安徽宣城茶亭铜金矿床地质和地球化学特征及成因

Geological and geochemical characteristics and genesis of the Chating copper-gold deposit in Xuancheng City,Anhui Province

安徽宣城茶亭铜金矿床是长江中下游成矿带浅覆盖区新发现的一个大型矿床,产于南陵-宣城中-新生代火山-沉积盆地中。矿床地质研究表明:矿体赋存于早白垩世石英闪长玢岩侵入体中,矿化强烈部位与隐爆角砾岩筒发育部位明显重叠而且范围略大;矿床围岩蚀变发育,以硅化、钾长石化、黑云母化、绢(白)云母化、黄铁矿化为主,分带性较为明显,矽卡岩化、绿泥石化、硬石膏化、碳酸盐化和高岭石化等蚀变仅在局部呈条带状、团块状分布;矿石具角砾状、脉状、细脉-网脉状、细粒浸染状构造和交代残余结构、固溶体分离结构等,其中,角砾状和脉状矿石中常常发育磁铁矿-赤铁矿和矽卡岩矿物石榴子石、透辉石等,硬石膏尤其发育,甚至形成硬石膏岩;矿石中金属矿物有黄铁矿和黄铜矿及少量斑铜矿、黝铜矿、砷黝铜矿等;隐爆角砾岩的角砾成分以石英闪长玢岩为主,胶结物主要为岩屑或岩粉及热液矿物两类。矿床流体包裹体研究表明:矿石石英中发育液相、气液相和含子晶气液相包裹体,气相成分以水为主;含子晶气液相包裹体主要见于成矿较早阶段与具有裂纹的黄铁矿共生的石英中,均一温度为360~420℃,流体盐度较高,而铜金主成矿阶段发育气液相和液相包裹体,均一温度介于270~300℃和190~220℃区间,相应的成矿流体的盐度和密度较低。矿床H-O、C-O和S、Pb同位素组成特征指示成矿流体、成矿金属元素及其化合元素主要来自于岩浆或岩浆岩,沉积地层和大气降水对成矿亦有一定的贡献,但所起作用相对较小。黄铁矿Re-Os同位素年龄与赋矿石英闪长玢岩锆石U-Pb年龄在误差范围内一致。综合矿床地质和地球化学特征,认为宣城茶亭铜金矿床与斑岩型和火山-次火山岩型(浅成低温热液型)矿床既有相似之处,也有明显的差异,因而确定为一个与早白垩世石英闪长玢岩浅成侵入体有关的隐爆角砾岩型矿床,该矿床是长江中下游成矿带中的一个新类型矿床。

The Chating Cu-Au ore deposit in the Xuancheng City of Anhui Province is a large deposit newly discovered in shallow overburden area of the Middle and Lower Yangtze River Valley Metallogenic Belt. It is located in the central uplift of NanlingXuancheng Meso-Cenozoic volcanic-sedimentary basin. The geological study of the deposit indicates that the ore bodies occur in the concealed quartz diorite porphyritic intrusion formed in Early Cretaceous. The strong mineralization keeps obviously with the cryptoexplosive breccia pipe and the range of the former is slightly larger than that of the latter. The alteration types are dominated by silication,potash-feldspathization,biotization,sericitization( muscovitization),and pyritization,with obvious zoning characteristics,while skarnization,chlorinization and kaolinization scattered in the mineralization area as strip belts or gobbets. The ores have brecciated,veined,veinlet-stockwork,fine granular disseminated structures,and replacement remnant texture,exsolution texture,etc. Magnetite-hematite and skarn minerals grow in the brecciated and veined ores. Especially anhydrite develops in veins and even forms anhydrock. The metallic minerals in the ores are dominated by pyrite and chalcopyrite as well as a small amount of bornite,tetrahedrite,tennatite,etc. The breccia in cryptoexplosion breccia pipe is mainly composed of quartz diorite porphyrite,and the cementation mainly has two types: one is rock debris or rock powder which has similar composition with quartz diorite porphyrite;another is hydrothermal minerals mainly including quartz and anhydrite. Three types inclusions like aqueous,vapor-aqueous,and daughter mineral-bearing inclusions are found in the quartz of the ores,and the component of vapor phase in the inclusions is mainly water. The daughter mineral-bearing inclusions are mainly found in quartz paragenesis with pyrite which has crakes formed in the earlier stage,with homogenization temperature range from 360℃ to 420℃ and higher salinity and density. There two homogenization temperature rages,270 ~ 300℃ and 190 ~ 220℃ were gained from the vapor-aqueous and aqueous inclusions developed at the main metallogenic stages of copper and gold,corresponding with lower salinity and density. The geochemical characteristics of H-O,C-O,S and Pb isotopes indicate that the ore-forming fluids,the ore-forming metal elements and the compound elements are mainly from magma or magmatic rocks,while sedimentary strata and atmospheric precipitation also contribute to mineralization,but play a relatively small role. The Re-Os age of pyrite is consistent with the U-Pb age of the zircon in quartz diorite porphyrite within the margin of error,indicating that its mineralization is closely related to the magmatic intrusion and hydrothermal activity of the metallogenic area in Late Yanshanian. Based on the comprehensive geological and geochemical characteristics of the deposit,comparing with the porphyry deposit and volcanic-subvolcanic( epithermal) deposit,it is considered that the Chating Cu-Au ore deposit in the Xuancheng City is a cryptoexplosive breccia type deposit related to the Early Cretaceous hypabyssal intrusive of quartz diorite porphyrite,which is a new type deposit in the Middle and Lower Yangtze River Valley Metallogenic Belt.