Granulite-Hosted Gold: Tectonic Setting and Lithogeochemistry of the Tropicana Deposit, Western Australia

Abstract

The Tropicana gold deposit (7.9 Moz) is hosted by the Neoarchean, amphibolite- to granulite-grade Tropicana Gneiss. These rocks form part of the Plumridge terrane, a tectonic unit that separates the in situ eastern margin of the Yilgarn craton from Proterozoic elements of the Albany-Fraser orogen, Western Australia, The gneissic host package to the orebody originally constituted a suite of medium-to high-K calc-alkaline and adakitic basaltic to rhyolitic lavas emplaced in a relatively thick crustal continental margin arc. Amphibolitized mafic intrusions have back-arc basin basalt-like compositions consistent with their having formed during extension and incipient rifting of the arc. Gold mineralization occurs mainly within a sheet-like, syenitic gneiss that ranges compositionally from basaltic andesite to rhyolite, and derived through metasomatism of compositionally equivalent lavas within an interpreted shear zone. Coarse perthite textures in microcline, ruffle exsolution in brown biotite, and occasional pyrite inclusions away from fractures in microcline porphyroblasts indicate that this shear zone predated peak metamorphism. Relative to the unmineralized host gneisses, the mineralized syenitic gneisses show significant enrichments in S and the ore elements (Mo, Te, Tl, Ag, Au, W and, occasionally, Bi and Sb, but not As, Cu, or Zn), in the K-group elements, and also in P, light rare earth elements (LREE), Th, U, Zr, and Hf. Depletions in most major elements (Si, Ti, Fe, Mg, and Ca), as well as V, Cr, Ni, and Co, are considered to reflect dissolution of relatively low volume mafic phases (hornblende, augite, Fe-Ti oxides) and plagioclase in the protoliths. Some of the distinctive enrichments (P, LREE, Zr, Hf) are linked to volume loss (in excess of 50%) associated with dissolution of silica and concentration of insoluble apatite and zircon in the mineralized unit, but the ore elements and strongly enriched (similar to X10) Th and U were added from the hydrothermal fluids involved. Exhumation of the Tropicana Gneiss into the upper crust was accompanied by fluid ingress and retrogression. This produced mm-to cm-scale, wavy biotite-pyrite shears, solution seams, and disseminations throughout the syenitic gneiss which were further retrogressed to sericite-chlorite assemblages in places, both of which host most of the gold in the deposit. Thus, although the present deportment of gold is unambiguously associated with the postpeak metamorphic, greenschist-grade retrogression at ca. 2520 Ma, there is compelling evidence that the paucity of quartz and carbonate veining, atypical element enrichments, and sheet-like nature of the Tropicana deposit in part reflect metasomatism associated with a prepeak metamorphic, relatively deep shear zone. At present, we have no direct evidence for the timing of introduction of gold, and a protracted history of gold precipitation, redistribution, and enrichment is implied.