Grain morphology of heavy minerals from marine and continental placer deposits, with special reference to Fe–Ti oxides

Abstract

The grain morphology of major heavy minerals from various placer-type mineral deposits (aeolian, beach, fluvial, alluvial, colluvial, residual–eluvial) in Mongolia, USA, Vietnam, Latvia, Germany, Spain, Cyprus, South Africa and Nepal was studied by means of SEM, conventional petrographic microscopy and visual inspection. Variation of grain morphology is first and foremost controlled by the physical and chemical processes operative in the environment of deposition and to a lesser extent by the climatic conditions. The sphericity of grains in the mineral assemblage strongly depends on the crystallographic system of the prevailing heavy minerals. It improves as minerals such as spinel and garnet, the most common representatives of the cubic crystallographic system, become more and more prominent among the heavy minerals in the placer material. Even if minerals belong to the same crystallographic system there may be significant differences as to their roundness. Analysis of the grain morphology of garnet shows an increase in angularity from the pyrope – through almandine-, grossularite- to the spessartite-enriched garnet solid solution series in the placers under consideration. Spessartite-enriched garnet solid solution series are widespread in pegmatites and low-grade metamorphic rocks where they normally form euhedral crystals. Such morphological differences in a solid solution series are inherited from the source rock and are not modified by sedimentary processes in proximal placer deposits. Thus isometric minerals such as spinel or garnet are not good marker minerals for depositional environments. Their strong points lie in the field of provenance rather than environment analysis as far as grain morphology is concerned. For a refinement of the morphology-based subdivision of depositional environments, particular groups of minerals have to be selected for the various types of placer deposits. There is no universal mineral ubiquitous to all placers whose morphological changes and surface textures are a direct indicator of a particular environment of deposition. Redox-sensitive minerals such as Fe–Ti compounds taking an intermediate position on the Mohs's hardness scale and being fairly resistant to weathering have proven to be a good tool for the distal placer types, spanning the full range from beach to fluvial placers, and including aeolian types. Ultrastable to stable mineral such as corundum sensu lato, kyanite or zircon are more appropriate for a morphology-based environmental analysis in proximal types, covering the full spectrum from fluvial to residual–eluvial and colluvial placer deposits.