Microstructure and crystallographic preferred orientation (CPO) of hematite ores from the Sishen Mine, Northern Cape Province, South Africa

摘要

Banded iron formations (BIFs) are described as banded chemical sedimentary rocks composed of major amounts of magnetite, hematite and quartz, with variable amounts of iron bearing minerals like carbonates (e.g. siderite, ankerite) and silicates (e.g. greenalite, grunerite). The BIFs are transformed to high grade iron ores by removal of quartz and transformation of almost all iron bearing minerals to hematite. On macroscopic and microscopic scale the high grade hematite ores generally reveal a more or less visible lamination caused by alternating layers of different grain sizes. In addition to the microstructure (grain size, grain shape etc.), the crystallographic preferred orientation is a powerful tool to characterize rocks and ores. The overall crystallographic preferred orientation (CPO, texture) of volumes can be determined by neutron diffraction and the local texture on small areas of polished sections by electron backscattering diffraction (EBSD). The CPO of volumes of all hematite ore types (including a conglomerate pebble) of the Sishen Mine is characterized by a weak, usually circular c-axis maximum aligned to the normal of the lamination of the ores, and seems to be typical for this essentially unmetamorphosed and undeformed ores. The crystallographic orientation of grains of the hematite ores determined locally by EBSD shows in general the same preferred orientation. But, it is remarkable that partially the CPO is quite different and inherited from grain orientations of preceding primary minerals (e.g. magnetite, goethite, siderite/calcite, ankerite/dolomite) of the Banded Iron Formation, without any microscopic visible indication.