Genesis Of Altered Slate Type Ores In The Huangjindong Gold Deposit,Jiangnan Orogenic Belt,South China

Altered rock ores,closely associated with fluid-rock interactions,represent a typical mineralization style in hydrothermal gold deposits.Jiangnan orogenic belt,located in the southeast margin of Yangtze block,is an important gold metallogenic belt in China.Huangjindong gold deposit is located in the middle part of Jiangnan orogenic belt in the northeast of Hunan Province(referred to as "northeast Hunan").It is one of the representative gold deposits with large reserves,high grade and abundant co-associated elements.The ore types of Huangjindong gold deposit can be divided into three: quartz vein,breccia and altered rock.The first two types of ores have high grade and quartz vein type ores received extensive research.However,altered rock ores are much less well studied.Compared with quartz veins,altered rock ores generally has a lower grade but larger reserve,which is of great significance for gold exploration.The altered slate ores are well-developed in the bleaching alteration zone in the Huangjindong gold deposit,but the genesis remains indistinct.Based on field work,petrographic observations and Tescan Integrated Mineral Analyzer(TIMA)analysis,the bleaching alteration is mainly characterized by the occurrence of siderite spots,as well as sericites and cryptocrystalline quartzes.Abundant gold-bearing pyrite and arsenopyrite in altered slates aggregate near carbonate spots,sharing similar geochemical compositions with those in quartz veins.These sulfides locally crosscut siderite grains,demonstrating that the bleaching alteration took place before gold mineralization.μ-XRF analysis and mass balance calculation show that Fe is not prominently gained or lost but aggregated to form siderite spots in the alteration zone.In addition,K contents increased during alteration,consistent with the presence of sericitization.Thermodynamic modeling of the dissolution of siderite manifests that the chemical reactions between siderite and S-Au-bearing fluids leads to the co-precipitation of gold,pyrite and arsenopyrite.Moreover,taking account of the siderite contents in altered slates and Au contents in sulfides,the water-rock interaction could generate altered slate ores with economic significance.These results show that large amounts of siderite spots are generated during the premineralization fluid-rock reaction,and thus provide favorable chemical traps for gold mineralization.Abundant siderites contribute to gold mineralization by triggering sulfidation,which is the major genesis for the altered slate ores in the Huangjindong gold deposit.