Abstract
The Bingham Canyon porphyry deposit is one of the world’s largest Cu-Mo-Au resources. Elevated concentrations of thallium (Tl) compared to average continental crust have been found in some brecciated and igneous samples in this area, which likely result from mobilization of Tl by relatively low temperature hydrothermal fluids. The Tl-enrichment at Bingham Canyon therefore provides an opportunity to investigate if Tl isotope ratios reflect hydrothermal enrichment and whether there are systematic Tl isotope fractionations that could provide an exploration tool. We present a reconnaissance study of nineteen samples spanning a range of lithologies from the Bingham district which were analysed for their Tl content and Tl isotope ratios, reported as parts per ten thousand (ε205Tl) relative to the NIST SRM997 international standard. The range of ε205Tl reported in this study (−16.4 to +7.2) is the largest observed in a hydrothermal ore deposit to date. Unbrecciated samples collected relatively proximal to the Bingham Canyon porphyry system have ε205Tl of −4.2 to +0.9, similar to observations in a previous study of porphyry deposits. This relatively narrow range suggests that high-temperature (>300 °C) hydrothermal alteration does not result in significant Tl isotope fractionation. However, two samples ~3–4 km away from Bingham Canyon have higher ε205Tl values (+1.3 and +7.2), and samples from more distal (~7 km) disseminated gold deposits at Melco and Barneys Canyon display an even wider range in ε205Tl (−16.4 to +6.0). The observation of large positive and negative excursions in ε205Tl relative to the mantle value (ε205Tl = −2.0 ± 1.0) contrasts with previous investigations of hydrothermal systems. Samples displaying the most extreme positive and negative ε205Tl values also contain elevated concentrations of Tl-Sb-As. Furthermore, with the exception of one sample, all of the Tl isotopic anomalies occur in hydrothermal breccia samples. This suggests that ε205Tl excursions are most extreme during the migration of low-temperature hydrothermal fluids potentially related to sediment-hosted gold mineralization. Future investigation to determine the host phase(s) for Tl in breccias displaying both chalcophile element enrichment and ε205Tl excursions can potentially provide new information about hydrothermal fluid composition and could be used to locate sites for future porphyry exploration.
Highlights
The formation of such deposits may be influenced by country rock lithology: for example, skarns require reactive, carbonate-rich rocks to form, whereas the genesis of epithermal deposits may be facilitated by permeable rocks, possibly aided by extensive fracturing and brecciation [1,2]
The current study aims to evaluate the potential of Tl isotopes for tracing fluid evolution, alteration, and sulfide precipitation in porphyry deposits by addressing the following questions: (1) Do Tl concentrations or ε205 Tl values correlate with elemental enrichments in porphyry deposits? (2) Can ε205 Tl be used to fingerprint hydrothermal fluids in porphyry systems? This latter question permits examination of the possible genetic link between the Bingham Canyon porphyry system and the nearby sedimentary-hosted gold deposits of Barneys Canyon and Melco
Thallium Concentrations Measured by MC-inductively-coupled-plasma mass spectrometer (ICP-MS) vs. ICP-MS
Summary
Porphyry ore deposits are large anomalies of sulfide-hosted mineralization caused by magmatic-hydrothermal processes, and constitute significant resources of Cu (~75% of the world’s reserves), Mo (~50%), Au (~20%) and lesser amounts of other metals such as Ag [1]. They are associated both genetically and spatially with porphyritic intrusions in the upper crust, which cool and exsolve hydrothermal fluids containing water-soluble Cl and S species that extract metals from the source magmas and wall-rocks [2]. Porphyry ore mineralization is centred upon the host intrusion, mineralization may occur elsewhere in the surrounding rocks due to metal transport in cooler hydrothermal fluids that form skarns, sediment-hosted mineralization, or epithermal deposits, which may be proximal or distal to the porphyry intrusions [1,4,5,6,7,8].
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