Neoproterozoic-early Paleozoic Tectothermal Events And Mineralization In Kuluketage Block,Xinjiang Province

Kuluketage block is located in the northeast of Tarim basin, with South-Tianshan orogen to the northwest, Middle-Tianshan block and Beishan orogen to the east, and Manjiaer depression basin to the south. Its north and south margins are controlled by the Xinger and Kongquehe faults. This block preserves the most complete Precambrian geologic units, which are composed of large area of extensively metamorphosed and deformed Pre-sinian basement rocks and weakly metamorphosed and deformed cover, leading to the typical bilayer structure. The Pre-sinian units comprise of a suite of medium to highly metamorphosed phyllite, schist, gneiss, hornblende schist, marble, metabasalt, metaandesite, metarhyolite and intrusive rocks. The Sinian units comprise of coarse clastic rocks and tillite with hundreds meters of medium-basic volcanic rocks and unconformably cover the Pre-sinian rocks. The Cambrian rocks are composed of a suit of phosphorous silicolites and carbonatites. The Ordovician units comprise of bathyalfacies cinerous medium to thin turbidites. The Silurian units comprise of shales and graptolite shales. The Devonian units comprise red clastic rocks and the Carboniferous units comprise of carbonatites and clastic rocks. All the Paleozoic groups are conformably or pseudo-conformably contacted. The Xingdi fault is the sole regional fault which divides the whole block into north and south parts. This fault is east-west trending and experienced multiple tectonic evolution since Mesoproterozoic.Kuluketage block is one of the most weakly studied area. It is an important part of the Middle-Asian orogen. Since2000, some important deposits are discovered, such as Xingdi No.Ⅱ Cu-Ni deposit, Dapingliang Cu-Mo deposit, Qiongtage Cu-Au deposit and Daxigou Fe-P deposit. This indicates the area is good prospecting of mineralization. However, compared to the other places in Middle-Asian orogen such as Kazakhstan, Kyrghyzstan and Russian, it is still needed to explore the ore-forming potention. Thus, further studies of the ore-forming theories, ore-forming mechanisms and metallogenesis are the crucial points for the breakthrough of minerals exploration. In this paper, the newly discovered Xingdi No.Ⅱ Cu-Ni deposit, Dapingliang Cu-Mo deposit, Qiongtage Cu-Au deposit and Daxigou Fe-P deposit are studied. Based on field work, detailed zircon U-Pb geochronology, geochemistry, isotope geochemistry and ore genesis studies were performed. With the thread of metallogenic system which are the combination of systematic and historical thinkings, the author synthesized the previous basic geology study and carried out the studies of the geodynamic background, ore-forming condition, ore-forming mechanism of the newly discovered four deposits to establish the metallogenic system. Then combining the geological condition, rock ore-bearing potential and regional geochemistry, the author made an assessment on the ore-forming potential. The main results and conclusions are summarized as follows:1.The magmatic events are systematically summarized.On the basis of the previous research achievements and the achievements of this paper, Kuluketage block experienced the following seven thermotectonic stages. They are:(1) Archean crust nucleus growing stage (3.6-3.3Ga, indicated by3.3Ga Sm-Nd age of Xinger plagio-amphibolite and the Archean inherited zircons in the granites);(2) crust growth and transformation stage (2.6-2.3Ga, indicated by widely distributed TTG granite gneiss and plagio-amphibolite);(3) crust transforming stage (2.1-1.8Ga, indicated by Paleo-to Meso-proterozoic continuous metamorphic events, medium to felsic magmatic activities and intensive strata folding and the magmatic rocks usually show very old Nd model age);(4) late Mesoproterozoic to early Neoproterozoic orogeny stage (1.1-0.86Ga, indicated by the folded strata and magmatic activities);(5) Neoproterozoic orogenic extensional stage (830-800Ma, with widely developed potassium-rich granite and adakite-like rocks, and alkaline mafic-ultramafic rocks);(6) Neoproterozoic intracontinental breakup stage (770-600Ma, with the tholeiitic mafic-ultramafic rocks and dykes, bimodal volcanic rocks and deep water sediments) and (7) Phanerozoic land building stage indicated by sedimentation interruption, but in the southeast margin, there are orogenic movements, such as Qiongtage cambrian arc magmatic activities and Ordovician collision magmatic activities.2.Proposed new geochronology data and discussed its ore-forming background(1) The three plutons in Dapingliang are intrusived at826Ma,816Ma and806Ma, respectively. Field geology shows that the first formed two plutons show weak oriention, while the last pluton shows typical granite structure, which indicates the tectonic transition from compression to extension. Geochemistry study shows that the plagiogranite, monzonitic granite and K-feldspar granite are high potassium calc-alkaline to alkaline, metaluminous, with CaO, FeO*/MgO and HFSE depleted and K2O enriched. Trace elements show that the Nb, Ta, P, Zr and Ti have negative anomalies, while Ba, K, Sm and Sr have positive anomalies. These geochemistry features show that they are formed in the post orogenic setting. Sr-Nd-Pb isotopes indicate that the magmas are derived from the lower crust, which inherit the arc magma geochemistry. Daxigou Fe-P deposit is located in the Qieganbulake-Tuanjiecun alkalescent mafic-ultramafic rock belt. The latest published data show that this belt formed during the820-800Ma, within the same period of potassium-rich granite activities. The geochemistry of this mafic-ultramafic belt also shows alkalescent features, which indicates the mafic-ultramafic belt and the potassium-rich granite formed at the same tectonic setting.(2) In this paper,761±31Ma Sm-Nd isochrone age was obtained for Xingdi No.Ⅰ pluton. On the basis of published data of Xingdi No.II, No.IV, the Xingdi mafic-ultramafic belt formed at 760-735Ma. These plutons show multiple intrusion. Field geology shows that Xingdi No.IV pluton has the lowest basicity and the dominant phases are gabbro and diorite, while No.Ⅰ and NoⅡ have the ultramafic phase and they show very similar geochemistry. Both the two pluton are tholeiitic, enriched in light rare earth elements (LREE) and large iron lithosphile elements (LILE), and depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE). However, the No.I experienced higher evolution, its incompatible elements and initial Sr isotope ratio are higher, the compatible elements andεNd(t) are lower, and it has more wall rock contamination. Geochemistry shows the plotons are formed in the rifting setting and their source is enriched.(3) The tonalite in Qiongtage is formed at Cambrian with the zircon LA-ICP-MS U-Pb of494±4Ma. Major elements suggest the pluton is medium to felsic, sodium and metaluminous. The REE content is low, with enriched LREE and weak δEu anomaly. The LILE elements are enriched and the HFSE are depleted. All the features show the affinity to arc magma which is related to subduction. This tectonic setting agrees with regional tectonic evolution.3.Synthetic ore genesis studies were performed(1) Dapingliang Cu-Mo deposit is a typical skarn deposit. Ore-forming substances and fluid are mainly from the pluton, while the wall rock mainly provides the geochemical barriers for metals deposition. The ore-forming process is that during830-800Ma, tectonic transition from compression to extension happened in the Kuluketage block and in such tectonic setting the potassium-rich adakite and A2-type granite are formed by the lower crust melting. Among them, the potassium-rich adakite carried numerous metals and fluid. The pluton is in oxide state with huge of fluid indicated by universal distributed microcline in the pluton. The magmatic fluid interacts with the marble and formed the skarn rock accompanied by the deposition of magnetite. Then in the later fluid stage, the sulfides are deposited and forms the Dapingliang skarn deposit.(2) Daxigou pluton are composed of hornblende-plagioclasite, pyroxenite, hornblende-pyroxenite, apatite-hornblende-plagioclasite and iherzolite. The Fe-P ore bodies are hosted in the hornblende-plagioclasite. According to the petrology, the pluton is divided into dark series (pyroxenite and peridotite) and white series (plagioclasite). The ore-forming mechanism is described as that during the post-collision tectonic setting, the enriched mantle-originating basalt is rich in P, Ti, alkaline and volatile matters. During its ascending, the temperature and pressure decree and come to the eutectic point, which leads to the magma liquation. The P and Ti are usually accompany the silica and volatile matter rich end member, such as plagioclasite.(3) Xingdi No.II pluton undergoes multiple evolution and extensive differentiation in the deep. It is formed in the rifting setting. The ore bodies are hosted in the lower part of peridotite facie or the contact between the pyroxenite and peridote. According to the microscope observation and geochemistry study, the magma experienced sulfide liquation in the early stage and the liquation is caused by fractional crystallization and wall rock assimulation. While in the later stage, fluid-rich is the crucial point for ore deposition. In the later stage of magma crystallization, fluid usually increases the oxygen fugacity and causes the formation of magnetite and water-containing minerals such as hornblende and biotite. (4) Qiongtage Cu-Au deposit formed at494Ma in the arc-subduction setting. During the intrusion of the magma, it assimilates the wall rock and extracting the metals from the wall rock. During the late stage, tectonic pressure release causes the fluid alteration and metals deposition in the top of the pluton and on the contact zone with wall rock. Magnetite, chalcopyrite and pyrite are developed in the quartz or/and calcite veins. After the ore formation, tectonic compression foliation makes the sulfide rearranged to coincide with the foliation and the ore gets further enriched. Thus, this deposit may be a magmatic hydrothermal transformed deposit.4.Recovered the Neoproterozoic-Paleozoic metallogenic system and then combining the ore-forming condition, rock ore-bearing potential and regional geochemistry, the author made an assessment on the ore-forming potential.During Neoproterozoic-Paleozoic, the Kuluketage block experienced post-orogenic stage (830-800Ma), rifting stage (774-735Ma) and subduction stage (ca.494Ma). According to the theory of metallogenic system, particular tectonic setting will have specific metallogenic system. The subduction stage will form the Cu-Pb-Zn-Mo-Fe-Au-W-Sn metallogenic system, the post-orogenic stage will form the Cu-Mo-Fe-Au-Cr-V-Ti-Nb-Ta-REE metallogenic system and the rifting stage will form Cu-Ni-Cr-Co-V-Ti-Pb-Zn metallogenic system. In the Kuluketage block, these tree stages are corresponding to the three ore-forming events. Those are potassium granite-related skarn Cu-Mo deposit and alkalescent mafic-ultramafic rock-related Fe-P deposit formed in the post-orogenic setting, tholeiitic mafic-ultramafic rock-related Cu-Ni sulfide deposit formed in the rifting setting and medium to felsic arc magmatic-related hydrothermal Cu-Au deposit formed in the subducting setting.Synthesizing the ore-forming condition, rock ore-bearing potential and regional geochemistry, the author suggests that KCS, HLGE, GSB and NSSK plutons in the west, and Akebulake pluton in the east have good Cu-Mo mineralization potential. Other more, the Baowenbulake and Yonghongshan Cu mineralization occurrences are corresponding well to the1:2million regional geochemistry anomalies. They have important Cu exploration potential. The uncovered alkalescent mafic-ultramafic rocks distributed along the Xingdi fault have the Fe-P mineralization potential. After study, there is impossible to find industrial Cu-Ni ore body in the shallow part of Xingdi No.Ⅰ pluton, but in the deep, there is very well Cu-Ni exploration potential. The Qiongtage area is a relative separated place, where mineralization is related to Cambrian tonalite. Combining with the regional geology and1:0.5million regional geochemistry anomalies, the area of Hy-3and Hy-5geochemistry anomalies have good mineral prospecting potential.