湘西北海相磷块岩的热液脱碳与铀多金属成矿作用

Hydrothermal decarburization and uranium-polymetallic ore-mineralization in marine phosphorite,northwestern Hunan

在扬子陆块东南缘湘西北武陵地区下寒武统牛蹄塘组海相黑色岩系下部鲕粒磷块岩中,首次发现纳米微米级硒铅矿。扫描电镜化学成分测定和微区解剖结果显示,海相沉积鲕粒状磷块岩的纳米微米级微裂隙和微孔隙中硒铅矿、闪锌矿、晶质铀矿、方铅矿、石英、磷灰石、叶蜡石、绢云母等矿物和残余碳共沉淀析出。研究认为在中高温热液脱碳过程中有机质经过热解、解聚、裂解形成CH4、H2S和残余碳,伴随的气还原作用是铀多金属和残余碳在微裂隙和微孔隙中共沉淀晶出的气还原机制。基于实体铀多金属矿床矿物空间定位关系的微观解剖,分析了湘西北海相磷块岩中铀矿物的气还原沉淀机制和地球化学机理,对重新认识热液型富铀矿床的成矿机制有重要意义。

The nano-miero clausthalite was discovered for the first time in oolitic phosphorite in the marine black rock series of the lower part of lower Cambrian Niutitang Formation,Wulin area, northwestern Hunan of the Southeast Yangtze Plate. By studying the chemical composition and the analysis of micro area through scanning electron microscope, it is recognized that the clausthalite, sphalerite, uraninite, galena, quartz, apatite, pyrophyllite, sericite and residual carbon were separated out by coprecipitation in the nanoscalemicron-sized micro pores and micro cracks of the marine deposited oolitic phosphorite. CH4, H2 S and residual carbon were produced by the pyrolysis and depolymerization of organic matter in the process of medium-high temperature hydrothermal decarburization; this process was accompanied by the gas reduction that could be reasonably interpreted as the cause of separate out of the uranium-polymetallic ores and residual carbon by coprecipitation in the micro pores and micro cracks. In this paper, the gas reduction precipitation mechanism and geochemical mechanism of uranium minerals in marine phosphorite are analyzed based on micro anatomy of spatial positioning of uranium polymetallic minerals. It is of great significance for restudying the metallogenic mechanism of hydrothermal uranium rich ore deposits.