Study On Geological Characteristics And Genesis Of Guoluolongwa Gold Deposit In Qinghai Province

The Guoluolongwa gold deposit is located geotectonically in the Nachitai Group inthe south belt within Eastern Kunlun orogenic belt. The ore bodies are controlled by thesecondary faults of NW-trending fault. The mineralization type consist of pyrite-dominantsulfide quartz vein type with minor sulfide alteration type. They strike45°⁷5°,nearly NE,and dip consistently with the regional stratigraphic.The main ore minerals are pyrite andchalcopyrity, with minor sphalerite, galena, and limonite. Gold occurs as electrum andnative gold, mainly in pyrite and quartz. The main gangue mineral is quartz. The hostrocks of guoluolongwa gold deposit are Nachitai group chlorite quartz phyllite, phyllonite,siliceous rock. Wall-rock alteration includes silicification, sericitization, choritization andcarbonatization. The gold ore-forming process experienced hydrothermal-mineralizationand supergene leaching-enrichment.According to petrological and geochemical researches, it was thought that maficdikes adjacent to AuⅠveins are Gabbro and diabase. They belong to sub-alkaline series,relatively depleted Ti and P, alkali-rich. REE patterns show that LREE is relatively richerthan HREE. Diagram showthey are of richlarge ion lithophile elements, depleted in highfield strength elements. Trace elements of the mafic dikes show that diabase is the productof enriched mantle partial melting related to Fluid metasomatism, without crustalcontamination and fractional crystallization. The gabbro is the product of partial meltingof mantle experienced crustal contamination and fractional crystallization. Mafic dikes inthe Nb×2-Zr/4-Y、La/Yb-Th/Ta and Ta/Hf-Th/Hf tectonic environmentdiscrimination diagram show diabase tectonic environment formed in extensional boardenvironment, and gabbro formed in the intra-oceanic arc environment. Geodynamicevolution in this area inferred gabbro and diabase formed in the late Indosinian. According to lithological researches and microthermometry of fluid inclusion inquartz-veins. Inclusions in quartz-polymetallic sulfide stage include typeⅠ （aqueoustwo-phase）, type Ⅱ （CO₂-bearing three-phase） and type Ⅲ （pure-CO₂）. Themicrothermometric study of inclusions indicate that typeⅠ inclusions, in which the vaporvolumes occupy10%⁴0%, their salinity is8%¹8%, homogenization temperature is130²80℃， density is0.87¹.03g/cm³. type Ⅱ inclusions, with a wide range of30%⁹0%for the CO₂volumes, in which their salinity is1.83%⁷.48%, homogenizationtemperature is222.4³57.1℃, CO₂of type Ⅱand typeⅢ homogenization temperature is14.5²9.9℃，density is0.62⁰.81g/cm³. Inclusions of quartz-barren sulfide stage is Ⅰtype （aqueous two-phase）. Their salinity is1.9%¹3.99%, homogenization temperature is179.9²04.6℃, density is0.89⁰.97g/cm³. It was considered that the ore-forming fluidsbelong to epithermal to mesothermal, low-salinity mixture fluid.Based on H-O isotopic studies, it was shown thatδ¹⁸O_H2Oof Quartz-polymetallicsulfide stage changes in6.4⁸.8‰, δDV-SMOWis-61^-89‰; δ¹⁸O_H2Oof quartz-barrensulfide stage is3.3⁴.5‰δDV-SMOWis-84^-101‰. It was concluded that fuild of quartz-polymetallic sulfide stage is mainly from the mixing of magmatic or metamorphic fluidwith minormeteoric water. Fluid of quartz–barren sulfide stage derived from obviousmeteoric water.According to the predecessor’s material, it was shown that sulfur isotopic of theGuoluolongwa gold deposit are characterized by the following: Galena average³⁴S_CDTis-3.463‰, the pyrite average³⁴S_CDTis2.208‰.³⁴S_CDTof pyrite> those of Galenaindicates that sulfur isotope fractionation basically reached a balance. The main sources ofsulfur are magmatic origin, maybe with a little of strata-coming sulfur. Pb isotopiccomposition of the Guoluolongwa gold deposit are relatively uniform. On the tectonicmap of Doe and Zartman （1979）, the Pb isotopic data are concentrated near orogenic beltsline, consistent with the actual tectonic setting, which reflect the the orogenic environment.Combining with S isotopic data, it was considered that the Pb sources maybe similar tothose of Sulfur, i.e. mafic dykes and strata among orogenic environment.Comparing with typical orogenic gold deposits, it was show that the geotectonicsettings, geology and geochemistry of mineral deposits, and fluid characteristics ofGuoluolongwa gold deposit are similar to those of typical orogenic gold deposit of theworld. Then the author think it belongs to orogenic gold deposit. Finally, the author buildan metallogenic model for it. Mafic dykes are related closely to gold mineralization. They can bring ore-forming materials （S, Pb, Au） and partial fluids, and drive shallow meteoricwater to circulate and mineralize.