AU-BI-TE-S ASSEMBLAGES FROM MALDON GOLD DEPOSIT, VICTORIA, AUSTRALIA

The Maldon goldfield (Victoria, Australia), situated in the contact aureole of the Late Devonian Harcourt Granite, comprises quartz reefs hosted by rocks of the Bendigo-Ballarat zone of the Lachan Fold Belt. Reinvestigation of maldonite type material shows that the mineralogy of gold and accompanying Bi-tellurides is more diverse than previously recognized. This includes jonassonite (Jon; AuBi₅S₄), aurostibite (AuSb₂), unnamed Bi₃(Te,S)₂ and hedleyite (Hed) in assemblages comprising native bismuth (Bi), native gold (Au), maldonite (Mld; Au₂Bi), joseite-B (JoB), joseite-A (JoA) and (Sb-bearing) bismuthinite (Bism; up to 0.35 apfu Sb). These minerals occur as (i) mm- to <50 µm patches/droplets in thin quartz-hosted veinlets; (ii) patches cementing/embedded within clots of chlorite/rock fragments. Native Bi and Au prevail, in variable proportions. All other mentioned minerals only occur as inclusions in such Bi-Au patches. Jonassonite (<50 µm), found along the boundary between Bi and JoB/-A, seems to form at equilibrium with the tellurides but is replaced by Bism. The latter, often associated with chlorite, mostly replaces minerals in the Bi-Au-Te-(S) patches. Relationships between native Bi, Au, Mld and Bi-tellurides in the Bi-rich droplets are illustrative for various associations in the Au-Bi-Te system, e.g., Hed+Bi, eutectic at 266ºC, Bi+Mld, eutectic at 241ºC. Maldonite is found however mostly as relicts within coarse symplectites of native Bi and Au. The symplectites are considered to form below 113ºC as decomposition products of Mld. The presence of both high-T (T=373-241ºC) and low-T (T= 241-113ºC) Mld can be inferred from the coexistence of coarse Mld, Au+Bi symplectites, and Bi in the same patch. The Au content measured by LA-ICP-MS in JoB, although <1 ppm, show that tellurides are genetically tied to the Au-Bi patches since they also carry residual amounts of this element. All the above suggest formation of reduced Au-Bi-Te-S assemblages (species with Bi>Te+S + Bi are stable in the Po/Mt fields) from melts exsolved from fluids at 241ºC < T < 373ºC. An overprint of these associations is seen in the Bism+chlorite replacement. These observations support the view that remobilization of an earlier Au-Bi mineralization formed prior to granite intrusion took place during contact metamorphism.