TRACE ELEMENT COMPOSITION OF PYRITE IN ACID-SULFATE ALTERED LITHOCAPS – A POTENTIAL EXPLORATION TOOL FOR CONCEALED ORE DEPOSITS

Acid-sulfate altered lithocaps (ASL) at Goldfield, Nevada and Summitville and Red Mountain Pass, Colorado host superimposed high-sulfidation (HS) Au-Ag deposits. In contrast, two large ASL's in SW Colorado (Lake City and Ohio Peak near Silverton) are generally “barren” or devoid of HS deposits. The alunite in each of these ASL's is largely magmatic hydrothermal in origin and is a product of vapor plumes emanating from dacitic to rhyolitic intrusions.

Pyrite compositions from ASL's in Colorado, Nevada, and Utah, as determined by LA-ICPMS, have widely varied compositions. Acid-sulfate altered lithocaps that host HS deposits (Goldfield, Red Mountain Pass, and Summitville) have bimodal compositions: one is roughly stoichiometric, whereas the other is trace-metal rich. The trace metal rich population is notably elevated in Cu (> 500 ppm), Zn (>185 ppm), As (> 120 ppm), Sb (> 19 ppm), and Pb (> 140 ppm). In contrast, disseminated pyrite from the two barren ASL's in Colorado is mostly stoichiometric.

The two barren ASL's overlie phyllic alteration with stoichiometric pyrite and deeper subeconomic Cu-Mo mineralization. Likewise, the phyllic halos of two other subeconomic Cu-Mo systems in Colorado contain stoichiometric pyrite.

Recent studies suggest that the ASL's in the NG Alunite Area and the coarse-banded alunite veins at Alunite Ridge in SW Utah, may overlie Climax-type Mo deposits. While there are no HS deposits at NG, pyrite from two locals is both stoichiometric and trace metal-rich (avg. ppm: Cu>500, Zn >170, As >290, Pb>215). In this respect, NG is like Goldfield, Red Mountain Pass, and Summitville permitting the existence of concealed HS deposits. At Alunite Ridge, the pyrite in an alunite altered dike adjacent to the veins is nearly stoichiometric with the exception of Pb, which had a mean of 220 ppm. Analyses of pyrite from phyllic zones of Climax-type porphyry systems are needed to better evaluate this possibility.

While NG may be related to a concealed Climax-type Mo porphyry and Goldfield, Red Mountain Pass, and Summitville may be related to concealed Cu-Mo porphyries, the geochemical signatures of their trace element rich pyrite are not much different.