论甘孜丘洛成矿带微细浸染型金矿的成矿条件

ON ORE-FORMING CONDITIONS OF THE FINE-DISSEMINATED TYPE GDLD DEPOSITS IN QIULUO METALLOGENETIC BELT,GANZI

甘孜丘洛成矿带微细浸染型金矿的成矿热液来自大气降水。矿化作用发生于燕山期的下渗雨水加热环流体系中。包裹体研究和热力学计算表明,成矿温度为93—306℃,成矿深度小于1km。矿化热液呈弱碱性,还原性较强,含盐度中等,贫氯富硫。Au以Au(HS)_2^-形式进行迁移,并在扩容减压带中沉淀富集。温度、压力降低和总硫活度减小是矿质沉淀的重要因素。

It is supposed that ore-forming solutions of the fine disseminated type gold deposits in Qiuluo metallogenetic belt were derived from meteoric waters. The gold mineralization was formed in the convecting hot meteoric water system during Yanshan tectonic period. Studies of fluid inclusions and thermodynamic calculation revealed that the gold ores were formed in a temperature ranging from 93 to 306℃,and in the depth less than 1 km. The average temperature and pressure in the early stage were 200℃ and 19. 5 MPa and in the latter stage 150℃ and 6 MPa,respectively. The Ore-forming solutions did pertained to Ca^(2+)-Mg^(2+)-(Na^+ )-HCO_3^--SO_()^(2-) type of alkalescent fluid which were lower in Eh values,poor in Cl^- ,rich in sulfur,and intermediate in salinity. The gold in rocks was mobilized and migrated into the solution through the following reaction:Au(s)+2 HS-(aq)+H^+(aq) +1/4O_2(g)(?)Au(HS)_2^-+1/2 H_2O(1) and migrated in the form of Au(HS)_2^-. When the ore-forming fluid converged into the low-pressure zone, i. e. , a brittle fractured zone,the temperature and pressure decreased drastically and made quartz and pyrite to precipitated, which in turn resulted in the total sulfur activity in system dreased so that the Au(HS)_2^- became unstable,and native gold was deposited.