Geological, sulfur isotopic, and mineral trace element constraints on the genesis of the Xiyi Pb-Zn deposit, Baoshan Block, SW China

摘要

The recently discovered Xiyi Pb-Zn deposit in Yunnan Province, SW China, contains 1.2 Mt of Pb and Zn metal. It is located within the Baoshan Block, in the Sanjiang Tethys orogenic belt and is hosted along fractures within carbonates of the Lower Carboniferous Xiangshan Formation. The deposit is hosted by veins within NE-SW trending strike-slip and steeply dipping NW-SE trending normal fault zones. The formation of the deposit involved sedimentary, hydrothermal, and supergene oxidation stages. The hydrothermal stage associated with mineralization include three sub-stages (i) a steeply-dipping quartz-calcite-rich sphalerite sub-stage; (ii) a polymetallic sulfide sub-stage; and (iii) a vein-host calcite-pyrite sub-stage. The deposit is enriched in Pb, Zn, Sb, Cd, Ag, and U, and depleted in Ni, Se, Ti, and Co. The ores have total rare-earth element (REE) concentrations of 17.4-186 ppm and are enriched in the light REE (LREE) with (La/Yb)(N) ratios of 3.75-9.23 and strongly positive Eu (Eu/Eu* = 1.42-4.60) and slightly negative Ce (Ce/Ce* = 0.70-0.88) anomalies. The positive Eu anomalies and LREE enrichment suggest that the ore-forming fluids were exsolved from a concealed intrusion during magmatic degassing. The S isotopic compositions of sulfides and sulfates indicate that S within the deposit was derived from both magmatic sources and barite within the hosting carbonates via thermochemical sulfate reduction. In-situ LA-ICP-MS analysis indicates that sphalerite within the deposit is texturally and chemically variable, with elevated Fe, Mn, Cd, Cu, Ag, Sb, Sn, Pb, and T1 contents. In comparison, galena within the deposit is enriched in Sb, Ag, Cd, Sn, and Tl, and pyrite has elevated Ag, As, Sb, and Pb contents. The trace element compositions of sphalerite and galena indicate that the Xiyi deposit may be the product of a distal hydrothermal event although the direct genetic relationship is not clear. The fracture-hosted nature, mineral assemblages, geochemistry, and mineral S isotopic and trace-element compositions of the Xiyi Pb-Zn deposit indicate that it is an epigenetic carbonate-hosted Pb-Zn deposit. This study also indicates that sphalerite is a more useful mineral than galena in terms of using mineral trace-element geochemistry to discriminate different deposit types.