TRACE ELEMENTS IN PYRITE FROM SIX AREAS AFFECTED BY LARGE SCALE-PERVASIVE HYDROTHERMAL ALTERATION: ACID ROCK DRAINAGE (ARD) AND SIGNATURES OF MINERALIZATION STYLE

Disseminated pyrite in areas of pervasive hydrothermal alteration (PHA) have been postulated to be trace-element sources of acid rock drainage (ARD). This study presents results from more than 400 pyrite analyses determined by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) from six areas with PHA in Colorado. Rocks in these areas have high acid-generating potential due to the abundance of finely disseminated and veinlet pyrite (> 10 weight percent), have been extensively leached, and have low acid-neutralizing potential. LA-ICPMS results demonstrate that pyrite from five out of six of these areas have low trace-metal abundances (median < 100 ppm). Mass balance calculations indicate that pyrite alone cannot account for even moderately elevated base-metal concentrations (< 500 µg/L) in related ARD. Fine-grained pyrite from acid-sulfate altered rocks from the Red Mountain Pass (RMP) hydrothermal system near Silverton Colorado—one of the six areas studied– show a bimodal pyrite distribution: they are trace metal rich and nearly stoichiometric. 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). This suite of disseminated pyrite is related to superimposed high- sulfidation mineralization that formed Ag, Cu, Sb, As, and Pb ores. Late-stage vein pyrite from this deposit contains anomalous Cu (4,840 ppm median), As (18 ppm median), Sb (11 ppm median), and Pb (182 ppm median). Pyrite from the RMP area contrast with those having near stoichiometric values from two other large acid-sulfate systems from this study. The low-trace element abundances reflect the absence or lack of superimposed high-sulfidation mineralization in these two systems. LA-ICPMS data indicate that pyite from two similar subeconomic Mo-Cu porphyry systems are anomalously enriched in Co (87 and 103 ppm median) and Ni (21 and 82 ppm median) relative to the acid-sulfate systems. These results suggest that pyrite composition could be used to identify geochemical signatures of mineralized systems of varied character and genesis.