Petrogenesis Of Mafic-ultramafic Intrusions And Minerogenesis Of Nickel Sulfide Deposit In The Periphery Of Qaidam Block, Qinghai China

Mafic-ultramafic intrusions are widely developed on the periphery of Qaidam Block,among which, some could bear Ni-Cu sulfide deposits, and they mainly distribute in themargin of Qaidam Block, Oulongbuluke Terrane and East Kunlun Orogenic Belt. To explorethe metallogenic characteristics of the Ni-Cu sulfide deposits on the periphery of QaidamBlock and provide theoretical basis for regional prospecting, two typical deposits,Niubiziliang and Xiarihamu, are selected to study in this paper. Based on the researches on thecharacteristics of geology, petrology, mineralogy, metallogeny and geochemistry, the origin ofthe intrusions, magmatic evolution, mechanisms of sulfide saturation and segregation arerevealed. Furthermore, the nature of magmatic source, mechanism of mantle dynamics andmetallogenesis are discussed.The main results of this study can be drawn as follows:(1) Niubiziliang nickel deposit is located in the Eboliang upfaulted, northwesternQaidam Block, and formed in the Late Devonian (367.0±2.0Ma). Regional geological datasindicate that during the Late Silurian and the Late Devonian, the northwest margin of QaidamBlock was in the continental margin rift environment.(2) The Niubiziliang intrusion, outcropped about8km2, is irregular shaped witheast-west extension and is a typical layered complex with clear cumulate layering and obviousvertical zoning. Rock types are abundant, and mainly composed of plagioclase lherzolite,plagioclase wehrlite, hornblende lherzolite, hornblende peridotite, olivine websterite,hornblende websterite, olivine pyroxene hornblendite, hornblende olivine gabbro,fine-grained gabbro, porphyaceous gabbro, melagabbro, gabbro, leucogabbro, quartz dioriteand tonalite. Rock structures show that the non-equilibrium fractional crystallizationdominates the crystallization and magmatic evolution at magmatic chamber, and thecrystallization temperature of main magma is1100~1178℃. The magma experiencedsignificant crustal contamination that increased gradually during magma evolution, giving riseto the chemical series changing from tholeiite series to calc-alkali series.(3) The MgO content of primary magma of Niubiziliang intrusion is10.8%. The Nd-Srisotopic composition and trace element geochemical characteristics indicate the depletedlithosphere mantle origin which is metasomatized by mantle flow/melt when the partialmelting mantle occurred or little earlier, resulting in the enrichment of the large ion lithophileelements and LREE.(4) The Niubiziliang intrusion is divided into three segments by F1and F2faults, among them No.â…  and No.â…¡ are mainly composed of ultramafic rocks, and they are the importantore-bearing intrusions. Ore type of No.â…  is chiefly hydrothermal, while No.â…¡ formedthrough the injection-pulp mineralization with two layers of ore-body, which are composed ofdisseminated and lump ore. δ34S in most of the sulfide from Niubiziliang range from-2.8‰to+0.32‰, indicating that sulfur mainly derived from the mantle. The integrated study indicatesthat fractional crystallization and assimilation process (AFC) are the main mechanism ofsulfide segregation of Niubiziliang deposit.(5) Xiarihamu nickel deposit locate in the island arc belt of the Central East Kunlun Faultzone, East Kunlun. The nickel deposit is hosted in No.â…  intrusion. The gabbro zircon U-Pbage is439.1±3Ma, suggestion its formation in the Early Silurian.(6) No. â…  intrusion consists of peridotite facies (dunire, harzburgite, lherzolite),pyroxenite facies (olivine orthopyroxenite, plagioclase-bearing pyroxenite) and noritic-gabbro(melagabbronorite, olivine gabbro, gabbro, leucogabbro). Except the gabbro and leucogabbro,all the rocks have cumulate texture, therefore fractional crystallization plays a leading role inthe magma evolution, and the main cumulate phases are olivine and orthopyroxene.(7) δ34S of sulfide from the Xiarihamu deposit is+4.44‰~+6.45‰, while the (187Os/188Os)iratio is0.1590~2.0097, with corresponding γOs values ranging from28to1520. Theisotopic composition shows the magma suffered strongly crustal contamination, which is alsodemonstrated by the field geological phenomena.(8) The Nd and Sr isotopic ratios are all in the range of enrichment mantle type, and haveEM II evolutionary trend. All rock types show the systematic enrichment of light rare earthelements and large ion lithophile elements, and depletion of Nb, Ta, Ti, P, which are similar togeochemical characteristics of magmatic rocks in the island arc environment. The mantlesource of Xiarihamu intrusion, therefore, should be the mantle wedge above the subductingslabs.(9) Fractional crystallization of a large number of olivine and orthopyroxene, and strongassimilation are the main mechanisms of sulfide segregation. In addition, more sulfur fromcrust source also promotes the sulfide segregation.(10) The metallogenic process of the Xiarihamu deposit can be divided into three stages,in-situ segregation, ore pulp injection and hydrothermal superimposition. It is: Partial meltingof the mantle wedge metasomatized by fluid/melts from subducting slabsâ†'Magma intrudesalong the second faults on the northern side of the Heishan-Nalinggele faultsâ†'Magmaticdifferentiation at deep magma chamber, which is the first enrichment of ore-forming materials, and then a part of the mafic magma moves away from magma chamber, residual magmaenters terminal magma chamber. Assimilating gneiss with Si and Al-riched and sulfur-richedcountry rock can promote the fractional crystallization and separation of sulfideâ†'ore-bearing magma continues to intrude and emplace in magma conduit and the space No.â… body exists, and then ore pulp injectedâ†'the late magmatic hydrothermal further activatesore-forming materials, and superimposes on the early mineralization.(11) Nickel ore on the periphery of Qaidam mainly formed during the Silurian-Devonianperiod. Therefore, the Paleozoic is the third epoch of nickel mineralization besideNeoproterozoic and Permian in our country.(12) The Heishan-Nalinggele fault controls the spatial position of mantle magma of No.â…  and magnesia peridotite of No.â…¢, so it must be a lithospheric fault. Many evidences showthat the island arc belt of the Middle belt of the East Kunlun is a tensive island arc belt.(13) Tensive island arc belt is the fifth tectonic environment besides of craton marginalrift zone, large igneous provinces, greenstone belt and stretching stage after collision orogenicbelt that are most easily to form nickel deposit.