Petrogenesis,metallogeny And Tectonic Settings Of The Saishitang Cu Deposit In Qinghai Province,China

The Saishitang Cu deposit,located in the Eastern Kunlun area,is one of the most representative Cu deposits with high exploration and exploitation level.Based on summary of previous studies,an systematic study of petrology,mineralogy and mineral chemistry,zircon U-Pb dating,molybdenite Re-Os dating,microthermometry of fluid inclusions and stable isotope was carried out to investigate the petrogenesis and material source,ore genesis and mineralization mechanism,and tectonic setting of the Saishitang Cu deposit.Skarn and Cu orebodies of the Saishitang deposit mainly occur along the contact zone between the quartz diorite and wall rocks.The study of ore-forming geological conditions shows the quartz diorite is composed of inner medium to fine-grained quartz diorite phase and outer porphyritic quartz diorite phase.The major ore-hosting rock is composed of metamorphosed tuff,marble containing minor intercalated metamorphosed sandstone and andesite.Four stages have been recognized in the skarn-and ore-forming process:a prograde skarn stage,a retrograde skarn stage,a quartz-sulfide stage and a quartz-carbonate stage.The skarn is classified as calcic skarn,which consists of garnet(grossular-andradite compositions),pyroxene(diopside-hedenbergite compositions),wollastonite in prograde skarn stage,amphibole(actinolite,ferrohornblende,hastingsite and ferropargasite)and epidote in retrograde skarn stage.There are obvious zonations from the intrsions to wall rocks.The garnet and epidote is richer in metamorphosed tuff and andesite close to the intrusions,where the garnet is dark red-brown and belongs to grossular;whereas pyroxene is richer near the marble,and the garnet is andradite compositionally,becoming pale brown or yellow green.Wollastonite,Mn-bearing pyroxene,phlogopite and magnetite occur close to the marble.Petrological and geochemical characteristics indicate that the metamorphosed tuff in ore-hosting wall rocks are calc-alkaline and show trend of volcanic rock transition to sedimentary rock in protolith reconstruction diagram.The andesite belongs to magnesian andesite(MA,MgO=3.5%?5.01%;Mg#=59?64),and zircons in andesite have ?Hf(t)values from-4.44 to 2.29.The andesite is best interpreted as resulting from the interaction between dehydration and melting of subducting slab and enriched mantle.The quartz diorite is calc-alkaline and metaluminous I-type granite,with ?Hf(t)values of zircons from-2.05 to 4.87,most likely derived from the partial melting of thickened lower crust with an additional input of mantle components.The minimum detrital zircon age of metamorphosed tuffaceous siltstone is 246±3Ma and zircon age of the andesite is 237.9± 1.2Ma,indicating ore-hosting strata were formed in Early to Middle Triassic.Zircons from medium-to fine-grained quartz diorite and porphyritic quartz diorite yield ages of 222.7±2.3 Ma and 222.6±2.4 Ma,respectively.Molybdenites of the quartz-sulfide stage yield a Re-Os age of 223.4±1.5 Ma.These ages constrain the mineralization age to Late Triassic.The study of fluid inclusions shows that the ore-forming fluids evolved from high temperature(400?460?),moderate-high salinity(42?48%NaCl equiv.and 8?14%NaCl equiv.)in the prograde skarn stage,to moderate temperature(280?340?),moderate-high salinity(36?40%NaCl equiv.and 6?10%NaCl equiv.)in quartz-sulfide stage,to moderate-low temperature(260?300?),low salinity(2?6%NaCl equiv.)in quartz-carbonate stage.Fluid immiscibility occurring at the early stage was related with the separation of ore-forming fluids,and fluid immiscibility(boiling)in late stage resulted in the precipitation of sulfides.The study of H,O,C isotope indicates ore-forming fluids were derived from magmatic water in prograde skarn stage,with the input of meteoric water in quartz-sulfide stage.The methane was detected as the predominated gas composition of fluid inclusions,indicating reduced characteristics of fluids.S and Pb isotope results show ore-forming materials mainly come from the magma and some of them may come from the wall rocks.The Saishitang Cu deposit is a skarn-type deposit,resulting from a reduced porphyry-skarn mineral system.The quartz diorite-dominated magmatism carrying plenty of heat and fluids intruded into Lower to Middle Triassic sequences and then caused widely metasomatism,accompanying the bimetasomatism between metamorphosed tuff,andesite and marble to form skarn and Cu mineralization.The andesite and volcanic-sedimentary rocks were formed in the final stage of the subduction of the A'nyemaqen ocean,and the quartz diorite and associated Cu mineralization were related to regional extension in a post-collision environment.