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Placer Analysis Limited, 2 London Street, Edinburgh EH3 6NA, UK
The objective of this paper is to show that the global distribution of placer deposits is largely a product of variation (both at present and in the recent geological past) in geomorphological processes acting at the earth's surface, given suitable primary mineral sources. The distribution of known placers of diamonds and gold as well as certain littoral placers is considered in relation to the occurrence and tectonic setting of the primary sources of the various minerals. Climatic controls of the many variables that are relevant to placer formation (e.g. weathering, rate of erosion, nature of sediment supply, opportunities for sediment reworking) are discussed in terms of broad morphogenetic regions. Five such regions are identified on the basis of distinctive combinations of the geomorphological processes responsible for placer formation. Global changes in climate and accompanying changes in the surface processes responsible for placer formation during both the Tertiary and, more particularly, the Quaternary have been important in producing the alternation between erosional and depositional conditions that is especially suitable to placer formation. Primary source tectonic setting and morphogenetic processes all interact to allow the identification of regions in which placer formation is favoured.
