Petrogenesis And Metallogenic Geochemistry Of Porphyry-skarn Type Copper-gold Deposit

In this thesis,we studied two typical Mesozoic porphyry-skarn copper polymetallic deposits in the Yangtze Block,eastern China,including the Yaojialing ZnCu-Au deposit in Tongling ore-cluster region,Middle-Lower Yangtze River metallogenic belt(MLYRMB)and the Tongchang Cu-Mo-Au deposit in Dexing orecluster region.The research content mainly includes the following three parts:firstly,the whole rock major,trace and isotope of the ore-forming granodiorite porphyry in Yaojialing deposit,the ore-forming granodiorite in Jinziwei Cu-Mo deposit,and late dykes in the mining area;secondly,the trace elements and S isotopes of sulfides(pyrite,chalcopyrite,sphalerite,and galena)in different orebodies of Yaojialing Zn-Cu-Au deposit.Thirdly,the geochemistry of zircon,apatite,and chlorite in granodiorite porphyries of Tongchang deposit.Detailed studies of petrology,geochronology,mineralogy,mineral deposit and geochemistry from two orefields,we try to unreveal their magmatic source and evolution,petrogenesis,mineralization mechanism and oreforming fluid source and evolution processes.The details are as follows:(1)Early Cretaceous magmatic activities in the Tongling ore-cluster region of eastern China produced a series of igneous rocks,which can be divided into early-stage ore-bearing intrusions associated with large-scale Cu polymetallic mineralization,and late-stage intrusions that are generally ore-barren.To understand petrogenesis and their relationship to mineralization of early-stage ore-bearing intrusive rocks(granodiorite from the Jinziwei deposit and granodiorite porphyry from the Yaojialing deposit)and late-stage ore-barren granodiorite dykes from the Jinziwei deposit,a series of studies were carried.Zircons from early-stage ore-bearing intrusions and late-stage ore-barren rocks yield U-Pb ages of 141～139Ma,and 129Ma,respectively,indicating that the gap between the two-stage magmatic events was approximately 10Ma.Altough the SiO2 contents of the two types of rocks are similar(62.15～68.11 wt%and 64.02～65.91wt%,respectively),their Sr/Y(49.26～140.54 and 17.90～21.24,respectively)and Y(7.4～16.2 ppm and 20.2～24.0 ppm,respectively)contents are significantly different,suggesting that the former is adakitic rocks,while the latter is non-adakite.Whole-rock Sr-Nd-Pb and zircon Hf isotope compositions indicate that the early-stage ore-bearing intrusions were derived from partial melting of the subducted slab(oceanic crust with marine sediments),followed by the interaction of lithospheric mantle peridotite and the later incorporation of Middle-Upper Yangtze continental crust materials during magma ascent/emplacement.The late-stage ore-barren dykes evolved from the early-stage intermediate-basic magma in the Tongling region,which was originated from the enriched lithospheric mantle metasomatized by slab fluids/melts followed by the addition of a small amount of crustal materials.The zircon geochemical results show that high oxygen fugacity of the early-stage intrusion is a key factor in regional Cu-Au mineralization.Our apatite geochemical study reveals the fact that magma with F-rich or Cl-rich characteristics directly controls the type of mineralization in this region.Furthermore,the F-rich magma is favorable to Cu-Mo deposits,and on the other hand,the Cl-rich magma is favorable to Cu-Au-Zn deposits.(2)The Yaojialing Zn-Cu-Au deposit is a porphyry-skarn-epithermal vein type compound deposit,which consists of epithermal vein type lead-zinc-silver orebodies,skarn Zn-Cu-Au orebodies and porphyry Cu-Au orebodies from shallow to deep.We decipher the source and metallogenic mechanism by studying trace elements and S isotopes of sulfides(pyrite,sphalerite,chalcopyrite,and galena)from three types of orebodies.Pyrite collected can be divided into two types.One is PyⅠ which is coexist with chalcopyrite.PyⅡ is coexist with sphalerite or galena.In addition,PyⅠ was further divided into PyⅠa(collected from porphyry copper body)and Py-Ib(collected from skarn copper body).PyⅡ can be further divided into PyⅡa(collected from skarn-type lead-zinc ore body)and PyⅡb(collected from vein lead-zinc ore body in strata).Sphalerite and galena in shallow veined Pb-Zn ore body and skarn type Pb-Zn ore body are named SpⅠ,GnⅠ and SpⅡ,GnⅡ respectively.The sulfide trace element thermometer showed that the temperature of ore-forming fluid decreased from 500～600℃ of the porphyry ore body to 300～360℃ of skarn ore body,and then to 240-280℃ of shallow vein ore body.The decrease of Co and Ni content of pyrite collected from deep to shallow may indicate the addition of meteoric water precipitation in the late ore-forming hydrothermal system.The euhedral Pyl pyrite particles have obvious oscillating zone,indicating that their growth rate is fast,and that the physical and chemical states(such as temperature,pH,fO2,geochemical composition,etc.)of the fluid change rapidly during their formation,which affects the solubility of trace metal elements and leads to the selective entry of these elements into pyrite.While some elements in Pyll also have obvious zonation characteristics,but do not have oscillating zonation,indicating that the growth rate of pyrite is relatively slow.Meanwhile,the content relationship of Au and As in the two types of pyrites is obviously different,the former is coupled,and the latter shows decoupling.We believe that this phenomenon is due to the high content of As in ore-forming fluid system may inhibit the absorption of Au on the surface of pyrite,rather than the decoupling of Au and As caused by rapid crystallization.According to the analysis of sulfur isotopes of various sulfides,we believe that the ore-forming materials of each type of orebody in Yaojialing deposit are derived from magmatic hydrothermal,and the fractionation of sulfur isotopes is the result of the change of physical and chemical state caused by hydrothermal evolution,rather than the addition of foreign sulfur sources.(3)The Tongchang granodiorite porphyry intrusion is an important ore body of the giant Dexing porphyry Cu-Mo-Au deposit in South China,and both mechanism of petrogenesis and mineralization processes still remain controversial.In this study,results of zircon U-Pb dating of three granodiorite porphyries yield ages of 169.8 ± 1.3 Ma,167.1 ± 1.1 Ma,and 166.2 ±1.3 Ma,respectively,indicating that their formation ages are contemporaneous.The zircon Hf isotopes of the three samples are consistent within the range of error(εHf(t)=2.5-4.8),indicating that they share the same magma source.Sr isotopes range from 0.70414 to 0.70759(av.0.70520)and 0.70530 to 0.70745(av.0.70624)for altered apatite and 0.70368 to 0.70493(av.0.70443)for fresh apatite.Major elements(F and Cl)of altered apatite(F=1.51-2.44 wt.%,Cl=1.332.32 wt.%)are significantly different from unaltered apatite(F=2.54-4.62 wt.%,Cl=0.004-1.20 wt.%).Trace element compositions,such as δEu,δCe,REE distribution patterns,and etc.,between different apatites are also diverse.All these apatite geochemical data indicate that some apatite grains were modified by hydrothermal fluids,which were derived from the crust characterized by higher oxygen fugacity than the initial magma,indicating that they have undergone the decomposition of plagioclase.Chlorite geothermometers show that the formation temperatures of chlorite from three granodiorite porphyry samples ranges from 221 to 269℃(av.240℃),236 to 254℃(av.246℃),and 196 to 232℃(av.215℃),respectively,indicating the temperature of hydrothermal alteration.The apatite and zircon data suggest that the Dexing porphyries were derived from partial melting of the subducted paleo-Pacific plate during the Jurassic rather than from the lower crust.Subducted oceanic crust with high oxygen fugacity and initial Cu content and Cl content are the key factors in forming the giant Dexing porphyry Cu-Mo-Au deposit.