Double Convective Hydrothermal System beneath Massive Sulfide Orebody in Gacun Deposit,Southwestern China

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-hounded 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 sulfde orehody; 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 plagiocluse. Sulfides often occur in the vein system and altered mafic volcanics. Quartz solubi