Mode of occurrence of arsenic and its influence on the precipitation of gold in the Jinlongshan gold deposit,southern Qinling

Located in Zhen'an-Xunyang basin of southern Qinling Mountains,the Jinlongshan gold deposit is a superlarge Carlin-type disseminated gold deposit discovered in the 1980's.The ore deposit occurs in Upper Devonian and Lower Carboniferous clastic and carbonate strata composed of calcareous siltstone,silty shale,silty limestone and argillaceous limestone intercalated with flyschoid formation.Ore bodies are strictly controlled by fault zones.Ores were examined by such means as optical microscope,electron microprobe analyzer(EMPA) and X-ray surface scanning for the purpose of probing into the content and distribution of Fe,As and S in arsenian pyrite and arsenopyrite in primary ore samples from the Jinlongshan ore deposit.The results show that As of low content is likely to be concentrated in pyrite,and As in pyrite is stable.When the As content is higher than ~6 wt%,two-phase mixtures of pyrite and arsenopyrite are stable.The dissolution of ferruginous limestone in the host rocks,the sulfidation of the dissolved iron by H2S,and the exsolution of partial metastable auriferous arsenian pyrite may provide favorable conditions for gold precipitation in the Jinlongshan gold deposit.