Geological And Geochemical Characteristics Of The Laojinchang Gold Deposit And Its Genesis In Beishan Orogen,Gansu Province

The Laojinchang gold deposit is one of the most representative magmatic hydrothermal gold deposit of the south Beishan metallogenic belt,with medium-size and medium-low temperature.Based on crosscutting relationships of different auriferous veins,mineral paragenesis,ore texture and structure,its mineralization stages can be divided into the quartz-pyrite stage???,quartz-arsenian pyrite-arsenopyrite stage???,quartz-pyrite-polysulfide stage???and quartz-calcite stage???.In this paper,electron microprobe analyzer?EMPA?is used to insight the concentration and compositions of elements in the gold-bearing minerals of different ore-forming stages.Pyrite and asenopyrite of different ore-forming stages have distinct features and element contents.Stage?,Pyrite is primarily coarsed-grained?0.50-1.50mm?euhedral cube,with chalacteristics of low As and Au.And with little fine-grained anhedral asenopyrite.Stage?and?are the main mineralization with fine-grained pyrite,respectively.Stage?is mainly characterized as arsenian pyrite arounded by a large number of arsenopyrites.Pyrite is dominately cube and pentagonal dodecahedron,rich in As and Au,with 0.30-1.00mm particle size.It is the most intense period of mineralization,and main period of the formation of asenopyrite.Asenopyrite in this stage is primarily appeared as rhombic-columnar,columnar and radiate-columnar aggregates,rich in S and depleted in As.Pyrite in stage?is commonly occured as veins of pyrite-chalcopyrite-sphalerite mineral paragenesis,with long strip and high S,Cu,Zn,Au and low Fe,As.Mineralization in stage?is relatively weak with little fine-grained anhedral pyrite and asenopyrite.The oxygen and hydrogen isotope shows that ore-forming fluid was mainly dirived from magmatic water and the mixture of magmatic water and meteoric water in the late stage.The studies on S and Pb isotope compositions indicate that the multi-sources of ore-forming materials in metallogenesis.The?³⁴S values of pyrite in stage?range from-3.8‰to-2.9‰with an average of-3.3‰.Pyrite and asenopyrite in stage?range from-4.7‰to 2.6‰with an average of-3.3‰.Whereas the?³⁴S values of pyrite and sphalerite in stage?have a?³⁴S range varying from-1.9‰to1.0‰?average 0.1‰?.Which suggesting a mantle-derived magmatic provenance for sulfur,and contaminated by sedimentary sulfur at the late stage,indicated by in-situ sulfur isotope compositions.Based on previous study,it is proposed that the ore-forming fluid evolved from S-rich and As-poor fluids to As-rich and S-poor fluids during the mineralization.Stage?,the ore-forming system was in neutral and stable environment with abundant sulfur.In the main stage,stage?is in As-rich,S-poor and high oxygen fugacity,As-rich fluids were injected into hydrothermal system due to the leaching-infiltration of meteoric water,which leads to the formation of Au-As complexes possibly and precipitating and accumulating in appropriate place.Whereas ore-forming system is characteristized by variety of metallogenic elements,S-rich,As-poor and weak reduction in stage?,Au may occured as[AuHS]or[AuS]^-complexes entering the pyrite lattic.