Abstract

The Goldfield district is a type locality for high-sulfidation epithermal gold deposit hosted by Tertiary volcanic rocks. Four principal mineralized zones, Main, Northern Main, McMahon Ridge and Gemfield, show mineralogical and chemical similarities. Three stages of mineralization are recognized: Stage I – pre-ore stage, Stage II – main gold stage and Stage III – supergene weathering stage.

Stage I consists of early hydrothermal alteration minerals such as alunite and kaolinite and gangue minerals such as quartz and barite. Precious and base metal mineralization occurred in Stage II. The first part of the main gold stage involves sequential mineralization of copper containing sulfide minerals such as chalcopyrite, enargitefamatinite, mohite, tetrahedrite and goldfieldite. Bismuthinite followed in the sub-stage IIe. Tellurium and silver-containing minerals such as hessite and argentite followed Stage III consists of chlorargyrite, native silver and various oxides of iron, tellurium, bismuth, tin and antimony. Among the sulfide minerals, argentite, matildite, emplectite and wittichenite are newly reported.

Electrum precipitated at least six times including late oxidation stage in various sulfide-associations (micro-assemblages). Isolated electrum in Stage IIa has low XAu El (avg. 0.87, std. 0.09). Most electrum-sulfide assemblages show high XAu El, Au composition in electrum (avg. 0.97, std. 0.01), while electrum-argentite assemblage shows low XAu El (avg. 0.74, std. 0.06). Wherever electrum occurs with iron oxide due to weathering, XAu El is 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 ratio (100) and log(aCl-/aH2S) = 2.5~3.5. Stage IIf hydrothermal fluids were characterized by (Au/Ag)aq ratio (10-4). All stages are estimated at pH ≈ 3.6 and log fO2 ≈ -32.

Correlation coefficients calculated from multi-element chemistry data are consistent with mineralogy. The Main area shows the highest enrichment in precious, semi-metal and base metal elements while the Gemfield area shows the lowest enrichment due to the effects of weathering.

The depletions of light REE and negative anomalies (0.30~0.42) of Eu resulted from hydrothermal alteration. Among four major mineralized areas, McMahon Ridge, which has the lowest LREE and largest negative anomaly of Eu, has experienced the most mass transfer.

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.