Geochemical Study On Metallogenetic Relationship Between Mesozoic Granitoids And Polymetallic Deposits In Southeastern Hunan,China

The southeastern Hunan Province is located in the Cathaysia Block where the Shi-Hang zone and Nanling belt meet,this area records intensive Mesozoic magmatism and hosts numerous coeval world-class polymetallic deposits.Baoshan skarn type Pb-Zn-Cu(Mo)deposit and Huangshaping skarn type W-Mo-Pb-Zn deposit are chosen as representatives in this paper in order to identify the petrogenesis of granitoids in these two deposits and the implications for associated polymetallic mineralization and regional metallogenesis.The ?34S values of pyrite disseminated in Baoshan granodiorite-porphyry range from+1.5‰ to+3.5‰ and are consistent with the ?34S values of sulfide ores in Baoshan deposit;in addition,the lead isotopic compositions of potash feldspar are also consistent with the sulfide ores.Thus,the sulfur and lead isotope features in Baoshan deposit indicate that granodiorite-porphyry dominates the ore-forming sulfur and metals precipitated in the sulfide orebodies.Moreover,the fluid inclusions in gangue fluorite have low homogenization temperature(130??150?)and low salinity(<8%),showing a signature of late stage magmatic-hydrothermal fluids with low temperature and salinity.The Huangshaping granitoids are characterized by two pulses of intrusive activity:a first-stage quartz porphyry and a second-stage felsite and granite porphyry,and our age data show that the quartz porphyry and felsite formed at 160.5±1.3 and 156.6±1.4 Ma,respectively,representing a period of Late Jurassic magmatism.Combining previous studies,the granite porphyry are thought formed at 155.2±0.4 Ma.Granitic enclaves within the quartz porphyry crystallized at 160.2±1.6 Ma,and zircons and apatites from the enclaves exhibit Hf isotopic and geochemical compositions that suggest a Paleoproterozoic lower crustal melt as one end-member of the magma that formed the quartz porphyry,whereas another likely end-member was coeval mantle-derived magma,as indicated by the geochemistry and Sr-Nd-Pb-Hf isotopes.However,both the felsite and the granite porphyry were probably derived from the melting of metamorphic basement rocks in the upper crust.The felsite clearly formed as a result of the rapid ascent and cooling of magma,whereas the granite porphyry underwent fractional crystallization because of its slower ascent and cooling.The garnet from Huangshaping deposit includes coarse grain,medium grain and light alteration rim,whereas scheelite was consist of a dark core and a bright mantle by cathodoluminescence(CL)observation,and can be further subdivided into four groups based on the CL feature and REE signatures analyzed by in situ LA-ICP-MS.The results show that garnet formed from distinct hydrothermal fluids with different components and redox states.However,the core of the scheelite shows the characteristics of LREE rich and HREE depleted with significant high Mo contents,whereas the mantle is rich in MREE with strongly negative Eu anomaly and low Mo concentrations,compositionally similar to the scheelite from typical skarn-type W(Mo)deposits and low-middle tempreture quartz vein-type Au-W deposits,respectively.This result indicates the transitions of ore-forming fluids from magmaitc fluids to large scale hydrothermal circulation systems during W mineralization within Huangshaping deposit and could be used to decipher the formation of this grand pollymetallic deposit.The present study reveals that the sulfides from deep levels of the Huangshaping deposit have lower and more consistent ?34S values(-96 m level:+4.4‰ to+6.6‰,n=13)than sulfides within the shallow part of the deposit(20 m level:+8.3‰ to+16.3‰,n=19).The ?34S values of deep sulfides are compositionally similar to those of magmatic sulfur within southeastern Hunan Province,whereas the shallower sulfides most likely contain reduced sulfur derived from evaporite sediments.The sulfide ores in the Huangshaping deposit have initial 87Sr/86Sr ratios that lie between the values of granitoids and sedimentary carbonate in the study area,but the ratios decreased with time,indicating that the ore-forming fluids were a combination of magmatic and formation-derived fluids,with the influence of the latter increasing over time.The lead isotopic compositions of sulfide ores do not correlate with sulfide type and define a linear trend in the 207Pb/204Pb vs.206Pb/204Pb plot that is distinct from those of the disseminated pyrite within sedimentary carbonates and Jurassic granitoids in the study area,but is similar to the lead isotopic composition of sulfides within coeval skarn Pb-Zn deposits in southeastern Hunan Province.In addition,the sulfide ores have old signatures with relative high 207Pb/206Pb ratios,suggesting that the underlying Paleoproterozoic basement within southeastern Hunan Province may be the source of metals within the Huangshaping deposit.The emplacement of granitoids did provide large amounts of heat and fluids to the hydrothermal system in this deposit and extracted metals from the basement rocks.Previous age data show that,the majority of Pb-Zn mineralization formed during two stages at?203 Ma and?155 Ma,and the associated magmatic rocks formed as the mixing of crustal melting and coeval mafic magmas.The sulfur isotopic compositions of sulfide ores indicate that the sulfur mainly derived from magmatic source,the influence of reduced sulfur derived from evaporite sediments can also be significant in some deposits.In addition,the lead isotopic compositions suggest that ore-forming metals can be directly from the magmatic rocks or as the results of extraction of underlying basement.Moreover,the C-O isotopic compositions of gangue calcite and H-O isotopic compositions of gangue quartz indicate that the ore-forming fluids mainly came from the mixing of magmatic fluids and meteoric weters.The apatite data indicate that granodiorite porphyry intrusions related to Cu-Pb-Zn mineralization are oxidized(with high 8Eu and low 8Ce values)and formed as a result of slab dehydration,melting of the mantle wedge overlying a subducted slab,and the partial melting of crustal material(with high Cl and low F contents and low 87Sr/86Sr values).The release of abundant Cl-and H2O-rich fluids from the slab triggered mantle melting and the extraction of metals that were precipitated within the deposits.In comparison,granites related to W and Sn deposits are moderately oxidized to reduced(with low ?Eu and high ?Ce values),and were generated by the partial melting of crustal material with only limited input from mantle-derived magmas(with high F and low Cl contents and high 87Sr/86Sr values).Granites associated with the W and Sn mineralization formed in a intra-arc rifting-related tectonic environment.The fact that these numerous polymetallic ore deposits formed in the same area during two successive periods of mineralization,the first from 180 to 160 Ma and the second from 160 to 140 Ma,indicates that the tectonic environment of southeastern Hunan Province evolved from a continental arc associated with the subduction of the Paleo-Pacific Plate and the formation of Cu-Pb-Zn deposits to a later intra-arc rifting environment associated with the formation of W and Sn deposits as a consequence of slab roll-back and mantle upwelling during the Late Jurassic.This study reveals that apatite compositions can be used as a proxy to reflect the differences between granitoids and their associated mineralization,and identify reginal metallogenesis and tectonic evolution.