The Yanshanian Magmatism And Mineralization In Shapinggou Porphyry Mo-polymetallic Deposit,Jinzhai,Anhui

The Shapinggou porphyry molybdenum(Mo)deposit,the super-large deposit discovered recently,is the largest one in Asia with the proven reserve of 2.45 Mt Mo metal in the East Qinling-Dabie Mo Metallogenic Belt.There are some pervious studies have been carried out and got some important conclusions.However,the understandings on magmatic resource and evolution,metallogenic contribution from different intrusions,hydrothermal alteration,metallogenic mechanism,and regional minerogenetic regularity are very limit and superficial.Therefore,this study carried out detailed field geological survey,drill-hole observation and sample collection.Then based on petrography(microphysiography)and mineragraphy,major elements,trace elements,rare earth elements,Sr-Nd-Pb-Hf isotope and U-Pb dating of intrusions;microphysiography,microthermometry,laser Roman spectra,gas and liquid composition and H-0 isotope of fluid inclusion;S-Pb isotope of ore minerals and gypsum have been employed to identify regional geologic features,geologic setting,temporal and spatial distribution and geochemical characteristic of intrusive rocks in ore district,geology and geochemical characteristic of ore deposit.In this study,I preliminarily reveal the magmatic resource and evolution,tectonic setting,metallogenic material resource and physico-chemical conditions,and even a lithogenetic and metallogenic model.Based on these conclusions,I preliminarily discussed the regional metallogenetic regularity in East Qinling-Dabie Mo Metallogenic Belt and compared with other porphyry deposits with different metal.Detailed field geological survey and petrography(microphysiography)observation indicate that intrusive rocks,widely distributed in the Shapinggou ore district as southeast part of Shangcheng batholith,range from ultramafic(olivine pyroxenite and plagioclase-bearing pyroxenite)to intermediate and felsic(diorite dikes,monzogranite,granodiorite,granite,quartz syenite,and granite porphyry).Their distribution and occurrence exhibit respective characteristic,olivine pyroxenite and plagioclase-bearing pyroxenite are in shape of rock walls and rock bosses;diorite is in shape of vein distributed in north of ore district;quartz syenite and granite porphyry are in shape of apophysis and small stock near by cryptoexplosive breccia pipe in the center of ore district;monzogranite,granodiorite and granite are in shape of stock and batholith in the periphery of ore district.LA-ICP MS zircon dating results suggest that all of the intrusions formed in Early Cretaceous and main focus on two stages(138.0?127.4 Ma and 116.3?110.0 Ma)except olivine pyroxenite(155.7 ± 2.7Ma)which formed in Late Jurassic.Combined with petrochemistry and geochemistry,the intrusions can be divided into two types.Type ? including olivine pyroxenite,plagioclase-bearing pyroxenite,diorite,monzogranite,granodiorite,granite and barren granite porphyry formed earlier than 120 Ma.They exhibit lower evolution degree,dramatically depleted high field-strength elements(HFSE)Nb-Ta,slightly depleted U-Th and Ti,flat REE pattern,lower ?Nd(t)and ?Hf(t)values and older two-stage Nd-Hf model ages;Type ? including quartz syenite and ore-bearing granite porphyry with coeval cryptoexplosive breccia,they are uniformly featured by higher evolution degree,alkaline-enriched,apparently enriched Zr-Hf,slightly enriched Nb-Ta and U-Th,distinctly depleted Ti,steep REE pattern with LREE enrichment and obvious negligible MREE anomalies,higher ?Nd(t)and ?Hf(t)values and younger two-stage Nd-Hf model ages.Detailed petrographic observations identified quartzes are xenomorphic a-quartz within quartz syenite,while the quartz phenocrysts within the granite porphyry,derived from the same magmatic sources and similar evolutionary processes,are pseudomorphous ?-quartz,characterized by a hexagonal bipyramid crystallography.Combined with titanium-in-zircon thermometry,quartz phase diagram,and granitic Q-Ab-Or-H2O phase diagrams,it is suggested that the quartz syenite and granite porphyry were formed under similar magmatic origins,including similar depths and magmatic crystallization temperatures.However,the ?-quartz within quartz syenite indicates that the crystallization pressure was more than 0.7 GPa,while the original?-quartz within the granite porphyry was formed under pressures of between approximately 0.4 GPa and 0.7 GPa.The groundmass of the granite porphyry which formed after the phenocryst indicated a crystallizing pressure below 0.05 GPa.This indicates that the granite porphyry was formed under repetitive and rapid decompression(>0.7 Gpa?0.4?0.7 Gpa ?<0.05 Gpa).The decompression was significant as it caused the exsolution of the ore-forming fluids,and boiling and material precipitation during the magmatic-fluid process.Detailed field geological survey,drill holes and thin sections observation indicate the diverse hydrothermal alteration widespread developed in the Shapinggou ore district.According to the altered minerals(mineral assemblage),correlation with ore minerals,spatial distribution of different alterations and cross-cutting relationship,these alterations can be divided into pre-mineralization(massive silicification),syn-mineralization(stage ?:K-feldspathization,biotitization and beresitization;stage ?:pyritization-silicification)and post-mineralization(chloritization and fluoritization).Stage ? in syn-mineralization is main Mo mineralization,including molybdenite vein,molybdenite-quartz vein,pyrite-molybdenite-quartz vein,pyrite-molybdenite-sericite(muscovite)-quartz vein;Stage ? in syn-mineralization is main Pb-Zn mineralization,including galena-sphalerite-pyrite-quartz vein,pyrite-quartz,and pyrite vein.The observation and study on fluid inclusions suggest that three types of fluid inclusions are recognized in the hydrothermal alteration-mineralization vein:liquid-gas two-phase(type ?),CO2-rich three-phase(type ?),and daughter mineral-bearing fluid inclusion(type ?).Pre-mineralization fluid exhibit medium-high temperature,medium salinity,and CO2-bearing.Stage ? fluid in syn-mineralization is unmixing of medium-high temperature,medium-low salinity,and C02-rich fluid and medium-high temperature,high salinity,and CO2-poor fluid;Stage ? fluid in syn-mineralization is medium temperature,low salinity,and C02-poor(-free).The fluid in post-mineralization is low temperature,low salinity,and CO2-free.The fluid compositional test suggest that hydrothermal fluid is dominated by H2O and CO2 with minor N2 and trace of H2,CO,and CH4 in vapor;and dominated by Na+,K+,Mg2+,Ca2+,F-,Cl-,SO42-,NO3-in liquid.From pre-mineralization to post-mineralization,the content of CO2 and N2 decreasing while H2O increasing in vapor,the concentration of Na+?K+?Cl-decreasing strongly while F-increasing slightly in liquid.In terms of H-O isotope of fluid,fluid in pre-mineralization was derived from magma;meteoric water begin to mix into original fluid gradually until syn-mineralization;fluid is dominated by meteoric water in the post-mineralization.According to S-Pb-C stable isotope of ore minerals,ore-forming materials were derived from magma,further,Mo originated from type ? intrusion,while Pb-Zn were co-derived from type ? and ? intrusion possibly.Combined with decompression during rock-forming process and CO2 variation during fluid process abovementioned,it can be inferred that decompression triggered CO2 gas escaping,then triggered further phase separation of medium-high temperature,medium-low salinity and C02-rich fluid and medium-high temperature,high salinity,and CO2-poor fluid from homogeneous fluid,and then changed the pH value of fluid system,finally caused the Mo unsteadiness and precipitation from hydrothermal fluid.Intensive study on the hydrothermal alterations which are close relationship with mineralization indicate that K-metasomatism within granite porphyry is noticeable and can be identified by the secondary enlargement of original K-feldspar,or plagioclase replaced by K-feldspar,with fine-grained and vermicular quartz and recrystallized albite around the edges of the K-feldspar grains.By comparing the altered samples from varying degrees of K-feldspathization reveal that the ore-bearing granite porphyry from different K-feldspathization intensity exhibit uniform Sm-Nd isotope but varying and high Rb/Sr ratio,discrete and even abnormally low(87 Sr/86Sr)0 value.This indicates that the Rb-Sr iscotopic system is sensitive for the hydrothermal fluid,while the Sm-Nd system is insensitive.Generally,great care must be taken on the influence to Rb-Sr system by the hydrothermal alteration when assessing the magmatic origin of porphyry bodies related to porphyry Mo deposits.It is the uppermost Mo mineralization stage between the late period of K-feldspathization to beresitization,suggesting the decline of pH value in fluid system trigger the Mo precipitation from fluid.Combined with the previous studies on the East Qinling-Dabie Mo metallogenic belt,the deposits in this belt share similar geologic characteristic,hydrothermal alteration features,geochemical characteristic,magmatic-hydrothermal origin,diagenetic mechanism,hydrothermal fluid evolution.In comparing with the porphyry Cu deposit and porphyry Cu-Mo deposit in China,we conclude that alkali-metasomatism occurs in porphyry deposits with different metals,but large differences exist in the degree of alteration,the intensity of Rb-Sr fractionation in the hydrothermal fluid,and the influence to intrusion.These differences imply that the different porphyry metal deposits have distinct sources of material for rock and ore formation,fluid origins,and tectonic settings.