Study On The Ore Fromation Fluid Characteristics And Genesis Of Shangxu Gold Deposit,Shuanghu,Tibet

Used the research thoughtway of comparison ore deposit geology, this paper concludes ore fromation fluid characteristics and discusses the ore genesis of Shangxu gold deposit on the base of deepen acknowledge of regional geological background and the deposit geological characteristics by a variety of technical meansShangxu gold deposit lies in the middle part of Bangongco-Nujiang suture belt, which has a good prospecting potential. The formation exposed within the feild is mainly the middle-low Jurassic Mugagangri group which is mostly composed of low metamorphic marine flysch. And three different stages of vein bodies are developed. Ore bodies are controlled by nearly EW-NWW faults, which occur as veins or lens. The major ore types are single quartz veins and structural alteration rock. Mineral components, texture and structure of ore are relativity simple. Adjacent wall rocks didn't undergo clearly hydrothermal alteration, and the main alteration types include silicification, sericitization, chloritization, carbonatization etc. Hydrothermal ore-forming period can be divided into three stages:I quartz-fine sulfides-gold stage, ? quartz-coarse sulfides-gold stage, ?quartz-carbonate minerals stage, and the former two stages are mainly ralated to gold mineralization.The research of fluid inclusions shows that there are four different types of primary fluid inclusions in auriferous quartz veins from Shangxu gold deposit. They are CO2 three-phase inclusion(?-type), rich liquid two-phase inclusion (?-type:it can be divided into II a and II b), rich gas two phase inclusion (III-type:it can be divided into?a and ?b) and single phase inclusion (?-type:it can be divided intoIVa and ?b), that can be reduced to C-type (CO2-H2O-NaCl system), W-type (H2O-NaCl system) and PC-type (pure CO2 system). The homogenization temperature of differernt fluid inclusions is 174-257?, salinity values 0.82-6.01% NaCleqv, and density values 0.82-0.91g/cm3. The content of gas phase primarily contains CO2 and H2O, with a small amount of CH4 and N2, and the liquid part mainly contains Cl". In a word, the ore-forming fluids of the Shangxu gold deposit is characterised by high content of CO2, low salinity, low to moderate homogenization temperature and low density, being similar to that of typical orogenic gold deposits. Basing on these results, the author speculates that the mineralization temperature is about 190?230?, and depth 1.4 km?2.1km, belongs to middle-low-temperature hydrothermal deposits.The Co/Ni ratio of pyrite suggests that it may result from wallrock metamorphic fluid. And REE characteristics of auriferous quartz veins are similar to those of pyrite, but differ from those of the regional magmatic rocks. All this indicates that the ore forming fluid of Shangxu gold deposit were derived from the wall rocks. On the other hand, the REE distribution patterns show obvious Eu anomaly while not obvious Ce anomaly, suggesting that the ore forming fluid is weakly reduced.According to the hydrogen and oxygen isotope test, the 8D values of fluid inclusions in auriferous quartz veins range from-104%o to-136‰, and ?18O from+5.17‰ to+7.45‰, indicating a mixture of metamorphic water and geothermal water. The The 834S values of ore forming fluid which are similar to that of strata range from-4.5%o to-2.0‰, and wall rock strata from-2.9‰ to-0.6‰, indicating the sulfur in ore-forming materials of the Shangxu gold deposit was derived from the wall rocks.By synthesis analysis, this paper pointed out that Shangxu gold deposit is mainly controlled by stratigraphic and structure factors, then belongs to orogenic gold deposit according to the genetic classification of native gold ore. A genetic model of Shangxu gold deposit is established for the first time on the base of discussing key issues in the mineralization process, that initial ore-forming fluid and materials mainly come from Mugagangr group, then migrate through structural fractures drived by tectonic stress, finally precipitate through boiling and mixing, forming ore bodies in faults. Combing mineral exploration, some major exploration indicators of the gold ore are summed up, including lithological and structural criteria, alteration criteria, sulfide mineral criteria, geochemical exploration anomaly and remains of previous mining tunnels etc. Then there is a better exploration potential to the outsider and deeper of known ore bodies in Shangxu gold field, and it is possible to find more primary gold deposits which similar to that of Shangxu in Bangongco-Nujiang suture belt.