Hydrothermal evolution of Au-bearing pyrite-quartz veins and their association to base metal veins in Central City, Colorado

Abstract

The Central City district played a major role in the early settlement and prosperity of Colorado, providing a major resource of Au, Ag, Cu, Pb, and Zn in the 19th century totaling more than $100 million during the time of production. Gold mineralization at Central City occurs in two main vein types: pyrite-quartz and base metal veins, which have been related to Tertiary intrusives. These veins occur in concentric zones with pyrite-quartz veins in the central zone of the district and base metal veins in the peripheral zone of the district. Although the broad vein distribution is well understood from previous studies, the evolution of the Central City district remains unclear, particularly when and how Au is introduced in the system. The goal of the study was to determine the hydrothermal evolution between the two major vein types, and their association to Au. This research combines textural, mineralogical and geochemical observations at the field and thin section scales across the district utilizing field emission scanning electron microscopy (FE-SEM), automated mineralogy (AM), electron microprobe analysis (EMPA), optical cathodoluminescence (CL), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. We have identified three significant alterations, four vein types, five types of pyrite, and two stages of mineralization: the pyrite stage and the base metal stage. These observations were used to construct a detailed paragenesis of both the pyrite and base metal stages of mineralization, which has allowed us to interpret the evolution of hydrothermal fluids in this district and under which conditions Au was introduced into the system. Fluids early in the pyrite stage were magmatic in origin with pyrite enriched in Ni and Co and related to Mo-bearing veins and K-feldspar alteration, followed by the formation of pyrite-quartz veins and phyllic alteration. Fluids of the base metal stage transition from Cu-Zn-(Pb) mineralizing chalcopyrite, sphalerite, and galena to Cu-Sb-As-(Au) mineralizing tetrahedrite and enargite and then to Pb-Zn-(Ag) mineralizing late base metal veins with argillic alteration. We conclude that Au was primarily introduced in the base metal stage of mineralization, together with the sulfosalts, and it occurs dominantly as electrum.