Genesis of the Xiyi Pb–Zn deposit, Yunnan Province, SW China: Evidence from trace element and fluid inclusion data

Abstract

The Xiyi Pb–Zn deposit lies in the central–northern part of the Baoshan Block in Yunnan Province, SW China, and represents a recent breakthrough in exploration for concealed Pb–Zn mineralisation in this area. The deposit consists of orebodies that are hosted by carbonate rocks of the lower Carboniferous Xiangshan Formation and are controlled by NE–SW-striking faults and associated secondary faults. The orebodies occur in layered, vein- and lens-shaped morphologies. The wall rocks were affected by calcite, pyrite, and barite alteration as well as silicification. Ores in the study area are predominantly massive, banded and vein-hosted, and can be texturally classified as replacement and interstitial ores with individual crystals of sulphides with subhedral–xenomorphic shape. The main metallic minerals in the ores are sphalerite, galena, pyrite, arsenopyrite and chalcopyrite. Four stages of the hydrothermal system are recognized based on crosscutting relationships and mineral assemblages: stage 1 (calcite – pyrite), stage 2 (sphalerite – galena – calcite), stage 3 (galena – sphalerite – arsenopyrite – calcite), and stage 4 (calcite – pyrite – quartz). Three types of fluid inclusion have been identified in calcite and barite: liquid-rich (TypeⅠ), pure-liquid (TypeⅡ) and pure-vapour (TypeIII). Fluid inclusions from stage 2, stage 3 and stage 4 display low homogenisation temperatures (101 °C–200 °C) and medium salinities (5.3–21.1 wt% NaCl equivalent). The trace element compositions of sulphides (sphalerite, galena, pyrite and arsenopyrite) were determined by electron microprobe analysis (EMPA) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS). The sphalerite (SpⅠand SpⅡ) contains elevated concentrations of Fe, Cd, Mn, Pb, Cu and Ag, but is depleted in Ge, Ga and In, whereas pyrite (PyⅠand PyⅡ) is enriched in As, Pb, Zn, Cu and Sb. Sphalerite trace-element compositions are indicative of formation at temperatures of 194 °C–235 °C, and the relationships between Ge and In in sphalerite suggest that metallogenic hydrothermal fluids of the Xiyi deposit is related to basinal brine activity. Pyrite has Co/Ni ratios close to 1, again suggesting that the metallogenic materials were derived mainly from the strata. The Y/Ho ratios of sphalerite and pyrite suggest that the metals of the deposit were derived from Devonian rocks in the area. All of these data indicate that the Xiyi Pb–Zn deposit is similar to Mississippi Valley-type (MVT) deposits elsewhere and represents stratabound epigenetic Pb–Zn mineralisation.