Gold to aurostibite transformation and formation of Au-Ag-Sb phases: the Krásná Hora deposit, Czech Republic
Abstract
Rare phases of the Au-Ag-Sb system were recognized in the Krasna Hora Sb-Au deposit (Sb 1.5-3 wt.%; Au 3-5 ppm), Czech Republic which correspond to auriferous dyscrasite (up to 7 at.% Au), auriferous allargentum (up to 34 at.% Au), and an unnamed phase with composition similar to the eutectics (E-1, E-2) of the experimental Au-Ag-Sb system. The dominant ore mineral is stibnite with rare native antimony, native gold and a Ag-Au alloy.
Textural relationships are well established: stibnite (early) -> gold -> aurostibite -> native antimony (late). Gold is present in four generations: Au-1 (0-15 at.% Ag) is the most abundant type; Au-2 (20-70 at.% Ag) forms thin rims along intra-grain boundaries of Au-1; Au-3 and Au-4 are rare and almost pure (similar to 0 at.% Ag).
The formation of most of the Au-2 and of Au-Ag-Sb phases is associated with Ag-mobilization coupled with the Au-1 to aurostibite transformation via dissolution-precipitation and solid-state diffusion processes at temperatures <200 degrees C.