralizing fluids contained more Sn and Cu, respectively, than the fluids analyzed from barren and Mo or U-Th REE mineralized intrusions. Positive correlations between fluid metal content and mineralization potential may exist also for W and REEs, whereas for Mo no such trend is evident. Therefore, at least for certain metals, the metal content of high-temperature fluid inclusions can be used as an indicator of the type and extent of mineralization. However, elevated metal concentrations are present also in some fluids from barren intrusions, which implies that the mineralization potential additionally depends on other factors such as the size of the intrusion and the development of structures that promote focused fluid flow.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Although less common than hematite ores, high-grade magnetite ores represent a distinct type of iron mineralization hosted by banded iron formations (BIFs). The Gongchangling iron deposit hosted in ~2.55 Ga BIFs in the North China craton represents one of the most economically important iron deposits in China. Located in mining area II, it is a high-grade (〉50 wt % Fe) magnetite deposit and one of the largest of its type in the world. However, the lack of reliable age constraints on iron mineralization has hindered the testing of competing genetic models for the formation of the Gongchangling deposit. In situ U-Pb geochronology of monazite and zircon intergrown with garnet from the proximal alteration zone of a high-grade iron orebody yielded an age of 1.86 Ga, which represents the timing of formation of high-grade magnetite mineralization. This age is coeval with a tectonic extension event recorded in the northeastern North China craton. Our results preclude the previously suggested genetic link between high-grade magnetite mineralization and ~2.50 Ga regional metamorphism. Growth of authigenic monazite and zircon is likely related to the breakdown of detrital zircon, which has undergone metamictization. In combination with previously published data, we propose that the development of zoned alteration associated with the deposit, which is characterized by the garnet-amphibole-magnetite assemblage in the proximal zone changing to a chlorite-quartz–dominated assemblage in the distal zone, can be attributed to a gradual decrease in temperature from 〉550° to ~250°C and to alteration minerals forming from leaching of the BIFs instead of by replacement of the wall rocks. Magnetite mineralization was controlled by the well-developed faults that cut the BIFs and provided conduits for silica-undersaturated alkaline meteoric fluids. Fluid flow likely took place in an extensional tectonic regime, similar to that invoked elsewhere for hematite mineralization but at greater depths.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Economically significant and geologically complex veined Cu-Co-Au mineralization was recently discovered at Carlow Castle in the Pilbara region of northwestern Western Australia. The inferred resource estimate for Carlow Castle as of March 2019 is 7.7 million tonnes (Mt) at 1.06 g/t Au, 0.51% Cu, and 0.08% Co, making it one of Australia’s most significant known Cu-Co-Au deposits. Here we provide the first account and scientific analysis of Carlow Castle. This analysis suggests that it is a hydrothermal Cu-Co-Au deposit, with mineralization hosted in sulfide-rich quartz-carbonate veins. The ore is hosted in veins that occur within a pervasively chloritized shear zone through the regionally significant Regal thrust. At Carlow Castle the shear zone associated with this thrust occurs within the Ruth Well Formation, an Archean mafic volcano-sedimentary sequence. Within the mineralized veins the dominant ore minerals are pyrite (FeS〈sub〉2〈/sub〉), chalcopyrite (CuFeS〈sub〉2〈/sub〉), chalcocite (Cu〈sub〉2〈/sub〉S), cobaltite (CoAsS), and electrum (Au,Ag). The genesis of the Carlow Castle deposit is still under investigation; however, the origin of the Cu-Co-Au mineralization is most likely related to the migration of metalliferous fluids along the Regal thrust. Based on Carlow Castle’s stratigraphic position within the Pilbara craton and the craton’s relative stability since the Archean, an Archean age of mineralization is most likely. The distinct Cu-Co-Au enrichment at Carlow Castle makes it unique among Archean ore deposits generally, as the majority of Cu-Co deposits are of maximum Proterozoic age. Therefore, understanding the genesis of the Carlow Castle deposit has important implications for understanding the unique processes through which Cu-Co-Au mineralization outside of basin-hosted ore deposits may be formed, particularly in Archean terranes.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Carlin-type gold deposits are one of the most important gold mineralization styles in the world. Despite their economic importance and the large volume of work that has been published, there remain crucial questions regarding their metallogenesis. Much of this uncertainty is due to the cryptic nature of the gold occurrence, with gold occurring as dispersed nanoscale inclusions within host pyrite rims that formed on earlier formed barren pyrite cores. The small size of the gold inclusions has made determining their nature within the host sulfides and the mechanisms by which they precipitated from the ore fluids particularly problematic.This study combines high-resolution electron probe microanalysis (EPMA) with atom probe tomography (APT) to constrain whether the gold occurs as nanospheres or is dispersed within the Carlin pyrites. APT offers the unique capability of obtaining major, minor, trace, and isotopic chemical information at near-atomic spatial resolution. We use this capability to investigate the atomic-scale distribution of trace elements within Carlin-type pyrite rims, as well as the relative differences of sulfur isotopes within the rim and core of gold-hosting pyrite.We show that gold within a sample from the Turquoise Ridge deposit (Nevada) occurs within arsenian pyrite overgrowth (rims) that formed on a pyrite core. Furthermore, this As-rich rim does not contain nanonuggets of gold and instead contains dispersed lattice-bound Au within the pyrite crystal structure. The spatial correlation of gold and arsenic within our samples is consistent with increased local arsenic concentrations that enhanced the ability of arsenian pyrite to host dispersed gold (〈a href="https://pubs.geoscienceworld.org/economicgeology#r14"〉Kusebauch et al., 2019〈/a〉). We hypothesize that point defects in the lattice induced by the addition of arsenic to the pyrite structure facilitate the dissemination of gold. The lack of gold nanospheres in our study is consistent with previous work showing that dispersed gold in arsenian pyrite can occur in concentrations up to ~1:200 (gold/arsenic). We also report a method for determining the sulfur isotope ratios from atom probe data sets of pyrite (±As) that illustrates a relative change between the pyrite core and its Au and arsenian pyrite rim. This spatial variation confirms that the observed pyrite core-rim structure is due to two-stage growth involving a sedimentary or magmatic-hydrothermal core and hydrothermal rim, as opposed to precipitation from an evolving hydrothermal fluid.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Porphyry copper deposits consist of low-grade stockwork and disseminated sulfide zones that contain characteristic vein generations formed during the evolution of the hydrothermal systems. The present contribution examines the influence of variable CO〈sub〉2〈/sub〉 concentrations on the solubility of quartz in single-phase hydrothermal fluids forming stockwork veins in porphyry deposits at temperatures of 150° to 550°C and pressures ranging from 100 to 2,000 bar at concentrations up to 8 mol % CO〈sub〉2〈/sub〉. The calculations demonstrate that quartz solubility in hydrothermal fluids decreases with increasing CO〈sub〉2〈/sub〉 content. Retrograde quartz solubility is less pronounced in CO〈sub〉2〈/sub〉-bearing fluids and is not observed in single-phase fluids having CO〈sub〉2〈/sub〉 concentrations exceeding 6 mol %.Despite the effects of CO〈sub〉2〈/sub〉, retrograde quartz solubility plays an important role in the formation of porphyry stockwork veins that contain little or no quartz as a gangue mineral. At high temperatures and lithostatic pressure conditions below 900 bar, early biotite veins can form as a result of quasi-isobaric cooling of single-phase hydrothermal fluids under conditions of retrograde quartz solubility or near-constant quartz solubility. Stock-work veins consisting of molybdenite or hypogene copper sulfide minerals lacking quartz could form at temperatures of up to 450°C under hydrostatic pressures ranging from ~250 to 900 bar. In the presence of CO〈sub〉2〈/sub〉, retrograde quartz solubility is shifted toward slightly lower temperatures at constant pressure. At temperatures below ≾375°C, quartz is precipitated during quasi-isobaric cooling irrespective of CO〈sub〉2〈/sub〉 content of the hydrothermal fluids, resulting in the formation of late porphyry quartz veins.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉ArcGIS was used to spatially assess and rank potential porphyry copper deposits using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data together with geochemical and geologic datasets in order to estimate undiscovered deposits in the southern Basin and Range Province in the southwestern United States. The assessment was done using a traditional expert opinion three-part method and a prospectivity model developed using weights of evidence and logistic regression techniques to determine if ASTER data integrated with other geologic datasets can be used to find additional areas of prospectivity in well-explored permissive tracts. ASTER hydrothermal alteration data were expressed as 457 alteration polygons defined from a low-pass filtered alteration density map of combined argillic, phyllic, and propylitic rock units. Sediment stream samples were plotted as map grid data and used as spatial information in ASTER polygons. Gravity and magnetic data were also used to define basins greater than 1 km in depth. Each ASTER alteration polygon was ranked for porphyry copper potential using alteration types, spatial amounts of alteration, stream sediment geochemistry, lithology, polygon shape, proximity to other alteration polygons, and deposit and prospects data. Permissive tracts defined for the assessment in the southern Basin and Range Province include the Laramide Northwest, Laramide Southeast, Jurassic, and Tertiary tracts. Expert opinion estimates using the three-part assessment method resulted in a mean estimate of 17 undiscovered porphyry copper deposits, whereas the prospectivity modeling predicted a mean estimate of nine undiscovered deposits. In the well-explored Laramide Southeast tract, which contains the most deposits and has been explored for over 100 years, an average of 4.3 undiscovered deposits was estimated using ASTER alteration polygon data versus 2.8 undiscovered deposits without ASTER data. The Tertiary tract, which contains the largest number of ASTER alteration polygons not associated with known Tertiary deposits, was predicted to contain the most undiscovered resources in the southern Basin and Range Province.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Submarine volcanic-hosted iron deposits are important sources of iron ore in northwestern China. Here we present the petrological characteristics and coupled Fe-O, C, and Si isotope data of iron ores from the Shikebutai submarine volcanic-hosted hematite deposit in the Western Tianshan region. Several stratiform and lenticular hematite-dominated orebodies occur in Carboniferous submarine volcano-sedimentary sequences in this region. The ores are mainly composed of hematite, quartz, and minor siderite with distinct alternating iron-rich and silica-rich bands. The hematite shows 〈span〉δ〈/span〉〈sup〉56〈/sup〉Fe and 〈span〉δ〈/span〉〈sup〉18〈/sup〉O values in the range of –0.31 to 0.80‰ and 2.2 to 7.0‰, respectively, and the jasper yields 〈span〉δ〈/span〉〈sup〉30〈/sup〉Si values of –1.90 to –1.20〈span〉‰〈/span〉. Iron and Si were both derived from hydrothermal fluids related to submarine magmatism/volcanism. The Fe-bearing minerals in the Shikebutai deposit define distinct formation pathways. Hematite is the primary dehydrated Fe(III) oxyhydroxide, and the Fe isotope data indicate fractionation resulting from the partial oxidation of Fe(II). The O isotope data reflect inheritance from the submarine hydrothermal fluids source. Jasper was produced during coprecipitation of silica adsorbed onto the Fe(III) oxyhydroxides. The siderite-rich iron ore/volcaniclastic rock samples with a high and variable total organic carbon content (0.14–5.57%) show negative 〈span〉δ〈/span〉〈sup〉13〈/sup〉C values (–3.0 to –1.1‰) and light 〈span〉δ〈/span〉〈sup〉56〈/sup〉Fe values (–1.11 to –0.84‰). Our isotope data, together with the common occurrence of hematite inclusions in siderite, suggest that siderite was mainly produced by microbial dissimilatory iron reduction during diagenesis. The geologic, petrological, and isotopic data suggest that the Carboniferous Shikebutai deposit was precipitated through chemical and biogenic processes.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Canadian Malartic stockwork-disseminated gold deposit is an Archean world-class deposit located in the southern Abitibi greenstone belt. It contains over 332.8 tonnes (t; 10.7 Moz) of Au at a grade of 0.97 ppm, in addition to 160 t (5.14 Moz) of past production (1935–1981). Although the deposit is partly situated within the Larder Lake-Cadillac fault zone, most of the ore occurs up to ~1.5 km to the south of the fault zone. The main hosts of the mineralized zones are greenschist facies turbiditic graywacke and mudstone of the Pontiac Group (~2685–2682 Ma) and predominantly subalkaline ~2678 Ma porphyritic quartz monzodiorite and granodiorite. These intrusions were emplaced during an episode of clastic sedimentation and alkaline to subalkaline magmatism known as the Timiskaming assemblage (〈2680–2670 Ma in the southern Abitibi). The orebodies define two main mineralized trends, which are oriented subparallel to the NW-striking S〈sub〉2〈/sub〉 cleavage and the E-striking, S-dipping Sladen fault zone. This syn- to post-D〈sub〉2〈/sub〉 ductile-brittle to brittle Sladen fault zone is mineralized for more than 3 km along strike.The ore mainly consists of disseminated pyrite in stockworks and replacement zones, with subordinate auriferous quartz veins and breccia. Gold is associated with pyrite and traces of tellurides defining an Au-Te-W ± Ag-Bi-Mo-Pb signature. The orebodies are zoned outward, and most of the higher-grade (〉1 ppm Au) ore was deposited as a result of iron sulfidation from silicates and oxides and Na-K metasomatism in carbonatized rocks. The alteration footprint comprises a proximal alteration envelope (K- or Na-feldspar-dolomite-calcite-pyrite ± phlogopite). This proximal alteration zone transitions to an outer shell of altered rocks (biotite-calcite-phengitic white mica), which hosts sub-ppm gold grades and reflects decreasing carbonatization, sulfidation, and 〈span〉a〈/span〉Na〈sup〉+〈/sup〉/〈span〉a〈/span〉H〈sup〉+〈/sup〉 or 〈span〉a〈/span〉K〈sup〉+〈/sup〉/〈span〉a〈/span〉H〈sup〉+〈/sup〉 of the ore fluid.Gold mineralization, with an inferred age of ~2664 Ma (Re-Os molybdenite), was contemporaneous with syn- to late-D〈sub〉2〈/sub〉 peak metamorphism in the Pontiac Group; it postdates sedimentation of the Timiskaming assemblage along the Larder Lake-Cadillac fault zone (~2680–2669 Ma) and crystallization of the quartz monzodiorite. These chronological relationships agree with a model of CO〈sub〉2〈/sub〉-rich auriferous fluid generation in amphibolite facies rocks of the Pontiac Group and gold deposition in syn- to late-D〈sub〉2〈/sub〉 structures in the upper greenschist to amphibolite facies. The variable geometry, rheology, and composition of the various intrusive and sedimentary rocks have provided strain heterogeneities and chemical gradients for the formation of structural and chemical traps that host the gold. The Canadian Malartic deposit corresponds to a mesozonal stockwork-disseminated replacement-type deposit formed within an orogenic setting. The predominance of disseminated replacement ore over fault-fill and extensional quartz-carbonate vein systems suggests that the mineralized fracture networks remained relatively permeable and that fluids circulated at a near-constant hydraulic gradient during the main phase of auriferous hydrothermal alteration.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Over 170,000 metric tonnes of high-grade clinoptilolite tuff were extracted from the open-pit mine in Nižný Hrabovec (eastern Slovak Republic) in 2018, making it one of the world’s major natural clinoptilolite producers. The mine is hosted in a Miocene volcanogenic-sedimentary deposit in the East Slovak basin, with estimated 150 million tonnes of clinoptilolite tuff—the economically most important reserve in the European Union. The 100-m-thick tuff horizon is under- and overlain by SW-dipping shallow-marine Badenian sediments (16.30–15.03 Ma ≅ Langhian stage). The tuff is rhyolitic (74.3–77.6 wt % SiO〈sub〉2〈/sub〉, 4.09–5.49 wt % Na〈sub〉2〈/sub〉O + K〈sub〉2〈/sub〉O) with a generally high-K calc-alkaline affinity (3.07–4.28 wt % K〈sub〉2〈/sub〉O), comparable to volcanic rocks in the central segment of the Carpathian-Pannonian region of similar age. Although some parts of the tuff underwent slight geochemical changes during formation, this did not significantly change the whole-rock composition of the strikingly homogeneous raw material present in the mine. Rare earth elements (ΣREE = 117–141 ppm) show a uniform pattern, with enriched light REEs and negative Eu anomalies of 0.42 to 0.6 (when normalized to chondrite), comparable to the upper continental crust composition. Mean 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr = 0.70880 and 〈sup〉143〈/sup〉Nd/〈sup〉144〈/sup〉Nd = 0.512463 indicate an enriched magmatic source, with a dominant crustal contribution. X-ray diffraction data and electron probe microanalyses show that clinoptilolite-Ca is the only zeolite phase present in the deposit. High-resolution electron probe microanalytical imaging and measurement techniques reveal that clinoptilolite is present as (1) coarse patches that either form pseudomorphs of volcanic glass shards or grew in voids and (2) ultrafine material making up the matrix of the tuff. Both textural types have Si/Al 〉4, and their evolution is connected to dissolution-(transport)-precipitation reactions from acidic volcanic ash under alkaline fluid influence and slightly elevated pressure-temperature (44°–84°C) conditions. Authigenic cristobalite, detected as ultrafine-grained matrix crystallites, formed during the zeolitization process from excess Si. The large-scale, remarkably homogeneous and monomineralic natural zeolite deposit formed from a well-equilibrated magmatic source, with no syn- to postsedimentary reworking and with essentially isochemical conditions during the zeolitization process.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Ntaka Hill Ni-Cu deposit in Tanzania lies within the Mozambique belt, a complex, long-lived Neoproterozoic suture zone that formed during the amalgamation of Gondwana. The deposit is hosted within the moderately deformed Ntaka Ultramafic Complex, which was emplaced at ~660 Ma, around 20 m.y. before the East African orogeny. Due to strain partitioning into amphibolites and metapyroxenites at the margins of the sills, magmatic cumulate textures are very well preserved. The Ntaka Hill mineralization is predominantly disseminated sulfides within orthopyroxenites and harzburgites. Sulfides show typical magmatic mineralogy and occur as isolated blebs and interstitial networks as well as in soft-walled segregation veins that form localized shoot-like high-grade zones. Deformation had relatively little effect on the deposition of sulfides other than minor localization of strain within some preexisting sulfide-dominant veins. Grade shells define elongate tubular channels that coincide broadly with the more magnesian cumulate rocks. Base and precious metal tenors are widely variable, ranging from 1 to 17% Ni and 0.5 to 10% Cu with no particular correlation with host-rock type, although the tenor range in the more magnesian cumulates is more restricted, with the low-tenor component (〈3% Ni, 〈1% Cu) missing. Olivine compositions vary widely, with forsterite contents ranging from Fo〈sub〉84〈/sub〉 to Fo〈sub〉89〈/sub〉 with up to 0.85% NiO. These unusually high Ni contents are attributed to equilibration with high-Ni-tenor sulfides percolating through the intercumulus pore space. Complex zoning patterns in the Cr content of orthopyroxenes record fluctuating crystallization conditions attributed to wall-rock assimilation. The intrusion emplacement setting is interpreted to be derived from a flow of xenolith-, crystal-, and sulfideladen magma through channelized sills where much of the sulfide was incompletely equilibrated with the host magma, resulting in highly variable R-factors. Despite the widespread presence of graphite in the marginal pyroxenites, olivine-sulfide Fe/Ni partitioning records typical redox conditions that are slightly more oxidized than quartz-fayalite-magnetite (QFM)—further evidence of a lack of extensive large-scale equilibrium within the intrusive complex. The Ntaka Ultramafic Complex preserves an arrested stage of the amalgamation of small sulfide-bearing sill-sediment complex intrusions into larger convecting magma chambers, forming one end of a process continuum including Kevitsa (Finland) as an intermediate stage and Mirabela (Brazil) as the layered intrusion end member.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The iron oxide-apatite (IOA) deposits of the eastern Adirondack Highlands, New York, are historical high-grade magnetite mines that contain variable concentrations of rare earth element (REE)-bearing apatite crystals. The majority of the deposits are hosted within sodically altered Lyon Mountain granite gneiss, although some deposits occur within paragneiss, gabbro, anorthosite, or potassically altered Lyon Mountain granite gneiss. The IOA deposits and the waste and/or tailings piles associated with them have potential as an unconventional resource for REEs. Reprocessing of these piles would have the advantage of partial recycling of the waste material to produce a set of critical elements.Thirty-four ore, nine rock, 25 waste-pile, and four tailings-pile samples were collected and analyzed for major, minor, and trace elements. At the tailings- and waste-pile sites, composite samples were collected by combining 30 to 〉50 subsamples randomly distributed over each pile. The total REE content of the waste and tailings piles varied from approximately 10 to 22,000 ppm, whereas the ore sample concentrations ranged from approximately 15 to 48,000 ppm total REEs. A positive correlation exists between the total REE content of ore and its associated waste pile. Median light REE/heavy REE values were 2.14 for waste/tailings piles and 2.25 for ore, which is a substantial relative enrichment in the heavy REEs in comparison to many developed REE mines, such as the mined carbonatites of Bayan Obo, China, and Mountain Pass, California. Importantly, the ore and waste samples are significantly enriched in both Y and Nd compared to other REEs in the samples. Other minor components such as Th are also elevated. Airborne radiometric surveys show large positive eTh and eU anomalies corresponding to tailings piles.Although it is a limited data set, geochemical data of unaltered and altered host rocks suggest a speculative new model for IOA ore formation in the Adirondack Highlands that is consistent with the geology and previously published data. The ferroan ore-hosting Lyon Mountain granite gneiss underwent localized potassic alteration that enriched the altered rock in Fe, REEs, Th, and other metals. A later sodic alteration event affected the previously potassically altered Lyon Mountain granite gneiss, which increased rock porosity and remobilized Fe, REEs, and other elements from the host rock into the iron ore seams. The sodic fluids responsible for ore formation were enriched in F and Cl.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Geometallurgy has developed since the 1970s, primarily on metallic ore operations. In parallel, industrial mineral operations have been optimized through detailed deposit knowledge and market development, without making specific reference to geometallurgical concepts. The Norwegian mining industry is dominated by industrial mineral and construction material operations, and, in this paper, key differences between the industrial mineral and the metallic ore sectors are investigated, along with their influence on the development and the use of economic block models and optimization methodologies. Further, the key levers and factors (mining method selection, processing route, scale, sequence, and cutoff policy) for value creation in industrial mineral operations are discussed, along with how and to what extent geometallurgy has been used. It is concluded that the five key levers cannot be used in industrial minerals operations as effectively as they are used in metallic ore operations. In industrial minerals, in situ strength variations are an important parameter in estimating key performance indicators such as recovery and product quality. When modeling the spatial variation in rock strength potential, additivity issues must be resolved by investigating the process the raw material is exposed to. The Norwegian industrial mineral sector has been using elements of geometallurgy but is facing unresolved issues related to strength variations and the use of measurement while drilling data.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Florida Canyon evaporite-related Zn-Pb sulfide deposit, in northern Peru, is one of the largest Mississippi Valley-type deposits in South America. Triassic carbonate and former evaporite-bearing rocks of the Pucará Group host the orebodies that constitute two different styles: (1) predominantly stratabound ore associated with hydrocarbon-rich porous dolostones and evaporite dissolution breccias and (2) high-grade ore associated with evaporite breccias representing diapiric injections along faults. A dome structure that controls the location of the ore deposit was defined by drill hole spatial data; the dome likely resulted from halokinetic processes during Andean deformation. NNE-trending, steeply dipping secondary faults linked to major northwest structures appear to control the distribution of ore grades in the deposit. Mineralization postdated hydrocarbon migration and accumulation. Strontium, carbon, and oxygen data isotope signatures allow distinction between pre- and synmineralization carbonate stages. The sulfur isotope composition of sulfides in the deposit suggests they precipitated as the result of mixing of a metal-rich fluid with resident hydrogen sulfide in the dome. Local thermochemical sulfate reduction may have contributed to the reduced sulfur budget during mineralization.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Pyrite (FeS〈sub〉2〈/sub〉) is a typical container of Pt in ores of magmatic and hydrothermal origin and in some carbonrich ores of sedimentary-diagenetic origin. Knowledge of the state of Pt disseminated in the matrix of pyrite, including local atomic environment (type of atoms in the nearest and distant coordination shells, coordination numbers, interatomic distances) and oxidation state, is necessary for physical-chemical modeling of platinum group element mineralization and for the improvement of Pt ore extraction and processing technologies. Here we report results of an investigation of local atomic structure of synthetic Pt-bearing pyrites by means of X-ray absorption spectroscopy (XAS). Synthesis experiments, performed at 580° and 590°C in a Pt-saturated system by means of salt-flux method, yielded crystals of pyrite with concentrations of Pt up to 4 wt %. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) showed that the distribution of Pt within the pyrite grains is of zonal character, but within the distinct zones Pt is distributed homogeneously. Negative correlation between the concentrations of Pt and Fe was observed in the synthesized pyrite grains. The slope of the correlation line corresponds to the formation of the solid solution in the Pt-Fe-S system and/or to the formation of PtS〈sub〉2〈/sub〉. The XAS experiments revealed the existence of two forms of Pt in pyrite. The main form is the solid solution Pt(IV), which isomorphically substitutes for Fe. The Pt-S distance in pyrite is ~0.1 Å longer than that of Fe-S in pure pyrite. The distortion of the pyrite crystal structure disappears at 〈span〉R〈/span〉 〉2.5 Å. The second Pt-rich form was identified by means of high-resolution transmission electron microscopy (HRTEM) as nanosized inclusions of PtS〈sub〉2〈/sub〉. Heating experiments with in situ registration of X-ray absorption spectra resulted in partial decomposition (dissolution) of PtS〈sub〉2〈/sub〉 nanosized inclusions with the formation of the solid solution (Fe〈sub〉1–x〈/sub〉Pt〈sub〉x〈/sub〉)S〈sub〉2〈/sub〉. Therefore, the PtS〈sub〉2〈/sub〉 nanosized particles can be considered as a quench product. Our data demonstrate that both Pt solid solution and PtS〈sub〉2〈/sub〉 nanosized inclusions (at high Pt content) can exist in natural Pt-bearing pyrites.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Geometallurgy is an interdisciplinary field aimed at describing potential ore deposits in terms that mine planners and economists can use to design and run profitable mining operations. The major geologic contribution to the field is defining the spatial variability of potential and active mining resources so that planning and scheduling can accurately predict the economic performance and environmental impact of mining in time to respond efficiently to variations in ore type. This information is needed at the feasibility stage and throughout the mine life. We review the available literature on how geologists have contributed to these predictions in the past. There have been substantial advances in predicting comminution behavior. Prediction of recovery and environmental impacts are less advanced. This introductory paper provides a brief review of geometallurgy and a synopsis of the papers in the Special Issue, along with suggestions on future directions.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Phoenix mine and predecessor operations in north-central Nevada have produced an aggregate of 5.2 Moz of gold and 550 million pounds of copper from an Eocene-aged Au-Cu porphyry-related skarn. The complex skarn mineralogy intimately associated with ore-grade mineralization poses significant challenges to blasting, mining, comminution, and process operations. These challenges are rooted in highly variable silicate mineralogy, which manifests as fine-grained, submillimeter grain-size, generally green colored rocks that inhibit accurate identification in the field. Prior to this study, all mineralogical data utilized in Phoenix mine ore control were sourced from blast hole cuttings mapped by ore control geologists in the field—the standard practice at many modern mine sites. At Phoenix, a direct link between mineralogy and mill performance was recognized; however, mineralogical data captured in the field was not sufficient to optimize process operations.To address this, it was determined that analytical work was necessary to quantify fine-grained mineralogy of variable ore types. A visible-near and short-wave infrared (VNIR-SWIR) hyperspectral imaging system provided the ideal tool, as it allows near real-time mineralogical data acquisition and semiquantitative determination of mineral abundances. Multiple iterative studies were conducted to prove that hyperspectral imaging of Phoenix ore types provides results suitable for process optimization. This six-month study described here included hyperspectral imaging of 3,008 blast hole cuttings samples from three pits, and 877 crusher feed, rougher feed, and rougher tails samples. Hyperspectral feature extractions derived from mill samples were paired with associated mill performance data and used to build predictive Au-Cu recovery, grade, and throughput models using multiple linear regression, partial least squares, and deep learning techniques with R-correlation values to observed data of 0.56 to 0.71. Blast hole hyperspectral data were then applied to recovery, grade, and throughput models to calculate predicted recoveries and throughputs that were spatially kriged with excellent correlations to geologic features.The application of VNIR-SWIR hyperspectral imaging to blast hole cuttings is a powerful predictive and diagnostic geometallurgical tool in operations where silicate mineralogy has a strong impact on process operations.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Cadia East porphyry deposit, located approximately 20 km south of Orange, New South Wales, Australia, contains a significant resource of copper and gold. This resource is hosted within the Forest Reefs Volcanics and is spatially and temporally associated with the Cadia Intrusive Complex. To extract ore, the underground mine currently uses the block cave mining method. The Cadia East geotechnical model provides data inputs into a range of numerical and empirical analysis methods that make up the foundation for mine design. These data provide input into the construction of stress models, caveability models, ground support design, and fragmentation analysis. This geotechnical model encompasses two commonly used rock classification systems that quantify ground conditions: (1) rock mass rating (RMR) and (2) rock tunneling quality index (Q index). The RMR and Q index are calculated from estimates of rock quality designation (RQD), number of fracture sets, fracture roughness, fracture alteration, and fracture spacing. Geologists and geotechnical engineers collect information used to produce these estimates by manually logging sections of drill core, a time-consuming task that can result in inconsistent data.Modern automated core scanning technologies offer opportunities to rapidly collect data from larger samples of drill core. These automated core logging systems generate large volumes of spatially and spectrally consistent data, including a model of the drill core surface from a laser profiling system. Core surface models are used to extract detailed measurements of fracture location, orientation, and roughness from oriented drill core. These data are combined with other morphological and mineralogical outputs from automated hyperspectral core logging systems to estimate RMR and the Q index systematically over contiguous drill core intervals. The goal of this study was to develop a proof-of-concept methodology that extracts geotechnical index parameters from hyperspectral and laser topographic data collected from oriented drill core.Hyperspectral data from the Cadia East mine were used in this case study to assess the methods. The results show that both morphological and mineralogical parameters that contribute to the RMR and Q index can be extracted from the automated core logging data. This approach provides an opportunity to capture consistent geologic, mineralogical, and geotechnical data at a scale that is too time-consuming to achieve via manual data collection.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Geometallurgy aims to develop and deploy predictive spatial models based on tangible and quantitative resource characteristics that are used to optimize the efficiency of minerals beneficiation and extractive metallurgy operations. While most current applications of geometallurgy are focused on the major commodity to be recovered from a mineral deposit, this contribution delineates the opportunity to use a geometallurgical approach to provide an early assessment of the economic potential of by-product recovery from an ongoing mining operation.As a case study for this methodology, possible rare earth element (REE) recovery as a by-product of Nb production at the Chapadão mine in the Catalão I carbonatite complex is used. Catalão I is part of the Alto Paranaíba igneous province in the Goias Province of Brazil. Currently, niobium is produced in the complex as a by-product of the Chapadão phosphates mine. This production is performed in the Tailings plant, the focus of this study. REEs, albeit present in significant concentrations, are currently not recovered as by-products.Nine samples from different stages of the Nb beneficiation process in the Tailings plant were taken and characterized by mineral liberation analyzer, X-ray powder diffraction, and bulk-rock chemistry. The recovery of REEs in each of the tailing streams was quantified by mass balance. The quantitative mineralogical and microstructural data are used to identify the most suitable approach to recover REEs as a by-product—without placing limitations on niobium production. Monazite, the most common rare earth mineral identified in the feed, occurs as Ce-rich and La-rich varieties that can be easily distinguished by scanning electron microscopy (SEM)-based image analysis. Quartz, Fe-Ti oxides, and several phosphate minerals are the main gangue minerals. The highest rare earth oxide content concentrations (1.75 wt % total rare earth oxides) and the greatest potential for REE processing are reported for the final flotation tailings stream. To place tentative economic constraints on REE recovery from the tailings material, an analogy to the Browns Range deposit in Australia is drawn. Its technical flow sheet was used to estimate the cost for a hypothetical REE production at Chapadão. Parameters derived from SEM-based image analysis were used to model possible monazite recovery and concentrate grades. This exercise illustrates that a marketable REE concentrate could be obtained at Chapadão if the process recovers at least 53% of the particles with no less than 60% of monazite on their surface. Applying capital expenditure and operational expenditure values similar to those of Browns Range suggests that such an operation would be profitable at current REE prices.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The juxtaposition of a Triassic evaporite diapir with the organic matter-rich Fahdene Formation (Albian-Vra-conian) along major faults in the Slata ore district raises the question of the roles played by halokinesis, hydrocarbons, and tectonics in mineralization. The Slata mining district, located in the Tunisian salt diapiric zone, contains Ba-Pb-(± Zn) ore hosted in the Aptian carbonates. The mineralogical paragenetic sequence consists of barite (Ba-1)–galena ± sphalerite ± calcite (Ca-1)–barite (Ba-2) and finally, late calcites (Ca-2 and Ca-3). Fluid inclusions from early barite reveal that it was precipitated from a warm (134°–157°C), H〈sub〉2〈/sub〉O-NaCl-KCl-CaCl〈sub〉2〈/sub〉, moderately saline (13.3–24.6 wt % NaCl equiv) basinal brine. This fluid is thought to have resulted from the mixing of a deep-seated, hot, metal-bearing fluid with a cooler, dilute SO42−-rich fluid. Early calcite and cogenetic sulfides (galena and sphalerite) precipitated from fluids of similar salinities and temperatures as the barite-forming fluids, but with the additional involvement of hydrocarbons. Sulfur isotope data suggest that thermochemical sulfate reduction of Triassic gypsum was the main source of reduced sulfur for sulfides. Late barite precipitated as a result of the mixing between a Ba-rich, hot, ascending fluid with a cooler, dilute Triassic sulfate-rich fluid in the absence of hydrocarbons. The homogeneous Pb isotope compositions of galena along with the Sr isotope compositions of barite point to a Paleozoic reservoir as the main source of metals with a contribution from the Triassic-Cretaceous rocks. The emplacement of the ore occurred during the Eocene-Miocene Alpine compressional tectonic activity that triggered the circulation of Paleozoic-derived metal-bearing fluids.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Mineralogy is a fundamental characteristic of a given rock mass throughout the mining value chain. Understanding bulk mineralogy is critical when making predictions on processing performance. However, current methods for estimating complex bulk mineralogy are typically slow and expensive. Whole-rock geochemical data can be utilized to estimate bulk mineralogy using a combination of ternary diagrams and bivariate plots to classify alteration assemblages (alteration mapping), a qualitative approach, or through calculated mineralogy, a predictive quantitative approach. Both these techniques were tested using a data set of multielement geochemistry and mineralogy measured by semiquantitative X-ray diffraction data from the Productora Cu-Au-Mo deposit, Chile.Using geochemistry, samples from Productora were classified into populations based on their dominant alteration assemblage, including quartz-rich, Fe oxide, sodic, potassic, muscovite (sericite)- and clay-alteration, and least altered populations. Samples were also classified by their dominant sulfide mineralogy. Results indicate that alteration mapping through a range of graphical plots provides a rapid and simple appraisal of dominant mineral assemblage, which closely matches the measured mineralogy.In this study, calculated mineralogy using linear programming was also used to generate robust quantitative estimates for major mineral phases, including quartz and total feldspars as well as pyrite, iron oxides, chalcopyrite, and molybdenite, which matched the measured mineralogy data extremely well (R〈sup〉2〈/sup〉 values greater than 0.78, low to moderate root mean square error). The results demonstrate that calculated mineralogy can be applied in the mining environment to significantly increase bulk mineralogy data and quantitatively map mineralogical variability. This was useful even though several minerals were challenging to model due to compositional similarities and clays and carbonates could not be predicted accurately.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Fluid inclusion studies give unique insights into physical conditions and composition of fluids involved in geologic processes. Most studies to date are performed on transparent minerals. Near-infrared (NIR) microscopy allows microthermometry to also be performed on minerals that are opaque to the visible light, such as pyrite, hematite, wolframite, enargite, and stibnite. The main drawback of this technique is the underestimation of the recorded phase-change temperatures with increasing light intensity—up to several hundred percent in the case of ice-melting temperatures. Although this issue has been known for a decade, it is poorly understood. We address this problem based on a systematic study of synthetic fluid inclusions in a variety of opaque minerals.For the first time, fluid inclusions have been cosynthesized with success in quartz and opaque minerals. Fracturing the host minerals by in situ quenching allowed for fluid-mineral equilibration prior to fluid inclusion formation. In this study, we assess the impact of mineral intrinsic parameters, mainly absorption and thermal conductivity, and experimental settings (light source operative power, diaphragm aperture, and the use of filters) on recorded phase-change temperatures. We show that these are underestimated due to local overheating of the sample caused by radiative heating from the light source. It affects all minerals, and the extent of the temperature shift of the observed phase changes depends on sample thickness, mineral characteristics, and the amount of light reaching the sample. Thus, any calibration of the temperature shift as a function of the amount of light is complicated and, in most cases, impracticable. However, we demonstrate, based on cogenerated inclusions in opaque minerals and quartz that yield similar values during NIR microthermometry, that for any mineral there is a range of light power and microscope settings for which no shift is noticeable within the thermal-stage accuracy. These are defined as “ideal measuring conditions,” ensuring reliability of acquired microthermometry data.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Karouni orogenic gold deposits are located in north-central Guyana 35 km to the west of the 5 Moz Omai gold deposit. They are hosted in a 2200 to 2100 Ma greenstone belt within the Paleo- to Neoproterozoic Guiana Shield. Karouni consists of two deposits, Smarts and Hicks, located 2 km apart along the NW-striking Smarts-Hicks shear zone. Both deposits are hosted within a sequence of greenschist facies mafic volcanic rocks and felsic intrusions. The hydrothermal alteration mineral assemblages vary according to lithology and are characterized by narrow selvages (〈1–4 m in width). A chlorite-talc-calcite assemblage dominates in high MgO basalt, whereas in high TiO〈sub〉2〈/sub〉 dolerite a progression toward the vein is seen from chlorite-calcite-rutile- to albite-dominated mineralogy. Karouni is anomalous among orogenic gold deposits for its dominant sodic alteration and distinct lack of potassic alteration. Gold is located within inclusions in coarse, disseminated pyrite associated with the proximal alteration zones and as coarse native gold within the quartz-carbonate veins. Minor gold is also located within Au-bearing telluride minerals. The high TiO〈sub〉2〈/sub〉 dolerites formed a favorable chemical trap due to their high magnetite content, suggesting sulfidation via redox reaction as a possible mechanism of gold deposition. Mass balance modeling of the hydrothermal alteration indicates a wall rock-dominated system with limited addition or subtraction of major elements with the exception of C, S, and Na. Modeling of the proximal alteration has also shown strong trace element enrichment of W-Bi-Ag-Te-Mo-Pb, all of which are correlative with gold. In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element geochemistry and secondary ion mass spectrometry (SIMS) S isotope analyses of pyrite from the gold-bearing hydrothermal system within the deposit indicate a geochemically and isotopically homogeneous system with only minor trace element variation due to differences in host rock, suggesting a single hydrothermal pulse correlative with the late stages of the Trans-Amazonian orogeny.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Fluid inclusions in clearly defined fluid inclusion assemblages (FIAs) from various geologic environments were examined to assess the uncertainty associated with determining the temperature of a fluid event based on fluid inclusion homogenization temperatures (T〈sub〉h〈/sub〉). A fluid event is defined as a physical or chemical process such as the healing of a microfracture or the formation of a growth zone in a crystal that occurs in the presence of a fluid phase and results in trapping of fluid inclusions to form an FIA. Examination of data from a large number of fluid events collected within a rigorous temporal and spatial (paragenetic) framework forms the basis for developing a complete fluid pressure-temperature-composition (PTX) history.The range in homogenization temperatures of fluid inclusions within well-constrained FIAs was determined, and the minimum (smallest) range in T〈sub〉h〈/sub〉, the median range in T〈sub〉h〈/sub〉, and the first quartile (Q1 at 25%) and third quartile (Q3 at 75%) of the median T〈sub〉h〈/sub〉 ranges were calculated for different fluid environments, including the following: 〈ul〉 〈li〉Low-permeability sedimentary environments: 49 out of 144 FIAs show a range in T〈sub〉h〈/sub〉 of ≤1°C; the median range = 2°C (from Q1 = 1°C to Q3 = 3.7°C).〈/li〉 〈li〉Mississippi Valley-type deposits: 11 out of 137 FIAs show a range in T〈sub〉h〈/sub〉 of ≤ 1°C; the median range = 4.1°C (from Q1 = 2.3°C to Q3 = 8.3°C).〈/li〉 〈li〉Epithermal deposits: 102 out of 923 FIAs show a range in T〈sub〉h〈/sub〉 of ≤1°C; the median range = 9°C (from Q1 = 3.8°C to Q3 = 19°C).〈/li〉 〈li〉Porphyry-type deposits: 24 out of 271 FIAs show a range in T〈sub〉h〈/sub〉 of ≤ 1°C; the median range = 15°C (from Q1 = 8°C to Q3 = 30°C).〈/li〉 〈li〉Orogenic Au deposits: 21 out of 231 FIAs show a range in T〈sub〉h〈/sub〉 of ≤1°C; the median range = 8.7°C (from Q1 = 4°C to Q3 = 20°C).〈/li〉 〈/ul〉While all environments show some FIAs in which all the fluid inclusions homogenize at essentially the same temperature (range = ≤1°C), we propose that the median range in T〈sub〉h〈/sub〉 reported here represents a reasonable and achievable constraint on the uncertainty associated with the temperature of a fluid event in the environments examined. In summary, the temperature of a fluid event, as represented by the range in T〈sub〉h〈/sub〉 of the fluid inclusions within an individual FIA, can be constrained to better than 15°C in all environments examined, and in Mississippi Valley-type and low-permeability (deep) sedimentary basin environments, the range in T〈sub〉h〈/sub〉 can be constrained to better than 2°C.The processes that produce variability in T〈sub〉h〈/sub〉 of fluid inclusions within an FIA are many and include natural variations in temperature, pressure, or fluid composition during trapping of the FIA, trapping of immiscible fluids, various forms of reequilibration in nature such as necking, stretching, and leakage, and modification of the inclusions during sample preparation and data collection. If the range in homogenization temperature for an individual FIA is found to be greater than the median range determined here for that environment, then assessment of the cause of the variability might provide useful information concerning the trapping and posttrapping history of the sample.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Better tools are needed to map the thermal structure of ore deposits. Here, carbonate clumped isotope thermometry is applied for the first time in epithermal, skarn, and carbonate-hosted deposits to identify the conditions involved in metal transport and deposition. Clumped isotope temperature calibrations were tested by measurement of carbonates from three geothermal fields in the Taupo volcanic zone, New Zealand, that record growth temperatures between 130° and 310°C. Results for modern Taupo volcanic zone calcites were paired with known fluid 〈span〉δ〈/span〉〈sup〉18〈/sup〉O values and these indicate precipitation in equilibrium with produced geothermal waters. Measurements carried out at the Waihi low sulfidation deposit in New Zealand, the Antamina polymetallic skarn in Peru, and the Mount Isa sediment hosted Pb-Zn and Cu deposit in Queensland, Australia, demonstrate that clumped isotope values are sensitive to temperature gradients defined using other methods. At Waihi, an andesite-hosted deposit, temperature controls the majority of variation in carbonate mineral 〈span〉δ〈/span〉〈sup〉18〈/sup〉O. At Mount Isa, ~300° to 400°C temperatures were recorded in a 1.5 Ga orebody, which are consistent with fluid inclusion values, highlighting the longevity of clumped isotope archives in dolomite minerals. Collectively, these results demonstrate the potential for clumped isotopes to delineate the heat footprint around deposits that contain carbonates, and to more effectively disentangle magmatic and meteoric fluid 〈span〉δ〈/span〉〈sup〉18〈/sup〉O signals.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Physical properties and compositions of ore-forming fluids in magmatic-hydrothermal systems have been mostly investigated by conventional fluid inclusion studies in transparent gangue minerals that are assumed to be co-genetic with the mineralization. However, ore-precipitating fluids can be directly studied by analyzing fluid inclusions in opaque ore minerals, such as wolframite, stibnite, pyrite, and enargite, by using near-infrared (NIR) petrography and microthermometry in combination with laser ablation-inductively coupled plasma-mass spectrometry analysis of individual inclusions. Although results of NIR fluid inclusion studies of ore minerals were first published in 1984, the technique is still not commonly used in fluid inclusion research due to a number of limitations related to the analytical equipment used, sample preparation, and NIR mineral transmittance.In this contribution, we present new data on the applicability of NIR fluid inclusion studies of ore minerals (pyrite, stibnite, enargite, and wolframite) from a series of magmatic-hydrothermal systems, according to their chemical composition and physical behavior during microthermometry. Our results reveal strong correlation between NIR transmittance and high trace element (Co, Ni, Cu, As) content in pyrite, enargite (Fe, Bi), and wolframite (Sc, V, Fe). Despite this, the restricted distribution of these elements in oscillatory and sector zoning has allowed observation of NIR mineral features and fluid inclusions. Energy absorption of opaque minerals, either as light energy during microscopy or as thermal conductive energy during fluid inclusion microthermometry, presents a second limitation for NIR fluid inclusion studies. Our results confirm the relevance of fluid inclusion studies in ore minerals by combining NIR microscopy and microthermometry. Despite some limitations due to trace element composition of the host mineral and its physical behavior at high temperature, a successful NIR fluid inclusion study can be performed on some ore minerals that are opaque in the visible light range, allowing the direct study of hydrothermal ore-forming fluids.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Grota Funda iron oxide copper-gold (IOCG) deposit is situated within a regional WNW-ESE–striking shear zone in the northwestern portion of the Carajás domain, an Archean (3.0–2.55 Ga) segment of the Carajás Province, Brazil. Metavolcano-sedimentary sequences of the Igarapé-Pojuca Group, comprising basalt, diabase, gabbro, dacite, and banded iron formations, are the main lithotypes recognized in the deposit area. In this sequence, mafic igneous rocks represent the main hosts to the copper (-gold) mineralization stages. The paragenetic evolution of the Grota Funda hydrothermal system encompasses an early high-temperature cal-cic-sodic alteration (albite-hastingsite-scapolite), followed by intense Fe metasomatism (magnetite-grunerite-almandine), potassic alteration with biotite, chlorite-quartz-tourmaline assemblage, and late carbonate-quartz veining. Copper (-gold) mineralization stages are spatially and temporally associated with iron-enriched (mineralization I), potassically altered (mineralization II), and chlorite-altered zones (mineralization III). Molybdenite from grunerite-magnetite veins yielded a Re-Os model age of 2530 ± 60 Ma, which is interpreted as the mineralization I age. The main sulfide ore (mineralization II) predominantly forms breccia bodies characterized by a chalcopyrite-magnetite-sphalerite-pyrrhotite assemblage. Chalcopyrite 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values (0.9 ± 0.9‰) point to a magmatic source for sulfur, probably leached from the host mafic rocks. Development of early and high-temperature (〉500°C) alteration assemblages is attributed to regional circulation of deep-seated hypersaline and metalliferous fluids. Mixing with moderate- to high-salinity (24–29 wt % NaCl + CaCl〈sub〉2〈/sub〉 equiv) and cooler fluids may have triggered ore precipitation in the main ore zone of mineralization II, due to a decrease in temperature and Cl〈sup〉–〈/sup〉 activity. Postore alteration assemblages resulted from considerable temperature and pH decreases.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Mount Carlton is a Paleozoic high-sulfidation epithermal deposit located in the northern segment of the Bowen Basin, northeast Queensland, Australia. The deposit is hosted in Early Permian volcanic and sedimentary rocks, and an open-pit mining operation includes the Au-rich V2 pit in the northeast and the Ag-rich A39 pit in the southwest. Mineralization at Mt. Carlton occurred during active rifting, partly contemporaneously with the deposition of volcanic sediments in localized half-graben and graben basins. Steep normal faults and fracture networks related to the rifting acted as fluid conduits and localized cores of silicic alteration. The silicic cores transition outward to zones of quartz-alunite alteration, which are, in turn, enveloped by a zone of quartz-dickite-kaolinite alteration. Epithermal mineralization at Mt. Carlton developed in three stages: Cu-Au-Ag mineralization dominated by enargite was overprinted by Zn-Pb-Au-Ag mineralization dominated by sphalerite, which, in turn, was overprinted by Cu-Au-Ag mineralization dominated by tennantite. Proximal Au-Cu mineralization in the V2 pit occurs in networks of steep faults associated with veins and hydrothermal breccias within a massive rhyodacite porphyry. Three distinct ore zones (Eastern, Western, and Link) are aligned, en echelon, along a broadly E trending corridor. The Western ore zone continues along ~600-m strike length to the southwest into the A39 pit, and it shows a metal zonation, from proximal to distal, of Au-Cu → Cu-Zn-Pb-Ag → Ag-Pb-(Cu) → Ag. Distal Ag mineralization in the A39 pit is concentrated in a volcanolacustrine sedimentary sequence that overlies the rhyodacite porphyry. It occurs in a stratabound position oriented parallel to primary sedimentary layering and locally exhibits synsedimentary ore textures. Such textures are interpreted to have formed as mineralizing fluids discharged into what most likely were lakes developed within localized rift basins, at the same time that the volcanolacustrine sediments were deposited. At depth, equivalent ore textures were produced within open spaces in the structural roots of the rift basins. 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar dating of hydrothermal alunite yielded an age range of 284 ± 7 to 277 ± 7 Ma, which links the formation of the Mt. Carlton deposit to the Early Permian back-arc rifting stage in the Bowen Basin. Prolonged extension provided rapid burial of the deposit beneath a postmineralization, volcanosedimentary cover, which was essential for the exceptional preservation of Mt. Carlton. The same extension caused displacement of the rock pile along a series of shallowly dipping detachment faults and segmentation and rotation of the ore zones across steeply dipping normal faults. This deformation would have displaced any underlying porphyry mineralization relative to the current location of Mt. Carlton.〈/span〉