The geology and structural influences on copper mineralisation, distribution and spatial occurrence in the Lubambe copper deposit, Central African Copperbelt, Zambia
Date
2022
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Abstract
The genetic relationship between copper mineralisation, stratigraphy and basin architecture in the Central African Copper Belt is very important as it plays a critical role in controlling and localising mineralisation. Evidence from basinal geometrical studies in most Zambian Copper Belt deposits suggests that major thrust faults and folded structures were developed during the Lufilian orogeny, which involved inversion of basement structures during periods of compression. The Konkola Dome and the Kirila Bombwe anticline are examples of major basement units where re-activation of extensional faults resulted in the generation of major structures later used as channel ways by copperbearing fluids. The presence of undeformed sulphides often hosted within structures shows that deformation predates mineralisation and critically played a major role in preparing the ground and structures for mineralisation.
An integration and re-interpretation of geophysical and geological data in the Lubambe Mining Area has resulted in a significantly improved understanding of the geology of the area and potential controls on ore distribution. Despite limited subsurface information from the geophysical datasets, the exercise has provided significant confidence to detailed mapping of poorly exposed parts of the area. Interpretation outcomes from this mapping and geophysical (aeromagnetic and gravity) suggest that the distribution of major folds reflects an early extensional, basement architecture which is thought to be key to ore deposit formation in the Lubambe License Area. Additionally, geological modelling of the Lubambe-Konkola Area has shown that a relationship potentially exists between ore distribution, fold axes and stratigraphic thickness trends. Specifically, the model suggests that both copper grades and Ore Shale 1 Member thickness trends are controlled by NE-trending growth faults which can be traced to the eastern-side of the property. There is modest correlation between the traces of faults interpreted from geophysical surveys and the occurrence of structures seen in drill core in the selected holes which provides evidence of faults during and immediately after sedimentation. This suggests that the depocentres were fault-bounded basin into which mineralising fluids flowed to deposit ores in the OS1 Member. Copper ore within the Lubambe License Area includes very fine-grained chalcopyrite and chalcocite with some bornite, malachite, chrysocolla and limonite staining. Hypogene copper mineralisation in the Lubambe license area occurs as disseminated and along bedding planes adopting similar grain sizes as the host rock. This is accompanied by secondary copper minerals and iron oxides filling veins, fractures and joints. The Ore Shale 1 Member within the Lubambe License area thickens towards the southeast and can be correlated to the regional structure. Copper grades in the rich zones can be correlated with two small sub-basins in the East and South limbs and another around the Extension Area. Significant structural control on copper distribution can be observed on copper grade distribution maps superimposed on structural maps showing major and intermediate structural features. There are significant concentrations of chalcocite near surface and also down dip in the East and South Limbs and the Extension Area. Chalcopyrite is widely distributed except near surface. Bornite is concentrated in the centre of the East Limb. Malachite and chrysocolla are well correlated with Acid Soluble copper and are found mainly near surface. Evidence also suggests that metal concentration and upgrading of the ore deposit was influenced by macro and meso-scale structural features during supergene processes as highly mineralised areas were coinciding with faulted and folds zones. A spatial relationship between the distribution of mineralisation and sedimentary architecture has been demonstrated to show a strong correlation in the Lubambe License Area. This interpretation is consistent with ore genesis models of sedimentary-hosted copper deposits.
Description
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science to the Faculty of Science, School of Geosciences, University of the Witwatersrand, Johannesburg, 2022
Keywords
Copper mineralisation, Spatial occurrence, Central African Copperbelt, Zambia