Geological And Geochemical Characteristics Of The Kaipinggou Ultrabasic Complex Indications For Prospecting In The Northern Margin Of The Qaidam

Ultrabasic complexes are different facies and different chemical compositions magmatic rocks with mantle-relation.Their petrological and geochemical characteristics not only record the dynamic processes of different stages of magma evolution and the mantle dynamics of magma formation,but also reflect the heterogeneity of small-scale geochemistry of mantle.Therefore,the studying of ultrabasic complexes has an important role in the exploration of the genetic model of magmatic rocks and the evolution of mantle.In addition,the ultrabasic complexes closely related to the magmatic nickel-copper sulphide deposit and chromite deposit,which is a unique product of mantle magma and metal accumulation in the continental environment.It is also an important window and research perspective for understanding the composition of mantle,deep processes,fluid action,migration and accumulation of ore-forming elements,crust-mantle interaction and geodynamics.More importantly,the studying of ultrabasic complexes is great significance to understand the genesis of magmatic nickel-copper sulphide deposits and chromite deposits then to guide the exploration of these resources.The North Qaidam UHP metamorphic belt is situated at the northeastern margin of the Qinghai-Tibet Plateau,in northwest China.It trends W-NW and is approximately400 km long.It is offset by the large Altyn Tagh sinistral strike-slip fault in the west.Paleozoic to Mesozoic sedimentary rocks deposited on Precambrian basement make up the Qilian block in the north.To the south lies the Qaidam block,which is a Mesozoic to Cenozoic intra-continental basin developed on the Precambrian crystalline basement.Several studies have suggested that the North Qaidam ocean basin was subducted during the Early Ordovician and then collision with Qilian block occurred during the Late Ordovician to Middle Devonian.The ultrabasic rocks in the North Qaidam can be divided into three types:the garnet-facies peridotite?Shenglikou?,the oceanic ophiolite-type mantle peridotite?Dulan?,and magma crystallization intrusion type after collision?Lenhu and Niubiziliang?.The Kaipinggou ultrabasic complex is located between the Yuka terrane and the Luliangshan terrane in the North Qaidam,and show nickel mineralization.At present,there are few studies on ultrabasic complex in this area.What is the relationship between its rock genesis and mineralization?Interestingly,Luofengpo ultrabasic complex contains chromite deposit,which is located in the west-northern to Kaipinggou ultrabasic complex.What is the relationship between these two ultrabasic rocks?Many scientific issues on the genesis and mineralization of these ultrabasic complexes still need to be systematically studiing.This paper mainly discusses the relationship between the Interaction of crust-mantle in subduction zone and the mineralization of the ultrabasic complexs.In this contribution,whole-rock major and trace and platinum-group elements,olive mineral elements,and the zircon U-Pb isotopes,trace elements and Hf isotopes for peridotite at Kaipinggou in the Qaidam orogen are studied.Kaipinggou peridotites are characterized by high contents of Mg#,Mg/Si and Ni.They have similar distribution patterns of REE and trace elements,depletion in HFSE and HREE,and slightly enrichment in LILE and LREE.Olivines have high Fo?90.11⁹2.77?and NiO?0.315⁰.45wt.%?contents,but low contents of CaO??0.02wt.%?.Whole-rock platinum group elements?PGEs?are similar to those of metamorphic and residual peridotites.There are two group metamorphic zircons in the Kaipinggou peridotites,which age of459.5±3.6Ma and 417.5±2.7Ma,corresponding to?Hf?t?of-0.71⁹.44 and-11.96^-1.2,respectively,indicating the nature and timing of the fluid derived from oceanic crust?or early continental crust subduction?and continental crust.In conclusion,the Kaipinggou peridotites are derived from the mantle wedge of the lithosphere and record the crustal features due to the metasomatism of the oceanic and continental crust fluids.The novel finding provides a new evidence for the evolution of oceanic lithosphere subduction to continental collision/subduction in the North Qaidam.Moreover,the petrographic observation and electron probe tests show that the chromite spinel has a distinct core-rim structure.The core is an aluminum chromite with high Al₂O₃,and low FeO^T,TiO₂,Cr^#characteristics,indicating that the host rock formed in the subduction zone?SSZ?environment that was the product of the reaction between mantle peridotite and melts of mid-ocean ridge basalt?MORB?.The edge is high-iron chromite with low Al₂O₃,high FeO^T,TiO₂ and Cr^#characteristics,indicating that the rim of the chromite is affected by serpentinization alteration and Fe-rich fluids.The aluminum chromite in the core is a magma chromite.The crystallization temperature is calculated to be about1372°C on average,the average crystallization pressure is 2.96 GPa,and the average?logfO₂ is-1.36,indicating that it is formed in the mantle asthenosphere.The Fe³⁺/Fe²⁺of the chromite decreases,besides,the Ni sulfide inclusions and the NiO component increase from the core to the edge indicating that Ni migrates and enriches with the increasing oxygen fugacity.In summary,it is speculated that the primary rock of peridotite in Kaipinggou is formed in the subduction zone,and the magma source area has MORB affinity.The later fluid metasomatism is transformed into serpentinized nickel mineralization and finally exposed to the surface.In addition,the study on the Luofengpo ultrabasic complex shows that some chromites have zoned texture with Al-rich,but Fe-poor cores.The chromites'cores have high Cr^#?81.54–85.72?and low Mg^#?25.27–36.00?.These together with their trace-element signatures are typical of ophiolitic chromites.Moreover,Cr-Mg-Ti-Al relationships obsreved in the cores of chromites indicate that the host rock should have been derived from a forearc lithosphere in a subduction zone environment.The magmatic zircon cores yielded an age of 483.1±3.5 Ma,which is interpreted to date the formation of the chromites in a supra-subduction-zone?SSZ?environment,whereas the metamorphic zircon rims yielded an age of 434.2±2.1 Ma and it is thought to date their growth contemporarily to the UHP metamorphism recorded in Qaidam.The?Hf?t?of relict magmatic zircons and metamorphic zircons range from-6.35 to 2.94 and-7.96 to2.58,respectively.The primitive mantle-normalized trace element spider diagram shows negative Nb,Ta and Ti anomalies,which is consistent with the patterns of CAA-OAB?CAA:Continental arc andesites,OAB:Oceanic arc basalts?.The chromites have low total platinum-group elements?PGE?,but they have high IPGE/PPGE?IPGE:Os,Ir,Ru;PPGE:Pt Pd,Rh?ratios.The chromites contain low-P silicate,chloride and carbonate inclusions?enstatite,Mg-calcite,bischofite,and dolomite?and higher metamorphic grade inclusions such as corundum and magnesite.In summary,the Luofengpo podiform chromite provides evidence regarding an arc occurred in the ocean between the Qaidam block and Qilian block.They originated from subduction of the oceanic crust and later experienced the metamorphism of continental collision.Summary collection of existing information,the results show that?1?the ratio of m/f from copper-nickel deposits mainly concentrate in 1.5⁸.5,and chromite deposits concentrated in 6.5¹2.5.In addition,the binary diagram of the major elements of whole rock also cannot effectively discriminate the mineralization specificity;?2?the Cu-Ni deposits basic-ultramafic rocks exhibit that high PPGE/IPGE ratios?0.06³43.75,average 16?,Pd/Ir>1,left inclined primitive mantle normalized pattern of PGE,and their chromite spinels show that high TiO₂,Fe#,and variety Cr^#and Mg^#.However,the chromite deposit basic-ultrabasic rocks show that low PPGE/IPGE ratios?0.0004²0.34,average 0.55?,Pd/Ir<1,right inclined primitive mantle normalized pattern of PGE;?3?their chromite spinel show that low TiO₂,Fe^#,Mg^#and high Cr^#.According to the above discriminant index,indicating that the Kaipinggou ultrabasic rock has nickel mineralization potential.