Metallogenesis Of Nickel Deposits In Eastern Kunlun Orogenic Belt, Qinghai Province

Eastern Kunlun Orogenic Belt，which belongs to an important part of the Qin-Qi-Kunmetallogenic domain on the global scale, is part of the southern branch of the CentralOrogenic Belt. Eastern Kunlun Orogenic Belt experienced a complex and lengthy historyof tectonic evolution and mineralization, riching in mineral resources. Based on systematicsummarization of the dynamical evolution history of the East Kunlun area, this paperselects the first, as well as the only one discovered super large copper-nickel sulfidedeposit-Xiarihamu nickel deposit for research. Combining with modern metallogenictheory, using methods of geochronology, petrology, mineralogy, geochemistry and isotopegeology with modern testing technology as support, this paper analyses geological andgeochemical characteristics of the deposit and explores the mineralization and deepmineralization process of Xiarihamu deposit by systematic studies on magma sourceregion, the nature of the parent magma, magmatic evolution and sulfide deep segregationmechanism. The main achievements of this study are summarized as follows:(1) Eastern Kunlun area developed syn-collision S-type granites with the age of915~928Ma which indicates that this area was in the environment of continent-continentcollision in early Jinning period, responding to the global convergence of Rodiniasupercontinent in time. Mafic-ultramafic intrusive rocks formed in the stretch environmentafter collision are mainly concentrated in the periods of Caledonian and late Indo since thePhanerozoic in the East Kunlun area.(2) The paper makes systematic chronology, geochemistry and tectonic setting studieson the intrusive rocks which developed in Xiarihamu ore district. Magmatic activity in thedistrict is divided into five periods of Jinning, early Caledonian, late Caledonian, early Hercynian and Indosinian. Copper-nickel ore and mineralization are mainly produced inthe mafic-ultramafic intrusive complex of late Caledonian-early Hercynian.(3) The preliminary determination is that the banded and lenticular garnet amphiboliterocks exposed in the area are retrograde metamorphic eclogite with the metamorphic ageof437.6±3.6Ma. The original rocks are fragments of oceanic basalts with characteristicsof E-MORB, on which eclogite metamorphism occurred due to the deep subduction ofoceanic plate to the continental plates, and amphibolite facies retrogressive metamorphismtook place during the later exhumation process.(4) No Ⅰ.Rock of Xiarihamu deposit emplaced in Jinshuikou group metamorphicrocks of Proterozoic and Neoproterozoic granitic gneiss containing garnet, with the lengthof about1.9km, width of700m and area of about1.33km2, E-W trending elongated stripwhich strike about60°. The rock mass displays features of layered mafic-ultramaficintrusive complex with clear lithofacies zonation and strong alteration, and the mainlithofacies include peridotite facies, pyroxenite facies and gabbro facies. The main rocktypes include harzburgite, iherzolite, plagioclase iherzolite, olive pyroxenite, pyroxeniteramps, gabbro-norite and so on. Peridotite and pyroxenite phase constitute the main bodyof the rock mass, also the main ore-bearing facies. Electron microprobe analysis showedthat olivine, spinele and orthopyroxene in the main rock-forming minerals are chrysolite,chrome spinel and bronzite respectively, and clinopyroxene are endiopside and augite,while hornblendes are allium amphiboles and magnesium amphiboles. Biotites in thePeridotite and pyroxenite phases are all phlogopite. No. Ⅱ-1rock mainly consists ofgabbro, with small amounts of pyroxenite and good mineralization of pyrrhotite,pentlandite and chalcopyrite.(5) The main ore bodies of Xiarihamu mining area were produced in the No. ⅠM afic-ultramafic rock system. The morphology of ore body mainly are stratoid and lenticular,partially are pure sulfide veins. The strike of the NEE extending ore bodies is about70°with the dip changing at0~35°, and the attitude is consistent with the rock mass. In deepit had a trend of plunging towards southwest with the plunge angle of about20°. Fromnorth to south along their dip, the thickness of ore bodies become gradually thinning and bifurcating with the grade decreasing. Electron microprobe analysis showed that oreminerals mainly consists of pyrrhotite, pentlandite and chalcopyrite, and theCo-containing independent minerals which founded in copper-rich massive ore werenickel cobaltite.(6) The δ34S ranges from2.7‰~7.6‰in the21samples of No1. Rock while the valuein the6samples from NoⅡ-1. Rock is4.9‰~6.1‰, both of which indicate the additionof crustal sulfur. The ratio of206Pb/204Pb is17.774~18.346in the13samples from No1.Rock, while207Pb/204Pb changes from15.536~15.598,208Pb/204Pb varies from37.529~38.209. The ratio of206Pb/204Pb is17.906~18.153in the3samples from NoⅡ-1.Rock, while207Pb/204Pb changes from15.570~15.604,208Pb/204Pb varies from37.865~38.595. Pb isotopic in the sulfide exhibits characteristics of both mantle andorogenic belt.(7) In the ore district, the ΣPGE content in the rocks is1.44×10-9~9.02×10-9whilethe value in the ores is16.11×10-9~56.08×10-9, generally higher than the rocks. Thetotal amount of PGE in ores is26.19×10-9~250.39×10-9after100%sulfide computing,significantly lower than the Jinchuan deposit. The ratio of Cu/Pd and the relationship of Niand Fo in olivine indicate that fractional crystallization of olivine and sulfide segregationoccurred in the early evolution of the original magma, and small amounts of sulfidesegregation may be the main reason for the relative loss of Cu and Ni.(8) The results of zircon LA-MC-ICP-MS U-Pb dating of mafic–ultramafic rocks inXiarihamu show that the crystallization ages of gabbronorite and talcized pyroxenite are(423±1) Ma and(422±1) Ma respectively in the NoⅠ.Rock, with the correspondingMSWD of0.14and0.07. The crystallization age of gabbro is (424±1) Ma with MSWD of0.15in the No Ⅱ-1rock. The age of gabbro is (427.4±7.3) Ma with MSWD of0.26inBinggounan complex intrusion.(9) Geochemistry characteristics show that the ratio of m/f is in the range of2.32~6.67, belonging to iron ultrabasic rocks. Samples are generally enriched in LILE elements(Rb, Th, U, K), and relatively loss Nb, Ta, P. The isotopic results of13analysis points onzircons from gabbronorite from NoⅠ Rock Mass show that the ratio of176Hf/177Hf is 0.282628~0.282833, εHf(t)=4.0~10.9, tDM1=610~875Ma, average age is788Ma.Theisotopic result of11analysis points on zircons from gabbro from NoⅡ-1Rock Mass,show that the ratio of176Hf/177Hf is0.282721~0.282912, εHf(t)=6.9~13.7, tDM1=497~775Ma, average age is608Ma.The result of Isotopic tests of4orthopyroxenite samplesfrom NoⅠ Rock Mass isεNd(t)=-3.1~-1.7（t=422）;(87Sr/86Sr)i=0.703105~0.710932，on average is0.705811. δ18OV-SMOW=5.2‰~7.0‰and3pyroxene samplesεNd(t)=-0.2~-1.6（t=422）.The result of Isotopic tests of5gabbro samples from No II-1Rock MassisεNd(t)=-2.1~-0.（7t=424）;(87Sr/86Sr)i=0.708200~0.708614，on average is0.708415.The foregoing analysis considered that Rock mass I and II have the same magma sourcewhich was derived from a depleted mantle, and may have been added metasomaticenriched lithosphe mantle components, and contaminated by crustal material.(10) According to Fo in olivine is up to88and the content of TFeO is10.11%in thesamples of harzburgite in the NoⅠ.Rock of Xiarihamu, we estimate the content of MgOis12.48%in the parent magma. The ratio of Ni/Cu and Pd/Ir suggest that the parentalmagma of the rock is high-MgO basaltic magma. Features above reflect that the parentmagma of the mafic-ultramafic rock in Xiarihamu may originated from a higher degree ofpartial melting of the mantle.(11) Based on the comprehensive analysis, we believe that Xiarihamu copper-nickelsulfide deposits was formed in the dynamic background of extensional after collision andthe diagenetic and metallogenic epoch is late Silurian while the deep dynamic process mayassociated with the subducted slab breakoff. Mantle-derived magma occurred in sulfursaturation and deep segregation and the main mechanism of sulfur saturation was thefractional crystallization of olivine and orthopyroxene and the addition of the crust sulfur.