Spatial and temporal distribution of ore deposits, in relation with thermal anomalies
Résumé
Ore deposits have formed over billion years of Earth's history with a discontinuous distribution in time and space.In time, their formation is episodical, and the data concerning the main deposit types show an alternation of periods with mineralisation followed by time gaps, with no large amount of ore generation. The "Snowball Earth" theory suggests that Banded Iron formations (BIF) are in relation with periods of intense global glaciations of Paleoproterozoic (2500-1600 Ma) and Neoproterozoic (1000-540 Ma) ages. During global glaciations, surface temperatures were of the order of-50 deg C and an ice layer of about 1km covered almost the entire Earth. We have performed analytical and numerical modelling of such events that shows the development of high thermal anomalies in the crust. These anomalies can reach several tens of degrees for a time lapse that depends on the duration of the glaciation. If the thermal impact is clear, the mechanical impact of the glaciations on the crustal rocks still needs to be evaluated to precisely assess the role of glaciations as a possible cause to some of the gaps in the temporal distribution of ore deposits.Ore deposits have also a heterogeneous spatial distribution: they are located in specific places of the Earth's crust, where thermo-mechanical and hydrothermal conditions have triggered their formation. The temperature pattern corresponding to these cases can prove to be particularly favourable to ore deposits. A good example of such deposits, is the Ashanti belt in Ghana of which we present a detailed study. It is the key district of gold mineralisation in the Paleoproterozoic terrane of West Africa. This is the second giant concentration of gold deposits after South Africa with a potential of about 2500 tons of gold. The Eburnean orogeny operated between 2.13 and 1.98 Ga. Two tectonic phases affected the area, a period of thrusting and a second one corresponding to tran-scurrent tectonism. A numerical modelling was performed to constrain the thermal evolution of the belt. Conditions of pressure and temperature given by metamorphism data, indicate that a temperature higher than 600 deg C is required at depth close to 18 km to produce the expected partial melting. According to regional lithology, crustal structure and thermal properties, such conditions can only be achieved when the basal heat flow is greater than 60mW/m-2. Lateral variations between rock units have to be considered to explain anomalous heat transfer of this area.