南秦岭大型钡成矿带中毒重石矿床成因新认识——来自单个流体包裹体证据

钡以BaSO_4形式存在的重晶石矿床在世界上广泛分布,但以BaCO_3形式存在的毒重石矿床极为罕见。在南秦岭早古生代硅岩建造中,发育大量层状毒重石和重晶石矿床,两类矿床在空间上表现出既共生又分离的分布规律,构成世界上极为独特的大型钡成矿带。本文对毒重石、钡解石、重晶石及石英等矿物流体包裹体进行了系统研究,结果显示,各矿物中流体包裹体的均一温度都变化于90～310℃,主要集中范围为120～220℃。尽管如此,毒重石、钡解石的形成温度峰值比重晶石高出40℃,而石英的形成温度分布较均一,没有出现明显的峰值。矿物流体包裹体的盐度虽变化于1%～15%NaCl_(eqv),但毒重石、钡解石和石英样品中的盐度值普遍大于5%NaCl_(eqv),而重晶石中的盐度值小于5%NaCl_(eqv)的样品数量占有相当大的比例。由于重晶石的流体包裹体主要以NaCl-H_2O型包裹体为主,而毒重石中富含大量N_2-CO_2-H_2S-CH_4等复杂组分水溶液型包裹体,显示出毒重石与重晶石在成矿环境上有较大差异。由此笔者明确提出,水溶液热化学硫酸盐还原作用是形成毒重石矿床主要机制的新认识。

A discussion on the origin of witherite deposits in large-scale barium metallogenic belt, southern Qinling Mountains, China: Evidence from individual fluid inclusion

The barite deposits in which Ba presents as BaSO_4 are widespread in the world, whereas the witherite deposits in the presence of BaCO_3 are rarely reported. Numerous layered witherite and barite deposits occur within the Early Paleozoic silicate formations of southern Qinling Mountains. These two types of deposits, representing a large-scale particular metallogenic belt in the world, exhibit paragenetic and separated distribution characteristics. This paper focuses on the fluid inclusion features of witherite, barytocalcite, barite, and quartz in the barium metallogenic belt. The results indicate that the homogenization temperature of fluid inclusions of each mineral ranges from 90 to 310 ℃, mainly concentrating at the range of 120 - 220 ℃. For all that, the peak of forming temperature of witherite and barytocalcite is higher than that of barite about 40 ℃, and quartz forming temperature distributes more homogeneously, and does not appear peak. The salinity of fluid inclusions of most minerals ranges from 1 to 15 wt% NaCl_(eqv), but the salinity of fluid inclusions of witherite, barytocalcite, and quartz generally is more than 5 wt% NaCl_(eqv), while for a considerable proportion of barite, the salinity of fluid inclusions is less than 5 wt% NaCl_(eqv). The fluid inclusions of the barite mainly are of NaCl-H_2O type, whereas the fluid inclusions of witherite are rich in aqueous solution with complicated components like N_2-CO_2-H_2S-CH_4 etc., which shows that there is a large difference between the metallogenic environments of witherite and barite. Based on these evidences, we suggest that the thermochemical sulfate reduction (TSR) of the solution was the main formation mechanism of witherite deposits.