Gold-bearing arsenian pyrite and marcasite and arsenopyrite from Carlin Trend gold deposits and laboratory synthesis

Invisible Au in natural and synthetic arsenian pyrite and marcasite correlates with anomalous As content and Fe deficiency, and high contents of invisible Au in most natural and all synthetic arsenopyrite correlate with excess As and Fe deficiency. As-rich, Fe-deficient arsenopyrite synthesized hydrothermally contains up to 3.0 wt.% uniformly distributed in growth zones. At the Deep Star Au deposit, Carlin Trend, Nevada, sulphide compositions span the full range of metastability from FeS (sub 2) to near FeAsS; arsenian pyrite has up to 0.37 wt.% Au, but arsenopyrite has excess S and is relatively poor in Au. Observed minimum Fe contents are 29.1 at.% in arsenian pyrite and marcasite from the Deep Star deposit and 31.3 at.% in synthetic arsenopyrite. The authors suggest that invisible Au in arsenian pyrite and marcasite from sediment-hosted Au deposits represents Au removed from ore fluids by chemisorption at As-rich, Fe-deficient surface sites and incorporated into the solids in metastable solid solution.