Cu isotope ratio variations in the Dikulushi Cu-Ag deposit, DRC: of primary origin or induced by supergene reworking?

Cu isotope ratio variations were studied in the high-grade, vein-type Dikulushi deposit, Democratic Republic of Congo. The deposit consists of Cu-Pb-Zn-Fe mineralisation comprising sphalerite, chalcopyrite, bornite and chalcocite which precipitated under reducing conditions. The mineralisation was partly remobilised in more oxidising conditions into a Cu-Ag mineralisation phase, which is dominated by Ag-rich chalcocite. The upper part of the deposit is strongly reworked by weathering. Sulphides from the two types of mineralisation and Cu carbonates/silicates from the weathering zone show significant Cu isotope ratio variations. Chalcopyrite and chalcocite from the Cu-Pb-Zn-Fe and Cu-Ag mineralisation types are characterised by variable low Cu isotope compositions. The Cu isotope composition for some chalcocite from the Cu-Ag mineralisation deviates to lower delta-65Cu values than those of chalcocite from the Cu- Pb-Zn-Fe mineralisation, probably reflecting remobilisation of the Cu-Pb- Zn-Fe mineralisation in an oxidising environment. The isotopic composition of Cu in chalcocite from both mineralisation types becomes higher toward the centre of the orebody and as a function of depth. The variation may have been related to physicochemical changes in the precipitation environment as a result of interaction of the reduced mineralising brine with the oxidised host rock or may have been induced by subsequent supergene processes. The supergene mineralisation consists of malachite, azurite and chrysocolla which are enriched in 65-Cu.
Cu isotope ratio variations were studied in the high-grade, vein-type Dikulushi deposit, Democratic Republic of Congo. The deposit consists of Cu-Pb-Zn-Fe mineralisation comprising sphalerite, chalcopyrite, bornite and chalcocite which precipitated under reducing conditions. The mineralisation was partly remobilised in more oxidising conditions into a Cu-Ag mineralisation phase, which is dominated by Ag-rich chalcocite. The upper part of the deposit is strongly reworked by weathering. Sulphides from the two types of mineralisation and Cu carbonates/silicates from the weathering zone show significant Cu isotope ratio variations. Chalcopyrite and chalcocite from the Cu-Pb-Zn-Fe and Cu-Ag mineralisation types are characterised by variable low Cu isotope compositions. The Cu isotope composition for some chalcocite from the Cu-Ag mineralisation deviates to lower delta-65Cu values than those of chalcocite from the Cu- Pb-Zn-Fe mineralisation, probably reflecting remobilisation of the Cu-Pb- Zn-Fe mineralisation in an oxidising environment. The isotopic composition of Cu in chalcocite from both mineralisation types becomes higher toward the centre of the orebody and as a function of depth. The variation may have been related to physicochemical changes in the precipitation environment as a result of interaction of the reduced mineralising brine with the oxidised host rock or may have been induced by subsequent supergene processes. The supergene mineralisation consists of malachite, azurite and chrysocolla which are enriched in 65-Cu.