西成地区碳酸盐岩REE特征及厂坝矿床白云岩成因

REE characteristics of carbonate rocks in Xicheng Devonian basin and origin of dolomite in Changba giant lead-zinc deposit

甘肃西成地区喷流沉积(SEDEX)铅锌矿床中广泛发育有碳酸盐岩,大量的稀土元素分析发现这些碳酸盐岩与铅锌矿石显示出不同的稀土元素地球化学特征。研究表明,铅锌矿层下部的生物碎屑灰岩和礁灰岩、厚层块状灰岩具有较明显的负Ce异常和负Eu异常,是正常海相沉积物,但在层状铅锌矿体附近的一些碳酸盐岩具有正的Eu异常,表明有海底喷流热水的参与,海水与喷流热水共同影响了碳酸盐岩的沉积,随着矿化程度的增高,岩(矿)石中δEu〔Eu×2/(Sm+Gd)N〕值也随之增大。厂坝_李家沟矿床矿体附近的厚层白云岩(石)是喷流热水与海水相互作用的产物,且受喷流热水的影响较大,多分布于喷口附近。

In the past two decades, many SEDEX lead-zinc deposits were discovered in Xihe-Chenxian Devonian basin, West Qinling Hercynian orogenic belt of Gansu Province. They cover over 100 million ton lead-zinc ore reserves [ w（Pb ＋ Zn）〉8% ]. The bedded lead-zinc ore bodies hosted along the boundary between the upper turbidite and lower carbonate rocks that often include some reef limestone. In some deposits, abundant dolomite and dolomite rocks occur near the bedded lead-zinc ore bodies, which make up an indicator for exploration. Nevertheless, their origin remains unclear. In this paper, the REE characteristics of the carbonates near or distant from the crater have been studied. It is suggested that thick limestone or marble and reef limestone in Middle Devonian below the bedded lead-zinc ore bodies are normal sea-floor sediments and have negative Ce （Ce/Ce^＊ ） and Eu （Eu/Eu^＊ ） anomalies. However, some of the carbonate rocks near the ore bodies and mineralized rocks were formed along the interface between the sea water and the hydrothermal fluids and have positive Eu anomalies. The Eu value of the rock increases with the rising sulfide content. The thick-bedded dolomite rock in the Changba-Lijiagou lead-zinc deposit resulted from the reaction between the hydrothermal fluids and the sea water, with the hydrothermal fluids playing the major role. It occurs near the exhalative center, being a product of exhalative accumulation similar to things of silica rock and albite rock.