The occurrence of cobalt and implications for genesis of the Pusangguo cobalt-rich skarn deposit in Gangdese, Tibet

Abstract

The Pusangguo cobalt-rich skarn deposit occurs in Gangdese, Tibet. In recent years, research on this deposit has detailed the geochronology, petrogenesis, fluid inclusions and skarn mineralogy, but there has been a lack of focus on the occurrence of cobalt. We present X-ray fluorescence, scanning electron microscopy and electron probe microanalyses that address this omission and help elucidate the genesis of cobalt-rich skarn deposits. Correlations between Co and other ore-forming elements (Cu, Pb, Zn and Ag) reveal that Co mineralization was synchronous with other metals. Photomicrographs and backscattered electron and X-ray maps of thin sections, confirm that cobalt mineralization is usually associated with chalcopyrite, sphalerite and other sulfides, which developed at the retrograde skarn stage with quartz, carbonate minerals, tremolite and other skarn-related minor phases. Three independent cobalt minerals are identified (cobaltite, carrollite, and linnaeite). The EPMA results further reveal two different occurrences of cobalt (independent cobalt minerals and isomorphous substitution Co in sulfides) in the Pusangguo deposit. Compared with chalcopyrite (mean Co content = 0.06 ± 0.06 wt.%, 1σ, n = 142) and galena (mean Co content = 0.02 ± 0.04 wt.%, 1σ, n = 82), both pyrite and sphalerite typically contain obviously higher Co contents (mean = 0.16 ± 0.17 wt.%, 1σ, n = 97 and 0.20 ± 0.10 wt.%, 1σ, n = 108, respectively). The correlations between Co and other elements indicate that Co + Ni replace Fe in pyrite via isomorphous substitution while Co + Fe replace Zn in sphalerite. These results further indicate that Co and Zn mineralization occurred at the same time. Combined with previously published Sr-Nd isotopic data on skarn minerals, the mineralization of Cu, Pb, Zn, Ag and Co were likely derived from, or share a common source with the intrusions at Pusangguo.