The transition from hypogene to supergene mineralisation at the Mashtu South Cu-co deposit, Katanga, Democratic Republic of Congo

Gigler, Gruffudd Morgan
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Mashitu South is a Cu-Co deposit situated in the Katanga Province of the Democratic Republic of the Congo (DRC), about 25km to the east of the city of Kolwezi. It lies within the northwestern portion of the Central African Copperbelt (CAC), an arcuate region with a world class abundance of copper deposits that straddles the external-fold-and-thrust-belt and the Domes region of the Lufilian arc, an orogenic belt that formed during the ∼600-500Ma Pan African orogeny. Cu-Co mineralisation at Mashitu South is hosted in the rocks of the Mines Series and Roches Argillo-Talceuses (RAT) subgroups of the Roan Group, deposited during the early stages of the opening of the Katangan basin, which is constrained to a maximum age of ∼880Ma. This study characterises the mineralisation stages and geochemistry of Mashitu South through a combination of core logging, geostatistical and petrographic techniques. The mineralogical and geochemical changes which occurred to the deposit throughout its paragenetic history are also explored. This is done with the purpose of linking the mineralisation at the deposit to regional metallogenic and geodynamic events, as well as developing vectors to Cu-Co mineralisation. Mineralisation at Mashitu South occurred in four stages. The first stage is characterised by the development of stratiform-disseminated, zoned Cu-Co sulphides restricted to the rocks of the Mines Series, in a manner comparable to the sedimentary hosted stratiform copper (SSC) deposit model. A protracted, syn-diagenetic timing is inferred for this stage, which caused the formation of hypogene, stratiform orebodies, primarily in the Kamoto Formation. Metals emplaced during the first stage were remobilised during the second mineralisation stage, which resulted in the development of vein-hosted hypogene Cu Co sulphides, but does not appear to have concentrated metals into significant orebodies at Mashitu South. The second period of mineralisation is inferred to be early/syn-orogenic in age. Hypogene mineralisation at Mashitu South is found to have an element association of Cu + Co + Bi + Ni + V + S ± Mo ± As ± Fe ± Zn. The deposit was subsequently affected by two discrete stages of supergene alteration and mineralisation. Supergene alteration has resulted in the in-situ oxidation of hypogene Cu-Co sulphides, and also the leaching and remobilisation of metals from hypogene mineralisation. The leaching and remobilisation process formed supergene, malachite-dominated orebodies which have an element association of Cu + Be + P ± Zn, the location of which is primarily controlled by metal availability and permeability of the host rocks. The RAT Subgroup is frequently host to such orebodies, underneath leached and eroded away Mines Series strata. These orebodies are usually found at a depth of ∼30 50m. An occurrence of bacillus-shaped features composed of Cu-Co sulphides was discovered in the lower Shales Dolomitique du Base (SDB) unit of the Mines Subgroup, and investigated with respect to the potential biogenicity and antiquity of the features as candidate microfossils. Further study found these features to be pseudofossils formed primarily by the replacement of diagenetic rutile crystals by sulphides, during the primary, stratiform mineralisation stage. The various elements of the hypogene geochemical association are shown to have different mobilities in the supergene leaching environment, with Cu being relatively mobile and Co + Bi + Ni + V ± Mo being relatively refractory. This variation in mobility between elements, which are associated with Cu in the hypogene environment, suggests a zonation of these elements around Cu in the supergene environment, controlled by the hydrological gradient. At Mashitu South, this has resulted in the relative enrichment of Bi, Mo, V and Co in the upper 10m of the rock profile. Nickel is not as depleted in this interval as Cu, which is strongly depleted in the near-surface environment. It is therefore hypothesised that Bi, Mo, V, Co and Ni would make better surface vectors towards buried Cu-Co mineralisation than Cu, in regions where the supergene leaching process is particularly effective. ‘Cobalt caps’ above Cu-Co deposits in Katanga are well known, but this study suggests that Bi, Mo and V may make even better surface vectors to mineralisation than Co.
A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, October 2018
Gigler, Gruffudd Morgan, (2018) The transition from hypogene to supergene mineralisation at the Mashitu South Cu-Co deposit, Katanga, Democratic Republic of the Congo, University of the Witwatersrand, Johannesburg,