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Abstract

Archaean cratons are fragments of old continents that are more richly endowed with mineral deposits than younger terrains. The mineral deposits of different cratons are also diversely enriched with useful (to humankind) chemical elements. Cratons are therefore mineral-diversity hotspots that represent regional geochemical heterogeneities in the early Earth, evidence for which remains encoded on each craton as unique metallogenic ‘fingerprints’. Some of the younger cratons (<3.0 Ga, e.g. Superior Province, Yilgarn and Zimbabwe) have strong Au, Cu, Pb and Zn imprints. Older (>3.0 Ga) cratons, however, are remarkably enriched in siderophile elements such as Ni, Cr, PGE, in both their crustal and mantle sections (e.g. Pilbara and Kaapvaal Cratons). Still other Archaean cratons are relatively enriched in Sn, W, U and Th (e.g. Amazonian, Leo-Man, Ntem and South China Cratons). How most of these fragments of old continents inherited their rich and diverse metallogenic characteristics is unresolved. Their dominant metallogenic inventories were formed near the time of their separation from the mantle; thereafter the inherited metals were frequently remobilized and redistributed during subsequent tectono-metamorphic, magmatic and erosion-deposition processes (e.g. tin in South America; platinum and gold in Southern Africa). Because different cratons are likely to represent only small remnants of once much larger and probably varied Archaean continents, part of the total metal inventories of Archaean continents must have been recycled back into the mantle. Using six selected element groups from our extensive in-house GIS database of Gondwana mineral deposits, we derive the metallogenic fingerprints of 11 Archaean cratons of the southern hemisphere, and compare these against metallogenic fingerprints of the same elements in younger crust of three continents (Africa, Australia and South America). We confirm that the mineral deposit density and diversity of Earth's continental lithosphere has decreased with time. We conclude that metallogenic elements were transferred more efficiently from the mantle to the continental lithosphere in the Archaean and/or that subsequently (<2.5 Ga) recycling of these elements (mineral deposits) back into the mantle became more effective.

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