Gossan mineralogy, textures, and gold enrichment over the Au (As, Bi, Ag) deposit in the Buracão Area (Brasília Fold Belt, Brazil): Implications for gold prospecting in weathering profiles

Abstract

The orogenic pyrite–rich Au (As, Bi, Ag) mineralization (up to 31 g/t) of the Buracão area is one of several orogenic deposits in the Brasilia Fold Belt (BFB; Central Brazil), exposed to tropical weathering that shows extensive oxidized profiles. The distribution and thickness of these gossan profiles lead to variable gold enrichments and uncertainties to gold prospecting in this region. In order to investigate physicochemical processes related to the evolution of the gossan cover and the supergene gold enrichment, we present mineralogical, textural and geochemical data from each zone of twenty gossan profiles in the Buracão area. These results show that the mineralogy and textures along the gossan profile reflect oxidation, hydrolysis, dissolution and precipitation processes controlled by different Eh–pH conditions, which are controlled by supergene conditions and hypogene ore composition. The geometry, distribution, and thickness of gossan profiles, as well as the style of gold occurrence, reflects textures of the hypogene orebodies, while the weathering has an important role in the gold chemical refining (i.e., increasing ca. 25% in gold fineness) and the distribution of gold grades along the profile.These points allowed us to identify that the leached spongy hematite–rich zone in the upper indigenous profile contains the higher fineness gold grades (i.e., the “bonanza level”), and Bi–minerals such as waylandite and bismutite that may be important pathfinders for high gold grades in these gossan profiles. Our results demonstrate that ore prospection in gossans over Au–As–Bi–Ag deposits should be done focusing on leached zones, different to that traditionally conducted in gossans related to base metal sulfide deposits that have focused on precipitation zones. The similarities between the hypogene mineralization of the Buracão area and many deposits in the BFB and worldwide encourage the application of this approach elsewhere, which improve gold prospecting in weathering profiles over orogenic deposits.