The Geological And Geochemical Characteristics Of Mafic-ultramafic Intrusions In The Hualong Area, Southern Qilian Mountains, And Its Ni-Cu Mineralization

Magmatic nickel-copper-PGE sulfide deposits form as the result of the segregation andconcentration of droplets of liquid sulfide from mafic or ultramafic magma and thepartitioning of chalcophile elements into these from the silicate magma.Magmatic sulfidedeposits fall into two major groups when considered on the basis of the value of theircontrained metals,one group in which Ni,and,to a lesser extent,Cu,are the most valuableproducts and a second in which the PGE are the most important. The magmatic Ni-Cu sulfidedeposits were the results from intraplate magmatism in the world,and concentrated in themargin of stable craton, rift environment and mantle plume activity region,from Archean toPaleozoic in formation geological history. In China,the magmatic Ni-Cu sulfide deposits werefocused on Proterozoic and late Paleozoic,but seldom other ages.These deposits were fosteredin the margin of stable craton and orogenic belt,but these magmatic Ni-Cu sulfide depositswere supposed to intraplate environment,why growed in orogenic belt,only because thedeposits from orogenic belt exposed? Jinchuan magmatic Ni-Cu sulfide deposit from Gansuprovince in China,the third largest deposit,was the result of very very large-scalemagmatism.The contemporaneous large-scale magmatic event may sweep more greaterarea,such as Qilian mountains,north margin of Qaidam basin and Arkin orogenic belt,thesearea maybe grow many mafic-ultramafic intrusions and mineralization. It is not unique,andsouth Qilian mountains with that of Longshoushan mountains containing Jinchuandeposit,developed Hualong mafic-ultramafic intrusions belt,its distribution nearly NW andabout length160Km,width10~30Km,was located in the middle part of the south margin ofLajishan mountains fault and fault of southern Qinghai mountains showing a wedge bodywith narrow west and wide east.The mafic-ultramafic intrusions that emplaced into Hualonggroup had different degree Ni-Cu mineralization,and even formed middle-small scaledeposits,therefore,it is a more important Ni-Cu mineralization belt in Qinghai province.The paper selected the Yulonggou, Lashuixia, Yaqu and Xiashentang intrusions of Hualongmafic-ultramafic belt for study, focused on the mineral deposit, petrology and geochemistry,through the depth research by the magma source and the characteristic of primary magma, theevolution process of magma, and the material exchange between with the magma andsurrounding rocks, the segregation mechanism of sulfide, and discuss the mineralization anddepth process of magmatic sulfide deposits in South Qilian mountains. The main results asfollowing:(1) All of mafic-ultramafic intrusions of Hualong rocks belt in Qinghai province intruded into Hualong rocks group of Hualong massif,and Hualong rocks group was a suit ofmiddle-deep metamorphic rock series,including quartzite,biotite quartz schist,muscovite andbiotite schist,quartz schist,hornblende schist,biotite plagioclase gneiss,compound gneiss andmigmatite.Ore-bearing intrusions from Hualong belt were made of mafic-ultramafic rocks,andits rock-forming minerals composed of olivine, augite, orthorhombic pyroxene, clinopyroxene,plagioclase, hornblende and biotite,and so on.Ore-bearing lithofacies were basicly gabbronorite,pyroxenite and peridotite, with intense alteration.(2) The forming age of Hualong rocks group was precisely determined firstly,the datafocused on893Ma and919Ma,and accompanied by ancient zircons of1000~1600Ma.Theamphibolites of Hualong group showed enriched LREE,undepleted Nb-Ta,it similared to traceelements distribution model and characteristic elements ratio,but different with relativedepleted high field strength elements(Th,Nb) of typical OIB,and belonged to intraplatecontinental basalt in the tectonic environment discrimination diagram.All of these reflectedthe original rock of amphibolites in Hualong group originated from melting source thatasthenosphere mantle metasomatic continental lithosphere mantle,it maybe belong to localcracking products of superlarge continent during convergence of Rodinia,or the superlargecontinent Rodinia’s convergence had not finished in the whole and the local begined cracking.(3) We obtained firstly crystallization ages of mafic-ultramafic intrusions in Hualong rocksbelt from Yaqu,Yulonggou,Xiashentang and Ashengong by ID-TIMS and LA-ICP-MS,theages were respectively440.74±0.33Ma (MSWD=1.1),442.4±1.6Ma (MSWD=0.59),449.8±2.4Ma (MSWD=0.31) and436.1±1.2Ma（MSWD=0.13）,commonly constrainting theformation time limitation of mafic complex.(4) The m/f ratio of mafic complex in Hualong rocks belt,Qinghai province,was focused on1.1,1.53and3.0,mostly1～3.5,belonging to ferruginous series mafic complex.They showedrelative enriched LREE,negative Eu and Nb-Ta anomaly,and almost negative Zr-Hf anomalyfrom spider diagram of chondrited REE and primitive mantle trace elements.And they had thecharacteristics of enriched large ionic lithophile elements(Sr,Rb,Ba) and relative depletedhigh field strength elements(Nb,Ta,Hf,Zr).All of these revealed geochemical characteristics ofarc magmatism.(5) The Nd and Sr isotopic composition changed greatly,suggesting open magma systemduring evolution.The initial ratio of187Os/188Os from sulfide of sparse disseminated ore was0.29~2.19,higher than that of mantle source deposits,but lower than that of crustaldeposits,maybe have more crustal contamination.The ratio of γOswas+80~+1183,together with ubiquitous depleted Nb and Ta,explaining furtherly the close relationship with crustalcontamination.(6) Geochemical characteristics showed that parent magma originated from mantle sourcemetasomatic enriched once,and its εNd(t) ratio was-7.74～+8.36,these suggesting the magmasource should had been located in asthenospheric mantle region that contaminated by mantlewedge material with subduction plate metasomatism.This metasomatic enriched event wasrelated with large scale plate subduction at early Paleozoic in Qilian mountains and northmargin of Qaidam basin.(7) On the covariant diagram of Th/Yb vs Nb/Yb,the mafic complex within Hualong rocksbelt was located in the above at the line of MORB-OIB,and falled into the region of modernisland arc basalt.Lajishan mountains,northern adjacent area of Hualong rocks belt,had manymafic-ultramafic intrusions with the age of440-450Ma and coeval granite,it furtherlyrevealed the close time on the Lajishan small ocean basin was about450Ma.Meanwhile,theHualong massif was at the orogenic stage after closing completely of Lajishan small oceanbasin and north margin of Qaidam basin ocean,the magma intruded into the position thatbelonged to postcollisional extension and formed the mafic-ultramafic intrusions of Hualongrocks belt.The riched silicon strata of Hualong rocks group created the condition ofassimilation and contamination for sulfide immiscibility,finally caused the addition on theriched silicon material,occurred the sulfide immiscibility,and last formed the different grademagmatic Ni-Cu sulfide deposits in Hualong rocks belt,Qinghai province.