Ore Genesis Of Late-Paleozoic Cu-Ni Sulfide Deposit In North Xinjiang, China:Constraints From Geochemical Data And Volatile Compositions

Numerous magmatic copper-nickel sulfide deposits are found in the Altai and Tianshan orogenic belts. There are different from other important magmatic deposits in the world in tectonic settings. The Kalatongke magmatic Cu-Ni sulfide deposit is the largest of important magmatic sulfide deposits in the Central Asian Orogenic Belt (CAOB). The Huangshan Cu-Ni sulfide deposit in eastern Tianshan orogenic belt is hosted by peridotites. These two deposits are representative in the regions, and contain useful information about regional metallogeny of magmatic copper-nickel sulfide deposits in orogenic belts.This dissertation presents whole-rock major elements, trace elements, and platinum group elements, and volatile chemical composition, carbon isotopic compositions and Sr-Nd isotopes in mineral separates. These data are used to evaluate the compositions and evolution of the parental magmas of the ore-bearings intrusive rocks, the mechanisms of sulfur saturation and concentration. The main conclusions are summarized below.1. Olivine in the Kalatongke and Huangshan copper-nickel sulfide deposits are chrysolites, with Fo ranging from70.93to81.39mol.%and from76.94to85.94mol.%, respectively. Olivine inclusions in clinopyroxene oikocrysts and distinct olivine in samples containing<2vol.%sulfide exhibit a positive correlation between Fo and Ni contents, suggesting fractional crystallization process. In contrast, olivine in samples containing>2vol.%sulfides show no clear relationships between Fo and Ni contents. Negative correlation between olivine Fo and Ni contents in the Kalatongke copper-nickel sulfide deposits indicate Fe-Ni exchange reaction between olivine and coexiting sulfide liquid, whereas variable Ni contents and lack of Fo-Ni correlation of olivine in the Huangshan copper-nickel sulfide deposits suggest sulfur seggregation during olivine crystallization.2. The Kalatongke and Huangshan copper-nickel sulfide deposits have similar primitive mantle normalized trace element patterns. They are characterised show enrichment of large ion lithophile elements (LILE) and LREE, negative Nb-Ta anomalies, positive Sr, anomalies and positive ENd (t) values. The Kalatongke and Huangshan copper-nickel sulfide deposits also have similar primitive mantle distribution patterns of platinum group elements, showing low PGE contents in the sulfide ores, relatively depleted in Os, Ir and Ru and enriched in Pt and Pd.3. The chemical and carbon isotopic compositions of volatiles released from olivine, pyroxene and sulfide mineral separates from norites, analyzed by means of vacuum stepwise-heating mass spectrometry, show high volatile contents at three temperature intervals,200-400℃,400-900℃and900-1200℃. The volatiles released from silicate mineral separates at400-900℃and900-1200℃have similar chemical and carbon isotopic compositions. These volatiles are mainly composed of H2O (av.～92vol.%) with minor H2, CO2, H2S and SO2. They likely represent the ore-forming magmatic volatiles.δ13CCO2values (from-20.86‰to-12.85‰) of pyroxene range between mantle and crust values. The volatiles released at200-400℃, which are dominated by H2O with minor N2+CO, show δ13CCO2values from-25.66‰to-22.98‰with the signatures of crustal components, suggesting secondary tectonic-hydrothermal activities. The volatiles released at400-900℃from sulfide separates are dominated by SO2(av.-75vol.%), with minor H2S. They possibly represent the volatiles from solidifying sulfide except the abundant SO2, which may have been derived from the decomposition of sulfide during heating experiment. The volatiles released from sulfide separates are dominated by SO2(av.～49vol.%) and CO2(34vol.%).4. Sm/Nd and Nb/Ta ratios vary from0.19to0.23and from16.33to18.77at Kalatongke and from0.21to0.34and10.67to19.00at Huangshan, respectively. These values are between the primitive mantle (0.33,17.83) and the upper crust (0.17,14.71), indicating involvement of crustal materials. Sr-Nd-C isotopes are also between the mantle and crust valuse. Mixing calculations show the contamination of upper crust and coeval A-type granites in the Kalatongke and Huangshan magma.5. The ore-forming magmas of the Kalatongke and Huangshan copper-nickel sulfide deposits experienced previous sulfide segregation at depth, resulting in depletion of platinum group elements in the parental magmas. Fractional crystallization of olivine and pyroxene, plus addition of crustal components during magma evolution caused sulfur saturation in the parental magmas and sulfide segregation. The volatiles released from intrusion1#have more oxidized gases (e.g. CO2and SO2) contents, higher CO2/CH4and SO2/H2S ratios and lighter δ13CCO2than intrusions2#and3#.6. The Kalatongke and Huangshan Cu-Ni copper-nickel sulfide deposits probably resulted from Early Permian plume activity or "slab window" of subducted Junggar Ocean (North Tianshan Ocean) in post-collision setting. The underplating of the Late Devonian-Late Carboniferous island arc magma relate to subduction of Junggar Ocean (North Tianshan Ocean) contribute to the crust growth. Subsequently, the Junggar Ocean (North Tianshan Ocean) probably closured during Early Permian (ca.300-290Ma). Therefore, the Kalatongke and Huangshan Cu-Ni copper-nickel sulfide deposits probably were generated in a post-collsion setting (ca..285Ma). Then, the plume or "slab window" resulted from break-off of the subducted oceanic crust caused the basatic magmatism and associated Ni-Cu ore genesis.