锡铁山矿床两类喷流沉积成因的铅锌矿体研究

锡铁山大型铅锌矿床发育有非层状和层状两种类型的铅锌矿体,通过对两类矿体产出地质条件与地球化学特征的研究,发现非层状矿体分布于代表喷口系统的网脉状蚀变带的上方附近,其中发育有大量的热水爆破角砾岩,而层状矿体分布于远离喷口的外侧,属于远端沉积的产物。两者具有相同的矿物组合和成矿元素组合,自非层状矿体→层状矿体,矿石品位下降,Zn/Pb比值增高,黄铁矿相对含量增加。揭示出非层状矿体不是层状矿体后期改造的产物,而是形成于喷流作用阶段。考虑到锡铁山矿床中规模巨大的喷口系统以及厚层状的喷流成因大理岩,推断锡铁山矿床及周边地区还有巨大的找矿潜力,找矿重点是层状铅锌矿床(体)。

Study of two types of ore bodies in Xitieshan lead-zinc SEDEX deposit, Qinghai Province

Located in the northern margin of Qaidam basin, Qinghai Province, the Xitieshan lead_zinc deposit contains about 50 million tons of ore with a grade of 8% Zn and 10% Pb. It was formed in a rift basin in Ordovician period. An exhalative_sedimentary system was found in the deposit. Ore bodies can be divided into two main types, i.e., unstratified and bedded ones. The unstratified ore bodies occur unconformably in thick marble formed during exhalation_sedimentation, and predominate near the altered stockwork zone that reflects the exhalation pipe. They possess more than 3/4 of the proven lead_zinc reserves, and consist predominantly of massive and coarse galena, sphalerite and pyrite, together with abundant hydrothermal eruptive and vent breccias. The bedded ore bodies occur near conformable banded marble far away from the stockwork zone. From unstratified to bedded ore bodies, the content of Pb_Zn gradually decreases, the grain size of sulfides becomes finer, and the Zn/Pb ratio gets higher. Geological and geochemical studies of the two types of ore bodies suggest that the unstratified ore bodies resulted from subsurface deposition, replacement, and zone refinement by upflowing hydrothermal fluids in the exhalation period, thus belonging to part of the SEDEX system. The bedded ore bodies were formed by dense brine sedimentation far away from the vent. The reserve ratio of the two types of ore bodies is obviously different from that of most SEDEX lead_zinc deposits in the world. It is pointed out that there exists a great vent facies, and hence this area might have remarkable bedded lead_zinc reserves which deserve further exploration.