Geological and thermal conditions before the major Palaeoproterozoic gold-mineralization event at Ashanti, Ghana, as inferred from improved thermal modelling

Abstract

Heat transfer processes before a major mineralizing event in the Paleoproterozoic continental crust of southern Ghana are the subject of a detailed regional thermal modelling study. The area of the Ashanti belt is the most mineralized for gold in West Africa, and it is believed that prior to the main gold mineralization event, crustal-scale thermal processes may have played a critical role in ore deposit formation. The thermal regime before and after the crustal shortening events that affected the region during the Eburnean orogeny (2130–1980 Ma) is calculated just before the main mineralization event which corresponds to late orogenic hydrothermal gold deposit formation. Measured thermal properties of lithological units are incorporated into the model, which is geometrically constrained by field studies. Computed pressure–temperature paths, compared to thermobarometric data, allow calibration of the model parameters. The temperature-dependence of thermal properties and the influence of compaction on vertically varying conductivities have been considered in the crustal-scale thermal modelling. The predictions from this model indicate that the most probable mantle heat flow value at this time in the region of the Ashanti belt is ∼30 mW m −2. Such a value is two to three times higher than present-day values below stable cratonic areas and could be considered as an upper limit in geodynamic models of the Paleoproterozoic mantle. This relatively high value might be related to the thermal input from a Paleoproterozoic mantle event such as a mantle plume, as previously suggested by several authors to explain metallogenic crises in West Africa. The P– T paths inferred from numerical modelling also allow discrimination between extreme geological scenarios. Indeed, the results of the modelling suggest that the basement is likely to be of continental rather than of oceanic type in the Ashanti area.