Mesozoic Magmatism And Its Relationship To Cu-Au Mineralization Potential At Jiaochong Oredistrict, Tongling, Anhui Province

The magmatic rocks(gabbro,pyroxene diorite and dioritic porphyry),amphibole-rich xenoliths,melt inclusions and sulfide inclusions in porphyry-skarn Cu-Au system at Jiaochong oredistrict,Tongling,were studied to constrain mesozoic magmatism and its relationship to Cu-Au mineralization potential.Petrographic observations show that gabbro is characterized by occurrence of abundant clinopyroxene phenocrysts and amphibole megacrysts.Compared to basic endmember—gabbro,less clinopyroxene phenocrysts and amphibole megacrysts are observed in pyroxene diorite and dioritic porphyry but abundant resorbed plagioclase and quartz phenocrysts occur in the most evolved endmember—dioritic porphyry.Based on their mineralogical and textural characteristics,amphibole-rich xenoliths are reaction-replacement hornblendite which are hosted in gabbro and consist mostly of hornblende and seem to have formed by reaction-replacement of a precursory clinopyroxene ± phlogopite assemblage in a crystal mush environment.Whole-rock chemistry results indicate that three magmatic rocks are all characteristically depleted in high field strength elements(Nb,Ta,and Ti)and enriched in large ion lithophile elements(Ba and Sr),but pyroxene diorite and dioritic porphyry possess more evolved Sr-Nd isotopic ratios than gabbros.Zircon U-Pb dating yield crystallization ages of 120.6±1.2 Ma(MSWD = 4.2),135.4±1.0 Ma(MSWD=1.6)and 128.6±1.2Ma(MSDW=1.9)for gabbros,amphibole-rich xenoliths and dioritic porphyry,respectively.Estimated crystallization temperatures and pressures for gabbro,reaction-replacement hornblendite,pyroxene diorite and dioritic porphyry are 1160-1206?,4.6-5.0 kbar;950-1150 ?,8-10.5 kbar;1112?,0.65 kbar and 791-798?,4.6-4.9 kbar,respectively,which record an evolution from basic endmember to felsic part.The mesozoic magmatisms at Jiaochong include underplating,reaction-replacement,assimilation,fractionation crystallization and magma mixing.Combined with the published whole rock data,it is concluded that Tongling intermediate-acid rocks are dominantly formed by polybaric crystallization involving early high-pressure intracrustal fractional crystallization of cumulates comprising hornblende and clinopyroxene,and late low-pressure fractional crystallization of hornblende and plagioclase.Magmatic sulfide inclusions occur mostly in the form of monosulfide solid solution but rarely also in the form of sulfide liquid.With the decreasing MgO content in amphibole,absolute concentration of Cu is decreased but Au and Ag is increased.Quantitative fractionation crystallization modeling reproduces well the Cu,Au and Ag behavior recorded in sulfide inclusions at Tongling.Based on the decreased Cu/Au and Cu/Ag ratios but increased Ag/Au ratios with increasing degree of differentiation,we concluded that once sulfide saturation is reached,it will decrease the Cu mineralization potential and increase Au(and Ag)mineralization potential.