大井银铜锡矿体—一个潜火山热液矿床的特征和成因

THE DAJING Ag—Cu—Sn DEPOSIT—THE CHARACTERISTICS AND GENESIS OF AN EPITHERMAL DEPOSIT RELATED TO THE SUBVOLCANIC INSTRUSIONS

大井银铜锡矿床是我国在金属组合上较为罕见的矿床之一。过去,该矿床曾被认为是与花岗岩有关的中深成热液矿床。作者从矿区潜火山侵入体与矿区东侧侏罗纪火山层序对比、矿床地质特征(包括矿体、矿石、金属矿物及金属分带)以及成矿作用地球化学等方面的资料和数据,论证了该矿床是一个浅成的潜火山热液矿床。

The Dajing deposit. Linxi County. Inner Mongolia, occurred in the edge of the fault upwelling area of the contact zone between the Mesozoic Linxi fault upwelling area and the Daban volcanic basin, is a large-sized Ag-Cu-Sn deposit. The deposit was considered as a plutonogenic hydrothermal deposit related to the Ma'anzi granite pluton by several workers. This paper demonstrates that the deposit is an epithermal deposit related to the subvolcanic intrusions. It is impossible that the Ma'anzi granite pluton is related to mineralizing of the Dajing deposit because the pluton (155Ma) is separated by 20 km from the deposit and is unaltered. A dike swarm of subvolcanie porphyries outcrops in the deposit. Both subvoleanic porphyies and the subaerial volcanic rocks of the Upper Jurassic to the eastern part of the deposit possess similar chemical composition, approximate age and conformable evolutional sequence. The intermediate-acid subvolcanic porphyries arc extensively altered and accompany spatially the Ag-Cu-Sn veins. Some Ag-Cu-Sn ore bodies are also in these subvolcanie intrusions. The Ag-Cu-Sn veins are controlled mainly by the fault system related to the volcanic apparatus. Ore petrology, ore mineral and wall-rock alteration of these veins reflect that the deposit is similar to the epithermal deposits related to the volcano—subvolcanism. The primary metalzoning in the deposit coincides with inferred volcanic apparatus and its heat halo. The study of fluid inclusions indicates that the ore-forming fluid is characterized by lower salinity and that the salinity decreases with reducing temperature. The analyses of inclusion composition indicate that the system of fluid is Na^+—Ca^(2+)—Cl^-—SO^(2-)—HCO^-. The δ^(18)O range of fluid for tin-mineralizing stage is from 6. 3 to 9.6‰. the δ^(18)O of copper-mineralizing stage and silver-mineralizing stage are 2.8～3. 4‰ and—10.6‰, respectively. The δD of fluid for silver—mineralizing is—108‰. The mineralizing depth calculated by the data of fluid inclusions and indimn content of cassiterite is 0.82～0.83 km, Above data suggest that the Dajing deposit is different from the tin-polymetallic deposits related to granite pluton in South China and that the Dajing deposit is an epithermal deposit related to the subvolcanic intrusions.