MINERALOGY AND GEOCHEMISTRY OF HIGH-GRADE GOLD ORES, GOLDFIELD, NV

Goldfield is a classic district that was used as a type locality to define epithermal “high-sulfidation” gold deposits hosted by Tertiary volcanic rocks (Heald et al., 1987). Collectively, three stages of mineralization are recognized: Stage I - pre-ore, Stage II - main gold, and Stage III - supergene / weathering in four principal sub-districts, Main, Northern Main, McMahon Ridge and Gemfield.

Stage I consists of early hydrothermal alteration minerals alunite and kaolinite, and gangue minerals quartz and barite. Precious and base metal mineralization occurred in Stage II. Native gold (IIa) is followed by electrum-sulfide assemblages (IIb-IIe) containing chalcopyrite, enargite-famatinite, mohite, tetrahedrite, goldfieldite and bismuthinite. Hessite and argentite (IIf) terminate this stage. Argentite, matildite, emplectite and wittichenite are newly reported. Stage III consists of chlorargyrite, native silver and various oxides of iron, tellurium, bismuth, tin and antimony.

Electrum precipitated at least six times in various sulfide-associations (micro-assemblages) and during late oxidation. Isolated electrum in Stage IIa has low X_Au^El (Av. 0.87). Most electrum-sulfide assemblages show high X_Au^El (Av. 0.97), whereas electrum-argentite assemblages have low X_Au^El (Av. 0.74). In electrum occurring with iron oxides, X_Au^El = 1.0.

Stage IIa hydrothermal fluids were characterized by (Au/Ag)_aq ratio (10^-0.8~10^-2.8) for log(aCl^-/aH₂S)= 1 and (Au/Ag)_aq ratio (10^-2) for log(aCl^-/aH₂S)= 3.2~4.2. Stage IIb-IIe hydrothermal fluids were characterized by (Au/Ag)_aq ≈ 10⁰ and log(aCl^-/aH₂S) = 2.5~3.5. Stage IIf hydrothermal fluids were characterized by (Au/Ag)_aq ratio (10^-4). pH and log ƒO₂ are estimated at ~ 3.6 and -32 for all stages, respectively.

Geochemically important pathfinder elements are Bi, Cu, As, Sb and Sn in the Main area, Cu, As, Sb and Zn in the Northern Main area and Ag, Te and Zn in the McMahon Ridge area. In the Main area, the most useful visual guide is the presence of bismuthinite.

The mineralogy and trace element geochemistry shows that sub-districts share many similarities. Systematic differences in mineral compositions (e.g., goldfieldite, enargite) from Main to Northern Main to McMahon Ridge area suggest that these deposits are part of a large zoned hydrothermal system.