Abstract
From a lithological, structural and metamorphic viewpoint, the Jacobina Basin in the Sa˜o Francisco province (Brazil) is representative of the foreland basins that mark a characteristic evolutionary stage in the development of the Palaeoproterozoic orogenic belts around 2000 Ma ago. In addition, it contains gold-bearing conglomerates of economic interest that are presently mined. The general structure of the basin is one of stacked detrital and schist formations along sinistral strikeslip/overthrusting shear zones, with exposed deep-seated units in the east and shallower and younger rocks in the west. New 40Ar/ 39Ar geochronological data give minimum ages of 1940-1910 Ma for the main thrusting and wrenching events. Shearing was accompanied by metamorphic recrystallization that indicates a high geothermal gradient and a progressive loading of the underthrust units. Despite this final major tectonic imprint, at least five stages of basin evolution are indicated by traces of tectonic instability, such as debris-flow deposits, chaotic breccias and early dyke swarms. In this context the conglomerate layers found in the basin formations correspond to periods of erosion of a preceding stage in the basin development. Multistage hydrothermal activity is shown by widespread alteration within mineralized conglomerates (sulphide, fuchsite, tourmaline, chromite, rutile), leaching of ultrabasic rocks, and phlogopite and/or emerald mineralization in contact metamorphic aureoles around late- to post-tectonic granites. This tectonic evolution is coherent with the development of the Jacobina Basin in a foreland position and its progressive involvement in the Transamazonian collision belt. Resedimentation processes, preservation of earlier stages of hydrothermal alteration in pebbles, extensive hydrothermal activity and a high geothermal gradient favour a model of epigenetic mineralization for the gold-bearing conglomerates with enrichment during the progressive propagation of the basin. Comparison with similar mineralized conglomerates shows that this model can be applied to other Precambrian foreland basins.
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