ORALS

The Gentio do Ouro Golden District is located on the western edge of the Chapada Diamantina, the central region of the State of Bahia, Brazil. The principal ore-related geological caracteristics of the study are include the presence of Proterozoic gabbroic sills in the lower portion of the Espinhaço Supergroup [1]. The mafic sills were subsequently affected by hydrothermal (metasomatic) alteration that is associated with the emplacement of a first phase of gold mineralization along the contacts between the sills and the wall rocks [2]. Quaternary surface weathering [3] led to the development of thick zones of supergene alteration predominantly developed along topographic highs and which include saprolit, latossol, duricrust and pisolite. Protolithe composition was established geochemically and more particularly through the sydtemstic use of the Fe2O3 (T)/MgO ratio that is considered to be representative of the initial composition of the rocks. The formation of supergene-altered layers through weathering under lateritic conditions, evolved from the parental rocks towards saprolit-latossol-duricrust-pisolite, predominantly in response to partial dessilicification and coprecipitation of trace elements with iron oxides/hydroxides. The dissolution of primary gold occurred in the presence of chemical complexes such as sulphates and sulphites, were produced by the oxidation hydrothermal auriferous sulphides in the presence of carbonates [4]. Mobilized gold was transported under oxidizing conditions as colloidal gold and subsequently precipitated in an array of ferraneous fractures, or filling micro-cavities [5]. The lateritic weathering processes responsible for the mobilization of gold and its secondary precipitation are responsible for its local enrichment, an increase of purity (all other associated trace metals being separated from gold), and, ultimately in the development of nuggets in lateritic materials. Samples of the parent rock and its lateritic products were search for geochemistry with the purpose to study the mobilization and concentration of major, trace and rare earth elements on profile of chemical weathering of gabbroic rocks. The geochemical analysis for major and trace elements was measured by atomic absorption spectrophotometry and ICP – inductively coupled plasma. The results for lanthanides was normalized to chondrites [6].

References:
[1] F.F.M. Almeida. Rev Bras Geociências 7(4) (1977) 349-364.
[2] F.A. Souza. UFBA (1992) 140P.
[3] I.G. Carvalho. UFBA (1985) 59p.
[4] I.G. Carvalho, S.S.P. Mestrinho, V.M.S. Fontes, O.P. Goel, F.A. Souza. Jour. Geochem. Exploration 40 (1991) 385-411.
[5] F.A. Souza, N.V.L. Souza, J.S.I. Santos. Gacmac-Quebec (2019) 175.
[6] N.M. Evensen, P.J. Hamilton, R.K. O’Nions. Geochim Cosmichim Acta 42 (1978) 4792-4805.