A geochemical study of gallium, germanium and indium, in sulfide mineralisation of the Kombat, Tsumeb and Khusib springs deposits, Otavi Mountain Land, Namibia
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Date
2024
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University of Namibia
Abstract
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
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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
Description
A dissertation submitted in fulfillment of the requirements for the Degree of Doctor of Philosophy in Science (Geology)
Keywords
Gallium, Germanium, Indium, Otavi Mountain Land, Sulfide mineralisation, Kombat deposit, Khusib Springs deposit, Tsumeb deposit, Trace elements