Study On Rock-forming And Ore-forming Of The Jinfosi Pluton, Western Part Of The North Qilian Mountains

The Jinfosi pluton,the biggest granite in the western part of the North Qilian Mountains,lies in about 40km southeast of Jiuquan city.Because of execrable transportation and natural conditions, the research level is still very low,and just a few works have been done there on the ore potentiality of the Jinfosi pluton.In order to research the age of the Jinfosi pluton and its tectonic settings as well as the age of corresponding ore deposit and metallogeny,we applied multidisciplinary methods of petrology, tectonics,ore deposit,remote sensing,radio geochemistry in this paper.Furthermore,the author discussed and studied the sources of ore-forming material and age of the deposits and the age of the Jinfosi pluton by isotopes of Re-Os,Rb-Sr,Sm-Nd,Pb,and the major elements,trace earth elements,and stable isotopes of sulfur,carbon,hydrogen and oxygen.We did some elaborate regional geological survey in this area,made sure that the distributions of granite and strata,structure and ore-controlling regularity and metallogenic geology condition. Additionally,we approximately ascertained the distribution range,occurrence,size,shape and component of the orebody.The above results will establish a reliable groundwork for the future researches.(1)The characteristics of the Jinfosi plutonTwo intrusive stages have been recognized.The earlier-staged granite is mainly composed of magnetite,apatite and zircon,and other secondary minerals such as sericite,carbonate and chlorite. The rocks contain abundant biotite plagiogranites and the inclusion of biotite quartz diorite without hornblende.The whole-rock isochron age of Rb-Sr and Sm-Nd are 421.9-426.1Ma,which indicates the intrusive age is the Wenlock to the Ludlow,with an inclusion temperature of 116-250℃,the corresponding depth of 0-0.9Km and pressure of 0-25 MPa.The later-staged granite is mainly composed of magnetite,and secondarily chlorite,sericite.The isochron age of Rb-Sr and Sm-Nd for the whole-rocks are 389.6-391.3Ma,which indicates that the intrusive age is the early-middle Devonian,with a inclusion temperature of 175-275℃,the corresponding depth of 0.28-1.5Km and pressure of 7.5-39 MPa. The original magma of the Jinfosi pluton which shows characteristics of S-type granite is derived from the crust.(2)Metallogeny of the Jinfosi plutonThe minerals sequences:1hematite-2pyrite-3chalcopyrite-4bornite-5galenitel-6wolframite-7molybdenite -8sphalerite-9galenite2-10scheelite-11anglesite-cerussite-limonite-goethite -chalcocite.The mineralization stages and mineral paragenesis orders:Hot liquid mineralization epoch:Ⅰmagnetite-hematite middle-high temperature phase;Ⅱquartz-pyrite low-middle temperature hydrothermal fluids phase;Ⅲchalcopyrite-molybdenite middle-high temperature hydrothermal fluids phase;Ⅳgalenite-sphalerite low-middle temperature hydrothermal fluids phase;Ⅴcalcite-galenite low temperature hydrothermal fluids phase.Surface oxidation epoch:parts of metallic compounds oxidizing(has been oxidized)into limonite,goethite,anglesite-cerussite and chalcocite.The ore-forming materials originate from the magma of crust and the ore-forming fluids are influenced by the atmospheric water,and their mineralization fluids have similar compositions and without visible differences in mineralization temperature.Based on Re-Os isotope dating,the age of Xiliugou W-Mo deposit is 422.2Ma,which indicates that the intrusive age is the Middle-Late Silurian,formed in the early period of the North Qilian mountains and the late period of the closure of the North Qilian residual ocean basin.The single source of the Suotonggou Pb deposit may be correlated with the Jinfosi pluton uplift and its wall rocks of the Ordovician volcano.(3)Ore-forming model;W-Mo deposit and the No.3 Pb-Zn deposit of SuoTonggou are similar to the early stage granite of Jinfosi in the rock geochemistry and inclusion characteristics.Moreover,the No.1 vein deposit is similar to the Ordovician Yingou Group(OY~a),and the No.3 Pb-Zn one possesses partial features of the former in geochemical studies and ore-forming fluids.The sources of the Jinfosi pluton and the correlative deposits are originated from the crust,and correlated with the mountain uplift.The No.1 Pb-Zn vein deposit was formed before the early stage granite of Jinfosi forming,and associated with the fluids of with the early stage of Jinfosi granite and a rocky team(OY~a),Yingou Group,Ordovician.Xiliugou W-Mo deposit was formed after the early stage granite of Jinfosi,and associated with the early stage granite and the base of Beidahe group.The No.1 Pb-Zn vein deposit was formed in the late Ore-forming of the Jinfosi pluton,which indicates that Ore-forming of the Jinfosi pluton ended.