Gold, wolframite, tourmaline-bearing lateritized gossans in the Amazon region, Brazil

Abstract

Several important supergene Au occurrences have recently been discovered in the Amazon region, representing a great impulse for mineral exploration in north Brazil (Costa, 1993). This follows what has already happened in the 1980's in lateritic environments of West Africa and Australia. These occurrences present ubiquitously the following important characteristics: (1) primary related sulfide mineralization associated with quartz veins that cut Proterozoic volcano-sedimentary sequences; (2) an upper oxidation zone (gossanous) altered by at least two main lateritization events — lateritized gossans and latosols; and (3) wolframite and tourmaline occurrences as resistate minerals (in addition to elevated As, Cu and Sn values) in the topmost horizons. The general structure of the upper part of these profiles comprises an upper latosol cover, a brecciated lateritic iron crust and ferruginous gossan. In the Águas Claras area, several studies have been conducted in order to characterize the Au distribution in the different supergene horizons as well as their mineralogical and geochemical associations. Gold mineralization is closely associated with the Fe oxy-hydroxides in the gossans, with a great range of composition in the different parts of the profile. SEM-EDS analysis of isolated grains from the gossans reveals highly heterogeneous Au compositions, especially in Ag contents, representing the presence of both primary grains and supergene/lateritic gold (higher fineness). Present understanding indicates that lateritization processes that took place in the Amazon region since early Tertiary times was superimposed on pre-existing gossans and gave rise to breccia-like Fe crusts, with a further dispersion of Au and other elements. The final products of lateritization are the widely distributed yellow, loose-clay latosols that invariably cover and mask these profiles and weaken Au signals. The ferruginous horizons (gossans, lateritic crusts, stone lines, etc.) are thus the better sampling media for Au geochemical exploration. The occurrence of coarse-grained tourmaline and wolframite can be an important auxiliary tool in the exploration, acting as true pathfinder minerals and also in distinguishing different Fe crusts and their probable primary parent rocks.