Lithogeochemistry, C-O isotopes and Stratabound Alteration Halos to Stratiform Copper Mineralisation, Zambian Copperbelt

Abstract

A major lithogeochemical study of the basin sediments surrounding the stratiform copper deposits of the Zambian Copper Belt has revealed extensive stratabound zones of both potassic and sodic alteration. The syn-ore potassic alteration is represented by enrichment of biotite and K-feldspar in the enclosing sediments, with whole rock K2O values varying from 3.5 to 10 wt%. The sodic alteration (albite-calcite), post-dates the copper mineralisation, and occurs in the Upper Roan siltstone-carbonate sequence overlying the potassic alteration zone. Na2O values vary from 1.5 to 6.3 wt%, corresponding to a maximum of 60 wt% albite in the altered rock. A comprehensive study of carbon and oxygen isotopes of various carbonate types throughout the sediments of the Zambian Copper Belt has shown a systematic relationship to alteration and copper mineralisation that has significant application in mineral exploration. Mineralised and potassically altered zones show strong coupled depletion in both carbon and oxygen isotopes, with d13C = ?6 to ?26 per mil and d 18O = 6 to 20 per mil. The strongly negative d 13C values of carbonate in the ore shale and potassic footwall clastics indicates the significant role of oxidation of organic carbon in the mineralising process. The fact that the relatively clean altered and mineralised footwall clastics also show this negative carbon isotope shift suggests that organic reaction products migrated out of the shale into the permeable footwall clastics prior to mixing with the ore fluid, leading to redox reactions causing coupled oxidation of organic matter and reduction of sulfate to H2S resulting in copper sulfide precipitation.