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

• Published in: Mineralogical magazine 2022-08, Vol.86 (4), p.606-618
• Main Authors: Henry, Dermot A, Birch, William D
• Format: Article
• Language: English
• Subjects: alloys; Australasia; Australia; azurite; Benambra Australia; Bismuth; bismuth ores; bismuthides; bornite; carbonates; copper ores; Creeks & streams; Devonian; Economic geology; eulytine; fluid phase; hedleyite; joseite; Limestone; Lower Devonian; magmas; malachite; metal ores; metamorphic rocks; metasomatic rocks; mineral assemblages; mineral deposits, genesis; Mineralization; Mineralogy; Minerals; molybdates; Morass Creek Gorge; mrazekite; namibite; orthosilicates; Paleozoic; pucherite; schumacherite; silicates; Silurian; Silver; Silver Flat Porphyry; skarn; sorosilicates; sulfides; sulfobismuthites; sulfosalts; sulphotsumoite; supergene processes; tellurides; Tellurium; tetradymite; Topography; Upper Silurian; vanadates; vesuvianite; Victoria Australia; Weathering; wittichenite; Wombat Hole Prospect; wulfenite
• ISSN: 0026-461X; 1471-8022
• DOI: 10.1180/mgm.2022.11

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 crystallized 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-XCO2 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 fS2/fTe2, with bornite crystallizing 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.