江西冷水坑斑岩型铅锌银矿床地质特征、热液蚀变与成矿时限

江西冷水坑铅锌银矿床是我国重要的铅锌银矿床之一,也是世界上少有的斑岩型铅锌银矿床.冷水坑斑岩型铅锌银矿床位于中国东部中生代月凤山火山盆地边缘,含矿斑岩为燕山中期碱性花岗斑岩,侵位于上侏罗统火山岩地层内,斑岩体边部发育大量的隐爆角砾岩.矿区发育两种矿化类型,即斑岩型矿化与层状改造型矿化,此两种均与碱性花岗斑岩有关.矿化以Pb、Zn、Ag为主,伴生少量的Cu、Au.斑岩型的铅锌银矿化发生在斑岩体内以及接触带中,具有面型矿化特点并显示出明显的矿化分带性.层状改造型以铁锰银铅锌矿化为主,矿体均为隐伏状分布于碱性花岗斑岩附近的火山岩含铁锰地层中,产状与火山岩地层一致.铁锰碳酸盐铅锌银矿体在靠近斑岩体时,银铅锌矿化相应逐渐增强,而远离花岗斑岩体,矿化明显较弱.围岩蚀变作用明显,主要为绢云母化、绿泥石化、碳酸盐化、硅化和黄铁矿化.矿化蚀变特征、蚀变类型与典型的斑岩型铜(钼)矿床不同,缺少斑岩铜(钼)矿床早期蚀变的钾交代作用(黑云母化与钾长石化),发育大量铁锰碳酸盐蚀变.矿田矿化蚀变具有一定的分带性,由岩体内向外蚀变可以分为三个带:绿泥石绢云母化带、绢云母化碳酸盐化硅化黄铁矿化带和碳酸盐化绢云母化带.斑岩型矿床的铅锌银矿化与绢云母化和绿泥石化蚀变密切相关.通过对蚀变矿物绢云母的~(40)Ar/~(39)Ar同位素测定,获得冷水坑斑岩型铅锌银矿化年龄为162.8 ±1.6Ma,与含矿斑岩形成时间一致,说明冷水坑斑岩型铅锌银成矿作用发生于中国东部燕山中期陆内环境.

Geological Characteristics and Mineralization Timing of the Lengshuikeng Porphyry Pb-Zn-Ag Deposit, Jiangxi Province

Lengshuikeng deposit in Jiangxi Province is one of the most important Pb-Zn-Ag deposits in China, which is also a rare, typical porphyry Pb-Zn-Ag deposit in the world. It is hosted in the Yuefengshan Mesozoic volcanic basin, eastern China. The ore-bearing porphyry is alkaline granite porphyry intruding into upper Jurassic volcanic rocks in the Middle Yanshanian period. A large number of cryptoexplosion breccia is developed around the edge of the ore-bearing porphyry. In the Lengshuikeng ore district, there are two types of mineralization, porphyry-type and layered transformation type, both of which are related to the alkaline granite porphyry. Porphyry-type Pb-Zn-Ag mineralization occurs in the porphyry and the contact zone of the porphyry to the volcanic rocks, showing a distinct mineralization zoning. The layered transformation type is characterized by Fe-Mn-Ag-Pb-Zn Mineralization, with the orebodies concealing in the volcanic rocks containing ferrous manganese carbonate, which is adjacent to the alkaline granite porphyry. Pb-Zn-Ag mineralization becomes weak from porphyry mass outward. The alteration of wall-rocks in the Lengshuikeng deposit includes sericitization, chloritization, carbonatization, silicification and pyritization. The Lengshuikeng porphyry Pb-Zn-Ag deposit differs from the typical porphyry Cu-Mo deposit in its abundant Fe-Mn carbonate alteration instead of potassium replacement appearing at the early alteration stage of porphyry Cu (Mo) deposits. The ore district, however, is of three distinct zones based on the altered mineral assemblages. Three zonations classified are chlorite sericite zone, the sericite carbonate silicate pyrite zone, and the carbonate sercite zone from the inner of the porphyry outward. The Pb-Zn-Ag mineralization of the porphyry deposit is closely related to the sercite and chlorite alteration. ~(40)Ar/~(39)Ar dating of sericite yields an isochron age of 162.8 ±1.6Ma, which is consistent with that of the ore-bearing porphyry, indicating that the deposit formed on the intracontinental setting in Eastern China during the mid-Yanshanian period.