Department of Geosciences

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Browsing Department of Geosciences by Advisor "Wanke, Ansgar"

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    A geochemical study of gallium, germanium and indium, in sulfide mineralisation of the Kombat, Tsumeb and Khusib springs deposits, Otavi Mountain Land, Namibia
    (University of Namibia, 2024) Shalimba, Ester; Wanke, Ansgar
    Carbonate-hosted base metal deposits are known to contain significant contents of critical metals such as Ga, Ge, and In. These metals occur mainly as inclusions or incorporated within the crystal lattice of the main sulfide minerals while they form rarely discrete minerals. The aim of this Doctor of Philosophy (PhD) study is to determine the geochemical characteristics of the critical raw metals that are associated with the sulfide mineralisation of the base metal deposits of the Otavi Mountain Land, with the emphasis on the Tsumeb, Kombat and Khusib Springs deposits. Tsumeb mine is an over the decades exploited breccia-pipe like polymetallic deposit, hosted by Neoproterozoic dolomite and limestones in the Otavi Group of the Damara Orogenic Belt. Exploitation took place between 1897 and 1996, with a production of about 30 Mt of ore, yielding 1.7 Mt Cu, 2.8 Mt Pb, 0.9 Mt Zn, and 80 t Ge. Mineralisation at this deposit consists of tennantite and sphalerite as the most abundant ore minerals, associated with galena, pyrite, bornite, chalcocite, digenite, and minor enargite, renierite, germanite, chalcopyrite, and traces of various arsenates. Kombat deposit, which is a stratabound, syntectonic Cu-Pb (Ag) sulfide deposit, is hosted in the dolomite of the Hüttenberg Formation, Otavi Group. Mining of the Kombat deposit is currently ongoing, with a total endowment of approximately 13.6 Mt, of which 12 Mt has been exploited until 2008. The ore mineralisation of the Kombat deposit is quite variable comprising massive, disseminated, veinlet-controlled, and brecciated mineralisation styles with abundant bornite, chalcopyrite, galena, covellite, and chalcocite set into quartz-calcite gangue. Khusib Springs deposit is a high-grade Cu-Pb Zn deposit which is classified as a “Tsumeb-type deposit”, hosted by Neoproterozoic dolomite, limestone and phyllites of the Maieberg Formation, Otavi Group. The main aim of this study is to show the siting of the most important minor and trace elements in the diverse sulfide mineral phases and to highlight the relationships between common trace elements and critical metals within the sulfide mineral phases. Various analytical techniques, including laser ablation inductively coupled plasma spectrometry (LA-ICP MS), electron microprobe analysis (EPMA), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray fluorescence (XRF), were employed to study the bulk ii geochemistry and the siting of the trace elements, for the sole purpose of answering the above-mentioned objectives. Additionally, portable X-ray fluorescence (pXRF) technique was utilized with the aim of evaluating the possible usability of a low-cost analytical technique in exploration for base and critical elements in carbonate-hosted sulfide mineralisations. The Kombat mineralization comprises three mineral assemblages (1) bornite-chalcopyrite-covellite, (2) bornite-galena-chalcocite, and (3) bornite-chalcopyrite, set into dolomite-quartz-calcite gangue, with chalcopyrite identified as the main carrier of Ga, Ge, and In in this deposit. The profound micro-analysis of Tsumeb ore revealed the presence of both Ga and Ge, associated with trace elements (Ag, As, Cd, Co, Cu, Fe, Ga, Ge, In, Ni, Pb, Sb, Zn), in the different sulfides. The most significant carriers of Ge are renierite (av. 8.21 wt.%) and enargite (0.78 wt.%), while Ga is mainly incorporated into renierite (av. 0.12 wt.%) and sphalerite (av. 0.11 wt%). Trace element abundance in sphalerite is characterized by elevated Cd, Ge, and Ga, suggestive of low formation temperatures, which corroborates with earlier research on the formation of Tsumeb ore. Reconnaissance Zn isotope data (-0.19 to 0.14‰) delineate Tsumeb ore as typical Zn-rich massive sulfide ore, with Zn isotope values overlapping with those of the Irish MVT deposits (-0.17 to 1.33‰), while Cu isotopes are comparable to isotopic data of hypogene mineralisation. In Khusib Springs ore, tennantite (Fe, Zn, Ag, Sb) and chalcopyrite (Cu, Pb, Ag) are the most important carriers of trace elements while all other sulfides are characterized by a surprisingly low trace element content. In contrast, Khusib Springs massive ore reveals a wealth of Ag-bearing minerals of the pearceite-polybasite mineral group. Modern portable tools are successfully applied for the analyses of selected elements in various fields. Portable XRF can be used as well for the analyses of selected elements (Cu, Fe, Mn, Pb, Zn) in Kombat-type ore, while these instruments fail for the analyses of elements like Ag, Sr, and, in particular, for the analyses of the target elements Ga, Ge, and In. Therefore, this study gives a detailed overview about the relationship between critical elements Ga, Ge, and In and associated elements in sulfide ores in the carbonate-hosted base metal deposits of the Otavi Mountain Land, while showing at the same time the limitations when access to modern analytical technologies is limited
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    Structural evolution and burial history of the Dolphin Graben, Northern Walvis Basin (block 1911), offshore Namibia
    (University of Namibia, 2021) Thomas, Frieda Kahewa-Ketu; Wanke, Ansgar
    This research is on the structural evolution and burial history of the the Dolphin Graben. The Dolphin Graben is a prominent graben structure situated on the continental slope in the northern Walvis Basin, immediately south of the Walvis Ridge and the Phoenix Volcanic Field. The Dolphin Graben is under-explored with only two wells located in it. Minor oil shows are reportedfrom well1911/15-1, while well 1911110-1 is labelled as dry. At least two regional source rock intervals are predicted to be present in the graben, one in the Aptian-Albian, the other in the Cenomanian-Turonian. None of the two exploration wells penetrated the Albian-Aptian source rock. The intersected Cenomanian-Turonian source rocks at the two well sites in the graben are recognized to be immature. Thus, the questions on the presence of a mature source rocks, reservoir charge access and hydrocarbon preservation within the graben remained pending. The project aims at providing the following three studies for the Dolphin Graben: a (1) detailed structural evolution analysis, (2) reconstruction of the burial history in 1D at three locations, and a (3) re-assessment of the hydrocarbon potential of the Dolphin Graben taking into account the structural evolution and burial history models. Seismic interpretation was carried out in Petrel softvvare, while 1 D burial hi tory models were created with PetroMod software. Seismic interpretation formed the foundation of the structural evolution study; it entailed marker horizon mapping, fault mapping and subsequently fault displacements measurements. JD burial models were created for three locations, namely well1911110-1, wel/1911115-1 andfor a pseudo well herein labelled as "New well". Subsequent thermal and maturity models were created with calibration data from the two 1911 wells. 11 A regional and local structural model were created from seismic data. The regional model gives a concept of the wider deeper crustal structure of the northern Walvis Basin, while the local model is rather a reconstruction of phases of fault activity specific for the structures defining the Dolphin Graben. A main outcome of the study is that the Dolphin Graben is a rift system that is independent from the general Namibian margin syn-rift extension. The graben formed after deposition of the syn-rift sequence and Seawards Dipping Reflectors (SDRs). In fact, most of the graben subsidence occurred during the early passive margin drift phase. The graben development commenced during the Barremian or early Aptian and two main extensional phases (Extension phases I and II) are recognized. The mechanism for this local rift is not fully understood but could relate to local thermal mantle anomalies associated with the volcanism at the adjacent Phoenix Volcanic Field and Walvis Ridge. Predicted lacustrine source rocks deposited during early extension and they are likely confined to ponds in smaller half grabens. During progressive extension, the smaller grabens became inactive, while extension vvas accommodated by a few main .faults that outline the Dolphin Graben. A continuous marker horizon in the Dolphin Graben has similar seismic characteristics and is located at a similar stratigraphic level as the known Aptian Albian source rock in the southern Walvis Basin. Hence, this seismic marker is understood as an indication that the Aptian-Albian source rock is well-developed in the Dolphin Graben. There, it is essentially laterally extensive, but condensed or absent on central structural highs and on the graben flanks. Another distinct seismic horizon ties well to the Cenomanian Turonian source rock in the 1911 wells and it extends over the entire Dolphin Graben. The lll ID burial history and thermal models revealed maturity of older presumed lacustrine as well as for the Aptian-Albian source rocks. A minor oil show is reported in drilled Lower Cretaceous carbonates occurring on a structural high in the central graben. It is possible that hydrocarbons used extensional faults as migration pathways to charge those carbonates. The Cenomanian-Turonian source rock has been identified as immature to early mature. With I 0. 6 % TOC it is a potentially excellent source rock, but burial depth within the Dolphin Graben was essentially insLifjicient for hydrocarbon generation. Therefore, fitture exploration should focus on petroleum plays that concentrate on the Aptian-Albian source rocks