ALBERT

Description: The Chahnaly low-sulfidation epithermal Au deposit and nearby Au prospects are located northwest of the intermittently active Bazman stratovolcano on the western end of the Makran volcanic arc, which formed as the result of subduction of the remnant Neo-Tethyan oceanic crust beneath the Lut block. The arc hosts the Siah Jangal epithermal and Kharestan porphyry prospects, near Taftan volcano, as well as the Saindak Cu-Au porphyry deposit and world-class Reko Diq Cu-Au porphyry deposit, near Koh-i-Sultan volcano to the east-northeast in Pakistan. The host rocks for the Chahnaly deposit include early Miocene andesite and andesitic volcaniclastic rocks that are intruded by younger dacitic domes. Unaltered late Miocene dacitic ignimbrites overlie these rocks. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology data yield ages between 21.8 and 9.9 Ma for the acidic-intermediate regional volcanism. The most recent volcanic activity of the Bazman stratovolcano involved extrusion of an olivine basalt during Pliocene to Quaternary times. Interpretation of geochemical data indicate that the volcanic rocks are synsubduction and calc-alkaline to subalkaline. The lack of a significant negative Eu anomaly, a listric-shaped rare earth element pattern, and moderate La/Yb ratios of host suites indicate a high water content of the source magma. Gold and electrum are temporally and spatially related to a series of structurally controlled, 030°-trending, subvertical hydrothermal breccias with chalcedony-adularia that cut porphyritic andesite and andesitic volcaniclastic rocks. Gold is associated with pyrite, a siliceous matrix of hydrothermal breccia, and previously formed vein clasts, as well as with iron oxides and hydroxides in oxidized zones. Rare silver minerals include Ag-bearing electrum and naumannite, iodargyrite, an unnamed silver diiodide, and hessite. Hydrothermal alteration is generally well developed surrounding the ore-bearing hydrothermal breccia. The main types of alteration in the area include an inner ~0.5- to 20-m-thick gold-bearing hydrothermal breccia composed of quartz-chalcedonyadularia-illite-pyrite, a ~5- to 50-m-thick zone of quartz, chalcedony, pyrite, illitic phengite, phengite, illitic muscovite, illite, illitic paragonite, paragonite, muscovite, montmorillonite and, rarely, siderite, and a 30- to 70-m outer propylitic zone of Fe-Mg chlorite, calcite, ankerite, dolomite, epidote, palygorskite, and pyrite. The Chahnaly Au deposit formed during the early stages of magmatism. LA-ICP-MS zircon U-Pb geochronology of host andesite and 40Ar/39Ar dating of two samples of gold-associated adularia show that the ore-stage adularia (19.83 ± 0.10 and 19.2 ± 0.5 Ma) is younger, by as much as 1.5 million years, than the volcanic host rock (20.32 ± 0.4 Ma). Therefore, either hydrothermal activity continued well after volcanism or a second magmatic event rejuvenated hydrothermal activity. This second magmatic event may be related to eruption of porphyritic andesite at ~20.32 ± 0.40 Ma, which is within error of ~19.83 ± 0.10 Ma adularia. The new LA-ICP-MS zircon U-Pb host rock and vein adularia 40Ar/39Ar ages suggest that early Miocene magmatism and mineralization in the Bazman area is of a similar age to that of the Saindak porphyry and Tanjeel porphyry center of the giant Reko Diq deposit. This confirms the existence of early Miocene arc magmatism and mineralization along the Iranian part of the Makran volcanic arc. Ore, alteration mineralogy, and alteration patterns indicate that the Chahnaly deposit is a typical low-sulfidation epithermal Au deposit, located in a poorly explored part of the Makran volcanic arc in Iran.

Description: The Glacier Creek volcanogenic massive sulfide (VMS) deposit, Alaska, is hosted within Late Triassic, oceanic back-arc or intraarc, rift-related bimodal volcanic rocks of the allochthonous Alexander terrane, known as the Alexander Triassic metallogenic belt. The Alexander Triassic metallogenic belt is host to the world-class Greens Creek Zn-Pb-Ag VMS deposit near Juneau in the south and the giant Windy Craggy Cu-Co VMS deposit in British Columbia, about 250 km to the north. The Glacier Creek deposit, located ~80 km southeast of Windy Craggy, consists of four tabular massive sulfide lenses within a bimodal mafic volcaniclastic and rhyolitic sequence. The mineralization-hosting stratigraphy is folded by a deposit-scale anticline and offset by a thrust fault near the axial surface of the fold. A resource of 8.13 Mt has been inferred from drilling, with grades of 1.41% Cu, 5.25% Zn, 0.15% Pb, 0.32 g/t Au, and 31.7 g/t Ag. Six main mineralization types are recognized, dominated by massive barite-sphalerite-pyrite, which is replaced at the base and center of the main lenses by massive and semimassive chalcopyrite-pyrite-quartz. The flanks and tops of the lenses are carbonate rich and consist of interbedded calcite-dolomite, barite and sulfide, resedimented massive barite-sulfide, and mineralized massive carbonate rocks. Tuffaceous hydrothermal sediment, with a distinct positive Eu anomaly, overlies the massive sulfide. Pyrrhotite and chalcopyrite in stringers constitute the main "feeder zone." Stringer-style sphalerite-pyrite mineralization occurs above and below the lenses. Fe-poor sphalerite is dominant throughout the lenses, whereas Fe-rich sphalerite occurs at the stratigraphic top and bottom of the lenses in pyrrhotite-rich zones. Galena, tennantite-tetrahedrite, and arsenopyrite are the most important trace minerals within massive barite-sphalerite-pyrite mineralization, which is generally enriched in Sb, Hg, and Tl. Mineralization-related gangue minerals include barite, quartz, barian muscovite, calcite, dolomite, albite, chlorite, hyalophane, and celsian. Four types of alteration are recognized in the dominantly basaltic host rocks: pervasive muscovite-rich alteration, quartz-pyrite alteration associated with sulfide stringers, stratabound carbonate-bearing alteration, and background epidote-bearing alteration. Mass balance calculations indicate gains of S, Fe, Si, and K with coincident losses of Ca, Na, and Mg in all of the alteration types. Trace elements, Tl, Sb, Hg, Ba, Zn, Cu, and As were added to the rocks, whereas Sr was lost. Short wavelength infrared (SWIR) spectroscopy shows an increase in the wavelength of the AlOH absorption feature toward mineralization at a scale of 30 to 50 m, coincident with a general decrease in the Na, K, and Al and increase in the Fe, Mg, and Ba content of muscovite. The Glacier Creek deposit is transitional in character between Greens Creek, which is more Zn, Pb, and precious metal rich, and the Windy Craggy deposit, which is more Cu and Co rich, reflecting differences in the basement rocks and depositional settings within the Alexander Triassic metallogenic belt. Mineral-chemical studies and sulfur isotope data suggest that the Glacier Creek deposit formed under initially oxidized and sulfate-rich conditions that evolved to more reduced conditions in the latest stages of mineralization. The abundant argillite and presence of hyalophane rather than barite in the immediate hanging wall of the deposit may be an indication of a deepening basin and development of local anoxia, similar to Greens Creek.

Description: The Caledonian orogenic belt of northern Britain hosts some significant quartz vein-hosted gold deposits. However, as in orogenic belts worldwide, the relationship between gold mineralization and regional tectonics, magmatism, and metamorphism is a matter of debate. This is primarily due to the absence of precise temporal constraints for the mineralization. Here we report high-precision 40 Ar/ 39 Ar and Re-Os ages for the largest known gold deposit at Curraghinalt (2.7 Moz) in Northern Ireland and use these ages to constrain the regional geologic setting of the gold mineralization and establish a genetic model. The gold resource is contained in a suite of quartz sulfide veins hosted by Neoproterozoic (Dalradian) metasediments, which have been thrust over an Ordovician island arc (Tyrone Igneous Complex). Previous studies recognized two generations of gold sulfide mineralization and we have identified a third in microshears that cut the veins. In the absence of precise geochronological data, mineralization ages from Ordovician to Carboniferous have been proposed. We have dated muscovite ( 40 Ar/ 39 Ar) in quartz vein-hosted clasts of Dalradian wall rock to 459.3 ± 3.4 Ma (all 40 Ar/ 39 Ar and Re-Os ages herein are reported at the 2 confidence level including all sources of uncertainty), an age that we interpret as representing the regional cooling path and which provides a maximum age constraint for all gold mineralization. This is consistent with the quartz veins postdating the end of main-stage deformation in the Grampian event of the Caledonian orogeny (ca. 465 Ma). Molybdenite (Re-Os) and sericite ( 40 Ar/ 39 Ar) from the newly identified gold-bearing microshears (third generation of gold mineralization) yield indistinguishable Re-Os models and 40 Ar/ 39 Ar ages, with a combined age of 455.8 ± 3.0 Ma. The radioisotope ages and field evidence temporally constrain gold mineralization at Curraghinalt to the lower Late Ordovician. Data show that the gold mineralization was emplaced during the Grampian event of the Caledonian orogeny. The ca. 10 Ma maximum possible mineralization interval (462.7–452.8 Ma) for all three episodes of gold emplacement is postpeak metamorphism and main deformation, coinciding with a period of rapid uplift and extensional tectonics following orogenic collapse. While previous studies have suggested the involvement of magmatic fluids in the deposition of the primary gold resource, the absence of magmatism throughout most of the mineralization interval and the nature of the geologic setting suggest that crustal orogenic fluids should also be considered. Overall Curraghinalt displays most of the characteristics of orogenic gold deposits but also some important differences, which may be explained by the geologic setting. The timing of mineralization at Curraghinalt broadly coincides with the shift from compressional to extensional tectonics. The extensional regime, rapid uplift, and a crustal profile comprising metasediments overlying a still hot island arc were ideal for creating large and long-lasting hydrothermal systems deriving heat, metals, and some of the fluids from the underlying arc.

Description: Mississippi Valley-type (MVT) deposits of the Viburnum Trend are typically lead dominant (Pb/Zn 〉5) and occur mainly in the reef-grainstone facies of the upper Bonneterre Dolomite (Cambrian). Recent drilling has encountered economic mineralization within the lower Bonneterre Dolomite of the Brushy Creek mine, more than 30 m below the main ore-bearing horizon of the district. These ores are more zinc and copper rich than typical deposits (Zn/Pb commonly 〉2 and Cu up to 8 wt %), with notable enrichments in Ni, Co, and Ag. There is pronounced zoning of Ni-Co, Cu, Zn, and Pb with increasing distance above the Lamotte Sandstone. Observations of drill cores and samples from active mine faces, and mineral textural relationships in thin section indicate that the lower ore mineralization contains dominantly early Zn- and Cu-bearing sulfides, which are overprinted by main-stage Pb-Zn mineralization. The lower ores contain multiple generations of chalcopyrite and sphalerite that display frequent breccia textures at the meso- and microscale. This style of breccia is uncommon for the Viburnum Trend and is composed dominantly of sulfides supported by clay (insoluble residue), which are cemented by subsequent generations of sulfides and carbonates, resulting in massive, high-grade ore. Cathodoluminescence microscopy reveals that ore is associated with two generations of dolomite cement that appear to predate the regional dolomite cement associated with main-stage Pb-Zn mineralization in the Viburnum Trend. The stratigraphic position, massive character, metal contents, mineral zoning and textures, and gangue dolomite cements of the orebody are unusual for the Viburnum Trend. Mineralogically and stratigraphically, these ores are more similar to the historically mined ores of the Indian Creek and Fredericktown-Mine La Motte districts to the north and east of the Viburnum Trend. In the Brushy Creek mine, the lower ores are not related to obvious stratigraphic controls, such as pinch-outs of the Lamotte Sandstone against Precambrian knobs, but appear to be localized along early zones of fracture-enhanced porosity and permeability within the Lamotte Sandstone, which promoted extreme dissolution of host rocks at the base of the Bonneterre Dolomite and development of high-grade Cu-(Ni-Co)-Zn-rich ore. The presence of distinct Ni- and Ni-Co-rich areas and the spatial variability of sphalerite types across the orebody indicate that there were likely multiple sites of ore fluid introduction whose influence varied with time during development of the ores.

Description: The Kapai Slate is a continuous, pyrite-rich carbonaceous shale horizon within the St. Ives Au district that is spatially related to high-grade Au mineralization. In situ laser ablation-inductively coupled mass spectrometry (LA-ICPMS) trace element analyses, in situ sensitive high resolution ion microprobe, stable isotope (SHRIMP-SI) S isotope analyses, and optical microscopy pyrite texture analyses were used to examine the different pyrite types in the Kapai Slate and Au deposits. These data were also used to confirm that the trace element signature of sedimentary pyrite can be preserved in rocks that underwent upper to mid-greenschist facies metamorphism and significant hydrothermal overprint. The data were further utilized to gain a more detailed understanding of the ocean conditions during deposition of the Kapai Slate and determine whether some of the Au and S in the St. Ives district could have been sourced from the Kapai Slate. Seven different types of pyrite were identified: fine-grained sedimentary pyrite (Py 1 ), nodular sedimentary pyrite (Py 2 ), remobilized sedimentary pyrite (Py 3 ), coarse-grained, inclusions poor late pyrite (Py 4 ), inclusion-rich magnetite series pyrite (Py 5 ), ore stage pyrite (Py 6 ), and pyrite associated with the mafic units (Py 7 ). Each type of pyrite was found to have distinctive trace element compositions and S isotope signatures. The results of the LA-ICPMS analyses provide evidence for early trace element enrichment in the Kapai Slate sedimentary pyrite (median values of 158 ppm Ni, 387 ppm Co, 82 ppm Cu, 727 ppm As, 1.91 ppm Mo, 13 ppm Se, 0.25 ppm Au, 7.72 ppm Te and 3.36 ppm Ag for Py 1 and 223 ppm Ni, 158 ppm Co, 99 ppm Cu, 856 ppm As, 1.27 ppm Mo, 10.2 ppm Se, 0.57 ppm Au, 10.09 ppm Te, and 6.62 ppm Ag for Py 2 ). Concentrations of Ni and Co are low, relative to other late Archean sedimentary pyrite (median of 813 and 465 ppm, respectively) and Mo levels are near that of the euxinic shales of the similar-aged Jeerinah Formation in the Hamersley Basin, Western Australia. These data suggest that the Kapai Slate was deposited in an anoxic to euxinic basin with relatively low biological productivity. The 33 S and 34 S signatures of the sedimentary pyrite suggest two different sources of S. Positive 34 S and negative 33 S signatures indicate bacterial reduction of SO 4 2– from seawater, whereas positive 34 S and positive 33 S signatures indicate an elemental S 8 source, indicating the pyrite formed later during diagenesis. This S isotope signature is consistent with a transition between a near-sediment environment to a more distal environment source. Analyses of the ore-phase pyrite yield weakly positive 33 S values. This suggests there was a minor contribution of sedimentary S to the more significant oxidized ore-forming fluids, which is consistent with a small contribution of Au from a sedimentary source. Approximations of the degree of sedimentary pyrite destruction in the pyrrhotite/pyrite dominated zones and pyrrhotite/magnetite/pyrite zones of the northern part of the St. Ives district were used to calculate the amount of Au released from the early sedimentary pyrite. The calculation suggests that a minor, though possibly locally significant, amount of Au could have been sourced from the Kapai Slate.

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).