扬子地块北缘大型钡成矿带中硫同位素组成及其意义

Sulfur Isotopic Compositions of the Large Barium Metallogenic Belt in the Northern Yangtze Block and its Significance

钡以BaSO4形式存在的重晶石矿床在世界上分布广泛，但以BaCO3形式存在的毒重石矿床极为罕见。在扬子地块北缘大巴山一带的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。本文研究了钡成矿带中的硫同位素组成特点，结果显示，在以毒重石矿石为主的成矿亚带中，重晶石样品的δ^34S值相对较低，变化范围较小（22．1‰～37．0‰），平均27．3‰（n=11），基本上与寒武纪海水硫酸盐的δ^34S值（δ^34S=27～32‰）接近或略低。反映了形成重晶石的硫来自海水，并有可能经历了热化学的硫酸盐还原作用；与毒重石共生的重晶石形成与热液流体有密切关系。在以重晶石为主的成矿亚带中，重晶石样品的δ^34S值相对较高，变化范围较宽（33．4‰～57．6‰），平均43．5‰（n=31）。与寒武纪海水硫酸盐δ^34S值相比发生了大的改变，反映了与同期海水硫酸盐相关的重硫的强烈富集。这种高正值特点超过了地史中海水硫酸盐的最高值，说明重晶石的形成与生物作用密切相关。强烈的重硫富集，归因于硫酸盐还原菌的活动。

The barite deposits in which Ba presents as BaSO4 are widespread in the world, whereas the witherite deposits in the presence of BaCO3 are rarely reported. Numerous layered witherite and barite deposits occur within the Early Paleozoic silicate formations of the Dabashan mountain, locating in the northern margin of the Yangtze Block. 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 sulfur isotopic features of this barium metallogenic belt. The results indicate that, the δ^34S values range from 22.1‰ to 37.0 ‰ （n= 11） with an average of 27. 3‰, for the barite samples from the predominantly witherite-bearing metallogenic subzone. The relatively low and limited δ^34S values are close to or slightly lower than those of the Cambrian seawater sulfates （δ^34S= 27- 32‰）, suggesting that the sulfur in the barites was derived from seawater sulfur and possibly experienced sulfate reduction caused by thermochemistry. The formation of the barites associated with the witherites is closely related to hydrothermal fluid. In contrast, the δ^34S values of the barites from the predominantly barite-bearing metallogenic subzone, range from 33.4 ‰ to 57.6 ‰ （n=31） with an average of 43. 5‰. These values are relatively higher and variable than those of the coeval Cambrian seawater sulfates, implying significant concentration of heavy sulfur related to the seawater sulfates. The positive values greater than the highest values of the seawater sulfates in the geological history, indicate that the formation of the barites was closely controlled by biogenetic agency. The obvious concentration of heavy sulfur could be due to the activity of reduced germs.