Geochemical exploration for gold in deep weathered lateritised gossans in the Amazon region, Brazil: a case histry of the Igarape Bahia deposit

The Igarape Bahia gold mine is located in the Carajds region, Para State, northern Brazil. The area is a typical example ofsupergene gold mineralization related to gossans, where superimposed lateritization processes obliterated their upper horizon and gave rise to new and wider dispersion of gold and of other metals, with meaningful consequences for geological exploration in this environment. An exploration program in this area was carried out by Docegeo, a subsidiary of "Companhia Vale do Rio Doce - CVRD", which is the sponsor of the Carajds mining district and, currently, is also minigfor gold at Igarape Bahia.

The main mineralized zone at Igarape Bahia is a ferruginous breccia-like horizon, or mixed zone, constituted by gossans, lateritised gossans, and fragments of Fe-rich metavolcano-sedimen-tary wall rocks. Gold contents average 5 g/t in a 12-million-tons ore reserve. Shallow to deep red clayey latosols cover invariably the gossans/laterite profiles which bear lower gold contents.

The geochemical exploration in this area was performed in two main phases, each one using different methodologies of soil sampling, what led to different results: (a) during first phase the soil materials were sieved and the coarse fraction with fragments of the gold-bearing ferruginous zone were put aside and the obtained results for the gold were around the detection limit of the method employed and did not highlight any anomalous zone; (b) during second phase, the whole soil sample was used without previous sieving, the results were noticeably enhanced and the 0.1 ppm gold isocontent line outlined the two mineralized ore bodies. The greatest contents are strictly related to the iron crust outcrops (without soil cover), while the lower contents are related to the main body (buried by a latosol cover).

The results show that gold was partly remobilized during the later itisation of gossans, mainly within the upper part of the ferruginous zone, where latosols have been developed. A high dispersion of gold values and a weakening of the gold signal in this zone were reported. These soil covers act as a barrier to the discovery of new ore deposits. New geochemical exploration models for gold prospection and analogous geological surveys in lateritic terrains which evolved in tropical rain forest environments, as the present Amazon region, must be developed taking into account the real structuration of the supergene profiles, as well as the role of the different soil covers which overlie them.