南非Postmasburg锰矿田白云岩的控矿作用

Dolostone Controls in Postmasburg Manganese Field,South Africa

Bishop、Paling两个矿区位于南非Postmasburg锰矿田的西矿带上,这一地区发育了典型的沉积—变质型矿床。在岩性序列中识别出了6个岩性单元,自下而上分别为:白云岩单元、铁锰矿体单元、铁质细砂岩/铁质砾岩单元、泥质粉砂岩单元、石英岩单元和页岩单元,其中白云岩单元构成了矿床发育的基底。白云岩有两种类型,分别为富锰白云岩及上覆的藻白云岩,它们是岩溶系统形成的基础。岩溶系统中的大量溶坑是沉积序列的主要赋存空间。原生矿体的形态、规模和发育位置均受溶坑的控制,侧向延伸不连续。锰土是富锰白云岩的风化残余物,被矿体底部的富锰矿石不整合覆盖。侧部的富铁矿石则直接覆盖在藻白云岩之上。堆积型矿体多分布于白云岩斜坡的边缘。将矿田的发育过程划分为三个时期,其中第一时期形成的N—S向溶蚀洼地为铁、锰的赋存奠定了基础;第三时期岩溶作用再次发生,原生矿体侧部坍塌,矿石在斜坡上富集形成堆积型矿体。

Bishop and Paling mining areas are located in the western belt of Postmasburg Manganese Field,South Africa,in which typical metamorphosed sedimentary deposits developed. Six lithology units are identified for the lithology sequence,they are dolomite unit,iron-manganese mineralization unit,ferruginous fine sandstone / ferruginous conglomerate unit,pelitic siltstone unit,quartzite unit and shale unit from bottom to top,respectively,among which dolomite unit constitutes the base for ore body. There are two types of dolostone. They are Mn-rich dolostone and algal dolostone overlying,and both of them contribute to the karst system. Mass of sinkholes in the karst system provide the main space for the occurrence of depositional sequence. The morphology,size and locality of the original ore are restricted by the sinkholes,resulting in discontinuous lateral continuity. The wad is the weathering residue of the Mnrich dolomite,and it is overlied unconformably by Mn-rich ores at bottom of the orebody,while the lateral part of the orebody is underlied directly by algal dolomite. Cummulate orebody excessively distributes at the edge of the dolomite slope. The whole development of the manganese field can be divided into three stages. The corroded depression in NS direction formed during the first stage lays a foundation for the iron and manganese sediment. During the third stage,karstification happens again,resulting in side collapse of original ore,which accumulated on the slope,and finally the cumulate orebodies were formed.