Willemite Mineralisation in Namibia and Zambia.
[Tesi di dottorato]
Willemite mineralisation of several Zn-Pb deposits hosted in the Proterozoic carbonates of Namibia and Zambia has been rather neglected in the literature on zinc ores of southern Africa. In fact, willemite (Zn2SiO4) is one of the main zinc carriers in several high-grade carbonate-hosted non-sulphide deposits and prospects, located in the southern African subcontinent. These deposits (Berg Aukas, Abenab West and Baltika in the Otavi Mountain Land, Namibia; Kabwe, Excelsior and Star Zinc, Zambia) are currently interpreted (Hitzman et al., 2003) to be all of the same hypogene-hydrothermal origin.
The Otavi Mountain Land (OML) broadly coincides with the Neoproterozoic carbonate platform of the Damara Orogen in Namibia. In the OML several ore deposits and prospects (both sulphides and non-sulphides), are hosted in the shallow-water carbonate successions of the Otavi Group. The primary sulphide ores occurring in the region have been subdivided between the ‘Berg Aukas-type’ (Zn-Pb MVT) and ‘Tsumeb-type’ (Pb-Cu-Zn pipes) deposits. A maximum age for ‘Tsumeb-type’ sulphide ores in the OML is 530 Ma, while the ‘Berg Aukas-type’ ores seem to be older and related to fluids expelled during the Damaran orogeny. In the few reported occurrences, willemite commonly replaces sphalerite in both ore types, but it may also occur as primary concentrations in the absence of sulphides or replace directly the carbonate host rock.
The Zambian deposits occur in the metasedimentary rocks of the Late Proterozoic Katangan Supergroup. The most important orebodies are located in the Kabwe area (Kabwe n.1-2-3/4-5/6, Airfield and Kashitu) and they contain both sulphides and willemite in dolomite host rocks affected by greenschist facies regional metamorphism. Based on the structural relationships, Zn-Pb sulphide mineralisation in the Kabwe district has been interpreted as having been formed before the last Lufilian deformational phase (ca 650 - 500 Ma) (Kamona and Friedrich, 2006), which would be broadly in the same age range as the OML sulphides. On the contrary, the Star Zinc prospect (Lusaka) is hosted by highly metamorphic lithotypes of the late Proterozoic Zambezi belt sequence. Willemite occurs in both Namibian and Zambian ores in several successive generations and variable paragenetic position. Fluid inclusion studies together with cathodoluminescence petrography and trace element geochemistry are important to distinguish between hypogene, high temperature willemites and low temperature (possibly) supergene ones. In fact in the high-temperature willemite, preferential green luminescent colours indicative of higher Mn values, are more common.
In the Namibian deposits, as well as in the Kabwe orebody, it was possible to detect a typical ‘willemitisation’ process affecting primary sulphides, where sulphur has been replaced by silica. This process led to an increasingly extended network of needle-thin willemite veins cutting through sphalerite until a complete removal of Zn-sulphide, whereas early established cerussite rims have preserved galena from total alteration. A hydrothermal dolomite halo, similar to those described at Vazante (Brazil) and Beltana (Australia) by Hitzman et al. (2003), is either very limited in space or absent in the African willemite occurrences. However, a further interaction of low temperature acid solutions with the carbonate hosts could have caused the deposition of post-willemite, widespread metal-bearing dolomite cements (Zn+Pb combined up to 10%), closely following a last generation of botryoidal willemite aggregates. In the host carbonates, it is also possible to observe a progressive removal of K and Al from the clay minerals in the stylolite seams, followed by the precipitation of Zn-smectite (sauconite). Smithsonite (ZnCO3) and hemimorphite [Zn4Si2O7(OH) 2•H2O)] occur as late phases filling the remaining porosity of the host rock. In the Star Zinc and Excelsior prospects (Lusaka area) willemite is associated with specular haematite and locally replaces franklinite and gahnite. This mineralogical association has many similarities with that occurring in the Franklin-Sterling Hill deposit (USA). Based on the occurrence of Zn-bearing spinels and other high-temperature minerals, the above-mentioned prospects should be considered of hypogene origin.
The fluid inclusions in the willemites in the Kabwe area, as well as in all the examined specimens from the Namibian mines, are monophase. A preliminary fluid inclusion study of the Star Zinc willemite by Sweeney et al. (1991) had produced temperatures ranging between 150-250° C, and salinities around 7-12 eq wt%. Our thermometric analyses roughly confirm the above homogenisation temperatures (Star Zinc and Excelsior Th: 200 - 250°C) and salinities (10 to 16 eq. wt% NaCl). Even if these temperatures are still well below those reached during the regional metamorphism of the Zambezi belt, they point to a hydrothermal origin for the willemite ores in the Lusaka area. The temperatures of the secondary inclusions range from 140 to 190°C for both deposits and the salinities are lower than in the primary ones (2 to 10 eq. wt% NaCl).
The mineralisation timing for most willemite ores is generally poorly constrained, because precise geochronological information on this kind of ore is rare. However, indirect evidence for the timing of the willemite deposits at Vazante (Brazil) and Beltana (Australia) may indicate mineralisation events in the range of ca 490 - 550 Ma (Hitzman et al., 2003). In the general absence of direct radiometric data, the first reliable Rb-Sr isochron model ages have been obtained for the willemite from Namibian deposits by Schneider et al. (2008). The measured willemites cover a possible interval of 490 - 550 Ma: these ages are consistent with the late stages of the Damaran orogeny and the uplift phases in the OML. Currently, Rb-Sr geochronology applied to Zambian willemites is still in progress, but the first results are also in the range of an Upper Proterozoic-Cambrian age. The assumed ages may point to a global period of willemite mineralisation extending from Uppermost Proterozoic to lower Ordovician in the southern hemisphere, under special tectonic and climatic conditions that favoured a circulation of low-temperature hydrothermal, oxygenated waters carrying Zn derived from supergene weathering phenomena. The precipitation of the hypogene high temperature Zn mineralisation occurring in the Lusaka mining area may be of the same age, but in this case the hydrothermal circulation of Zn-carrying fluids was associated with the waning stages of the regional metamorphism in the Zambezi belt.
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