大水式金矿床地质特征及成因探讨

GEOLOGICAL CHARACTERISTICS AND GENESIS OF THE DASHUI TYPE GOLD DEPOSITS

大水式金矿床是新近发现的一个新类型金矿 ,其矿化特征独特、埋藏浅、品位高、化学成分简单、易采选、具有很高的经济价值。矿体呈脉状、不规则囊状或漏斗状产于西秦岭造山带三叠纪碳酸盐岩中。矿石主要为一套热液成因的红色、褐色硅化岩和硅质岩 ,极端贫硫化物。矿石组构和矿物组成等反映出的成矿作用方式主要为热液渗滤交代、构造裂隙和角砾岩孔隙等有限空间的充填沉淀作用以及古构造岩溶内相对开放空间中的沉积或堆积作用 ,构成比较完整的热液交代 -充填 -沉积成矿体系。成矿环境为近地表 ,成矿时代为燕山中晚期 ,成矿物质来源与燕山陆内造山阶段中酸性岩浆活动密切有关 ,金矿化和岩浆岩可能是统一的受深部构造制约的燕山期构造 -岩浆

The Dashui ore deposits, being a new type of gold deposits recently discovered in West Qinling orogenic belt, have very unique ore\|forming characteristics and extremely high economic values. Mineralization mainly occurred in a set of silicified rocks and silicalites formed by hydrothermal activities. Orebodies are hosted in Triassic limestones and are apparently controlled by faults. Ore minerals include gold, hematite and chalcedony and minor sulfide minerals. Silicified limestone, silicalite, siliceous breccia and silicified granodiorite are four major types of ores, which are serial products of the hydrothermal system at different geological environments through different ways of hydrothermal\|metasomatism, infilling and sedimentation. In space, from the center toward peripheral parts of the hydrothermal active system, the hydrothermal sediments are produced in order of hydrothermal explosive breccia+hydrothermal silicalite+calcite veins→hydrothermal metasomatic jasperoids+silicified granodiorite→silicified limestone. From the lower to upper part, the products are successively hydrothermal filling veinlet\|stockwork silicalite+calcite veins→hydrothermal sedimentary laminated silicalite+oolitic\|pisolitic conglomerates+horizontal calcite crust. In time, hydrothermal metasomatic mineralization took place a bit earlier than hydrothermal infilling and sedimentary mineralization, and the composition of the hydrothermal system evolved from iron\|silica\|rich one in the earlier stages towards carbonate rich one in the late period.\;The temperature and pressure measured for fluid inclusions show that the whole mineralization might have undergone a short process characterized by a wide range of temperatures, and the metallogenic depth was very shallow, probably near the surface. The mineral assemblage fabrics and chemical composition of the ores and the very low gaseous reduction parameter R of fluid inclusions suggest a high oxidizing metallogenic environment. Geological and geochemical characteristics of the deposit and gold abundance of regional strata indicate that ore\|forming meterials mainly came from deep sources closely related to Yanshanian tectono\|magmatism. The metallogenic epoch probably belongs to the middle\|late Yanshanian period.