湘中锡矿山锑矿床巨量矿石堆积机制:来自方解石流体包裹体的约束

The accumulation mechanism of giant ores in the Xikuangshan antimony deposit,central Hunan:Constraints from fluid inclusions hosted in calcite

湘中锡矿山锑矿床目前研究程度已经很高,但对该矿区锑矿石发生巨量堆积的原因仍不清楚。热液矿床巨量矿石的堆积过程,实际上就是大规模流体汇聚并发生有效沉淀的过程,因此,对锡矿山矿区流体演化过程和沉淀机制的研究有助于解决其巨量矿石的堆积机制问题。锡矿山矿区方解石非常发育,且与成矿关系密切,是研究该区流体作用的理想对象。对该区不同期次方解石进行流体包裹体岩相学和显微测温研究,初步刻画了该区流体演化过程和矿石沉淀机制。研究表明,不同期次方解石中流体包裹体类型基本相同,主要为Ⅰ型(纯液相包裹体)和Ⅱ型(富液相两相包裹体),但流体包裹体的岩相学特征存在明显差异:主成矿期流体包裹体不发育且个体小,成矿晚期流体包裹体发育程度中等且个体较大,成矿后流体包裹体非常发育且个体很大。流体包裹体显微测温分析显示,该区经历了2次独立的成矿事件:主成矿期成矿流体为中高温、中低盐度的热液,这种热液有利于锑的大规模迁移,矿石沉淀机制为流体混合作用;成矿晚期流体为中温、低盐度热液,这种热液中锑的浓度明显小于主成矿期,矿石沉淀机制为冷却作用;成矿后还存在一期热液事件,其流体为一种低温、低盐度热液。锡矿山矿区矿石巨量堆积主要与主成矿期成矿作用有关,该期成矿流体中锑的浓度高,流体规模巨大,且流体混合作用这种矿石沉淀机制非常有利于超大型锑矿床的形成。

Although a lot of efforts have been made on the Xikuangshan deposit,the key scientific problem why such giant ores were accumulated in the narrow Xikuangshan area still remains unclear.The detailed investigation on the evolution of the ore-forming fluid and on the mechanism of its ore precipitation will be helpful for solving the above problem,since the giant ore accumulation in the hydrothermal deposit resulted from the convergence and effective precipitation of the large-scale hydrothermal fluid.Calcite is common in the Xikuanghan deposit and is closely related to antimony mineralization,thus it is an ideal mineral to study the mineralization process in the Xikuangshan ore district.Based on the researches on the petrography and microthermometry of fluid inclusions hosted in calcite,the fluid evolution process and ore precipitation mechanism are preliminarily depicted,and then the accumulation mechanism of giant ores in the Xikuangshan deposit is discussed.It is shown that,various calcites share similar fluid inclusion types,but their fluid inclusions display the distinct differences in view of petrography,the fluid inclusions hosted in those calcites of the early mineralization are rare in amount and small in size,the inclusions hosted in calcite of the late mineralization are relatively developed and relatively large in size,and the inclusions in the post-ore calcites are the most in amount and the largest in size.It was revealed that there are twice independent mineralization events happened in the Xikuangshan ore district.The early mineralization is characterized by the hydrothermal fluid with relatively high temperature and moderate salinity,which is beneficial to the large-scale migration of antimony,and the fluid mixing is responsible for the ore precipitation of early mineralization;the late ore-forming fluid is a kind of moderate-temperature and low-salinity hydrothermal solution,the antimony concentration in this kind of hydrothermal solution is significantly lower than that in the early mineralizing fluid,and the ore precipitation resulted from the fluid cooling.Moreover,there is another hydrothermal event after antimony mineralization with low-temperature and low-salinity solution.The giant accumulation of ore in the Xikuangshan ore district is mainly ascribed to the early mineralization,with high-concentration antimony in the ore-forming fluid and the giant flux of fluid,and the ore precipitation mechanism of fluid mixing in the early mineralization stage are favorable for the formation of the giant antimony deposits.