Age and sources of the ore-forming material for the Qibaoshan Cu-polymetallic deposit in Hu'nan Province:Evidence from quartz vein Rb-Sr isotopic dating and S-Pb iso-topes

The Qibaoshan Cu-polymetallic deposit, located in the west section of the Qinzhou-Hangzhou metallogenic belt, is the largest Cu-polymetallic deposit in northeastern Hu'nan Province.However, the data of the deposit age are very insufficient.Based on quartz minerals Rb-Sr isotopic dating of the Qibaoshan deposit, the authors tried to obtain the age.The results show that the Rb-Sr isochron age is 153.4±2.0Ma (MSWD=1.8), initial ⁸⁷Sr/⁸⁶Sr=0.71849±0.00026, nearly identical with the quartz porphyry (153Ma to 155Ma) in the ore district in age, which suggests that the genesis of the Qibaoshan deposit was closely related to the quartz porphy-ry.The diagenesis and mineralization were both in the Yanshan period.The authors analyzed S, Pb isotope data of the pyrite to iden-tify the mineral sources of the Qibaoshan Cu-polymetallic deposit and obtained the following data:δ³⁴S=3.24‰~4.84‰ for the ore, with an average of 4.198‰; δ³⁴S=2.22‰~3.86‰ for the rock, with an average of 2.805‰.The δ³⁴S values change overall in a small range, and the δ³⁴S values of the rock change even in a smaller range, more close to the value of 0, which suggests that the sul-fur in the rock was most possibly derived from the mantle; and the sulfur in the ore came from the main mantle mixed with a small amount of crustal sulfur.The changes of lead isotopes are also in small ranges:²⁰⁶Pb/²⁰⁴Pb change in the range of 18.315~18.396 for the ore, with an average of 18.359;²⁰⁷Pb/²⁰⁴Pb change in the range of 15.629~15.737, with an average of 15.675;²⁰⁸Pb/²⁰⁴Pb change in the range of 38.376~38.856, with an average of 38.609.The values for the rock change in a similar range.These data show that lead isotope composition of the Qibaoshan Cu-polymetallic deposit has a lower crust (thorium-rich and uranium-depleted) characteris-tics.A comprehensive analysis of the Sr-S-Pb isotopic system of the Qibaoshan deposit indicates that the ore-forming material was derived from the ore gas-liquid fluid, resulting from the deep magma evolution and differentiation in the same evolution process as the quartz porphyry.At the same time, the host rock might have extracted a small amount of minerals from the wall rock to take part in the mineralization process through the circulating convection effect of heating groundwater, resulting in quartz porphyry intrusion and crystallization.