矿床地球化学国家重点实验室知识库

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 100 km 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 +/- 23 Ma, indistinguishable from the age of the associated felsic volcanic rocks. The associated felsic volcanic rocks are characterized by negative Nb-Ta anomalies and positive epsilon(Nd)(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 delta S-34 values of the sulfides from the deposit vary from -1.24 to + 4.32 parts per thousand, indicating a predominantly magmatic source for the sulfur. The sulfides are also characterized by homogeneous and relatively low radiogenic Pb isotope compositions (Pb-206/Pb-204 = 18.310-48.656, Pb-207/Pb-204 = 15.489-15.643 and Pb-206/Pb-204 = 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 delta C-13(PDB) and delta C-18(SMOW) values of the associated hydrothermal calcite crystals vary from -2.3 parts per thousand to +0.27 parts per thousand and from + 14.6 to +24.4 parts per thousand, respectively. These values are between the mantle and marine carbonate values. The narrow range of the delta C-13(PDB) values for the calcite indicates that carbon-bearing species in the hydro thermal fluids were primarily derived from marine carbonates. The delta O-18 values for the hydrothermal fluids, calculated from the measured values for quartz, are between -2.1%0 and +3.5%.. The corresponding delta D 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.