Geology, Geochemistry Of Xiaoliugou W-Mo Deposit In The Western Of The North Qilian

The Xiaoliugou W-Mo deposit is located in metallogenic belt of western ofNorth Qilian, which is a typical deposit related to granitoids. On the basis of thedetailed study of field geological, using by electronic probe, inductively coupledplasma mass spectrometry, microthermometry and isotope mass spectrometry, doingsome studies included mineralogy, U-Pb geochronology, trace element geochemistry,fluid inclusion and stable isotope etc., and compared geological characteristics of theW deposit with those of other typical deposits. This paper has proposed several pointsas follows:1. The mineralization can be divided into three types, porphyry types, skarntypes and quartz vein types. The formation of the deposit experenced four stages:skarn stages, retrograde stages, quartz-sulphide and quartz-carbonate.2. Xiaoliugou granitoids are comprised of monzogranite and granodiorite.Electron microprobe analysis indicates that biotite from monzogranite granodiorite isannite, Plagioclase from monzogranite is albite and that biotite from granodiorite isfeldspar. The high oxygen fugacities provide suitable conditions for the formation ofthe W-Mo deposit.3. Zircon U-Pb dating and geochemistry of the Xiaoliugou granite are studied toconstrain its geochronology and petrogenesis. Zircon LA-ICP-MS dating yieldsconcordant ages of454.0±2.0and417.7±1.7Ma respectively indicating that the twoplutons were formed at Caledonian，later affected by Hercynian and Yanshanianmagmatic-thermal event. Geochemical data shows that，Xiaoliugou granite is silicariched in composition with high content of alkali. It’s a peraluminous granite andbelongs to the high-K calc-alkaline series，enriched in Rb，Th，U，K，Pb，depletedin Ba，Sr，Ti，P with strong negative abnormal of Eu，especially，the monzogranitehas intensely negative Eu abnormality and should be ascribed to highly fractionatedI-type granite. The Hf isotope shows that the εHf(t) value of the monzogranite rangefrom-4.45to4.04，with tDM2ages between1176Ma and1714Ma，while the εHf(t) value of the granodiorite range from-4.18to4.43，with tDM2ages between1124Maand1670Ma，indicating that the Xiaoliugou granitoids were most likely generated bymixing of a depleted mantle-derived and an induced crustal-melted felsic magma inthe deep crust. we suggested that the source region of granite were likely from thePaleoproterozoic-Mesoproterozoic ancient crustal rocks. Comprehensive studiesindicate that Xiaoliugou W-Mo deposit has a closely temporal relationship with themonzogranite. Combine with the tectonic evolution of the Qilian, We speculate themonzogranite was generated in a continental arc tectonic setting triggered by slabsubduction, the granodiorite probably have been generated in syn-collison setting.4. Fliud inclusions study indicates that there are four dominant types of fluidinclusions, namely aqueous two-phase inclusions, CO2-bearing inclusions, daughtermineral-bearing inclusions and gas-rich inclusions. The ore-forming fluid belong toMedium-high temperature, medium to low salinities of NaCl-H2O-CO2-(CH4) systemin theⅡand Ⅲ mineralization stage, and medium to low temperature, low salinities ofNaCl-H2O system in the Ⅳ mineralization stage. From the early to the latemineralization stages, the temperature and salinities reduced gradually.5. Hydrogen-oxygen isotopic compositions indicate that the hydrothermal fluidwas mainly of magmatic hydrothermal fluid at early mineralization stage, mixed withmeteoric water in various degrees at the late mineralization stage. The characteristicof fluid inclusions and hydrogen-oxygen isotopic indicate that the fluidimmiscibility(boiling) and fluid mixing are actors that controlling the precipitation ofW、 Mo and the formation of the ore deposit.6. C-O isotope reveals that the carbon is from magmatic hydrothermal andstratum，the sulfide isotope indicates the sulfide is from magmatic hydrothermal.Through this work combined with previous research results，we view that the W isfrom monzogranite and stratum, the Mo mainly come from the monzogranite.7. Compared with the Ta’ergou tungsten deposit, we view the differenatiation ofthe rock is the main reason for the different mineralication. Through comparing Qilianand Huanan tungsten deposit, even China and word tungsen-Skarn deposit, wesuggest deepen understanding of the formation conditions of tungsten ore in different tectonic environments, such as granitoids controlling the tungsten mineralization insubduction and collision environment of Caledonian and Hercynian.