江西大背坞金矿床矿石物质组成及金的赋存状态

ORE MATERIAL COMPONENT AND GOLD OCCURRENCE OF DABEIWU GOLD DEPOSIT,JIANGXI PROVINCE

大背坞金矿床属贫硫化物（糜棱岩）石英脉型，该矿床有用组分唯有Ａｕ，自然金是Ａｕ最主要的赋存形式，几乎集中富集了全部Ａｕ组分．虽然矿石中有极微量的银金矿，并在方铅矿中发现可能还存在次显微金，但含金量却微不足道．石英、黄铁矿、毒砂是自然金的主要载体．自然金成色高，多以中粗粒裂隙金形式产出，粒间金次之，包裹金较少．本矿床中黄铜矿、方铅矿、闪锌矿不发育，含量少，但它们与自然金关系密切，镜下常常发现与自然金共生赋存于较粗粒的黄铁矿、毒砂等载金矿物中．这３种硫化物是发现富矿化的标志．铅同位素结果表明本金矿成矿物质来源于前震旦纪变质沉积岩，中元古界双桥山群是矿源层．金以络合物的形式迁移．当温度降低（低于３００℃），含矿溶液进入容矿空间压力降低，金发生沉淀．矿化早期石英脉包裹体ｐＨ值４．９１，Ｅｈ值１６４．５２，到矿化主期ｐＨ升高到６．３８～６．７２，Ｅｈ降低到５７．４４，从而使金在溶液中的溶解度大大降低，促使金发生沉淀．

Dabeiwu gold deposit,where Au is the only useful element,belongs to sulfide poor mylonite quartz vein type Natural gold is the dominant occurrence form,in which almost all Au component concentrate There is utmost small amount of electrum in the ore,and secondary micro gold may be found in galena,but the content of gold is very little Quartz,pyrite and arsenopyrite are the main carriers of natural gold The fineness of the natural gold is quite high Most of the gold occur as medium macrograined crevice gold,with minor intergranular gold and less inclusion gold Chalcopyrite,galena and sphalerite are closely related to natural gold,although their contents are low They are usually found intergrowing with natural gold in macrograined pyrite and arsenopyrite The mineralization of the three sulfides is the indicator to rich ores The Pb isotope result shows that the ore forming material is derived from pre Cambian metasedimentray rocks Mesoproterozoic Shuangqiaoshan group is the source bed of ore Au migrates as complex and precipitates as the decreasing of temperature (<300℃) and prssure At the early stage of mineralization,the pH value of inclusion in quartz vein is 4 91, E h 164 52 During the main stage of mineralization,the pH value gets up to 6 38～6 72,while the E h value lowers down to 57 44 The lowered dissolubility of Au in solution makes the precipitation of Au