浙江拔茅银矿成矿地球化学条件及成矿物质来源

THE GEOCHEMICAL CONDITIONS AND THE ORIGIN OF ORE--FORMING MATERIALS OF BAMAO SILVER DEPOSIT, ZHEJIANG

拔茅银矿为一赋存于中生代陆相火山岩中的浅成热液矿床。本文通过该矿床矿物中的气—液包裹体,S、Pb、H、O等稳定同位素组成,矿物共生组合稳定存在的热力学条件,稀土元素地球化学特征,岩石的含矿性等方面的研究,阐述了其成矿地球化学条件和成矿物质来源。

Bamao silver deposit is an epithermal deposit hosted in volcanic rocks of the Mesozoic volcanic belt in Eastern China coast. Based on the studies of gas—fluid inclusions, isotopic compositions of S, Pb, H, and O, stability thermo—dynamic conditions of mineral assemblage, geochemical characteristics of REE, and compositions of ore—forming elements in rocks, the followings are showed. Bamao silver deposit is an epithermal deposit, the homogenization temperatures in the early stage of metallogenism range from 200℃ to 300℃, and in'the late stage range from 100℃ to 150℃. The nature of the ore—forming fluid ranges between weak acid and weak alkline(pH=5.2 ～6.65). According to the thermo—dynamic calculation about the equilibrium equations of mineral assemblage, the f CO_2 of the hydrothermal system is equal to or larger than 10^(-0.7)×10~5Pa; the f O_2 ranges from 10^(-51.3)×10~5Pa to 10^(-32.2)×10~5Pa; and the f S_2 ranges from 10^(-18.4)×10~5Pa to 10^(-10.2)×10~5Pa. The f S_2 is much higher than f O_2. The values of δ^(34)S of sulfides range between -9.81 and +3.83 per mil, which shows the sulfur is polygene; some of that is organic or crustal origin, and the other is mantle—derived. The compositions of lead isotopes are relatively stable, and the Homls—Houtermans model ages are in two groups. One is about 145. 8Ma,and the other is about 226. 1Ma—312. 8Ma,showing that the lead mainly came from the volcanic rocks of J_3 and the Pre—Mesozoic fundamental rocks. The δ^(18)O_(H_2O) values range from—7.47 to + 6.63 per mil, and the δD values range from—109.0 to—59.6 per mil, showing the hydrothermal fluid is a mixture, and the majority is meteoric water. The characteristics of REE show that there is somewhat homogeneity between the ores and wallrocks in the origin. Those also show that part of the ore—forming materials came from the volcanic rocks. The analyses of minor elements in rocks show that there are two rock units which are rich in Ag, Pb. One is the Mesozoic volcanic rocks,and the other is the Pre—Mesozoic fundamental rocks. The ore—forming elements exist as complexes in hydrothermal fluid. The changes of temperature and pressure in the hydrothermal system,and the addition of meteoric water into the hydrothermal fluid are the main factors leading the division of complexes and the deposition of sulfides.