ALBERT

Description: The Shagou vein-type Ag-Pb-Zn deposit in the Xiong’ershan district, southern margin of the North China craton, is hosted within amphibolite facies metamorphic rocks of the Late Archean to early Paleoproterozoic Taihua Group. The Ag-Pb-Zn veins are localized in NE- to NNE-trending brittle faults and typically display symmetrical zoning consisting of siderite, quartz + sphalerite, galena, and quartz + calcite from the margin toward the center of each vein. Ore-related hydrothermal alteration is well developed on both sides of the veins, dominated by silicification, sericitization, chloritization, and carbonatization. Sericite separates extracted from a major Ag-Pb-Zn vein yield a 40 Ar/ 39 Ar plateau age of 140.0 ± 1.0 Ma (1 ) and isochron age of 141.1 ± 1.6 Ma (1 ), indicating that mineralization occurred at the beginning of Early Cretaceous. Field and textural relationships indicate four hydrothermal stages marked by assemblages of quartz + siderite (stage I), quartz + sphalerite + ankerite (stage II), quartz + galena + silver minerals + ankerite (stage III), and quartz + calcite (stage IV), respectively. Silver minerals are abundant in all veins and are composed of, in paragenetic order, argentiferous tetrahedrite, polybasite, jalpaite, argentite, and native silver. These silver minerals commonly occur as replacements of galena, chalcopyrite, and other sulfides, or as fillings of microfractures in sulfides and quartz. Microthermometric measurements of primary fluid inclusions in quartz, carbonates, and sphalerite from various hydrothermal stages indicate that ore minerals were deposited at intermediate temperatures (267°–157°C) from aqueous-carbonic to aqueous fluids with moderate salinities (7.2–15.9 wt % NaCl equiv). Coexisting galena-sphalerite pair yields sulfur isotope equilibrium temperatures of 205° to 267°C, consistent with the overall homogenization temperatures of fluid inclusions. The microthermometric data also indicate that both fluid mixing and fluid-rock interaction were important mechanisms for ore precipitation. Carbonate minerals (siderite, ankerite, calcite) spanning the entire mineralization history have 13 C V-PDB values of –5.2 to –1.4 and 18 O V-SMOW of 10.9 to 15.0, corresponding to calculated values for the ore fluids of –6.5 to –1.8 and 1.4 to 5.4, respectively. 34 S V-CDT values of sulfide minerals (pyrite, sphalerite, galena) range from 1.1 to 5.5, consistent with a deep-seated sulfur source. Galena separates have 206 Pb/ 204 Pb ratios of 17.472 to 17.813, 207 Pb/ 204 Pb ratios of 15.411 to 15.498, and 208 Pb/ 204 Pb ratios of 38.178 to 38.506. The isotope data, together with geological and geochronological evidence, favor a primary metamorphic source for sulfur and other components in the ore fluids. A synthesis of available data suggests that the Shagou deposit is a typical vein-type Ag-Pb-Zn deposit that formed under an extensional geodynamic setting associated with tectonic reactivation of the North China craton during the late Mesozoic, a time period that is manifested by pervasive magmatism, widespread formation of metamorphic core complexes, and development of faulted basins throughout the eastern part of the craton. Metamorphic devolatilization of the Meso-Neoproterozoic marine sedimentary rocks previously subducted beneath the Xiong’ershan district, facilitated by extensive magmatism and elevated heat flow due to lithospheric extension, could have provided large amounts of ore fluids responsible for the Ag-Pb-Zn mineralization. The NE- to NNE-trending faults affiliated with the transcrustal Machaoying fault may have acted as pathways for the upward migration of deep-seated metamorphic fluids. Mixing of the metamorphically derived fluids with meteoric waters ultimately resulted in deposition of the Ag-Pb-Zn veins in brittle extensional structures.

Description: The Poyi magmatic Ni-Cu sulfide deposit is situated in the Beishan fold belt in the northeastern rim of the Tarim craton. Many Permian magmatic Ni-Cu sulfide deposits, such as those in East Tianshan, are present in the adjacent Central Asian orogenic belt to the north. The Poyi deposit is hosted in a small dike-like ultramafic-troctolitic body that was emplaced into a much larger gabbroic intrusion. Our new zircon U-Pb isotope data reveal that these two intrusions formed ~6 Ma apart. The ultramafic-troctolitic intrusion was emplaced at 269.9 ± 1.7 Ma, whereas the gabbroic intrusion was emplaced at 276.1 ± 1.9 Ma. The results show that the Poyi deposit is the youngest among the major magmatic Ni-Cu sulfide deposits (≥0.2 Mt Ni) of Permian ages in the Beishan-Tianshan region. Sulfide mineralization in the Poyi deposit occurs as steeply dipping disseminated sulfide lenses mostly associated with wehrlites in the center of the dike. Olivine from the Poyi ultramafic rocks has Fo content up to 91 mol %, which is similar to the lower limit of mantle olivine and the most primitive within the Permian Beishan-Tianshan nickel belt. Like other magmatic sulfide deposits in this belt, olivine from the Poyi deposit is depleted in Ca (〈1,000 ppm). The estimated parental magma for the Poyi most primitive ultramafic rocks contains 15 ± 2 wt % MgO. Cotectic olivine-sulfide segregation from the Poyi magma is inferred from systematic variation of Fo-Ni contents in olivine from some sulfide-barren ultramafic rock samples, and supported by the occurrence of sulfide droplets as small inclusions in olivine (Fo 90 mol %) in these rocks. The involvement of multiple pulses of sulfide-charged magma with different compositions is indicated by the abrupt change of olivine Fo content with depth and the presence of olivine with similar Fo contents but dramatically different Ni abundances in the different parts of the deposit. The ( 87 Sr/ 86 Sr) i ratios and Nd(total) of the Poyi ultramafic rocks and troctolites range from 0.7042 to 0.7052 and from 4.9 to 6.0, respectively, which are close to the isotope compositions of depleted mantle and within the ranges of major magmatic Ni-Cu sulfide deposits of Permian ages in East Tianshan. No more than 5 wt % of bulk crustal contamination is required to explain the variations of Sr-Nd isotopes in the Poyi ultramafic-troctolitic intrusion. The abundances of incompatible trace elements in whole rocks and clinopyroxene crystals indicate very weak light REE enrichments coupled by significant negative Nb-Ta anomalies in the parental melts. Bulk sulfides in the Poyi deposit are characterized by positive correlations between any pair of platinum-group elements (PGE), indicating that PGE tenor variations in the deposit are mainly controlled by variable R-factors (magma/sulfide mass ratios). The estimated initial concentrations of PGE in the parental magma for the Poyi deposit are almost two orders of magnitude lower than the abundances of PGE in some continental picrites. Given that the parental magma for the Poyi deposit is as primitive as a primary mantle-derived magma, the depletion of PGE in the Poyi deposit is most likely due to previous sulfide segregation at depth. Based on these observations, we conclude that sulfide saturation in the Poyi PGE-depleted, Mg-rich magma was triggered by addition of crustal sulfur during magma ascent and that the Poyi deposit was a dynamic conduit used by multiple pulses of olivine- and sulfide-charged magma.

Description: The large Dahongshan Fe-Cu-(Au-Ag) deposit in the Kangdian iron oxide copper-gold (IOCG) metallogenic province, southwest China, contains approximately 458.3 Mt of ore at 41.0% Fe, 1.35 Mt Cu (metal) at 0.78% Cu, and significant amounts of Au (16 t), Ag (141 t), Co (18,156 t), and Pd + Pt (2.1 t). The deposit consists mainly of two types of ores: (1) lenses of massive or banded magnetite-(hematite) hosted in extensively Na metasomatized metavolcanic rocks, metaarenite, and brecciated rocks, and (2) strata-bound disseminated, stockwork, and banded magnetite-chalcopyrite-(bornite) in mica schist and marble. Both types of orebodies and country rocks underwent extensive hydrothermal alteration, resulting in a similar paragenesis. Pervasive stage I sodic alteration formed widespread albite and local scapolite. It was subsequently replaced by Ca- or K-rich minerals represented by actinolite, K-feldspar, biotite, sericite, and chlorite of stages II and III. Magnetite is slightly younger than and partly overlaps the sodic alteration assemblages. Hematite is texturally later than magnetite, is locally abundant within the massive Fe oxide orebody, and is closely associated with sericite. Copper sulfides are coeval with quartz, biotite, sericite, and chlorite in stage III assemblages. Widespread siderite and ankerite predominate in stages II and III, respectively. Quartz-calcite veins mark the result of waning stage IV hydrothermal alteration. In addition to widespread alteration during the major ore-forming event, the deposit has also undergone extensive overprinting and remobilization during post-ore magmatic and metamorphic events. The Dahongshan orebodies are intimately associated with abundant doleritic dikes and sills that have hydrothermal mineral assemblages similar to those in the ore-hosting rocks. One dolerite sill that cuts a massive Fe orebody has a laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb age of 1661 ± 7 Ma, which is, within uncertainty, consistent with the age of 1653 ± 18 Ma determined for hydrothermal zircons from stockwork chalcopyrite-magnetite ore. The zircon U-Pb ages are thus considered to mark the timing of major mineralization that formed the Dahongshan deposit. Post-ore modification is recorded by an Re-Os isochron age of 1026 ± 22 Ma for pyrite in discordant quartz-carbonate-sulfide veins, and by younger Neoproterozoic mineralization dated at ca. 830 Ma using Re-Os isotopes on molybdenite. The former age is contemporaneous with late Mesoproterozoic magmatism in the region, whereas the latter is coeval with regional Neoproterozoic metamorphic events in southwest China. Carbon and oxygen isotope values of albitized marble are between those of mantle-derived magmatic carbon and dolostone end members. The ore-forming fluids that equilibrated with stage II magnetite have 18 O values of 9.1 to 9.5, whereas fluids linked to the deposition of quartz and ankerite during stages III and IV have lower 18 O values of 2.9 to 7.3. The oxygen isotope data indicate that the ore-forming fluids related to stage II are chiefly magmatically derived and mixed with abundant basinal brine during stages III and IV; this interpretation is consistent with sulfur isotope values of sulfides in the deposits. Pyrite and chalcopyrite from the Dahongshan deposit have a large range of 34 S values from –3.4 to +12.4, implying mixing of magmatic and external sulfur (likely from basinal brines) in sedimentary rocks. The Dahongshan deposit formed in an intracratonic rift setting due to underplating by mafic magmas that induced large-scale fluid circulation and pervasive sodic-calcic metasomatism in country rocks. Ore metals were derived mainly from a deep-seated magma chamber and partly from country rocks. Hydrothermal brecciation of the country rocks formed at the top of the dolerite intrusions and along zones of weakness within the country rocks owing to overpressure imposed by the ore fluids. Magnetite and hematite precipitated early near the dolerite intrusions, whereas Cu sulfides formed later in country rocks where sulfide saturation was favored. We propose that this genetic model may be widely applicable to Precambrian IOCG deposits elsewhere that formed in intracratonic rift settings.

Description: Geologic samples are extremely diverse and share a tendency for both heterogeneity and complexity. This is especially true for ores, which commonly result from a complex interplay of processes in highly dynamic environments. In recent years, a number of tools allowing the chemical mapping of major (e.g., mineral liberation analysis, MLA), minor (e.g., electron microprobe, EPMA), and trace (e.g., laser ablation-inductively coupled plasma-mass spectrometry, LA-ICP-MS) elements in geologic samples at ~1- to 50- μ m resolution and over mm2 areas have seen rapid development and have become readily available. To date, the application of synchrotron-based X-ray fluorescence (SXRF) mapping has been limited to addressing key questions because of low availability and high cost. This paper demonstrates how recent advances in X-ray fluorescence detector technology are bringing new possibilities to ore petrology. Millisecond dwell times allow collection of thin section size maps at resolutions of a few μ m in hours, while improvements in data analysis software simplify the production of quantitative elemental maps. Based on the imaging of six samples representative of different commodities (Pt, U, Cu, Ge) and different geologic contexts (PGE deposit; sandstone-hosted U deposit; vein-type polymetallic hydrothermal deposit; iron oxide-copper-gold (IOCG) deposit), we demonstrate that megapixel SXRF (MSXRF) can efficiently provide the information necessary to understand metal speciation in the context of thin section-scale textural complexity. Image analysis revealed a number of new results for the studied deposits, for example, (1) the distribution of micrometer-sized Pt-rich grains and Ti mobility during the formation of schistosity at the Fifield Point prospect (New South Wales, Australia); (2) the presence of Ge contained in organic matter and of Hg minerals associated within quartzite clasts in the Lake Frome U ores (South Australia); (3) confirmation of the two-stage Ge enrichment in the Barrigão deposit, with demonstration of the presence of Ge in solid solution in the early chalcopyrite (Portuguese Iberian pyrite belt); and (4) the enrichment of U during late dissolution-reprecipitation reactions in the bornite ores of the Moonta and Wallaroo IOCG deposits (South Australia). These results illustrate that MSXRF is a powerful technique for locating nano- to microparticles of precious metals (Pt) and trace contaminants (e.g., Hg) that form distinct (micro) minerals. In addition, it is a powerful tool for understanding commodities with relatively low ore grades and complex distribution (100–1,000 ppm; e.g., U, Ge).