Genesis Of The Weibao Cu-Pb-Zn Deposit In Xinjiang,China

The Weibao Cu-Pb-Zn deposit is located in Xinjiang,NW China(latitude 37°07'58" N,longitude 91'07'08' E,elevation 5000 m),with a total ore resource of 33 million metric tons(Mt)at 0.15%Cu,1.01%Pb,1.22%Zn,and 9.01 g/t Ag.This deposit is one of several in the Qimantagh metallogenic belt(QMB),East Kunlun Mountains,northern Tibetan Plateau.In this study,detailed lithologic,structural,alteration,and mineralization time-space relationships of the Weibao deposit,as well as the whole-rock major and trace element,geochronological,mineralogical and isotopic data were present.The objective of this study is to(1)define the temporal constraints on the sequence of igneous rocks in the Weibao deposit,(2)evaluate the origin of these magmas and the tectonic setting during their emplacement,(3)examine the different geochemical characteristics of ore-bearing and barren magmas in the Weibao deposit,(4)constrain the composition and sources of ore-forming fluids,as well as the sources of metals,(5)clarify the mechanism of alteration and metal precipitation,and(6)suggest an advanced genetic model for the Weibao deposit.Spatially,the Weibao deposit comprises three ore blocks from west to east,known as Weixi,Main and Weidong.The host rocks are the Mesoproterozoic Langyashan Formation which is mainly composed of carbonate rocks,and to a lesser extent,basaltic volcanic rocks.The common intrusions include the diorite porphyry(Weixi),quartz diorite(Weixi),pyroxene diorite(Weidong),and diabase.U-Pb dating indicates that the basaltic volcanic rocks formed in Middle Devonian(393.0 ± 5.5 Ma to 392.0 ± 5.0 Ma),whereas other intrusions were collectively emplaced during Late Triassic.Specifically,the Late Triassic intrusive activity broadly began with emplacement of barren diorite porphyry with a U-Pb age of 232.0 ± 2.0 Ma,and was followed by emplacement of ore-bearing quartz diorite and pyroxene diorite with nearly consistent U-Pb ages of 223.3 ± 1.5 and 224.6 ± 2.9 Ma,respectively.This is consistent with intrusive activity in the Qimantagh metallogenic belt which manifests economic mineralization is predominantly related to intrusions of Middle to Late Triassic age.Subsequently,the diabase was emplaced with a U-Pb age of 225.2 ± 2.4 Ma.The Middle Devonian volcanic rocks show compositions mainly characterized by high potassium,light rare earth element(LREE)-enriched patterns in chondrite-normalized rare earth elements(REE)diagrams,and evident enrichment of Rb,Ba and K and depletion of Th,U,Nb and Ta contents in primitive mantle-normalized patterns,although the degrees of enrichment and depletion are variable.These characteristics of major and trace element data highlight a hornblende-dominated fractionation during ascent of magmas.The diorite porphyry,quartz diorite and pyroxene diorite belong to high-K calc-alkaline granitoids,and are similar in both major and trace elements,except that two ore-bearing intrusions are characterized by lower SiO2 and K2O,and higher TFe2O3 and MgO.The ?Hf(T)values of zircons range from-2.6 to +7.5 for the Weibao volcanic rocks and-4.1 to +2.5 for these three Late Triassic rocks,indicating that all these magmatic rocks were probably derived from a hybrid origin involving combined depleted mantle(and hence asthenospheric mantle)and melts and/or fluids inherited from an early subduction event.Besides,the new geochronological and geochemical data,together with previously published data in the East Kunlun Orogenic Belt,suggest that the Weibao basaltic lavas and intrusive rocks formed in a post-collisional setting,and that the Qimantagh area underwent strong interactions between mantle and crust in both Early Paleozoic-Middle Devonian and Late Paleozoic.Compared with zircons from barren diorite porphyry,those from ore-bearing quartz diorite and pyroxene diorite are characterized by high Ce4+/Ce3+ ratios that indicate higher oxygen fugacity.Unlike zircons,Ce anomalies of apatite are indiscriminate between barren and ore-bearing intrusions.Nevertheless,apatite from ore-bearing intrusions displays much lower Mn contents than that from barren intrusions,which also indicates higher oxidation state of ore-bearing intrusions.Besides,apatite from quartz diorite and pyroxene diorite shows higher Cl contents and lower F/Cl ratios.This,combined with the high oxidation state,indicates the high productivity of quartz diorite and pyroxene diorite in the Weibao deposit.When compared with ore-bearing intrusions from porphyry Cu deposits in world,those from skarn Cu-Pb-Zn deposits display much lower Ce4+/Ce3+ and EuN/EuN*ratios of zircon(and thus lower oxygen fugacities)and lower Cl and higher F/Cl ratios of apatite.Combined with published studies,we argue that these differences are general,although the exact cause is still unknown.Since both zircon and apatite are widespread minerals in a variety of igneous rocks,this work demonstrates that combined zircon Ce4+/Ce3+ ratios and apatite halogen contents might provide a sensitive means to infer economic potential of magmas and the resulting mineralization types in intrusion-related exploration targets.Mineralization in the Weibao deposit is intimately related to Late Triassic intrusions.Weixi and Weidong,which are proximal to intrusions,are mainly characterized by Cu mineralization,whereas Main,which is distal to intrusions,is predominantly characterized by Pb-Zn mineralization.Correspondingly,skarns from Weixi and Weidong are characterized by a high garnet/pyroxene ratio and diopside-and andradite-rich composition of pyroxenes and garnets,and therefore indicate a proximal,oxidized type.In contrast,skarn mineralogy ofMain is characterized by a lower garnet/pyroxene ratio.This feature,as well as the hedenbergite-rich and grossular-rich composition of pyroxenes and garnets at Main,is consistently indicative of a less oxidized or slightly reduced environment.At least five hydrothermal mineralization stages can be identified,i.e.,early skarn,late skarn,Fe-Cu sulfide,Pb-Zn sulfide and late quartz-carbonate stages.Petrographical observation manifests that four types of fluid inclusions occur in the Weibao deposit,i.e.,liquid-rich,vapor-rich,halite-bearing and CO2-bearing fluid inclusions.The microthermometric study indicates a general trend of cooling and dilution of the magmatic-hydrothermal fluids.The early skarn and late skarn stages are characterized by liquid-rich,vapor-rich and halite-bearing inclusions,with the homogenization temperatures ranging from 440-560? and 430-505?,respectively,and the salinities ranging from 16.0-59.8 wt.%NaCl equiv and 11.5-42.4 wt.%NaCl equiv.The Fe-Cu sulfide stage is characterized by liquid-rich and vapor-rich inclusions,with the homogenization temperature and salinity ranging from 340°-448? and 2.1-15.0 wt.%NaCl equiv.The Pb-Zn sulfide stage is characterized by liquid-rich,vapor-rich and CO2-bearing inclusions,with the homogenization temperature and salinity ranging from 288-398?(mostly 320-370?)and 0.4-12.6 wt.%NaCl equiv.Only liquid-rich and CO2-bearing inclusions occur at the late barren quartz-carbonate rocks with much lower homogenization temperatures(mostly<240?)and salinities(mainly 3-4 wt.%NaCl equiv).The evidence from oxygen,hydrogen and carbon isotopes of hydrothermal minerals(quartz and epidote)supports that the ore-forming fluids of the Weibao skarn system were originally derived from magmatic water,whereas the late quartz-carbonate stage was predominantly controlled by meteoric water.Although the fluids from Cu-Fe sulfide and Pb-Zn sulfide stages were still mainly composed of magmatic water,similar to those from the skarn-forming stages,they were strongly modified by the fluid-rock interaction.The sulfur and lead isotopes of sulfides indicate a mixed source combined by Triassic granitoids and ore-bearing strata.Besides,combined with previously published data,it can be deduced that the metal precipitation within the Weibao deposit mainly resulted from the fluid boiling,thermal decline and fluid-rock interaction.Overall,the results of this study have two important implications:(1)proximal and distal skarn can form by a single magmatic-hydrothermal system;(2)the Weibao deposit shows many similarities with skarn deposits in the QMB,and its genetic model therefore can be extrapolated to other skarn deposits.