Magmatism And Genesis Of The Giant Pulang Porphyry Cu-Au Deposit,Yunnan,SW China

The giant Pulang porphyry Cu–Au district(446.8 Mt at 0.52% Cu and 0.18 g/t Au)is located in northwestern Yunnan Province,SW China.Tectonically,it lies in the southern segment of the Yidun arc,part of the Sanjiang Tethyan orogenic belt.Spatially,the district consists of three separate deposits,South Pulang(?96% of the total ore reserves),and the smaller East and North Pulang deposits.Four intrusive phases hosting the three Pulang deposits have been recognized.These are,in order of emplacement,premineral fine-grained quartz diorite and coarse-grained quartz diorite;intermineral quartz monzonite;and late-mineral diorite porphyry.The complex intruded the Late Triassic reduced Tumugou slates.Zircon U–Pb LA–ICP–MS dating shows that intrusive activity occurred around 216 ± 2 Ma.The diorite porphyries are geochemically similar to mafic magmatic enclaves(MME)hosted in coarse-grained quartz diorite,and both are characterized by low Si O2 and high total alkali,K2 O,Mg O and compatible trace element concentrations.They are enriched in large-ion lithophile and light rare earth elements(LILE and LREE,respectively),but depleted in high field-strength and heavy rare earth elements(HFSE and HREE,respectively),and yield variably high(La/Yb)N ratios with weak to negligible Eu anomalies.Geochemical data and whole-rock Sr–Nd,and zircon Hf–O isotopic compositions indicate that the diorite porphyry and MME formed through crustal assimilation of a magma produced during low-degree partial melting of metasomatized phlogopite-rich subcontinental lithospheric mantle.In contrast,geochemical and isotopic(Sr–Nd–Hf–O)data of the coarse-grained quartz diorite and quartz monzonite suggest that they represent high-K calc-alkaline to shoshonitic adakitic magmas that were sourced from subduction-modified juvenile lower crust.Detailed petrographic observations,mineral chemistry,and/or whole-rock major and trace element studies indicate that the Pulang porphyry complex evolved in an open magma storage system in the upper crust through combined crystal fractionation and mafic magma recharge.The coarse-grained quartz diorite and quartz monzonite likely formed by crystal fractionation of a common parental magma within a felsic magma chamber at a paleodepth of ?11 km.Whereas the diorite porphyry and MME represent relatively mafic magma derived from a deeper magma reservoir situated at a paleodepth of ?19 km.Repeated mafic magma recharge into the felsic porphyry magma chamber and mixing between them are recorded by a series of microstructural and mineralogical observations:(1)quench-textured MME in both the coarse-grained diorite and quartz monzonite;(2)reverse zoning(An-poor core overgrown by An-rich rim),sharp zoning(An-rich core with abrupt rimward An decrease)and patchy-core(An-rich and An-poor patches)textured plagioclase;(3)coexistence of high-Al and low-Al amphibole;(4)biotite inclusion zones in K-feldspar within the quartz monzonite;(5)skeletal biotite in the diorite porphyry and MME;(6)embayed or rounded feldspar(K-feldspar and plagioclase)and quartz phenocrysts or xenocrysts,quartz ocelli in the diorite porphyry and/or MME;(7)apatite inclusion zones in plagioclase from the coarse-grained quartz diorite and diorite porphyry.Repeated mafic magma recharge would have supplied significant sulfur,metals(e.g.,Cu and Au)and H2 O to the porphyry system,and such recharge would also rejuvenate the porphyry system and aid the formation of large,long-lived magma chambers,both helped lead to formation of the giant porphyry Cu deposits at Pulang.Hydrothermal alteration at Pulang includes five main types: potassic,propylitic,chlorite–sericite,phyllic and argillic alteration.Potassic alteration,subdivided into early K-feldspar alteration and late biotite alteration(dominant)occurs mainly in the central quartz monzonite and locally in the immediately adjacent coarse-grained quartz diorite and fine-grained quartz diorite.Propylitic alteration,the most intensive alteration in the district,occurs from the deposit core outwards and has overprinted potassic alteration,within and ?500–800 m around the central quartz monzonite at South Pulang and within and ?200–500 m around the central quartz monzonite at North Pulang.Late chlorite–sericite,phyllic and argillic alteration have overprinted preexisting potassic and propylitic alteration assemblages,and are locally developed within all four intrusive phases.Copper and gold mineralization show a positive correlation of ore grades,and both are mainly associated with propylitic alteration and to a lesser degree with potassic and chlorite–sericite alteration,unlike the more usual association with potassic alteration typical of porphyry Cu deposits globally.In addition,some hypogene pyrrhotite,especially at East Pulang,is intergrown with chalcopyrite and occurs with the late chlorite–sericite and phyllic alteration minerals in the coarse-grained quartz diorite.Molybdenite Re–Os dating shows that the main mineralization at South Pulang occurred between 216.54 ± 0.87 and 216.13 ± 0.86 Ma.The sequence of intrusive emplacement,alteration and veining,and sulfide associations at the three deposits suggest that South and North Pulang are two independent copper–gold mineralized porphyry systems,whereas East Pulang is probably a distal product of fluids from the South Pulang system.The primary magnetite over ilmenite and mineral assemblage titanite + magnetite + quartz in the causative quartz monzonite,and the abundant hydrothermal anhydrite veining associated with early potassic and main-mineralization propylitic alteration are testament to the oxidized nature of the felsic intrusion and resultant early hydrothermal fluids.The presence of reduced pyrrhotite related to late chlorite–sericte and phyllic alteration suggests fluid reduction during these two stages,probably through interaction with the reduced wall-rock Tumugou slates.The atypical,peripherally and centrally located propylitic alteration and Cu mineralization at Pulang,both characteristic features of diorite-hosted porphyry copper systems,can be reasonably attributed to fluid interaction with the Pulang intrusions containing relatively high initial contents of calcium,magnesium and iron.