摘要
The Gacun Kuroko-type deposit, Southwestern China, is hosted in rhyolitic rocks associated with the underlying mafic rocks occurred in the - 1000 m deep fault - bounded basin within the intra -arc rifting zone which formed on the Triassic Yidun island - arc. Two vertically separated alteration systems are recognized: one is conformable or semiconformable alteration zone developed in - 150 m thick mafic unit 1-1.5 km below the massive sulfide ore body; the other is discordant alteration pipe directly surrounded around stockwork ore within rhyolitic unit. The lower conformable alteration zone extending for several kilometers along strike is characterized by silicification and epidotization which result in the development of quartz vein and quartz-epidote vein systems in mafic lava flows and replacement of primary minerals and groundmass in spilitized mafic volcanics and dikes by quartz, epidote - group minerals and sodic plagioclase. Sulfides often occur in the vein system and altered mafic volcanics. Quartz solubility relation indicates that silicification is a consequence of interaction of Si- saturated fluids with mafic rocks in a higher temperature system (T>340℃), intensifying by intrusion of mafic dike or high-level acidic magma chamber. The alteration pipe of diameter about 2 km shows a similar mineralogical zoning to Kuroko deposits of Japan. The sequence is quartz + hyalophane; sericite + chlorite + quartz and zeolite-like zones from core to margins of the pipe. The chlorite core only occurs in the root part of the alteration pipe and downwards transfers into epidote - chlorite and epidote - quartz vein swarm extending 500 m downwards. The felsic rocks away from the orebody and alteration pipe took place district-scale alteration, which has typical low-temperature mineral association: illite + albite + quartz + calcite. Whole -rock and quartz δ18O values indicate that district - scale alteration is a result of interaction of seawater with rocks at lower temperature (T<200℃)under water-dominated condition. However, the altered rocks from the pipe show remarkably δ18O enrichment, and bulk -rock δ18O values decreased gradually toward stockwork orebody from 15.1‰-l5. 75‰ in zeolite-like zone and 12. 05‰-14. 2‰ in sericite - quartz zone to 11.3 ‰ - 14. 4‰ in quartz - hyalophane zone. The filled temperatures of fluid inclusions in quartz and sphalerite lie in the ranges of 280 -320 ℃ for quartz - hyalophane zone and 250 ℃ to 297 ℃ for sericite-quartz zone. The estimated δ18O values of hydrothermal fluids are 7. 98‰ and3.2‰, respectively, based on quartz δ18O data in the deposit. The lower conformable alteration is considered to be approximately coeval with the alteration pipe, based on the SiO2 concentration in the fluids, which restrict the main fluid - rock reaction zone to be located in mafic horizon by quartz barometer, and metal element flux calculation and sulfide - epidote vein system developed both in alteration systems. High - salinity fluid inclusions in gangue quartz (>8% eq. NaCl) from stockwork ore and in quartz phenocryst (>40% eq. NaCl) in footwall rhyolite strongly suggest the existence of hot-saline brine to react with mafic complex and leach metal components, which probably originates mainly from magmatic fluid derived from high-level acidic magma chamber. The brine layer located in mafic unit possibly heats and drives the overlying single -pass convective seawater reacting with felsic rocks. The 'density window' may be expected to occur on the interface between seawater and brine layer, when the brine becomes to be gravitationally instability by the turbulent entrainment of seawater during magmatic and/or tectonic activities. The sulfide mineralization and alteration pipe is inter preted as an effect of the 'density window' through which the mixed fluids of brine with seawater adiabatically discharges upwards.
The Gacun Kuroko-type deposit, Southwestern China, is hosted in rhyolitic rocks associated with the underlying mafic rocks occurred in the - 1000 m deep fault - bounded basin within the intra -arc rifting zone which formed on the Triassic Yidun island - arc. Two vertically separated alteration systems are recognized: one is conformable or semiconformable alteration zone developed in - 150 m thick mafic unit 1-1.5 km below the massive sulfide ore body; the other is discordant alteration pipe directly surrounded around stockwork ore within rhyolitic unit. The lower conformable alteration zone extending for several kilometers along strike is characterized by silicification and epidotization which result in the development of quartz vein and quartz-epidote vein systems in mafic lava flows and replacement of primary minerals and groundmass in spilitized mafic volcanics and dikes by quartz, epidote - group minerals and sodic plagioclase. Sulfides often occur in the vein system and altered mafic volcanics. Quartz solubility relation indicates that silicification is a consequence of interaction of Si- saturated fluids with mafic rocks in a higher temperature system (T>340℃), intensifying by intrusion of mafic dike or high-level acidic magma chamber. The alteration pipe of diameter about 2 km shows a similar mineralogical zoning to Kuroko deposits of Japan. The sequence is quartz + hyalophane; sericite + chlorite + quartz and zeolite-like zones from core to margins of the pipe. The chlorite core only occurs in the root part of the alteration pipe and downwards transfers into epidote - chlorite and epidote - quartz vein swarm extending 500 m downwards. The felsic rocks away from the orebody and alteration pipe took place district-scale alteration, which has typical low-temperature mineral association: illite + albite + quartz + calcite. Whole -rock and quartz δ18O values indicate that district - scale alteration is a result of interaction of seawater with rocks at lower temperature (T<200℃)under water-dominated condition. However, the altered rocks from the pipe show remarkably δ18O enrichment, and bulk -rock δ18O values decreased gradually toward stockwork orebody from 15.1‰-l5. 75‰ in zeolite-like zone and 12. 05‰-14. 2‰ in sericite - quartz zone to 11.3 ‰ - 14. 4‰ in quartz - hyalophane zone. The filled temperatures of fluid inclusions in quartz and sphalerite lie in the ranges of 280 -320 ℃ for quartz - hyalophane zone and 250 ℃ to 297 ℃ for sericite-quartz zone. The estimated δ18O values of hydrothermal fluids are 7. 98‰ and3.2‰, respectively, based on quartz δ18O data in the deposit. The lower conformable alteration is considered to be approximately coeval with the alteration pipe, based on the SiO2 concentration in the fluids, which restrict the main fluid - rock reaction zone to be located in mafic horizon by quartz barometer, and metal element flux calculation and sulfide - epidote vein system developed both in alteration systems. High - salinity fluid inclusions in gangue quartz (>8% eq. NaCl) from stockwork ore and in quartz phenocryst (>40% eq. NaCl) in footwall rhyolite strongly suggest the existence of hot-saline brine to react with mafic complex and leach metal components, which probably originates mainly from magmatic fluid derived from high-level acidic magma chamber. The brine layer located in mafic unit possibly heats and drives the overlying single -pass convective seawater reacting with felsic rocks. The 'density window' may be expected to occur on the interface between seawater and brine layer, when the brine becomes to be gravitationally instability by the turbulent entrainment of seawater during magmatic and/or tectonic activities. The sulfide mineralization and alteration pipe is inter preted as an effect of the 'density window' through which the mixed fluids of brine with seawater adiabatically discharges upwards.
基金
The study is supported by Agence of International Science and Technology, Japan
