Abstract
In supergene Zn non-sulfide deposits, the Fe-oxy-hydroxides (FeO/OH) are mainly concentrated in the residual zones (gossan) on top of the oxidized ore bodies, although they can also be found throughout the whole weathering profile coexisting with the primary and secondary ore assemblages. Fe-oxy-hydroxides are rarely pure as they form in systems where a wide range of metals, most of them of economic importance (e.g., Zn, Pb, Co, REE, Sc, Ga, Ge, V, etc.), freely circulate and can be “captured” under specific conditions. Although their occurrence can be widespread, and they have a potential to scavenge and accumulate critical metals, FeO/OH are considered gangue phases during the existing processing routes of Zn non-sulfide ores. Moreover, very little is known about the role of the deposit type on the geochemistry of FeO/OH formed in a specific association. Therefore, this paper provides a comprehensive assessment of the trace element footprint of FeO/OH from a number of Zn non-sulfide deposits, in order to define parameters controlling the metals’ enrichment process in the mineral phase. To achieve this, we selected FeO/OH-bearing mineralized samples from four supergene Zn non-sulfide ores in diverse settings, namely Hakkari (Turkey), Jabali (Yemen), Cristal (Peru) and Kabwe (Zambia). The petrography of FeO/OH was investigated by means of scanning electron microscope energy dispersive analysis (SEM-EDS), while the trace element composition was assessed using laser ablation-ICP-MS (LA-ICP-MS). Statistical analyses performed on LA-ICP-MS data defined several interelement associations, which can be ascribed to the different nature of the studied deposits, the dominant ore-formation process and subsequent evolution of the deposits and the environmental conditions under which FeO/OH phases were formed. Based on our results, the main new inferences are: (A) Zinc, Si, Pb, Ga and Ge enrichment in FeO/OH is favored in ores where the direct replacement of sulfides is the dominant process and/or where the pyrite is abundant (e.g., Cristal and Hakkari). (B) When the dissolution of the host-rock is a key process during the supergene ore formation (i.e., Jabali), the buffering toward basic pH of the solutions favors the uptake in FeO/OH of elements leached from the host carbonate rock (i.e., Mn), whilst restricting the uptake of elements derived from the dissolution of sulfides (i.e., Zn, Pb, Ga and Ge), as well as silica. (C) The input of exotic phases can produce significant enrichment in “unconventional” metals in FeO/OH (i.e., Cr and Co at Kabwe; Y at Cristal) depending on whether the optimal pH-Eh conditions are attained. (D) In the Kabwe deposit, FeO/OH records heterogeneous geochemical conditions within the system: where locally basic conditions prevailed during the alteration process, the V and U concentration in FeO/OH is favored; yet conversely, more acidic weathering produced Zn- and Si-bearing FeO/OH.
Highlights
Non-sulfide Zn-Pb deposits, formerly known as Calamines from the Polish word “galman” [1,2,3], consist of ores formed by the oxidization of primary sulfide bodies
The petrography of FeO/OH was investigated by means of scanning electron microscope energy dispersive analysis (SEM-EDS), while the trace element composition was assessed using laser ablation-ICP-MS (LA-ICP-MS)
Statistical analyses performed on LA-ICP-MS data defined several interelement associations, which can be ascribed to the different nature of the studied deposits, the dominant ore-formation process and subsequent evolution of the deposits and the environmental conditions under which FeO/OH phases were formed
Summary
Non-sulfide Zn-Pb deposits, formerly known as Calamines from the Polish word “galman” [1,2,3], consist of ores formed by the oxidization of primary sulfide bodies. The meteoric waters, commonly mixed with groundwaters, react with primary sulfides and leach out the metallic elements (Fe, Zn, Pb, As, etc.), which precipitate as secondary Zn and Pb hydrous silicates and carbonates, such as hemimorphite, sauconite, smithsonite and cerussite, when suitable T, pH and Eh conditions are attained [2] During this process, large amounts of Fe-oxy-hydroxides (hereafter, FeO/OH) form, consisting of minerals such as goethite, lepidocrocite, maghemite, ferrihydrite and hematite, formed as a result of the breakdown of Fe-rich hypogene mineralogical species (e.g., pyrite, marcasite, pyrrhotite and Fe-rich sphalerite) [4]. The results indicate that the adsorption can occur either by simple replacement of surface hydroxyl groups by an anion (such as silicate, phosphate, chloride, arsenate anions) or via formation of complexes where at least one H2 O molecule is retained by the adsorbing species between the surface and the anion (such as nitrate and perchlorate anions) [8]
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