Cu And Zn Isotopic Features Of Typical Magma-related Deposit Systems In Tibet And Their Geological Significances

The Tibet metallogenic belt contains various types of deposits with complex mineralization processes and unique preservation condition.Three different types of deposits in the Tethys metallogenic domain of Tibet are selected for this study.The first type is the porphyry and high-sulfidation epithermal Cu-Au deposit（Tieglongnan Cu-Au deposit）formed in the continental arc background of the Neo-Tethys oceanic subduction process.Generally,the high sulfidation epithermal Cu-Au deposit is overprinted on the porphyry system,which forms a typical Cu-S binary system superimposed on the Cu-Fe-S ternary system,resulting in a unique spatial distribution of copper minerals;Second type is porphyry-skarn copper polymetallic deposit（Jiama copper polymetallic deposit）formed in the continental-continental collision and extension environment of Gangdise metallogenic belt.It formed typical porphyry,hornfels（chalcopyrite）and skarn ores（both chalcopyrite and bornite）.The distribution characteristics of copper minerals show the coexistence of both Cu⁺and Cu²⁺minerals;The third type is the vein-type zinc-lead-silver-antimony deposit（Zhaxikang zinc polymetallic deposit）in the northern Himalayan metallogenic belt.In the belt,there is a spatial distribution feature that antimony-silver in the north,middle Lead-zinc-silver and zinc-lead metals in the south.These three types of deposits are ubiquitous and their spatial distributions represent cases evolved from the Neo-Tethys Ocean to the continental-continental collision surroundings.In this paper,systematic studies on mineralogy,copper and zinc isotope geochemistry are carried out,which is of important theoretical and practical significance.The results of Cu-Zn isotopes and their geological significances are summarized as follows:1.Based on a systematic study of Rongna area which is proved to be a porphyry overprinted by high-sulphidation copper deposit,the copper isotopic values in South Tiegelong display progressive increase with depth,having an overall variation of?⁶⁵Cu up to 8‰.The large?⁶⁵Cu variation is likely of supergene origin and the systematic increase of?⁶⁵Cu with depth can be explained by the release of isotopically heavy Cu at the top and its transport downward.This is supported by the coupling of positive?⁶⁵Cu values with high Cu grade and the occurrence of both negative and positive?⁶⁵Cu values in two drill holes.Such mineralized targets,if buried at depth,could be located using Cu isotopes.2.Data of zinc,sulfur and lead isotopes of Zhaxikang epithermal Pb-Zn polymetallic deposit in northern Himalaya metallogenic belt,indicate that the formation of ore deposits is related to magmatic fluids.The deposit has a typical zinc isotope characteristic of magmatic hydrothermal mineralization.Generally,sphalerite and galena have similar Zn isotopic compositions.The?⁶⁶Zn of the fluid is calculated to be 0.39‰.This indicates a magmatic-hydrothermal origin of the Zhaxikang Pb-Zn deposit.Zn isotopic compositions of the sulfides significantly extend the range of regional basement rocks,suggesting that sedimentary rocks（e.g.,shales）are also a significant source of Zn.The results of our investigations indicate that the Zhaxikang Pb-Zn deposit is most likely a magmatic-hydrothermal deposit.3.The ore-barren and the ore-bearing intrusions both formed at 15 Ma.The ore-bearing intrusions have lower SiO₂,higher MgO,lower Sr and Sr/Y ratios,higher Y and more enriched LREE and Sr-Nd-Hf isotopic compositions compared with the barren rocks.The ore-bearing intrusions resulted from magma mixing between juvenile lower crust-derived felsic magmas and the enriched mantle-derived mafic magmas.Mixing between mafic and felsic magmas is also strongly supported by the occurrence of mafic xenoliths and the reversed zoning of major elements observed in hornblende of the ore-bearing rocks.The input of metasomatized mantle-derived magmas enriched in volatile and with high oxygen fugacity into the crust may be a key requirement for the formation of PCDs in continental collisional belts.Based on Cu isotope features of three mineralization types in the Jiama porphyry deposit,different stages of hornfels mineralization show limited Cu isotope difference due to late-stage fluid events which had overprinted on former ones.Hornfels and skarn mineralization have different Cu isotopic compositions,withδ⁶⁵Cu_horfel of-0.8 to 1.9‰andδ⁶⁵Cu_skarnkarn of 0.2 to 1.0‰.Sulfides in skarn have a slightly heavierδ⁶⁵Cu than those in porphyry and horfels.Temperature and pH control the fractionation of Cu isotopes during hydrothermal fluid evolution.The variation of Cu isotopes in Jiama is from-0.68 to 2.06‰.Because of various stages of hydrothermal activities,it is difficult to get a clear implication for tracing the source of metals.In this dissertation,fluid migration,the source and the ore genesis in the metallogenic process have been discussed detailedly,based on the study of copper and zinc isotopes in three deposits from magmatic-hydrothermal metallogenic system.All types of deposits have shown detectable variations of copper and zinc isotopes.The case study from Zhaxikang shows that zinc isotopes may be used to trace the sources of metals,in particular trace whether metals（e.g.,Zn）are derived from the contribution of sedimentary carbonates.Copper isotopes show great variations in low temperature condition and epigenetic processes.Copper isotopes may help interpret fluid evolution and can be further developed as a guiding tool for deep exploration.