上扬子地块早震旦世大塘坡期锰矿成因和沉积学

晋宁运动以后的上扬子地块,在早震旦世为陆内拉张阶段。其西缘有火山喷发,而其东南边缘为拉张裂谷带,沿黔东、湘西形成北东向的地堑盆地、沉积了黑色炭质页岩、锰矿和硅质岩。锰矿层由枕状富锰矿体和粒序层状、层纹状锰矿体组成,具有皮壳和胶状结构,并由菌藻团粒、团块、硅质放射虫以及粘土等悬浮沉积物组成层纹和显微粒序。其结构和构造特征显示锰质由海底热水补给,经化学、生物化学作用,沉积在低能深水还原环境中。

The Sedimentollgy and Origin of Early Sinian Manganese Deposits form the Datangpo Formation,South China

Early Sinian black manganese carbonate deposits of the Datangpo formation are distributed in eastern Guizhou and western Hunan Provincce. The formation comprises a belt which extending for 200 km along depositional strike and 40 km perpendicular to the strike. The manganese precipitation is located in an extension in the passive continental margin of the Yangtze plate. The Datanpo formation is interbedded between two sequences that have been interpreted sa glacial diamictites. The formation comprises manganese ore and Mn- rich shale which show fine. graded laminations, with A-B-C, A-B-E Bourna sequences. The Mn-rich shale contains algae and fungi and acritarchs as reported previously; but the radiolaria have been found for the first time, and the radiolaria from Early Sinian has not been reported in the world. These features suggest that the deposition in a relatively deep water marine setting under conditions of anoxia which could have been caused by melting and a sea-level rise. The manganese ore bodies comprise lens and pillow-shaped structure which show a structure of successive concentric layer, and are cut by many fine baritite veinlets. Particularly, veinlets are concentrated in individual layer, and usually terminated at layer boundaries. This indicates that veins filled the fissures of shrinkage which was produced during the manganese ore cooling and separating phases. It is suggested that the manganese originates from hydrothermal exhalation and extrusion at the sea floor. It is precipitated as the successive concentric layers around a nucleus, thus building- up a pillow-shaped body. The δ34S signature (average 48. 3‰) of the manganese ore is typical of a hydrothermal origin. We suggest that glacial melting at the begining of Dagangpo formation brought about a major sea-level rise and the resulting transgression established a relatively deep marine environment. Hydrothemal exhalattion at the sea bottom could have resulted in a divsion of the sea water column with anoxic bottom water conditions and Precipitated the syngenetic manganese deposits.