Early weathering of palladium gold under lateritic conditions, Maquine Mine, Minas Gerais: Brazil

Since the 80's, studies have shown that Au is mobile in supergene lateritic surficial conditions. They are based either on petrological, thermodynamic studies, or experimental works. In contrast, few studies have been done on the mobility of the Pt group elements (PGE). Moreover, at the present time , no study has addressed the differential mobility of Au, Ag and Pd from na tural alloys in the supergene environment. The aim of this study is to unde rstand the supergene behavior, in lateritic conditions, of Au-Ag-Pd alloys of the Au ore locally called Jacutinga at the Maquine Mine, Iron Quadrangle , Minas Gerais state, Brazil. The field work shows that the host rock is a "Lake Superior type" banded ir on formation (BIF) and that the Au mineralization originates from sulfide-b arren hydrothermal processes. Primary Ag-Pd-bearing Au has developed as xen omorphous particles between hematite and quartz grains. The petrological st udy indicates that the most weathered primary Au particles with rounded sha pes and pitted surfaces were found, under the duricrust, within the upper f riable saprolite. This layer, however is not the most weathered part of the lateritic mantle, but it is where the quartz dissolution resulting porosit y is the most developed. The distribution of Au contents in the weathered r ocks are controlled by the initial hydrothermal primary pattern. No physica l dispersion has been found. Most of the particles are residual and very we al;ly weathered. This characterizes early stages of Au particle weathering in agreement with the relatively low weathering gradient of the host itabir itic formations that leads essentially to the development of isostructural saprolite lateritic mantle. Limited dissolution of primary Au particles iss ued from the friable saprolite induces Pd-Ag depleted rims compared to prim ary Au particle Pd-Ag contents. In addition, limited very short distance in situ dissolution / reprecipitation processes have been found at depth with in the primary mineralization, as illustrated by tiny supergene, almost pur e, Au particles. The supergene mobility order Pd > Ag > Au as reflecting ea rly weathering stages of Au-Ag-Pd alloys under lateritic conditions is prop osed. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.