Metallogenicprocess Of Porphyry-Skarn Cu-Au Deposits In Tongling District,Eastern China

Tongling district is one of the most important ore districts in the Middle-Lower Yangtze River metallogenic belt(MLYB)and is the cradle of "Strata-bound skarn and superimposed mineralization".The mineralization systems are characterized by Cu-Au skarn deposits,which dominantly consist of massive skarn ore with stratabound ore in Upper Devonian to Triassic formations.Recently,porphyry type mineralization has also been discovered in the Tongling district.Although the magmatism and metallogeny of the Tongling district have been studied extensively,the metal source and genesis of stratabound ore and the spatial and temporal relationships of porphyry/skarn ore remain controversial.Xinqiao and Dongguashan deposits are typical examples of the porphyry,skarn and stratabound ores in the Tongling distract,and offer an excellent opportunity to examine the metal source and relationships between the three ore types.This study of these deposits aims to achieve a comprehensive understanding of their magmatic-hydrothermal mineralization systems.There are several intrusion phases developed at Xinqiao and Dongguashan deposit,including pyroxene diorite,quartz monzodiorite and quartz diorite porphyry.The presence of disseminated chalcopyrite and pyrite show the quartz monzodiorite is the main fertile magmas in these deposits.Ti-in-zircon thermometry show that the pyroxene diorite crystallized at a temperature around 828?,and the quartz monzodiorite crystallized at about 784?.Study of magmatic zircon from the quartz monzodiorite,and hydrothermal titanite,rutile and garnet from hydrothermally-altered porphyries and skarn can be used to constrain the magmatic and hydrothermal alteration events.LA-ICP-MS U-Pb geochronological results reveal that porphyry and skarn mineralization are associated with quartz monzodiorite and formed at about 140Ma,which makes Xinqiao and Dongguashan consistent with other Cretaceous porphyry-skarn deposits in the MLYB.The ratios of Ce/Nd,Ce4+/Ce3+and Eu anomalies of magmatic zircons in the quartz monzodiorite show higher values than in pyroxene diorite and quartz diorite porphyry,which indicates that the fertile quartz monzodiorite was more oxidized and rich-water than the barren magmas.Moreover,apatite in the quartz monzodiorite has high SO3 and Cl contents,and lower F/Cl than barren magmas.Xinqiao and Dongguashan have well-developed porphyry alteration zoning,including potassic,propylitic and phyllic zones;the potassic and phyllic zones are close to mineralization.There are many hydrothermal minerals developed in different altered porphyry intrusions and skarn,such as apatite,titanite,rutile,magnetite,pyrite and quartz.The petrology and chemical characteristic of these minerals reflects changes in the hydrothermal fluid.The stable coexistence of rutile,apatite,magnetite and quartz shows the potassic alteration stage had high temperature and high fO2,and the major and trace elements of apatite and rutile reveal that the fluid contained more volatiles(Cl-,F-,OH-)and rare earth elements in the potassic stage.Furthermore,the variation of HFSE and V in titanite also indicates the fluid had high F that facilitated mobility of the HFSE(Zr,Hf)in the potassic stage.The abundant pyrite in the phyllic alteration zone indicates this stage had high fS2 with lower fO2.The decreasing REE content of apatite may indicate that an externally-sourced fluid was involved in the ore forming process.In this study,we found that apatite from both potassic altered rock and garnet skarn show similar mineral CL textures and major/trace elements,which indicates that the potassic stage and early skarn stage has similar fluid composition and physical conditions.Apatite and titanite in exoskarn and retrograde skarn show complex mineral oscillation textures and distinctive trace elements,suggesting that the exoskarn and retrograde skarn mineralization process was totally different from the porphyry alteration and mineralization.These differences indicate more intensive fluid-rock reaction and a different fluid system;this is also supported by enrichment of Mg,Mn and Sn in magnetite from skarn ore.Xinqiao and Dongguashan both have endoskarn and exoskarn zones;the endoskarn is garnet skarn dominated by grossularite.REE and Eu anomalies in grossularite indicate that the endoskarn formed under high temperature neutral-pH conditions in an mostly closed system.The garnet in exoskarn shows variable Fe/Al content and multiple internal texture;these characteristic indicate involvement of fluids from multiple sourcesduring exskarn formation and mineralization.Stratabound orebodies are most important at Xinqiao and Dongguashan;skarn and porphyry orebodies are minor.Colloforn pyrite in stratabound ore has attracted most research attention,and although the genesis of the stratabound ore has been intensively studied but remains controversial.Geological and chemical(trace elements and in situ S isotope)evidence strongly suggest that the colloform pyrite is magmatic-hydrothermal in origin and the metals in stratabound ore come from the quartz monzodiorite.In addition to Xinqiao and Dongguashan,there is a variety of skarn type mineralization and several typical skarn deposits in the Tongling district,e.g.,Chaoshan Au,Tongguanshan or Fenghuangshan Cu-Au,and Yaojialing Pb-Zn deposit.These skarn or porphyry-skarn deposits are associated with diorite,though compared to the composition of magmatic apatite from fertile intrusions,the trace elements in magmatic apatite(Eu/Eu*,Ce/Ce*,REE,Mn,Fe,Sr,Cu,Zn)can as an indicator for different porphyry-skarn type mineralization,e.g.,magmatic apatite from skarn Pb-Zn deposits is enriched in Mn,Fe,and Sr;magmatic apatite from skarn Au deposits is enriched Cu,and Zn,but apatite from porphyry-skarn Cu-Au deposits has compositions intermediate between those.Compared with porphyry deposits in magmatic arc settings(the Andes),the geochemistry of porphyry-skarn deposits from the Tongling district are similar,such as the fertile magmas also show oxide,rich-water and volatiles feature;but the wall rock and alteration intensity/type has difference.The wall rocks in the Tongling district appear to have controlled the skarn formation and supplied part of the metals in ore,but the porphyry deposits from Andes is mainly controlled by magmatism.