Study On Ore-forming Mechanism And Property Of Metallogenic Fluids Of The Huaheitan Molybdenum Deposit At Beishan,gansu Province

The Beishan orogenic belt characterized by multi-stages,multi-times and multi-cycles,experienced multistage plate collision and large scale extension,and developed a series of nearly EW-trending ductile deep fault and ductile shear zone,which control the northern polymetallic metallogenic ore belt at Beishan area.The Huaheitan hydrothermal molybdenum deposit is located in the Liuyuan region of Gansu,the southern Beishan area and adjoined the late Triassic Huaniushan granite.The Huaheitan molybdenum deposit was hosted in the granite and along the contact with hornfels of the Mesoproterozoic Stenian Pingtoushan Formaion.Based on field works and previous research data,the regional geological background,and orebody characteristics are discussed in this paper.Meanwhile,the ore genesis on the basis of ore-forming fluid and geochemical data are also discussed.The Huaheitan molybdenum deposit mainly shows disseminated ore structures.According to characteristics of ore-bearing veins and intercalation relationships of different veins,the oreforming processes can be divided into hydrothermal ore-forming period and surface oxidation period.The hydrothermal ore-forming period can be further subdivided into 3 stages:(?)quartz-molybdenite-pyrrhotite stage;(?)quartz-molybdenite-arsenopyrite stage;(?)quartz-pyrite-chalcopyrite-sphalerite stage.The crystalline sequence of minerals of molybdenum ore were fromed from high temperature to low temperature,and Molybdenite is the main Mo-bearing mineral.The mineralization elments geochemistry of bedded formation and plutons shows that metallogenic materials are mainly from the Huaniushan granite of the late Triassic,partly from the bedded formation.The ?34S value of pyrite in the main orebody ranges from 2.6‰ ~ 4.6‰ with a mean of 3.72‰,similar to those of the Huaniushan granite of the late Triassic.The ?DH2O values of the ore forming fluid ranging-117.4‰ ~-103.9‰,with average-110.8‰,combining with the ?18OH2O values 1.4‰ ~ 13.4‰,average 9.5‰,infers that the ore-forming fluids are originated mainly from magmatic water during the early period,and mixed with meteoric water in the late stage.Date plotted in salinity versus homogenized temperature diagram of the fluid inclusions in Huaheitan molybdenum deposit shows the same result.So it can be concluded that magmatic water as Huaheitan molybdenum deposit initial ore-forming fluids carried some minerals,then this fluids flowed throught the deep fault and rised to shallow space,and extracted mineral elements from the hornfels of the Mesoproterozoic Stenian Pingtoushan Formaion;finally at late stage the mineral elements from formation together with mineral elements from magma make up material source of this deposit.The homogenization temperatures salinities and densities of the fluid inclusions in stage ? minerals are 383? ~ 583?,12.39%NaCleqv ~ 17.26%NaCleqv,and 0.43g/cm3 ~ 1.02g/cm3,respectively.For the fluid inclusions in stage ? minerals,the homogenization temperatures salinities and densities are 105? ~ 342?,0.71%NaCleqv ~ 20.22%Na Cleqv,and 0.77g/cm3 ~ 1.09g/cm3,respectively.While,those in stage ? minerals are 86? ~ 232?,1.22% NaCleqv ~ 17.61%Na Cleqv,and 0.90g/cm3 ~ 1.06g/cm3,respectively.Laser Raman spectroscopic analysis show that the liquid components and the gas components of fluid inclusions in three stages are mainly H2 O.There are amounts of CO2 detected from the gas components of fluid inclusions in the stages ? and ?;while minor amounts of CH4? SO2 and C6H6 are detected from the gas components of fluid inclusions in the stage ?,of which minor amounts daughter minerals(halite)are contained.The evolutional characteristics of ore-forming fluid of The Huaheitan molybdenum deposit are the following: ore-forming fluids in stage ? contain large amounts gas-phase CO2 in H2O-CO2-Na Cl system with high homogenization temperature and high to moderate salinity,experienced transformation from supercritical fluid to non-supercritical fluid;in stage ?,a large amounts of gas-phase and liquid-phase CO2?CH4 and SO2 in H2O-CO2-Na Cl system with high to moderate homogenization temperature,high to low salinities,experienced ore-forming fluid boiling;in stage ?,lacking of gas-phase and liquid-phase CO2 in H2O-NaCl system with moderate to low homogenization temperature and moderate to low salinities.The main mineralization's homogenization temperature(quartz-molybdenite-pyrrhotite stage)are between 383? to 583?,which is high-temperature mineralization;The mineralization depth is about 0.26 km ~ 2.40 km,belongs to middle-shallow depth.According to the results mentioned above,it is concluded that the minerals of Huaheitan molybdenum deposit mainly originated from magmatic hydrothermal fluids,the ore-riched magmatic hydrothermal fluids flow through the deep fault and rise to shallow place,and the high temperature and abrupt pressure drop ensuing in the process may result in transformation from supercritical fluid to non-supercritical fluid,with the ore-forming elements to trigger litho metallogenesis.The fluid extracted mineral elements from the hornfels of the Mesoproterozoic Stenian Pingtoushan Formaion,and the mineral elements from formation together with mineral elements from magma make up material source of this deposit.The ore-forming fluids(mainly ore-forming magmatic hydrothermal fluids)mixing meteoric water result in ore-forming fluid boiling and the ore-forming materials triggering litho metallogenesis again.The orebodies occur in the contact zone between the Huaniushan granite pluton and the hornfels of the Mesoproterozoic Stenian Pingtoushan Formaion.The type of the Huaheitan molybdenum deposit is medium-deep and nearusurface magmatic hydrothermal vein typed deposit.