ALBERT

Description: The Kalgoorlie goldfield (~50 Moz Au produced), famous for its long mining history and diversity of precious metal telluride minerals, is a world-class Neoarchean Au-Ag-Te district, which includes the Golden Mile Super Pit, the largest single gold deposit in the Eastern Goldfields of Western Australia, and the smaller but nonetheless significant Mount Charlotte deposit, 3 km to the north. The gold ore at Kalgoorlie is of two types—Au- and Te-rich first stage (Golden Mile, or Fimiston, ore), which constitutes the bulk of the Au endowment, followed by a relatively Te-poor second stage (Mount Charlotte ore). Fimiston-stage ore is characterized by deformed quartz-carbonate structures termed "lodes:" thin (1–10 cm) zones of quartz/ankerite/gold/telluride-rich vein breccias with halos of fine-grained pyrite, muscovite, ankerite, and tourmaline. Charlotte-stage ore is primarily hosted by ankerite-pyrite-rich alteration selvages around flat-sided, undeformed bucky quartz veins and is the only ore style present in the Mount Charlotte mine itself. The primary host unit for both mineralization styles is the Golden Mile Dolerite, one of several dolerite intrusions in the mafic-ultramafic volcanic succession of the Kalgoorlie terrane. Along with the large amount of mafic metavolcanics, consistent with typical greenstone belt stratigraphy, the Kalgoorlie goldfield contains at least three fine-grained carbonaceous (meta)black shale units (from oldest to youngest: the Kapai Slate; an unnamed interflow shale near the top of the Paringa Basalt; and black shale forming the base of the Black Flag Group). Each of these units contains varying amounts of synsedimentary, diagenetic, and hydrothermal-metamorphic pyrite and pyrrhotite, including well-preserved pyrite nodules. Nodules at the Golden Mile Super Pit vary in diameter from a few millimeters to several centimeters, can have several concentric zones of pyrite with internally variant textures, and are commonly deformed into ovoid shapes. There are also horizons of pyrrhotite nodules within certain sections of these units; like their pyrite counterparts, these are commonly concentrically zoned and show evidence of later deformation. Rare examples of thin massive sulfide beds are also present in the interflow shale near the top of the Paringa Basalt. LA-ICP-MS imaging of pyrite nodules from each of the three black shale units reveals complex (and sometimes spectacular) concentric compositional zonation that parallels the growth zones. Trace element concentrations vary within different nodule bands in a coherent pattern, with Au, Ag, Te, and As typically enriched together in certain zones. Gold content is particularly high in the Paringa Basalt interflow shale nodules, which average 3 to 4 ppm Au as well as 30 to 40 ppm Ag, 30 to 40 ppm Te, and 1,000 ppm As. Samples taken several kilometers to the south (along strike) and west of the Golden Mile of the Kapai Slate and Black Flag Group shale also contain disseminated and nodular pyrite enriched in Au, Ag, Te, and As at levels comparable to samples of those formations within the deposit. However, in distal samples of the Paringa interflow shale, there is only laminated and nodular pyrrhotite, marked by enrichments in Au, Ag, Sb, Te, Tl, Pb, and Bi relative to a later (and presumably metamorphic) pyrrhotite which crosscuts and partially replaces the earlier pyrrhotite. Lead isotope studies of nodules from the three shale units, as well as pyritic ore samples from two separate Fimiston-stage lodes and one Mount Charlotte-stage sample, have been undertaken to help resolve relative timing issues. Nodular pyrite from each shale formation has a distinct isotopic composition, with the Kapai Slate samples being the least radiogenic, followed by those from the Paringa interflow shale and, lastly, the Black Flag shale. These data result in progressively younger Pb-Pb model ages, in keeping with the established stratigraphic order. In contrast, ore pyrites contain a wide spread of relatively unradiogenic to radiogenic isotope compositions, partially overlapping with the nodular pyrites. Sulfur isotope studies ( 32 S, 33 S, and 34 S) have provided evidence on S source(s) for the nodules and ore-stage pyrites. Whereas the cores of most nodules contain pyrite with negative 33 S, a signal thought to be derived from seawater sulfate, the rims of the same have positive 33 S, which may result from metabolization of atmospheric elemental S. By contrast, ore-related pyrites (both Fimiston- and Mount Charlotte-stage) have no or little 33 S anomalies. The shape, internal textures, and distinct trace element enrichment and zonation, evidently little affected by ore-forming processes, suggest the nodules are synsedimentary to early diagenetic. There is virtually no evidence that gold or other elements have been added to the nodules during hydrothermal ore events; gold, along with many other elements, remains a coherent part of the primary nodule structure. Lead and S isotope studies on the pyrite nodules provide strong supportive evidence of an early marine sedimentary age for the nodules: the Pb isotopes give an age roughly equivalent to progressive sedimentation of the black shale host rocks, and the S isotopes are best explained by marine sulfate being the original S source for the nodules. The evidence is compelling that there was enrichment of Au-Ag-Te-Hg-As during intervolcanic sedimentation and diagenesis in the Kapai Slate, the interflow shale near the top of the Paringa Basalt, and Black Flag shale, before the formation of the Fimiston-stage gold-telluride lodes. While this work does not permit us to comment on the gold source issue in the Kalgoorlie deposits, the fact remains that syngenetic/diagenetic gold preconcentration in fine-grained, sulfidic, moderate- to deep-water sediments likely occurred across the Eastern Goldfields between ~2700 to 2680 Ma.

Description: Gold mineralization at the Tropicana mine occurs within the Plumridge terrane along the eastern margin of the Archean Yilgarn craton in the Albany-Fraser orogen, Western Australia. Mineralization is hosted in a favorable syenitic lithofacies of the Tropicana Gneiss with a minimum igneous age of 2638 ± 4 Ma (2 ) and which was metamorphosed to mid-amphibolite to lower granulite facies in the period ca. 2638 to 2520 Ma. The Tropicana Gneiss was exhumed to crustal levels equivalent to greenschist-facies conditions by the time of economic gold mineralization. The major gold-bearing pyrite-biotite-sericite mineralization formed in association with shear zones during northeast-southwest compression (D 3 ) that postdated W- to NW-verging thrusting (D 2 ). The late fluid-induced event (Tropicana event; Doyle et al., 2013 ; Blenkinsop and Doyle, 2014 ) produced a mineral assemblage indicative of greenschist-facies conditions. The paucity of water in granulite-facies gneisses under retrograde conditions suggests that fluids were introduced from an external source for both mineralization and the younger metamorphic event. This occurred at ca. 2520 Ma as determined from biotite (ca. 2515 Ma, 40 Ar/ 39 Ar age), pyrite (ca. 2505 Ma, Re-Os, Pb/Pb ages), and tungsten-rich rutile (2521 ± 5 Ma, U-Pb age): the latter is considered to provide the best direct measurement age of gold mineralization. The Tropicana Gneiss was derived from the upthrusted easternmost margin of the underlying Yilgan craton, or represents a relatively small (160 km long x 50 km wide) remnant crustal block accreted to the Yilgarn craton sometime between ca. 2.6 and 2.5 Ga. The timing of the Tropicana mineralization is distinctly younger than greenstone-associated gold deposits elsewhere in the Yilgarn craton. As in the Dharwar craton, India, and the Limpopo belt, Zimbabwe, economic gold mineralization at Tropicana postdates the major peak of Late Archean world-class gold mineralization (ca. 2.65 Ga) by more than 100 m.y. The Tropicana Gneiss was relatively unaffected during the younger Albany-Fraser orogeny, with only minor remobilization of gold. Deformation may have been localized at the margins of the domain, between constituent ridged structural blocks, and/or within discrete high-strain shears.

Description: Trace element concentrations in marine pyrite, measured by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), have the potential to open a new window into deep-time ocean chemistry, atmosphere oxygenation, and genesis of basin-hosted ore deposits. Only early-formed syngenetic and early diagenetic marine pyrite preserves the trace element chemistry of the past oceans, as late diagenetic and metamorphic recrystallization of pyrite changes the trace element budget. A database of over 5,000 marine pyrite trace element analyses by LA-ICP-MS has enabled the development of deep-time proxies for nutrient supply, productivity, ocean pH, and atmosphere oxygenation. These proxies suggest that the Archean ocean was enriched in Fe, Ni, Co, As, Au, and Hg compared with modern oceans, probably related to composition of erosive flux from the continents and active seafloor hydrothermal activity. This was also a time for major iron, gold, and nickel ore formation in sedimentary and greenstone settings. In the Paleoproterozoic, there was a decrease in Ni, Co, As, and Au, replaced by increasing Cu, Zn, and $${\mathrm{SO}}_{4}^{2-}$$ in the oceans and O 2 in the atmosphere. The first appearance of red beds and evaporites is a response to the rise in O 2 and $${\mathrm{SO}}_{4}^{2-}$$ , and provided the conditions necessary for sediment-hosted Cu and stratiform Pb-Zn-Ag sedimentary exhalative (SEDEX) deposits. Through 1700 to 1500 Ma, phosphorous, gold, and most other nutrient trace elements dropped to a minimum in the ocean, possibly related to tectonic stasis and changes in atmosphere O 2 and/or ocean pH. Sediment-hosted Au, orogenic Au, and volcanic-hosted massive sulfide (VHMS) deposits are virtually absent from this period, whereas mineral systems that required relatively oxidized ore fluids, such as SEDEX Zn-Pb, iron oxide copper-gold (IOCG), and unconformity uranium became more abundant, due to these changed conditions. All redox-sensitive and nutrient trace elements rose dramatically in concentration at the Proterozoic-Phanerozoic boundary and peaked in the mid- to Late Cambrian oceans, accompanied by black shale deposition enriched in Mo, Se, Ni, Ag ± Au, and platinum group elements. Cyclic variation in nutrient trace elements increased in frequency through the Phanerozoic on a wavelength of 50 to 100 m.y., compared with 500 to 1,000 m.y. in the Proterozoic. The more frequent Phanerozoic cycles relate to repeated episodes of continent collision, mountain building, and increased erosive flux of trace elements into the oceans. Ore deposit cycles in the Phanerozoic of SEDEX Zn-Pb, orogenic sediment-hosted Au, and VHMS have a time frame similar to the tectonic and seawater chemistry cycles.

Description: Laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) trace element maps of pyrite and gold from the Carbon Leader Reef in the Witwatersrand basin provide new evidence that a significant proportion of the pyrite and gold was intrabasinal, derived from the West Rand Group or equivalent shales stratigraphically below this reef. Rounded detrital pyrite grains in the Carbon Leader Reef vary from compact to porous to sooty, with similar textures and composition to diagenetic pyrites reported from sedimentary rocks in the West Rand Group. Detrital porous and sooty pyrites contain 0.4 to 11.3 ppm solid-solution gold, sulfur isotope 34 S values from –17 to +16%, and high levels of As and Te, plus a wide range of other trace elements (in decreasing order: Ni, Co, Sb, Cr, Cu, U, Pb, Bi, Mo, Zn and Ag). The sooty pyrite is the most Au, Te, and Mo rich and is intergrown with alumino-silicates and organic matter. The detrital pyrite textures, S-isotopes and geochemistry resemble diagenetic pyrite developed under suboxic to anoxic bottom water conditions. The LA-ICP-MS maps also show that the detrital pyrite grains have euhedral hydrothermal pyrite overgrowths containing micro-inclusions of gold, brannerite, and Ni-As sulfides. The pyrite overgrowths are also enriched in solid-solution gold, tellurium, and other trace elements including, in decreasing order, As, Co, Ni, Cr, Cu, Mn, Pb, Bi, and Ag. Fractured and brecciated pyrite associated with brittle bedding-parallel fracture zones in the Carbon Leader Reef are also enriched in these elements due to alteration of pyrite surrounding the fractures. Laser ICP-MS Pb isotope determinations on the cores of detrital pyrite indicate an age between 2750 and 2950 Ma with hydrothermal overgrowths originating between 2100 and 2020 Ma incorporating highly radiogenic Pb. This study demonstrates that both of the two competing theories for the origin of the Witwatersrand gold reefs are likely to be correct. We suggest that the hydrothermal event was widespread (kilometer scale) and involved basinal fluids that scavenged gold, tellurium, arsenic, and trace elements (Co, Ni, Cr, Cu, Mn, Pb, Bi, Ag) from gold-bearing sedimentary units in the Central Rand Group.