Geological And Geochemical Characteristics Of Hydrothermal Alteration In The Dawan Deposit In The Jiangnan Orogenic Belt And Implications For Gold Mineralization

Hydrothermal alteration commonly occurs in lode gold deposits,and contributes to metal precipitation through water-rock interactions.The wall rock alteration is the main manifestation of hydrothermal alteration,which is distributed on both sides of the ore vein.Besides being used as an important prospecting indicator,the alteration zone is also important for the precipitation of metal elements.The identification of the alteration zone can also improve improve the economic value of metal deposits.In addition,abundant mineralization information such as the nature of ore-bearing fluids,physical and chemical conditions as well as migration and enrichment of ore-forming elements during mineralization are also recorded in the altered rocks.Therefore,it is very necessary to carry out research on hydrothermal alteration for guiding the prospecting and exploration,increasing the deposit reserves and establishing the genetic model of the deposit.In the Dawan gold deposit of the northeastern Hunan Province,South China,bleaching of slates enclosing auriferous quartz veins widespreadly occurred,with the colours transformed from dark-grey into pale yellow-grey.The bleaching has been used to guide gold exploration,but its nature and genesis,as well as its relationship with gold mineralization,remains unclear.In this paper,based on the fact that bleaching alteration is closely related to gold mineralization in space,detailed field investigation and petrography,Tescan Integrated Mineral Analyzer（TIMA）,X-ray fluorescence analysis（μ-XRF）,whole-rock major,trace element analysis,mass balance calculation and geochemical reaction path modelling were applied to clarify the geological and geochemical characteristics of hydrothermal alteration in the Dawan gold deposit by combining with the previous research results of fluid inclusion,metallogenic structure and geological chronology.Field and petrographic works,combined with Tescan Integrated Mineral Analyzer（TIMA）,demonstrate that the bleaching was mainly caused by the combination of sericitization and carbonatization.Combined with fluid inclusion studies,it is inferred that the bleaching was caused by the pre-ore hydrothermal event characteristic of CO₂-bearing fluids.Large amounts of siderite are distributed within the bleaching zones,and they are locally crosscut by Au-bearing sulfide veins,suggesting that the bleaching happened before gold mineralization.Mass balance calculation andμ-XRF analysis indicate that despite the abundance of siderite,Fe is not gained during alteration.Geochemical path modeling further suggests that siderite is efficient in precipitating Au by decreasing S contents of the ore fluids and destroying Au-bearing complexes.In addition,the contents of siderite in the altered slates and of Au in sulfides suggest that the chemical interaction between siderite and ore fluids can produce ores with economic significance.Based on the above research,this paper holds that the siderite developed during the pre-mineralization alteration,provided ideal chemical traps and prepared the ground for gold mineralization.When the gold-bearing fluids overprinted the altered rocks,sulfides formed through reaction with siderite.Therefore,it can be concluded that the sulfidation,which resulted in the destruction of Au-S complexes,is the major mechanism for gold mineralization within the bleached slates.