GOLD-BEARING ARSENIAN PYRITE AND MARCASITE AND ARSENOPYRITE FROM CARLIN TREND GOLD DEPOSITS AND LABORATORY SYNTHESIS

Invisible gold in natural and synthetic arsenian pyrite and marcasite correlates with anomalous As content and Fe deficiency, and high conte nts of invisible gold in most natural and all synthetic arsenopyrite c orrelate with excess As and Fe deficiency. As-rich, Fe-deficient arsen opyrite synthesized hydrothermally contains up to 3.0 wt% Au uniformly distributed in growth zones of light backscattered electron contrast. At the Deep Star gold deposit, Carlin Trend, Nevada, the sulfide comp ositions apparently span the full range of metastability from FeS2 to near FeAsS (40 at% S); arsenian pyrite contains up to 0.37 wt% Au, but arsenopyrite has excess S and is relatively Au poor. Observed minimum Fe contents are 29.1 at% in arsenian pyrite and marcasite from the De ep Star deposit and 31.3 at% in synthetic arsenopyrite. We suggest tha t invisible gold in arsenian pyrite and marcasite and arsenopyrite fro m sediment-hosted gold deposits represents Au removed from ore fluids by chemisorption at As-rich, Fe-deficient surface sites and incorporat ed into the solids in metastable solid solution. However, the oxidatio n state of invisible gold (Au-0, Au1+) remains uncertain because the c hemisorption process is intrinsically nonsystematic in terms of crysta l-chemical parameters and does not result in definitive atomic substit ution trends.