广西东平锰矿赋矿地层归属及其时代探讨

THE AGE AND NATURE OF THE STRATA HOSTING THE DONGPING MANGANESE ORE DEPOSIT IN GUANGXI

广西东平锰矿是一个大型红土型锰矿床,分为原生锰矿含矿层和次生锰矿含矿层。由于历史原因,绝大部分学者将原生锰矿含矿层的地层归属浅水台地相的北泗组。区调中,根据矿区实测剖面的岩石组合、沉积相特征及古生物特征与标准剖面的对比,将赋矿地层归属于石炮组,并新建东平层,代表含锰地层。在赋矿地层下部的泥岩层中采获Claraiasp.等早三叠世双壳类及菊石化石,LA-ICP-MS法测得赋矿地层顶部火山碎屑岩中锆石U-Pb年龄为251.5±2.5Ma,在火山碎屑岩层之上采获中三叠世双壳类和菊石,确定锰矿赋矿地层的时代为早三叠世中晚期。

The Dongping manganese ore deposit in Guangxi is a large--scale lateritic type Mn ore deposit. The ore deposit consist consists of primary and secondary manganese ore-bearing layers. However, the na- ture and age of the strata hosting the primary manganese ore-bearing layer are uncertain. Traditionally, the hosting strata were regarded as part of the Beisi Formation deposited in shallow-water platform facies.Based on field survey, sedimentary facies analysis, and biostratigraphic data, we redefine the ore-bearing strata as the Dongping unit （T1^d） which belongs to the Shipao Formation. Early Triassic bivalves （e. g Claraia sp. ） and ammonoids were obtained from mudstone layers in the lower part of the ore-bearing stra- ta. A LA-ICP-MS zircon U-Pb age of 251.5±2.5 Ma was obtained from pyroclastic rocks in the upper part of the ore-bearing strata. Together with Middle Triassic bivalves and ammonoids from the pyroclastic rocks, the new data suggest that the manganese ore-bearing strata are middle-late Early Triassic in age.