A Preliminary Description of the Nkamouna Cobalt-Manganese-Nickel Laterite Deposit, Southeast Cameroon
Gideon Dzemua Lambiv, Sarah A. Gleeson, W. Buckovic, Benard Abei Ayongaba, Emmanual Simo, Lin Christophe Thierry Omgba Ahanda, Pierre Christain Mikolebeh, 2009. "A Preliminary Description of the Nkamouna Cobalt-Manganese-Nickel Laterite Deposit, Southeast Cameroon", Supergene Environments, Processes, and Products, Spencer R. Titley
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Seven Co mineralized laterite deposits have been identified in southeast Cameroon. The deposits are developed over serpentinite bodies of unknown age and origin. This paper focuses on one deposit, Nkamouna, which currently has delineated proven and probable reserves of 52 million tonnes of 0.24 percent Co and 0.72 percent Ni (43-101 compliant), making it the first Co laterite deposit with economic potential.
The weathering profile at Nkamouna is thick and company geologists have identified the following units in the profile (from the base): protolith (serpentinite), saprolite, silcrete, ferralite, ferricrete breccia unit, and the granular unit. The ore zone at Nkamouna is found at the top of the ferralite and at the base of the ferricrete breccia unit and has an average thickness of 8 m. The ore mineralogy is dominated by very coarse absolane that contains significant concentrations of Co. The coarse nature of the asbolane allows for physical upgrading of the ore before processing, which has a significant impact on the economics of the deposit.
The deposit is different from most Ni laterite deposits in that it has a very high Co/Ni ratio and very low Mg contents. The profile at Nkamouna is complex and, in particular, in the ferricrete breccia unit there is evidence for multiple phases of profile formation. This, along with a potential pre-enrichment of the protolith, may have resulted in the high Co endowment seen at Nkamouna.
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Supergene Environments, Processes, and Products
At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years.
Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting.
As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.