Some unusual geochemical features of the oxidized zone at the central sector of the Singbhum copper belt, India

Abstract

Incipient removal of Cu even from the caprocks, in situ transformation of sulphides to oxidized compounds, fixation of substantial Cu by supergene 'clay' minerals, virtual absence of secondary sulphide enrichment, and non-congruence between present water table and the 'bottom' of oxidized zone are the unusual features observed in the oxidized profile at Chapri mining-block within the Singhbhum copper belt. Petromineralogical studies of ore and host-rock samples from drill cores, and in situ measurement of Eh-pH values of mine and surface water furnish an explanation for the observed features. Occurrence of low-pyrite orebodies as sheeted veins and disseminations accounts for localized, inadequate generation of strong acids that was readily consumed by hydrolysis reactions in host rock. This inhibited any significant migration of Cu, outwards or downwards, by: (a) precipitating oxidized compounds; and (b) fixation of Cu in supergene, neoformed clay minerals. Vertical zoning of copper minerals in the oxidized profile is consistent with stability diagrams and with measured Eh-pH values. The 'bottom' of the oxidized zone presumably represents a palaeo-water table, tilted in sub-recent time. Preliminary leaching experiments indicate feasibility of exploitation of Cu left out in the oxidized zone.