ALBERT

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: 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.