A combined fluid inclusion and S–Pb isotope study of the Neoproterozoic Pingshui volcanogenic massive sulfide Cu–Zn deposit, Southeast China

Abstract

The Pingshui Cu–Zn deposit is located in the Jiangshan–Shaoxing fault zone, which marks the Neoproterozoic suture zone between the Yangtze block and Cathaysia block in South China. It contains 0.45million tons of proven ore reserves with grades of 1.03wt.% Cu and 1.83wt.% Zn. This deposit is composed of stratiform, massive sulfide ore bodies, which contain more than 60vol.% sulfide minerals. These ore bodies are hosted in altered mafic and felsic rocks (spilites and keratophyres) of the bimodal volcanic suite that makes up the Neoproterozoic Pingshui Formation. Metallic minerals include pyrite, chalcopyrite, sphalerite, tennantite, tetrahedrite and magnetite, with minor galena. Gangue minerals are quartz, sericite, chlorite, calcite, gypsum, barite and jasper. Three distinct mineralogical zones are recognized in these massive sulfide ore bodies: a distal zone composed of sphalerite+pyrite+barite (zone I); an intermediate zone characterized by a pyrite+sphalerite+chalcopyrite assemblages (zone II); and a proximal zone containing chalcopyrite+pyrite+magnetite (zone III). A thin, layer of exhalative jaspilite overlies the sulfide ore bodies except in the proximal zone. The volcanic rocks of the Pingshui Formation are all highly altered spilites and keratophyres, but their trace element geochemistry suggests that they were generated by partial melting of the depleted mantle in an island arc setting. Homogenization temperatures of the primary fluid inclusions in quartz from massive sulfide ores are between 217 and 328°C, and their salinities range from 3.2 to 5.7wt.% NaCl equivalent. Raman spectroscopy of the fluid inclusions showed that water is the dominant component, with no other volatile components. Fluid inclusion data suggest that the ore-forming fluids were derived from circulating seawater. The δ34S values of pyrite from the massive sulfide ores range from −3.6‰ to +3.4‰, indicating that the sulfur was primarily leached from the arc volcanic rocks of the Pingshui Formation. Both pyrite from the massive sulfide ores and plagioclase from the spilites have similar lead isotope compositions, implying that the lead was also derived from the Pingshui Formation. The low lead contents of the massive sulfide ores and the geochemistry of their host rocks are similar to many VMS Cu–Zn deposits in Canada (e.g., Noranda) and thus can be classified as belonging to the bimodal-mafic subtype. The presence of magnetite and the absence of jaspilite and barite at the −505m level in the Pingshui deposit suggest that this level is most likely the central zone of the original lateral massive sulfide ore bodies. If this interpretation is correct, the deep part of the Pingshui Cu–Zn deposit may have significant exploration potential.