A hydrocarbon replacement model for the Zambian Copperbelt deposits

Abstract

A systematic relationship between the distribution grossly stratiform Cu-deposits, basin architecture, and facies associations in the Zambian Copperbelt (ZCB) suggests the deposits formed at sites of former significant accumulations of both mobile and/or in situhydrocarbons.Deposits occur within a 200 m stratigraphic window containing the first laterally-extensive, fine-grained, originally organic-rich strata within the host Katangan Supergroup. Arenite-hosted deposits occur within, or immediately overlie confined or peripheral portions of sub-basins formed during the initial stages of sedimentation and are overlain by relatively impermeable strata; sites with classical hydrocarbon trap geometries. Direct evidence for the former presence of hydrocarbons (e.g. pyrobitumen) is lacking in the ZCB, however anomalously light carbon isotopic signatures for both vein and matrix carbonate, spatially associated with the ores, are interpreted to record the oxidation of organic matter synchronous with ore formation. Antithetic distributions of (early diagenetic) wall-rock anhydrite and sulfides at many ZCB deposits suggest sulfur in the deposits was largely sourced and reduced in situ. A local source for F10migrated hydrocarbons and evidence that extensive in situ thermochemical sulfate reduction accompanied or preceded ore formation requires pre-mineralisation burial of the host rocks to depths approaching or exceeding the maximum preserved thickness of the Katangan succession in the ZCB. This indicates Cu-mineralisation occurred late in the depositional history, perhaps during initial stages of basin inversion at the onset of onset of Lufilian orogenesis after ~600 Ma, but prior to significant folding of the host succession.