Geochemistry Of Late Mesozoic Magmatic Rocks And Related Mineralizations And Mineral Prospecting In The Jiurui District Of Jiangxi Province

Large-scale Cu-Au mineralization is associated with the Late Mesozoic granitoids in the Jiurui mineralization district,Middle-Lower Yangtze mineralization belt,Eastern China.In order to constrain the petrogenesis of these granitoids and coeval mafic rocks,a detailed geochemical and Sr-Nd-Pb-Hf isotopic study has been carried out.The Jiurui granitoids dominantly consist of granodiorite porphyry and quartz diorite porphyry.They are characterized by SiO2 and K2O contents of 58.8 wt.%to 68.6 wt.%,and 1.9 wt.%to 5.7 wt.%,respectively.They show relatively high MgO contents from 1.0 wt.%to 3.1 wt.%,with an average of 2.1 wt.%,and high Mg#value ranging from 39 to 70 with an average of 54.The Jiurui granitoids are enriched in light rare earth elements(LREE),large ion lithophile elements(LILE),and compatible trace elements(Cr,Ni and V),but relatively depleted in Nb,Ta,Y and Yb,with negligible negative Eu anomaly(Eu/Eu=0.76-1.13,with an average of 0.91)and almost no negative Sr anomaly.The initial whole-rock 87Sr/86Sr ratios range from 0.7060 to 0.7092 and the ?Nd(t)values vary from-5.4 to-2.0.Plagiodases of granitoids show radiogenic Pb isotopic ratios with 206Pb/2O4Pb(17.937-18.215),207Pb/204Pb(15.545?15.583),208Pb/204Pb(38.165?38.564).The zircon Hf isotopic compositions show a ?Hf(t)value from-11.8 to 2.4.The coeval mafic rocks consist of lamprophyre,diabase and fine-grained mafic dyke.They are characterized by SiO2 contents from 47.6 wt.%to 54.8 wt.%,with negative Eu anomaly and positive to negative Sr anomaly.The initial whole-rock 87Sr/86Sr ratios range from 0.7059 to 0.7071 and the ?Nd(t)values vary from-3.8 to-1.4.By comprehensively considering the geochemical and isotopic compositions of Jiurui granitoids and coeval mafic rocks,we come to the conclusion that granitoids are similar to adakites that were related to delamination,and coeval mafic rocks are directly originated from enriched lithospheric mantle.The thickened lower continental crust and underlying enriched lithospheric mantle delaminated into asthenospheric mantle simultaneously.This process lead to the partial melting of thickened lower crust and enriched lithospheric mantle,and the melts of them experienced asthenospheric mantle assimilation during ascend.Mafic rock melts could be generated from enriched lithospheric mantle without delamination,which was heated by upwelling asthenospheric mantle.The geochemical and isotopic compositions of mafic rocks were probably controlled by AFC process.A geochemical and isotopic comparison of the granitoids in the Jiurui district with Angui and Tongling of the same mineralization belt indicates that from the southwest to northeast in this metallogenic belt,the proportion of mantle component shows a decreasing trend from the Jiurui to Angui and Tongling districts.It is suggested that the Juirui district has the highest potential for huge amount of Cu-Au accumulation since mantle is an important supply for these metals.The major types of mineral deposit in Jiurui ore district are porphyry,skarn and massive sulfide type.Porphyry and skarn types are typical magmatic-hydrothermal deposits,and massive sulfide deposits of Jiurui area are located near intrusive rock,which shows that magmatic-hydrothermal fluids have played a crucial role in the mineralization process.In Jiurui area,petrographic characters of ore-barren intrusive rocks are almost the same as ore-bearing samples,and parameters for identify ore-bearing intrusive rock as well as how the parameters affect mineralization are still unknown.In order to solve above problems,a detailed study on geochronology,major elemental geochemistry and mineral chemistry was performed for the ore-bearing and ore-barren quartz diorite-porphyrite and granodiorite porphyry of the Jiurui ore district,Jiangxi Province.Zircon U-Pb age results demonstrates that extensive magmatism was coeval and intensive in this area,and suggests emplacement ages of 138.2+1.8Ma to 148.0±1.OMa for the ore-bearing intrusive rocks,and 139.0±1.3Ma to 149.6±3.OMa for the ore-barren intrusive rocks,which are consistent within the error range.The variation ranges of SiO2 and other 9 major elements are quite similar between ore-bearing and ore-barren intrusive rock,which means that their major geochemical compositions are similar.The average temperatures of ore-bearing and ore-barren intrusive rocks are 769? and 723?respectively,which are consistent within the error range.But the variation range of ore-bearing samples(580?-915?)is obviously greater than ore-barren ones(656??796?).It is worth noting that the average pressure of ore-barren intrusive rocks(5.7kbar)is evidently greater than ore-bearing samples(1.4kbar).Therefore,the ore-bearing intrusive rocks of Jiurui area have experienced a more distinct decreasing pressure process,which is quite important to form magmatic aqueous phase and brittle failure for hydrothermal circulation.However,decreasing pressure process of intrusive rock is not the sufficient condition of mineralization,but a necessary condition.Based on the data of Jiurui area,the mineralization potential is weak when the pressure of intrusive rock exceeds 4kbar.The fO2 of ore-bearing intrusive rocks has a wider variation range than ore-barren samples and half of the data higher than HM buffer line,but the fO2 data of ore-barren samples are all lower than HM buffer line,which means that high fO2(higher than HM buffer line)is beneficial to mineralization in Jiurui ore district.The skarn type copper deposits of Jiurui mineralization district are composed of Wushan,Dongleiwan and Dengjiashan.Garnet-dominated skarn alteration developed in Wushan deposit,which phenomenon is not only different from Dongleiwan and Dengjiashan but also scarce in the whole metallogenic belt.In order to discuss the relationship between geological characteristic and ore forming process of skarn deposits in Jiurui area,we carried out mineral chemistry,H,O,S and Pb isotope analyses on skarn silicates and sulfides.The average andradite%and hedenbergite%values are 83 and 10 for Wushan,59 and 15 for Dongleiwan,43 and 22 for Dengjiashan.The pistacite component(Ps value)defined as Fe3+/(Fe3++Al)and Fe3+/Fe2+value of epidote are 0.12 and 1.63 for Wushan,0.30 and 32.73 for Dongleiwan,0.17 and 42.85 for Dengjiashan.Average prograde and retrograde temperature calculated by coexisting garnet-clinopyroxene pairs and chlorites are 574? and 281?for Wushan,573? and 246? for Dongleiwan,560? for Dengjiashan(without chlorite).The mean values and variation ranges of ?34S value are as follows:Wushan 0.0‰and-2.8?1.4‰;Dongleiwan-0.3‰ and-2.6?1.1‰;Dengjiashan-1.9‰ and-2.9?-0.5‰.Variation ranges of 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb of all the three deposits are 17.900?18.205,15.538?15.649 and 38.170?39.025 respectively.Mean ?18OFluid values of prograde and retrograde stages are 8.13‰ and 7.81‰ for Wushan,6.47‰ and 2.33‰ for Dongleiwan,8.27‰ and 4.43‰ for Dengjiashan.?DFluid value of Jiurui skarn deposits are-65.2?-30.9‰ for progade minerals and-64.0?-32.8‰ for retrograde minerals.Based on the above analyses,we can conclude that skarn type alteration of Wushan,Dongleiwan and Dengjiashan are all formed in a relatively oxide environment,and oxidation extent of prograde skarns from stronger to weaker is Wushan>Dongleiwan>Dengjiashan,however,in retrograde skarns Dongleiwan and Dengjiashans own a higher oxidation degree than Wushan.The ore-forming material source constrained by sulfide ?34S values and Pb isotopic ratios show evident magmatic origin feature.Considering?18OFluid and ?DFluid features,the hydrothermal fluid of prograde skarns of all three deposits are derived from magmatic source,but Dongleiwan and Dengjiashan have meteoric water input in the retrograde stage.In ore-forming process aspect,the garnet-dominated alteration and sulfide precipitation of Wushan skarn deposit are mainly caused by system cooling as intrusive rock crystallization.The Dongleiwan and Dengjiashan skarn deposits own a developed retrograde skarn minerals such as epidote,chlorite,quartz and calcite,which probably caused by hydrothermal fluid mixing with cooler,oxidizing and dilute meteoric water.In order to improve the exploration situation of Jiurui district,we have enhanced the fundamental geological research,and studied the effect of NEE faults and SN faults on controlling the magmatic and ore-forming processes in detail.The Shanshangwan area is selected for specific research.There are several dykes appearing in EW direction,and alteration and mineralization phenomenon are observed in the dykes and surrounding rocks.Combined with petrology,geochemistry,geophysics exploration,and remote sensing spectrum researches,we have discussed the petrogenesis,location of concealed rocks,alteration characteristics,and come to the conclusions that Shanshangwan magmatic rocks own the same petrogenesis as other mineralization in Jiurui district.And the dykes extended to east underground.Based on above analysis,we set four drill holes at positions which are good for mineralization,in order to expose these locations:1)interior of northern two dykes;2)the contact zone of dykes and surrounding Silurian sandstones;3)boundary between Ordovician and Silurian which nearby the dyke;4)massive sulfide ore deposits developed at the boundary between Devonian and Carboniferous.