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Synsedimentary to early diagenetic gold in black shale-hosted pyrite nodules at the Golden Mile Deposit, Kalgoorlie, Western Australia
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 (32S, 33S, and 34S) have provided evidence on S source(s) for the nodules and ore-stage pyrites. Whereas the cores of most nodules contain pyrite with negative Δ33S, a signal thought to be derived from seawater sulfate, the rims of the same have positive Δ33S, which may result from metabolization of atmospheric elemental S. By contrast, ore-related pyrites (both Fimiston- and Mount Charlotte-stage) have no or little Δ33S 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.
Australian Research Council
AMIRA International Ltd
ARC C of E Industry Partner $ to be allocated
Anglo American Exploration Philippines Inc
AngloGold Ashanti Australia Limited
Australian National University
BHP Billiton Ltd
Barrick (Australia Pacific) PTY Limited
CSIRO Earth Science & Resource Engineering
Mineral Resources Tasmania
Minerals Council of Australia
Newcrest Mining Limited
Newmont Australia Ltd
Oz Minerals Australia Limited
Rio Tinto Exploration
St Barbara Limited
Teck Cominco Limited
University of Melbourne
University of Queensland
Zinifex Australia Ltd
Publication titleEconomic Geology
Department/SchoolSchool of Natural Sciences
PublisherEconomic Geology Publ Co
Place of publication5808 South Rapp St, Ste 209, Littleton, USA, Co, 80120-1942
Rights statementCopyright 2015 Society of Economic Geologists, Inc.