Geochemistry of gold and selenium in hydrothermal ore deposits.

Abstract

Field observations, theoretical calculations and laboratory measurements were used to determine why selenide minerals are less common in ore deposits than sulfide and telluride minerals, where gold is located mineralogically in Carlin-type gold deposits (CTDs), and how much gold enters bornite and chalcopyrite under conditions typical of porphyry copper deposits. The rarity of selenide minerals in ore deposits is mainly due to the strongly chalcophile character of selenium, which allows it to substitute for sulfur in sulfide minerals. Thermodynamic considerations show that highly oxidizing conditions are required to oxidize sulfide to sulfate, thus preventing early sequestration of selenium in sulfide minerals. But, these conditions are rarely attained in nature. Most gold in CTDs is in arsenian pyrite. Mineralogic and BSE study of arsenian pyrite shows that there are several different types that differ in gold content, chemical composition, and grain size. XANES, EXAFS, SIMS and TEM study of gold in arsenian pyrite shows that it is present both as free gold (Au$\sp0)$ and structurally bound gold (Au$\rm\sp{I}),$ but not as Au$\rm\sp{III}.$ Au$\rm\sp{I}$ was probably incorporated into arsenian pyrite either in a vacancy position on a cation site in the pyrite lattice or adsorbed as Au(SH)$\sp0$ or Au$\sb2$S compounds. Neither depositional mechanism requires that the hydrothermal solution was saturated with respect to native gold. As much as 50% of the contained gold at Twin Creeks is structurally bound, suggesting that economically important gold deposits do not necessarily form from solutions saturated with respect to native gold. Thermodynamic calculations show that the assemblage gold-bornite-magnetite is stable in the high temperature core of porphyry copper-gold (PCG) deposits. Experimental studies in the system Au-Cu-Fe-S show that bornite incorporates one order of magnitude more gold than chalcopyrite/iss. The amount of gold in both bornite and chalcopyrite varies with chemical composition and decreases with decreasing temperature. These observations suggest that if the high-temperature potassic core of PCG deposits is preserved, gold would be closely correlated with bornite. Because bornite and chalcopyrite can contain very little gold at lower temperatures typical of phyllic and argillic alteration, these events could redistribute original sulfide-hosted gold.