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The Wombat Hole Prospect, Benambra, Victoria, Australia: a Cu–Bi–(Te) exoskarn with unusual supergene mineralogy

Part of: Minerals, crystal structures and geochemistry - Peter Williams special issue

Published online by Cambridge University Press:  28 January 2022

Dermot A. Henry  and
William D. Birch
Dermot A. Henry
Affiliation:
Sciences Department, Museums Victoria, GPO Box 666 Melbourne, Victoria 3001, Australia
William D. Birch*
Affiliation:
Sciences Department, Museums Victoria, GPO Box 666 Melbourne, Victoria 3001, Australia
*
*Author for correspondence: William D. Birch, Email: bbirch@museum.vic.gov.au
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Abstract

The Wombat Hole Prospect is a small copper–bismuth–(tellurium) exoskarn cropping out in the Morass Creek gorge, near Benambra, in eastern Victoria, Australia. Its main primary sulfide constituent is bornite in a grossular‒vesuvianite matrix. The skarn formed in a megaclast of Lower Silurian limestone from metal-bearing fluids accompanying the high-level emplacement of the Late Silurian–Lower Devonian Silver Flat Porphyry. Though the primary bornite mineralisation has been nearly obliterated by weathering, there are small relict patches containing exsolved grains of wittichenite (Cu3BiS3) and chalcopyrite, as well as inclusions of bismuth tellurides in the tetradymite group, namely sulphotsumoite (Bi3Te2S) and hedleyite (Bi7Te3). Joséite-A (Bi4TeS2), a mineral with a formula Bi3(Te,S)4, several unnamed Cu–Bi‒Te phases and minute grains of native bismuth have also been detected. Pervasive veining by chrysocolla throughout the garnet‒vesuvianite host contains a range of unusual secondary bismuth minerals that have crystallised at various times. These include mrázekite, namibite, pucherite, schumacherite and eulytine. Other secondary minerals present include wulfenite, bismutite, azurite, malachite and a poorly-crystalline bismuth oxide containing several weight percent tellurium. Rare grains of gold (electrum) containing up to 23 wt.% Ag are also present. The assemblage of grossular–vesuvianite with minor diopside is indicative of formation in a low-${\rm X}_{{\rm C}{\rm O}_ 2}$ environment under fluid-buffered conditions. A temperature range between ~650°C and as low as ~150°C can be estimated from the exsolution of wittichenite and chalcopyrite from the bornite. The tetradymite-group inclusions formed first under low values of $f_{{\rm S}_ 2}$ /$f_{{\rm T}{\rm e}_ 2}$, with bornite crystallising as values increased. The primary Cu‒Bi‒Te mineralogy and the unusual secondary mineral assemblage makes the Wombat Hole skarn unique in southeastern Australia. The deposit provides scope for studying the mobility of elements such as Bi and Te over short distances during weathering of hypogene ore minerals.

Keywords

skarnborniteCu–Bi–Te mineralssupergene bismuth mineralsbismuth vanadatesmagmatic fluidsMorass CreekVictoria
Type
Article
Information
Mineralogical Magazine , Volume 86 , Issue 4 , August 2022 , pp. 606 - 618
Copyright
Copyright © The Author(s), 2022. Published by Cambridge University Press on behalf of The Mineralogical Society of Great Britain and Ireland

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Footnotes

This paper is part of a thematic set that honours the contributions of Peter Williams

Associate Editor: Peter Leverett

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