Geological-Geochemical Characteristics And Genetic Mechanism Of Nailingou Au Deposit In Aohan Banner, Inner Mongolia

Located in the north geosynclinal side of the large and deep Chifeng-Kaiyaun fracture,Nailingou Au deposit is a small Au deposit currently explored and exploited. Since medium-and large-size Au deposits in this area are all located in the platform side, the geosynclinalside with few or small-scale Au deposits hasn’t received enough attention in terms of oreexploration and theoretical research. Moreover, as the only Jurassic deposit with industrialsignificance in this area, the Nailingou Au deposit plays an important role in studying geneticmechanism, and in exploring new types of mineralization and new areas of resources in thegeosynclinal side north the Chifeng-Kaiyuan fracture.In terms of its genetic mechanism, there hasn’t any special field investigation. Based onthe fact that this deposit was hosted in Jurassic Zhangjiakou-Group volcano strata, someresearchers defined it as "volcano hydrothermal"; others classified it as "magmatichydrothermal", according to its metallogenic mechanism.Through geological field investigation and indoor observations, over140quartz-veinbelts were identified, which can be divided into three groups according to their distributionaldirections: NE, NW and NNE; some individual veins were close to EW or SN; thecharacteristics of the ore-control fracture revealed tension fracture; the mineral compositionand structural fabrics both displayed the properties of a typical low-temperature deposit.The fluid inclusions in the Nailingou Au deposit were dominated by gas and fluid phases,with gas/fluid ratios mainly between10%and30%. The homogenization temperatures were157.2～279.6℃, all within the range of medium and low-temperature hydrothermal fluids. Interms of salinity, it generally ranged from5.02%～13.88wt%NaCl （average10.37wt%NaCl）;in G1compacted grainy-quartz inclusions, it had a wider range （6.92～12.61wt%NaCl,average11.00wt%NaCl）, and was in relation to the commixture of the high-temperaturehigh-salinity magmatic hydrothermal fluids and the low-temperature low-salinity atmosphericprecipitation; in G2grainy-quartz inclusions, it ranged11.36～13.88wt%NaCl, with a largeraverage （12.54wt%NaCl） than G1, probably due to the boiling metallogenic fluids; in G3pectinate-quartz inclusions, it ranged5.02～10.27wt%NaCl, with a low average （7.35wt%NaCl）, possibly because of their isothermal blending with the low-salinityatmospheric precipitation. The capture pressures of the inclusions in this deposit ranged14.8～30.4Mpa, and the metallogenic depth was1.48～3.04km, indicating hypabyssal type.The hydrogen and oxygen isotopic compositions varied greatly among differentgenerations of quartz fluid inclusions; the hydrogen and oxygen isotopes fell close to themagmatic water side in early generations, to the precipitation-line side in young generations;and to linear arrangement from top to down in the same generation, indicating that themixture of magmatic hydrothermal fluids and atmospheric precipitation was dominated by theformer in early generations, but by the latter in young generations. The analyses of carbon andoxygen isotopes in calcite indicate that, δ¹³C（‰PDB） ranged between-4.158‰～-4.868‰（average-4.54‰）, close to the value in mantle （-5.5‰）; δ¹⁸O（‰SMOW） varied between7.025‰～8.595‰（average7.86‰）, consistent with the range in granite （+7～+13‰）, butdifferent from andesite, and thus providing favorable evidence that the metallogenic fluidswere the derivates of granite.The rocks related to the formation of Nailingou Au deposit were Early CretaceousKelidai granodiorite, in petrochemistry and petrogeochemistry, were indicated high-K andhigh-Ca alkaline granite, belonging to type I; in the R1-R2tectonic discrimination diagram,the projection points mostly fell into late orogenic phase. These results indicate that theKelidai granodiorite rocks were formed under the setting of Early Cretaceous tectonicactivities turning from NW-SE extrusive action to NW-SE extensive action, and due to theupward intrusion of crust-mantle mixed magma caused by the thinning of crust, thedelamination of lithosphere, the upwelling of mantle and the remelting of crust.In general, our study indicated that the Nailingou Au deposit was hypabyssal Yanshanianmedium-low-temperature magmatic hydrothermal Au deposit.