Mineralogical, textural and paragenetic studies of the leadzinccopper mineralization in the lower Benue Trough (Nigeria) and their genetic implications

Abstract

Leadzinccopper mineralization in the lower Benue Trough of Nigeria occurs as discordant fracture controlled veins within the Lower Cretaceous shales of the Asu River Group. Three stages of events are proposed for the evolution of the veins in this district: 1. (i) pre-ore fracturing and brecciation of Albian shales accompanied by the precipitation of framboidal and colloform aggregates of pyrite, siderite and quartz; 2. (ii) ore stage formation of sphalerite, galena, copper bearing minerals (e.g. chalcopyrite, bornite, chalcosite, wittichenite and digenite) tetrahedrite, bournonite and boulangerite; and 3. (iii) final deposition of octahedral galena, sphalerite, bravoite and marcasite in the hanging wall of the veins. Quartz precipitation appears to be at several stages although a large amount of quartz accompanied the ore stage sulphide precipitation. A mineralogical zoning from a prite-galena-sphalerite rich top, to chalcopyrite rich base is shown by the larger veins in the district. Post ore oxidation and lateritic weathering of primary vein constituents led to the supergene enrichments of limonite, cerrusite, smithsonite, malachite and azurite which are sometimes preserved with the primary minerals. These mineralogical associations, compositions of the ore and gangue minerals, and the previusly reported temperatures of vein minerals are compatible with those of ore deposits formed from basinal brines. The mineralization is considered to be epigenetic and vein formation appears to have accompanied the Cenomanian tectonic event in the Benue basin.