Abstract
ABSTRACTThe Kabwe Zn-Pb deposit (central Zambia) consists of a cluster of mixed sulfide and non-sulfide orebodies. The sulfide ores comprise sphalerite, galena, pyrite, chalcopyrite and accessory Ge-sulfides (±Ga and In). The non-sulfide ores comprise: (1) willemite-dominated zones encasing massive sulfide orebodies and (2) oxide-dominated alteration bands, overlying both the sulfide and Zn-silicate orebodies. This study focuses on the Ge, In and Ga distribution in the non-sulfide mineralization, and was carried out on a suite of Kabwe specimens, housed in the Natural History Museum Ore Collection (London). Petrography confirmed that the original sulfides were overprinted by at least two contrasting oxidation stages dominated by the formation of willemite (W1 and W2), and a further event characterized by weathering-related processes. Oxygen isotopic analyses have shown that W1 and W2 are unrelated genetically and furthermore not related to supergene Zn-Pb-carbonates in the oxide-dominated assemblage. The δ18O composition of 13.9–15.7‰ V-SMOW strongly supports a hydrothermal origin for W1. The δ18O composition of W2 (−3.5‰ to 0‰ V-SMOW) indicates that it precipitated from groundwaters of meteoric origin in either a supergene or a low-T hydrothermal environment. Gallium and Ge show a diversity of distribution among the range of Zn-bearing minerals. Gallium has been detected at the ppm level in W1, sphalerite, goethite and hematite. Germanium occurs at ppm levels in W1 and W2, and in scarcely detectable amounts in hemimorphite, goethite and hematite. Indium has low concentrations in goethite and hematite. These different deportments among the various phases are probably due to the different initial Ga, In and Ge abundances in the mineralization, to the different solubilities of the three elements at different temperatures and pH values, and finally to their variable affinities with the various minerals formed.
Highlights
THE term ‘non-sulfide’ is used to define a type of Zn-Pb ore deposit dominated by ‘oxidized’ Zn and Pb ore minerals, and is applied to distinguish such deposits from entirely sulfide deposits (Large, 2001; Hitzman et al, 2003)
Petrography has confirmed the complex paragenesis reported in previous studies on the Kabwe deposit
The present authors suggest that this original sulfide association was altered by at least three oxidation stages: two stages that were dominated by the formation of willemite, and another characterized by deep weathering-related processes
Summary
THE term ‘non-sulfide’ is used to define a type of Zn-Pb ore deposit dominated by ‘oxidized’ Zn and Pb ore minerals, and is applied to distinguish such deposits from entirely sulfide deposits (Large, 2001; Hitzman et al, 2003). This paper is part of a special issue entitled ‘Critical-metal mineralogy and ore genesis’. Departmental funds granted by DiSTAR (Napoli, Italy) to N. Mondillo at the Università degli Studi di Napoli “Federico II” have contributed to the costs of Open Access publication for this paper
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