Comparison Of Ore-forming Systems Of Cu-Ni-PGE Sulfide Deposits Associated With The Emeishan And Siberian Mantle Plumes,and Jinchuan Cu-Ni-PGE Sulfide Deposit

Nickel, copper and platinum group elements (PGE) are important metals. These metals mainly come from magmatic sulfide deposits. Some of these deposits are related to mantle plume activity in a continental setting. The Emeishan and Siberian large igneous provinces (LIP) are both formed by mantle plume in the Permian. However, these two LIPs host different types of magmatic deposits. The former mainly hosts many world-class Fe-Ti-V oxide deposits whereas the latter hosts several world-class Ni-Cu-PGE sulfide deposits. Several magmatic Ni-Cu-PGE sulfide deposits have been also found in the Emeishan LIP but they are much smaller than those in the Siberian LIP. Nonetheless, the Ni-Cu-PGE sulfide deposits in the Emeishan LIP are useful for a study of genetic relationships between mantle plume activity and Ni-Cu-PGE sulfide mineralization. Furthermore, a better understanding on the origin of these deposits is helpful to the on-going mineral exploration in the region. Hence, a comparative study of the Ni-Cu-PGE deposits in the Emeishan LIP, the Noril’sk deposits in Siberian LIP and the Jinchuan Ni-Cu deposit in North China Craton have been carried out using integrated approaches including geochronology, mineralogy, petrology and geochemistry of major, trace elements, PGE and C-He-Ar and Sr-Nd-Hf isotopes. The data of volatiles, noble-gas isotopes and carbon isotopes from this study provide new insight into magma-crust interaction during continental basaltic magmatism, which forms another part of this thesis. The most important conclusions from this study are summarized below.1. The Binchuan picrites associated with Emeishan flood basalts in NW Yunnan have been confirmed as a part of Emeishan LIP by zircon U-Pb SHRIMP age of256.2±1.4Ma for the andesitic tuff interlayer. Hence, these picrites can be used to investigate the nature of the Emeishan mantle plume. Some researchers believe that the Binchuan picrites associated with flood basalts in NW Yunnan are the integral parts of the Permian Emeishan LIP whereas others believe that they are Triassic picritic porphyrites or Cenozoic olivine-rich dikes. The zircon U-Pb SHRIMP age of256.2±1.4Ma for the andesitic tuff within the Binchuan volcanic succession which contains the age-disputed picrites is within the range of ages for the Emeishan LIP. The εHf values (-0.5to6.6) of comagmatic zircons (-256Ma) from the andesitic tuff of the Binchuan volcanic succession are within the range (-5to10) of zircons from the Emeishan LIP. The age-disputed picrites and the undisputed Emeishan picrites have similar olivine Fo-Ni-Mn compositions and whole-rock trace element ratios such as Th/Nb and Ti/Dy. The Cenozoic olivine-rich dikes in SW Sichuan and NW Yunnan can be distinguished from the picrites associated with the Emeishan flood basalts by different olivine Fo-Ni-Mn correlations and whole-rock trace element ratios. The results from this study confirm that the age-disputed picrites in NW Yunnan and SW Sichuan belong to the Permian Emeishan LIP.2. Depletion of Ni is olivine from the Ni-Cu-PGE sulfide deposits such as Zhubu, Limahe and Qingkuangshan in the Emeishan LIP indicates a previous sulfide segregation event which took place at depth. Olivine compositions show that the most primitive parental magmas of these deposits have MgO/FeO ratios similar to the transporting magma (i.e., groundmass) of the Emeishan picrites. Both positive and negative Fo-Ni correlations of olivines are observed in the Zhubu and Limahe deposits. Only positive Fo-Ni correlation has been found in the Qingkuangshan deposit. At a given Fo content, the Ni contents of olivines from these deposits are significantly lower than that of olivines from the Emeishan picrites with the same Fo contents, indicating the olivines in these deposits crystallized from a Ni-depleted magma due to previous sulfide segregation at depth.3. The whole-rock major element compositions of the Emeishan Cu-Ni-PGE sulfide deposits are controlled by the types and abundances of major minerals. Whole-rock trace elements and Sr-Nd isotopes indicate that their parental magmas originated from mantle plume and were subsequently contaminated by variable proportions of crustal materials during magma ascent and emplacement. The compositions of ultramafic rocks in the host intrusions are mainly controlled by the abundances of olivine and pyroxenes; the compositions of coexisting gabbros are mainly controlled by the abundances of pyroxenes and labradorite; the compositions of coexisting gabbrodiorites are mainly controlled by the abundances of andesine, clinopyroxene, hornblende, biotite and Fe-Ti oxides. The intrusive rocks are all characterized by enrichments of large ion lithophile elements (LILE) plus light rare earth elements (LREE) and by significant Nb depletion. The Sr-Nd isotopes of clinopyroxene separates show the parental magmas have mantle-plume signatures. The Pd/Ir ratios in the sulfide-bearing intrusive rocks are variably lower than that in the Emeishan picrites, consistent with fractional crystallization of olivine and Cr-spinel from the parental magmas prior to the sulfide segregation which formed the deposits. Mixing calculation using Sr-Nd isotope data indicate15to20wt.%crustal contamination for the parental magmas of the Zhubu and Limahe intrusions. The ratios of Nb/Th and Th/Yb in whole rocks show that the Qingkuangshan parental magma was contaminated by～10wt.%crustal materials.4. The PGE contents in parental magmas of the Ni-Cu-PGE sulfide deposits in the Emeishan LIP are variably lower than that in the coeval Emeishan picrites. which indicate the parental magmas of the different deposits experienced various degrees of previous sulfide segregation at depth. The ratios of whole-rock Pd/Ir in these deposits are slightly higher tlian that of the Emeishan picrites, indicating that fractional crystallization of olivine and Cr-spinel played a role in PGE fractionation in the parental magmas. The estimated PGE contents in the parental magma of the Zhubu Ni-Cu-PGE deposit are slightly higher than that in the other deposits. The contents of PGE in the parental magma of the Zhubu deposit are estimated to be7ppb Pd,9.3ppb Pt and0.8ppb Ir. The compositional variations of bulk sulfides in the Zhubu deposit can be modeled by sulfide segregation from such magma with R factor (magma/sulfide mass ratio) varying from100to6000. The PGE tenors (i.e.,recalculated to100%sulfide) of the sulfide ores in the Qingkuangshan and Limahe deposits are much lower than those of the sulfide ores in the Zhubu deposit. This indicates that the parental magmas of the Qingkuangshan and Limahe deposits are more depleted in PGE than that of the Zhubu deposit.5. The volatiles released from olivine and clinopyroxene separates from the Zhubu and Nori’sk Ni-Cu-PGE sulfide deposits indicate that the parental magmas of these two deposits are similarly originated from reduced sources, and the mechanism for sulfur saturation may have been different for the different deposits. H2O is a dominant component in the volatiles released from the mineral separates from both deposits. The estimated H2O contents in the parental magmas of the Zhubu and Noril’sk deposits are0.39wt.%and0.6wt.%, respectively. H2and CO2contents are relatively high in the Zhubu and Noril’sk volatiles. Sulfur is low in the volatiles from both deposits. He is detectable in Noril’sk volatiles. The total contents of volatiles released at temperature>400℃are2589mm3.STP/g on average. N2(27%) and CO2(26%) are main components of volatiles released from the Zhubu samples. H2O is the main component of volatiles released from the Noril’sk samples. Sulfur contents in the volatiles increase with releasing temperatures. The total volatile contents at each releasing stage are up to10171.34mm3.STP/g. H2(53%) and CO2(23%) are main components of the volatiles from the Zhubu samples. The H2and H2S contents are higher in the volatiles released from the Noril’sk sample. The variation of CO/CO2with releasing temperature indicates that the parental magmas of both deposits changed from relatively reduced to more oxidized on cooling. The results also show that sulfur-bearing volatiles were transported by ascending magma. The carbon isotopes of CO2and CH4of volatiles from the Zhubu samples are from-44.85%o to-22.83‰and from-22.9‰to-7.01‰, respectively. These data indicate mixing between mantle and crust. The isotopic values of the Noril’sk samples are much lower, indicating a dominant crustal origin.6. The different Cu-Ni-PGE sulfide deposits associated with mantle plume magmatism could have formed by sulfide saturation in magma induced by fractional crystallization, sulfur addition, or/and crustal contamination. Based on trace elements and isotopes, it is proposed that sulfide saturation in the parental magma of the Emeishan Cu-Ni-PGE sulfide deposits was mainly triggered by crustal contamination, although the sulfur saturation in Noril’sk parental magma may have been caused by assimilating anhydrite. The formation of the Zhubu intrusion can be explained by two stages:a conduit stage for the sulfide ore-bearing marginal zone and an in situ differentiation stage for the layered sequence. Important sulfide segregation mainly occurred at the conduit stage. Highly variable metal tenors of bulk sulfides in the marginal zone indicate a dynamic magma-passing system for this zone. Comparatively,sulfur saturation was triggered by significant amounts of crustal fluids in Jinchuan Cu-Ni-PGE sulfide deposit. This is indicated by the results of He and Ar isotopic mixing calculations, which show that about20%crustal fluid and63%air saturated fluid were added into mantle-derived magma for the Jinchuan intrusion.7. Mantle plume magmatism in continental settings is ideal for the formation of giant magmatic Cu-Ni-PGE sulfide deposits. The Cu-Ni-PGE sulfide deposits in the Emeishan and Siberian LIPs and the Jinchuan Ni-Cu sulfide deposit share some similarities in sulfide concentration mechanisms. In these deposits, flow differentiation played a critical role in sulfide-liquid accumulation. The zircon U-Pb SHRIMP age for the Zhubu deposit is263.2±5.6Ma. The εHf values (-3.05to3.61) of comagmatic zircons (～263Ma) from this deposit indicate a mantle plume-derived magma with minor crustal contamination. The lower part of the marginal zone of the Zhubu intrusion could have been brought up by faulting and hence future exploration in Zhubu area should look for such a target. At the regional scale, it is important to look for mafic-ultramafic intrusions with ages and petrological characteristics that are similar to that of the Zhubu deposit. The εHf values (-11to-5) of comagmatic zircons with U-Pb age of821±11Ma from the Jinchuan ore-bearing mafic-ultramafic intrusion indicate interaction of mantle plume with sub-continental lithospheric mantle during magmatism.