Geological Setting And Genetic Model Of The Yinshan Cu-Au-Pb-Zn-Ag Deposit,Jiangxi Province

The Yinshan Cu-Au-Pb-Zn-Ag deposit is located in the southeastern margin of the Yangtze Block,and in the northwest of the Northeastern Jiangxi Deep Fault.The ore-bearing strata are the Neoproterozoic Shuangqiaoshan Group and the Middle Jurassic felsic volcanic rocks.The main structures are the Yinshan volcanic edifice,NE-trending Yinshan anticline and a series of nearly EW-and SN-trending faults.Mineralization related rocks are the Middle Jurassic felsic sub-volcanic rocks.There are two types of mineralization at Yinshan,including the early Pb-Zn-Ag mineralization controlled by the rhyolitic quartz porphyries(172 Ma),and the late Cu-Au mineralization associated with the dacitic porphyries(170 Ma).The main ore minerals are pyrite,chalcopyrite,galena,sphalerite,tetrahedrite-tennatite,gold,silver,and silver sulphosalt,and the principal gangue minerals are quartz,sericite,calcite,and chlorite.The Dexing porphyry copper deposits are adjacent with the Yinshan deposit,and are controlled by the Middle Jurassic granitoids.Orebodies are veinlets along the contact zone between the granitoids and wallrocks.Ore minerals mainly are pyrite,chalcocite,chalcopyrite,molybdenite,and native gold.Principle gangue minerals are K-feldspar,biotite,quartz,sericite,calcite and chlorite.In this study,we present systematic zircon LA-ICP-MS zircon U-Pb dating,element and Sr-Nd-Hf isotopic data for Yinshan and Dexing igneous rocks,and fluid inclusions and H-O-S-Pb isotope data for the ore veins.Our results show that the Yinshan igneous rocks were produced during the Middle Jurassic(ca.176-166 Ma).Geochemically,the Yinshan igneous rocks are characterized by calc-alkaline and show some adakitic affinities with high Sr contents(up to 903 ppm),Sr/Y(up to 93.2)and LaN/YbN ratios(12.4 to 65.1)and low Y(2.53 to 15.8 ppm)and Yb contents(0.22 to 1.39 ppm).The absence of a negative Eu anomaly,extreme depletion in Y and Yb,relatively low MgO contents(0.2 to 2.9 wt%),and relatively high 207Pb/204Pb ratio(15.555-15.575),indicate that Yinshan igneous rocks were likely derived from the partial melting of thickened lower continental crust.In addition,the Yinshan igneous rocks show similar element characteristics to arc-derived igneous rocks with bulk Earth-like ?Nd(t)values(-1.1 to+1.8)and initial 87Sr/86Sr(0.7036-0.7090),relatively depleted ?Hf(t)values(-1.5 to+5.8),and Hf model ages of 0.8-1.2 Ga.However,there is no arc-related evidence in the study area in the Middle Jurassic,which indicates that these arc-related elemental and isotopic compositions are possibly inherited from Meso-to Neo-Proterozoic juvenile continental crust formed by the Neoproterozoic oceanic crust subduction along the Jiangnan Orogen.Dexing porphyries show similar zircon ages,geochemical and isotopes,except the relatively higher MgO contents and Mg#values,which suggesting involvements of the mantle materials during rock-forming process.Two-phase liquid-rich(type ?),two-phase vapor-rich(type ?),and halite-bearing(type ?)fluid inclusions can be observed in the hydrothermal quartz-sulfides veins.Type ? inclusions widespread occur in the Yinshan deposit,and show homogenization temperatures of 187-303 ? and salinities of 4.2-9.5 wt.%NaCl equivalent in the Pb-Zn-Ag mineralization,and homogenization temperature range of 196-362 ? and salinity values of 3.5-9.9 wt.%NaCl equivalent in the Cu-Au mineralization.The pervasive occurrence of type I fluid inclusions with low-moderate temperatures and salinities imply the mineralizing fluids formed in epithermal environments.The type II and coexisting type III inclusions from deeper level below the Cu-Au ore bodies share the similar homogenization temperatures of 317-448 ?,and contrasting salinities of 0.2-4.2 and 30.9-36.8 wt.%NaCl equivalent,which indicate that boiling process took place.The oxygen and hydrogen isotope compositions of inclusion water in quartz imply that ore-forming fluids were mainly derived from magmatic sources.Sulfur and lead isotopic compositions of sulfides suggest a homogeneous magmatic sulfur source.The Dexing porphyry deposit is different from the Yinshan deposit and show more widespread boiling fluid inclusions with higher temperatures.Therefore,the Yinshan deposit is distinct from the typical porphyry or epithermal deposit,and probably is the transition from porphyry to epithermal deposit.Local boiling process beneath the epithermal Cu-Au ore-forming system indicates a possibility that porphyry-style orebodies may exist at even deeper zones.The possible ore deposit model is that:Neoproterozoic Cu-Au rich juvenile crust was formed resulting from oceanic subduction along the East Jiangnan orogen.During Middle Jurassic,probably a localized intra-continental extensional environment,partial melting of Cu-Au rich Neoproterozoic juvenile crust in the Dexing region caused the formation of adakitic rocks and large-scale Cu-Au polymetallic deposits.Ore-forming fluids were moved to the shallow fault zones from the deep magmatic source regions.Temperatures decreased in the shallow zones and it caused metal deposition.