Mineralogy and mineral chemistry of Bi-minerals: Constraints on ore genesis of the Beiya giant porphyry-skarn gold deposit, southwestern China

The Beiya deposit, located in the Sanjiang Tethyan tectonic domain (SW China), is the third largest Au deposit in China (323 t Au @ 2.47 g/t). As a porphyry-skarn deposit, Beiya is related to Cenozoic (Himalayan) alkaline porphyries. Abundant Bi-minerals have been recognized from both the porphyry- and skarn- ores, comprising bismuthinite, Bi-Cu sulfosalts (emplectite, wittichenite), Bi-Pb sulfosalts (galenobismutite, cosalite), Bi-Ag sulfosalt (matildite), Bi-Cu-Pb sulfosalts (bismuthinite derivatives), Bi-Pb-Ag sulfosalts (lillianite homologs, galena-matildite series), and Bi chalcogenides (tsumoite, the unnamed Bi2Te, the unnamed Ag4Bi3Te3, tetradymite, and the unnamed (Bi, Pb)(3)(Te, S)(4)). Native bismuth and maldonite are also found in the skarn ores. The arsenopyrite geothermometer reveals that the porphyry Au mineralization took place at temperatures in the range of 350-450 degrees C and at log fS(2) in the range of -8.0 to -5.5, respectively. In contrast, the Beiya Bi-mineral assemblages indicate that the skarn ore-forming fluids had minimum temperatures of 230-175 degrees C (prevailing temperatures exceeding 271 degrees C) and fluctuating fS(2)-fTe(2) conditions. We also model a prolonged skarn Au mineralization history at Beiya, including at least two episodes of Bi melts scavenging Au. We thus suggest that this process was among the most effective Au-enrichment mechanisms at Beiya. (C) 2016 Elsevier B.V. All rights reserved.