The geochemical association Au-As-B-(Cu)-Sn-W in latosol, colluvium, lateritic iron crust and gossan in Carajas, Brazil: importance for primary ore identification

Carajas, in the state of Para, eastern Amazon, is the most important minera l province in Brazil. Its high plateau landscape contrasts with the lowland domain of the Amazon region. It is developed on old lateritic formations a nd covered by dense tropical rain forest under a hot and humid tropical cli mate. The bedrock is made up of high-grade metamorphic terrain of Archean a ge, greenstone belts and post-tectonic granites. The laterite profiles are complex, partly transformed into yellow to red latosols on the plateau and colluvium on the slopes. The lateritic terrain hosts the gold rich gossans of the Igarape Bahia and Aguas Claras gold mines. Gold is found in red lato sols, iron crust, gossans and colluvium. In order to investigate the capaci ty of these materials as geological sampling media and as indicators of pri mary ore, a multi-element geochemical survey was carried out at Aguas Clara s. Seventy-four samples were analysed for 27 elements by AAS (Au, Cr, Cu, M n, Ni, Pb and Zn), XRF (Cl, W), OES (B, Be, Ga, Mo, Sc, Sn, Sr, V, Y, Zr an d Bi), AA-HG (As, Hg, Se and Sb), ICP-AES (Te), and the mineralogy identifi ed by XRD. The supergene materials are mainly kaolinite, quartz, hematite, maghemite, gibbsite and some goethite. Dravite, muscovite, wolframite and c hromite are accessory minerals. In the latosols, As, An, B, Cr, Cu, Ga, Mo, ph, Se, Sn, V, W and Zr are concentrated by a factor greater than three ti mes compared to crustal abundance levels, whist gossan and colluviums conta in more than ten times the crustal average levels of As, Au, B, Mo, Sn and W. Multivariate R-mode analyses distinguish several associations according to the nature of the supergene materials. The As-Cu-Mn-Pb-B-Au-W-F cluster corresponds to primary signatures still preserved in latosols, iron crust a nd colluvium; and the Cr-V-Ga-Mo-Ni-Zn-Se-Sc-Hg-Sn-Cl-Y-Zr cluster reflects the general laterite evolution. The latosols preserve the geochemical sign ature and signal of underlying primary ore, and, together with the colluviu m, they can be used as good sampling media. (C) 1999 Elsevier Science B.V. All rights reserved.