Metallogenic fingerprints of Archaean cratons

Abstract

z a ) 'The duty of the geologist and the prospector is in fact to deliver the goods' 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 metallogenlc 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.

Secular changes recorded in mineralization of African crust

“…These results are consistent with the results of de Wit and Thiart (2005), Thiart and de Wit (2006) on a near-global scale (Gondwana Continents) for Archean cratons (>2.5 Ga) compared to younger crust (<2.5 Ga). de Wit and Thiart (2005) show that each craton appears to have a unique metallogenic fingerprint and that a significant mineral diversity exists in the crust of different cratons of similar ages, and that Archean crust has a greater mineral diversity than that of younger crust.…”

“…In order to compare the density of mineralization of different areas we use a measure of spatial association known as the spatial coefficient (Mihalasky and Bonham-Carter, 2001;de Wit and Thiart, 2005;Thiart and de Wit, 2006). The spatial coefficient (q ij ) is based on the weight of evidence approach (Bonham-Carter, 1994) and it is normalized with regards to the area of the shields/craton thus making it a comparable measure…”

“…Such evolutionary changes in mineral deposits from Archean to Recent have been suggested to provide guides to the evolution of Earth's tectonic evolution and related processes (e.g. Hutchinson, 1981;Windley, 1993Windley, , 1995Barley et al, 1998;Condie, 1998Condie, , 2000Kerrich et al, 2000;Goldfarb et al, 2001;Groves et al, 2005;de Wit and Thiart, 2005;Thiart and de Wit, 2006). These changes are most likely in response to changes in the conditions of the earth's crust, mantle, hydrosphere, atmosphere, or biosphere under which these deposits were formed.…”

Temporal relations between mineral deposits and global tectonic cycles

“…Some 25% of gold resources occur in Archean deposits (Goldfarb et al 2001), largely in the range 2.7-2.5 Ga, yet Archean crust constitutes less than 6% of the current continental crustal volume ( Fig. 1; Goodwin 1996). De Wit & Thiart (2005 highlight secular variation in metallogenic potential with Archean cratons more richly endowed in some types of mineral deposits than younger terrains, reflecting more efficient transfer of metallogenic elements from the mantle to the continental lithosphere.…”

The Hoggar Gold and Rare Metals Metallogenic Province of the Pan-African Tuareg Shield (Central Sahara, South Algeria): An Early Cambrian Echo of the Late Ediacaran Murzukian Event?

“…More than 100 rare-metal and 400 gold occurrences have been identified. Nevertheless, the total amount of gold known in the Hoggar part of the Tuareg shield (totalling between 120 and ≥200 t Au, according to various sources) is very low, compared to the endowment of other Precambrian shields worldwide (e.g., de Wit and Thiart 2005). In the same way, the ca.…”