Abstract
The continental lithosphere consists of crust and subcontinental lithospheric mantle (SCLM), which is the non-convective portion of the uppermost mantle. In great part of the globe, both layers were formed simultaneously during the Archean and gradually evolved structurally and compositionally, according to the frequency and intensity of tectonic events. Therefore, the SCLM not only played an active role in the development of mineral systems that shape deposits in the continents, but also preserved the thermal, physical, compositional, chronological and metallogenetic clues required to discover them.
Lithosphere mapping approaches based on the multidisciplinary investigation of mantle fragments (i.e., xenoliths and xenocrysts), brought to surface by mantle-derived magmatism (e.g., kimberlites), can identify SCLM features such as geothermal gradients, rock types, metasomatic signatures, oxygen fugacity and ore-forming potential. The lateral and vertical recognition of these characteristics in lithospheric blocks with various modification degrees, allow the separation of individual domains reflecting different tectonothermal ages. These are usually recognised as: Archons (A), representing the best-preserved parts of the lithosphere (≥ 2.5 Ga); Protons (P), indicating reworked portions of Archean SCLM (2.5 – 1 Ga); Tectons (T), composed of very fertile and thin SCLM younger than 1 Ga; and A, P, T combinations.
A case study shows the products of lithosphere mapping from a geologically complex terrain in the southwestern margin of the São Francisco Craton in Brazil, which was affected by multiple tectonic events. Many mantle xenocrysts (>3000) in the area indicated an Archean SCLM that have been gradually modified and eroded, resulting in a cold (37.5-42.5 mW/m²) but thin (~140 km) lithosphere, mostly fertile and metasomatised. This work solved a “diamond enigma” about unknown primary sources of alluvial diamonds in the region, suggesting the gemstones as survivors of SCLM processes, sampled by numerous low-grade pipes/dykes. Additionally, the study provided evidence for several other mineral systems including gold, nickel, copper and PGE, based on mantle mineralogy and seismic tomography integration. This demonstrates how lithosphere mapping is a valuable tool for both exploration and prospectivity.
