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Tellurium and selenium in mafic volcanogenic massive sulfide hydrothermal systems: evidence from the Troodos ophiolite, Cyprus

Martin, Andrew J. 2019. Tellurium and selenium in mafic volcanogenic massive sulfide hydrothermal systems: evidence from the Troodos ophiolite, Cyprus. PhD Thesis, Cardiff University.
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Abstract

Tellurium (Te) and Selenium (Se) are identified as elements critical to the European Union’s Strategic Energy Plan: both essential components in the manufacture of thin-film photovoltaic solar cells. Their strategic importance is however paralleled by a poor understanding of their distribution and enrichment in a range of ore-forming environments. The significance of Te and Se in green energy production, their rapidly increasing demand and by-product nature of supply has led to their classification as critical elements by the EU and rest of the world. To address concerns over the supply of Te and Se, the UK National Environmental Research Council established the SoS Minerals (Security of Supply) initiative, within which the ‘TeaSe’ consortium aimed to characterise Te and Se in a range of ore-forming environments. This current study, which forms part of the larger TeaSe project, focuses on the distribution and enrichment of Te and Se and associated elements in mafic, Cyprus-type or Cu-Zn Volcanogenic Massive Sulfide (VMS) deposits in one of their principal on-land analogues; the Troodos ophiolite of Cyprus. Exceptional exposure, preservation of original seafloor spreading structures and the lack of any apparent metamorphic overprint in this Late Cretaceous (92 Ma) fragment of Tethyan spreading ridge make the Troodos ophiolite the ideal natural laboratory to investigate processes associated with mafic VMS deposits. VMS deposits occur within the extrusive sequence of the Troodos ophiolite, at the periphery of the complex, within a suite of basaltic to andesitic lavas that once formed the palaeo-seafloor. In this study new geochemical, isotopic and mineralogical data from more than 25 VMS and other mineralised localities spanning the entire Troodos ophiolite is presented, thereby ensuring a representative range of ore-forming processes that control the distribution of Te and Se in Troodos VMS deposits are characterised. Because of a lack of historic data the concentration of Te and Se in mafic VMS deposits has never previously been characterised. This is especially true in the case of Troodos VMS deposits. This study finds that both Te and Se are enriched in Troodos VMS to levels comparable to actively-forming back-arc and intra-oceanic arc-related hydrothermal systems. Furthermore, data highlights the highest Se concentration in pyrite (9565 ppm; n=1514) yet recorded in any VMS deposit regardless of tectonic environment. Additionally, the mineral-scale distribution of Te and Se between different sulfide minerals is not uniform: Se preferentially partitions into chalcopyrite (n=195) whilst Te partitions into pyrite (n=1514). Selenium is most likely incorporated in pyrite and chalcopyrite in a lattice bound substitution whilst Te can occur as either nano to micro-scale inclusions or as a lattice bound substitution. A correlation between Bi and Te in some Troodos VMS (e.g. the Skouriotissa deposit: R2=0.35) suggests Bi-Te inclusions may host appreciable Te. On a deposit scale, localised seafloor oxidisation leads to the extreme enrichment of Se. In the Apliki VMS deposit, a hematite-pyrite-chalcopyrite-rich zone termed the South Apliki Breccia Zone (SABZ) was identified via regional geochemical screening as containing high Se (>1000 ppm). The paragenesis of sulfides and oxides in the SABZ suggest the remobilisation of Se during hematite formation and its incorporation in late-stage euhedral pyrite. This demonstrates the significance of fluctuating redox in leading to the extreme, albeit localised, enrichment of Se. Furthermore, whole-rock geochemical data shows that Te and Se may be enriched through supergene oxidation-related processes, leading to the concentration of Te and Se in Fe (oxy)hydroxide crusts. The distribution of Te and Se between individual VMS deposits on a regional scale (10’s of km) across the Troodos ophiolite is extremely variable. Selenium/sulfur ratio, magmatophile trace-element signature and sulfur isotopes have been used to assess variation in magmatic volatile influx and source-rock composition between different VMS deposits. Two processes are proposed that explain the observed distribution of metals and sulfur isotope values in Troodos VMS: i) The leaching of igneous lithologies; or ii) a magmatic volatile influx. The VMS deposits of the Troodos ophiolite occur at different stratigraphic levels within the extrusive sequence: either within the Lower Pillow Lava (LPL), or overlying the Upper Pilllow Lava (UPL). Lava geochemistry between the ‘UPL’ and ‘LPL’ is markedly different with the UPL distinctly more primitive in composition, and enriched in Cu+Se+Au relative to the LPL suite. Therefore, assuming that the leaching of igneous lithologies provided the only source of metal in the VMS hydrothermal system, those VMS that postdate the accretion of the UPL can be expected to show an enrichment in Cu+Se+Au (e.g., Skouriotissa, Phoucasa) relative to those that predate them (e.g. Agrokipia). However, some LPL hosted VMS deposits exhibit an unusual enrichment in Cu, Se and Au (e.g. Mala); this could indicate the addition of a magmatic volatile phase. Sulfur isotope (δ34S) measurements of hydrothermal sulfides indicate that the majority of sulfur in the hydrothermal system for a ‘typical’ Troodos VMS is sourced through the thermochemical reduction of seawater sulfate and the leaching of primary igneous lithologies and formation of epidosites in the sheeted dykes underlying the VMS deposits. Consequently, δ34S values for all Troodos sulfides average +4.7‰ (n=180). However, this study identifies, for the first time, an immature magmatic-dominated sulfur isotope signature in an ancient VMS deposit: the exceptionally preserved Mala VMS. This deposit exhibits a light sulfur isotope signature in pyrite averaging -3.4‰ (n=7), a value significantly less that the Troodos magmatic mean (<0-1‰). Considered together with its stratigraphic position, deep within the LPL, geochemical and isotopic data suggest that Mala was preserved – probably through rapid burial by subsequent lava flows – during the immature stages of sulfide accumulation, and therefore retains a distinct magmatic volatile-dominated signature that would normally be overprinted and modified during VMS maturation and off axis fluid flow. Evidence strongly suggests that metals in immature VMS deposits, even in the relatively volatile-poor mafic VMS system, are sourced from a magmatic volatile phase during the initial stage of VMS formation. Immature or magmatic end-member VMS deposits (e.g. Mala) are characterised by fluids that probably underwent SO2 disproportionation (leading to δ34S values of <0‰ in sulfides), an enrichment in magmatophile elements, and apparent lack of zone-refining in the VMS mound. Consequently, Immature VMS deposits exhibit an enrichment in Te, Se and Au, demonstrating a robust link between magmatic volatile influx and the enrichment of Te and Se. In contrast, a ‘typical’ Troodos VMS deposit exhibits a δ34S signature that is consistent with sulfur derived from the leaching of primary igneous lithologies and the thermochemical reduction of seawater sulfate (δ34S +4.7‰, n=180). The preservation of an immature magmatic signature has only previously been observed in actively forming seafloor massive sulfide deposits in arc-related environments.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: Hydrothermal, VMS, SMS, Troodos, LA-ICP-MS, Sulfides, Volatiles, Ophiolite, Te, Se
Funders: National Environmental Research Council (NERC), School of Earth and Ocean Sciences, Cardiff University
Date of First Compliant Deposit: 19 July 2019
Last Modified: 19 Jul 2019 09:07
URI: http://orca-mwe.cf.ac.uk/id/eprint/124346

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