FORELAND BASINS AND GOLD-BEARING CONGLOMERATES - A NEW MODEL FOR THE JACOBINA BASIN (SAO-FRANCISCO PROVINCE, BRAZIL)

From a lithological, structural and metamorphic viewpoint, the Jacobin a Basin in the Sao Francisco province (Brazil) is representative of th e foreland basins that mark a characteristic evolutionary stage in the development of the Palaeoproterozoic orogenic belts around 2000 Ma ag o. In addition, it contains gold-bearing conglomerates of economic int erest that are presently mined. The general structure of the basin is one of stacked detrital and schist formations along sinistral strike-s lip/overthrusting shear zones, with exposed deep-seated units in the e ast and shallower and younger rocks in the west. New Ar-40/Ar-39 geoch ronological data give minimum ages of 1940-1910 Ma for the main thrust ing and wrenching events. Shearing was accompanied by metamorphic recr ystallization that indicates a high geothermal gradient and a progress ive loading of the underthrust units. Despite this final major tectoni c imprint, at least five stages of basin evolution are indicated by tr aces of tectonic instability, such as debris-flow deposits, chaotic br eccias and early dyke swarms. In this context the conglomerate layers found in the basin formations correspond to periods of erosion of a pr eceding stage in the basin development. Multistage hydrothermal activi ty is shown by widespread alteration within mineralized conglomerates (sulphide, fuchsite, tourmaline, chromite, rutile), leaching of ultrab asic rocks, and phlogopite and/or emerald mineralization in contact me tamorphic aureoles around late-to post-tectonic granites. This tectoni c evolution is coherent with the development of the Jacobina Basin in a foreland position and its progressive involvement in the Transamazon ian 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 m ineralization for the gold-bearing conglomerates with enrichment durin g the progressive propagation of the basin. Comparison with similar mi neralized conglomerates shows that this model can be applied to other Precambrian foreland basins. (C) 1997 Elsevier Science B.V.