A INFLUÊNCIA DAS CONDIÇÕES GEOQUÍMICAS NA OXIDAÇÃO DA ARSENOPIRITA E NA MOBILIDADE DO ARSÊNIO EM AMBIENTES SUPERFICIAIS TROPICAIS

Revista Brasileira de Geociências

Ricardo Perobelli Borba & Bernardino Ribeiro Figueiredo

34(4):489-500, dezembro de 2004

A INFLUÊNCIA DAS CONDIÇÕES GEOQUÍMICAS NA OXIDAÇÃO DA
ARSENOPIRITA E NA MOBILIDADE DO ARSÊNIO EM AMBIENTES
SUPERFICIAIS TROPICAIS
RICARDO PEROBELLI BORBA1 & BERNARDINO RIBEIRO FIGUEIREDO2
Abstract THE INFLUENCE OF GEOCHEMICAL CONDITIONS ON ARSENOPYRITE OXIDATION AND ON THE MOBILITY
OF ARSENIC IN TROPICAL SURFACE ENVIRONMENTS Geochemical processes that govern weathering conditions of auriferous deposits in the Iron Quadrangle, Minas Gerais, Brazil were studied through X-Ray Diffraction analysis, Scanning Electronic
Microscopy, and chemical analysis of groundwater collected in mines. Natural oxidation of arsenopyrite in quartz-arsenopyrite veins
(without carbonates) promotes the formation of scorodite, while the oxidation of arsenopyrite in veins of quartz-carbonates-arsenopyrite leads to the formation of low-crystalline iron arsenates which contains Ca, and in smaller amounts, the formation of scorodite, kolfanite, yukonite, and farmacossiderite. The release of As to surface and groundwaters occurs in steps: (i) during the oxidation of arsenopyrite a fraction of As is solubilized and another fraction participates in the formation of iron arsenate or scorodite, (ii) during total or incongruent dissolution of secondary As minerals there is retention of a fraction of arsenic adsorbed in iron oxide and another fraction is solubilized. The presence of carbonates in sulfurous mineral deposits neutralizes acid drainage formed from the oxidation of sulfides. However, in the case of As, neutral to alkaline pH induces its release into solution by dissolution of iron arsenates and scorodite, increasing As mobility in a surface environment.
Keywords: arsenopyrite, oxidation, scorodite, kolfanite, yukonite, farmacossiderite arsenic, mobility, gold, Iron Quadrangle
Resumo
Os processos geoquímicos que regem o intemperismo de depósitos