Wed 24th January @ noon, Rm 312.207

Abstract:

After over a century of intensive research, the Kalgoorlie goldfields, home to the giant Golden Mile Au-Te deposit, still hold undiscovered treasures.

The Hidden Secret Ag-Au-Te deposit, located 1 km northwest of the Golden Mile Superpit and 350 m east of the Mt Charlotte mine, is a resource recently discovered by Kalgoorlie Consolidated Gold Mines (KCGM). More specifically, the historical upper Hidden Secret orebody, with bonanza grades of 49.45 g/t Au and 139.45 g/t Ag from 9676 t, was mined in 1904-1925 – but the continuation of the ore (the lower Hidden Secret orebody), with a reported reserve of 665 t at 3.56 g/t Au, wasn’t located until 2011. The Hidden Secret orebodies (both upper and lower) host unusually silver-rich ores in comparison to the greater Golden Mile. Gold is hosted in silver-rich native gold (mostly electrum, >20% Ag) and the Ag-Au telluride minerals hessite and petzite. The deposit also contains a diverse suite of non-economic Bi, Hg, Ni and Pb telluride minerals, assemblages of which were used in thermodynamic modelling of the ore deposition conditions. The assemblage electrum-hessite-coloradoite was used to derive an ore geothermometer for Au-Ag telluride deposits, and produced temperatures of 188-239°C for Hidden Secret ores, which are relatively low compared to typical mineralization temperatures of orogenic gold deposits. Further, the high-Ag telluride ores at Hidden Secret were linked to similar localized Ag-Au-Te occurrences at Mt Charlotte and Mt Percy, pointing to a district scale, lower-T and higher-Ag mineralizing event.

Elsewhere in the Golden Mile, from a location that is now empty space in the centre of the Superpit, a new telluride mineral was discovered in a historical WASM museum specimen from the Associated gold mine. Officially approved by the International Mineralogical Association in March 2016, the mineral was named kalgoorlieite after the type locality. Kalgoorlieite’s empirical formula is [As_1.59Sb_0.41]_2.00Au_0.02Ag_0.01[Te_2.95S_0.01Se_0.01]_2.97, or more simply As₂Te₃. Substantial concentrations of Sb in the mineral structure suggest the existence of a series between kalgoorlieite and tellurantimony, Sb₂Te₃. Observed only in thin section, kalgoorlieite occurs as grains ranging in size from <10 to 50 µm, intergrown with other telluride minerals and native tellurium. The grain size was too small for the traditional XRD characterization of the crystal structure usually required for a new mineral definition, but fortunately, synthetic As₂Te₃ was already well-described and the structure of natural kalgoorlieite was successfully matched to that of the synthetic form using Kikuchi diffraction. Thermodynamic modelling shows that kalgoorlieite has a relatively small stability field, which shrinks dramatically in the presence of tetrahedrite (a common sulphosalt in Golden Mile ores), explaining the rarity of this mineral in telluride ores.

Bio:

Kirsten is originally from Vancouver, where she began her voyage in the earth sciences and completed a BSc at the University of British Columbia in 2002. She then headed to the French side of Canada, where she earned an MSc and PhD in experimental geochemistry, with a focus on the transport of molybdenum in ore fluids, at McGill University. After a brief sojourn as a geologist with a Quebec exploration company in 2008, she began a postdoc at the GFZ German Research Centre for Geosciences in Potsdam, where she did experiments on subsurface CO₂ storage and metal transport in fluids. She moved to the southern hemisphere in 2012, where she began working as a lecturer in ore deposits at the Western Australian School of Mines, Curtin University. Kirsten’s research has focused on various topics in ore genesis, including experiments on metal transport in aqueous and hydrocarbon ore fluids, as well as studies of the gold deposits around Kalgoorlie, where she discovered a new telluride mineral called kalgoorlieite.