The origin of the Dapingzhang volcanogenic Cu–Pb–Zn ore deposit, Yunnan province, SW China: Constraints from host rock geochemistry and ore Os–Pb–S–C–O–H isotopes

Abstract

The Dapingzhang volcanogenic Cu–Pb–Zn sulfide deposit is located in the Lancangjiang tectonic zone within the Sanjiang region, Yunnan province of southwestern China. The deposit occurs within a felsic volcanic dome belonging to a mid-Silurian volcanic belt stretching for more than 100km from Dapingzhang to Sandashan. The mineralized volcanic rocks are predominantly keratophyre and quartz keratophyre with subordinate spilite. The Dapingzhang deposit is characterized by well-developed vertical zonation with stockwork ores in the bottom, disseminated sulfide ores in the middle, and massive sulfide ores in the top, overlain by a thin layer of chemical sedimentary exhalative rocks (chert and barite). The Re–Os age of the pyrites from the deposit is 417±23Ma, indistinguishable from the age of the associated felsic volcanic rocks. The associated felsic volcanic rocks are characterized by negative Nb–Ta anomalies and positive εNd(t) values (+4.4–+6.5), similar to the coeval calc-alkaline volcanic rocks in the region. This observation supports the interpretation that the felsic volcanic rocks associated with the Dapingzhang deposit are the derivatives of arc basaltic magma by extensive fractional crystallization. The δ34S values of the sulfides from the deposit vary from −1.24 to +4.32‰, indicating a predominantly magmatic source for the sulfur. The sulfides are also characterized by homogeneous and relatively low radiogenic Pb isotope compositions (206Pb/204Pb=18.310–18.656, 207Pb/204Pb=15.489–15.643 and 208Pb/204Pb=37.811–38.662), similar to the Pb isotopic compositions of the associated volcanic rocks. The Pb isotopic data indicate that mantle-derived Pb is more prevalent than crust-derived Pb in the deposit. The S–Pb isotopic data indicate that the important ore-forming materials were mainly derived from the associated volcanic rocks. The δ13CPDB and δ18OSMOW values of the associated hydrothermal calcite crystals vary from −2.3‰ to +0.27‰ and from +14.6 to +24.4‰, respectively. These values are between the mantle and marine carbonate values. The narrow range of the δ13CPDB values for the calcite indicates that carbon-bearing species in the hydrothermal fluids were primarily derived from marine carbonates. The δ18O values for the hydrothermal fluids, calculated from the measured values for quartz, are between −2.1‰ and +3.5‰. The corresponding δD values for the fluids range from −59‰ to −84‰. The O–H isotopic data indicate mixing between magmatic fluids and seawater in the ore-forming hydrothermal system. Similar to a typical volcanogenic massive sulfide (VMS) deposit, the ore-forming fluids contained both magmatic fluids and heated seawater; the ore metals and regents were derived from the underlying magma as well as felsic country rocks.