Porphyry-related polymetallic Au-Ag vein deposit in the Central City district, Colorado: Mineral paragenesis and pyrite trace element chemistry

Abstract

The Colorado Mineral Belt is a ~500 km long segment of calc-alkaline to alkaline Laramide-age intrusions and related porphyry and polymetallic vein deposits. The Central City historic Au-Ag mining district in the Front Range of Colorado, USA, has been one of the most productive Au deposits in the Colorado Mineral Belt, with a total of 119.1 t of Au produced between 1859 and 1959. Development of NE-trending Laramide strike-slip faults and re-activation of pre-existing Proterozoic brittle structures in Proterozoic gneiss provided the structural controls for the mineralization of Au and Ag in re-opened pyrite-quartz and base metal veins. Previous studies have related ore formation in the district to the emplacement of Late Cretaceous-Paleogene Laramide porphyritic alkaline intrusions. It remains, however, unclear how to reconcile the Au-Ag enrichment of this deposit with the historic vein descriptions and the evolution of a deep magmatic-hydrothermal porphyry system. Here, we document the crosscutting relationships and the mineral paragenesis of the different vein types and use the trace element chemistry in pyrite to fingerprint Au mineralization and the succession of different veining styles in the Central City district. Based on comparisons to porphyry and polymetallic-style veins worldwide, a revised vein classification is proposed, which distinguishes porphyry, transitional pyrite-quartz, and polymetallic veining styles. These veining styles led to the development of an almost concentric district-wide metal zoning, comprising a central and an intermediate zone, both enriched in Cu-Au-Ag, grading into a peripheral zone enriched in Zn-Pb-Ag. Rare porphyry-style veins include K-feldspar veins and quartz veins with molybdenite ribbons. The transitional-style veins include abundant pyrite-quartz veins that crosscut the earlier formed porphyry-style veins, and disseminated pyrite accompanied by phyllic alteration. Formation of the Au-Ag-bearing polymetallic-style veins was associated with the brecciation and re-opening of pyrite-quartz veins. Based on mineral paragenetic relationships, the polymetallic-style base metal veins can be subdivided into four subtypes, comprising sphalerite-chalcopyrite veins, tetrahedrite veins, enargite-pyrite-quartz veins, and galena-sphalerite-carbonate veins. The textures, mineral inclusions and trace element concentrations in pyrite are distinct for the different vein types and associated alteration halos. Pyrite in pyrite-quartz veins is enriched in Co and Ni, and contains quartz inclusions. In contrast, pyrite in polymetallic base metal veins is characterized by elevated Au (≤20 ppm), Cu, As, Pb, Ag and Sb concentrations, and contains inclusions of electrum and galena. The observed veining styles, paragenetic relationships, and associated pyrite chemistry in the Central City district confirm that ore formation is porphyry-related. The Central City district also shares several similarities with Cordilleran polymetallic-style Cu-Zn-Pb-Ag vein deposits associated with Cu-(Mo) porphyries. However, the high Au and low Cu-(Mo) grades make Central City an example of a newly recognized porphyry-related polymetallic Au-Ag vein deposit type.