The Minerogenesis Of Heishan Nickel-copper Sulfide Deposit, Gansu Beishan, China

The Central Asian Orogen Belt (CAOB) is a massive, complex Phanerozoicaccretionary collage and also is an important metallogenic zone hosting manynickel–copper magmatic sulfide deposits in the world. It has been the focus of domestic andforeign geologists for the mineralization, and mafic-ultramafic complex are known to occuracross CAOB. Permian mafic–ultramafic intrusions have been discovered in the EasternTianshan (e.g., Tudun, Huangshan, Xiangshan, Hulu etc.) and the Beishan Fold Belt inXinjiang (including Pobei, Luodong, Bijiashan, Hongshishan etc.), NW China, at thesouthern margin of the CAOB. The Heishan mafic–ultramafic rocks belt is located in theeastern part of Beishan, Near the Hongliuhe–Xichangjin ophiolite, along the Hongliuhe andHeishan-Jianquanzi fault. In this rocks belt, there are a series of mafic-ultramaficintrusions, such as Dashantou, North Heishan, Shuangjinzi, Guaishishan, Yingmaotuo andMazongshan. The recent study suggested that Late Devonian is an important metallogenicperiod for the nickel-cupper sulfide deposit in this region. The paper selected the Heishandeposits that is the only large-scale nickel–copper deposit for detailed study, focusing onpetrology, mineral deposit and geochemistry, through the depth study by the magma sourceand characteristic of primary magma, Parental magma composition, the magma evolutionprocess, the material exchange between with the magma and surrounding rocks, thesaturation and segregation mechanism of sulfide, and discuss the mineralization and depthprocess of magmatic sulfide deposits in Heishan deposit. Conclusions are as following:1) The zircon SHRIMP U-Pb dating for olivine gabbros and diorite of Heishanintrusion yielded274.5±3.9Ma, combined with the existing zircon age of Dashantou(359.3±5.7Ma SHRIMP U-Pb) and Guaishishan (353~405Ma SHRIMP U-Pb) in this region,Which is interpret as the forming age of Heishan mafic-ultramafic rocks. The forming ageof these mafic-ultramafic rocks in Beishan reveals there is an important metallogenic periodfor the nickel-cupper sulfide deposit in the active continental margin of the Tarim cratonduring Late Devonian.2) Heishan is a typical small-scale mafic-ultramafic ore-bearing intrusion, which iscomposed by different mafic-ultramafic lithofacies with the same age. Harzburgite, iherzolite, olivine gabbro norite, gabbro dyke; re the mainly economic mineralized rocks.Olivine, orthopyroxene, plagioclase, clinopyxene, amphibole and biotite are the mainpetrogenic minerals. The Cu-Ni ore bodies are at the bottom of margin of the intrusion.3) The Mineralogical characteristics indicate that Fo values between81.54and86.98,and the high content of Ni in olivine which belong to chrysolite. The trapped silicate andsulfur liquid had changed the origin composition of olivine Significantly. The species ofOpx and Cpx are bronzite and diopside respectively, and the plagioclase mainly belonglabradorite and feldspar. MELTS simulation results show the accurate crystallizationtemperature and sequence of the main petrogenic minerals.4) The geochemical characteristics show that it is rich in iron (m/f=4.66) in Heishanmafic-ultramafic intrusion and has tholeiitic evolution trend of its parental Magma. Therocks of the Heishan intrusion are characterized by enrichments of large ion lithophileelements, strong negative Nb (Ta) anomalies and positive K and Pb anomalies. And TheHeishan intrusive rocks have restricted εNd(t)(+0.34to+3.95) and a large range of ratios(87Sr/86Sr)i(0.7041to0.7087). These similarities suggest that the primary magma of theHeishan intrusion was derived from the depleted mantle and has both MORB and aec-likefeatures, which is modified by subducted altered oceanic crust. The high-Mg primarymagmas of Heishan intrusion were derived from partial melting of the asthenosphere andmetasomatized subcontinen-tal lithospheric mantle modi fied by subducted materials.5) Similar PGE distribution patterns of mafic-ultramafic rock show that complexoriginated from the same parental magma, which depleted PGE.∑PGE contents of themafic-ultramafic rocks range from1.87to12.05ppb, meanwhile∑PGE contents in Cu-Niore are7.32to173.9ppb, generally higher than in rocks. The variable PGE contents in100%sulfide reveals that distribution of metals elements is inhomogeneous. The featuresthat∑PGE contents of mafic-ultramafic rock and ore is lower, enrichment in PPGE and thehigh Ni contents are similar to Jinchuan deposit.6) The features of the major elements, trace elements and Nd-Sr isotope implying aminor contamination of middle-upper crust (about5%).7) The composition of parental magma are calculated by Olivine and the selectedsamples are:51.5%SiO2，0.96%TiO2，15.1%Al2O3，10.26%TFe2O3，0.16%MnO， 11.63%MgO，7.37%CaO，1.79%Na2O，1.09%K2O，0.14%P2O5；The PGE contents inparental magma be estimate:：Os0.085ppb，Ir0.018ppb，Ru0.072ppb，Pd0.956ppb，Pt4.053ppb，Cu28ppm，Ni152ppm. Pd/Ir and Ni/Cu ratios indicate that high Mg basalticmagma, consistenting with inference of whole-rock geochemistry, isotope data andcalculated parental magma composition.8) The parental magma of Hershan mafic-ultramafic intrusion derived from the lowerdegree (about8%~14%) of mantle partial melting through the calculation of the Ni, MgOand PGE in parental magma.9) Modeling R value (the mean value is1000) and the degree of monosulfide solidsolution crystallization, we find that the PGE and Cu-Ni ores are enriched in liquid sulfideand only individual samples completely control by monosulfide solid.10) The Ni and Fo in olivine, Cu/Zr and Cu/Pd suggest that the sulfide segregation(about0.12%~0.17%) and olivine fractional crystallization occurred almost at the same timeduring the magma evolution.11) The sulfides pre-segregation, low degree of partial melting caused by PGEdepleted in parental magma and the smaller R-factor, lower degree of partial meltingprobably be the main reason of differentiation of PPGE and IPGE.