TELLURIDE AND Bi-SULFOSALT MINERALOGY OF THE YANGZHAIYU GOLD DEPOSIT, XIAOQINLING REGION, CENTRAL CHINA

Abstract

Abstract The Xiaoqinling vein-style gold district (reserves of >630 t Au) has a very pronounced tellurium signature. The Yangzhaiyu gold deposit is one of the largest gold deposits in this district. The ore-mineral assemblage consists of pyrite, chalcopyrite, galena, bornite, sphalerite, gold, and native tellurium together with a wide variety of telluride and Bi-sulfosalt minerals. The telluride mineral assemblage comprises Phase A, sylvanite [(Au,Ag) 2 Te 4 ], hessite [Ag 2 Te], petzite [Ag 3 AuTe 2 ], calaverite [AuTe 2 ], stutzite [Ag 5− x Te 3 ], rucklidgeite [PbBi 2 Te 4 ], altaite [PbTe], volynskite [AgBiTe 2 ], tellurobismuthite [Bi 2 Te 3 ], tetradymite [Bi 2 Te 2 S], and buckhornite [AuPb 2 BiTe 2 S 3 ]. The Bi-sulfosalt minerals observed are Phase B, aikinite [PbCuBiS 3 ], felbertalite [Cu 2 Pb 6 Bi 8 S 19 ], and wittichenite [Cu 3 BiS 3 ]. We observed two unnamed phases: Phase A and Phase B. Phase A, with the empirical formula of AgTe 3 , occurs in small intergrowth aggregates with sylvanite and chalcopyrite, and locally with bornite, galena, and altaite. Phase A and its associated minerals occur along healed micro-fractures in pyrite. It has a distinctly golden reflection color, without discernable reflectance pleochroism or anisotropy. AgTe 3 was synthesized in 1982, but its natural occurrence has not been reported to date. Phase B, with the empirical formula of Cu 20 FePb 11 Bi 9 S 37 , was observed in an intergrowth aggregate together with hessite, galena, chalcopyrite, bornite, and buckhornite. It has a pinkish grey reflection color, without discernable reflectance pleochroism or anisotropy. The telluride and gold mineralization took place at or below ca. 220 °C and evolved towards decreasing f Te2 . The ore-forming fluids and components of the Yangzhaiyu gold deposit may ultimately stem from refertilized upper mantle, either directly from devolatilization or from a magmatic system resulting from partial melting.