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Licensed Unlicensed Requires Authentication Published by De Gruyter May 6, 2017

X-ray spectroscopy study of the chemical state of “invisible” Au in synthetic minerals in the Fe-As-S system

Alexander L. Trigub, Boris R. Tagirov, Kristina O. Kvashnina, Dmitriy A. Chareev, Maximilian S. Nickolsky, Andrey A. Shiryaev, Nina N. Baranova, Elena V. Kovalchuk and Andrey V. Mokhov
From the journal American Mineralogist

Abstract

Minerals of the Fe-As-S system are the main components of Au ores in many hydrothermal deposits, including Carlin-type Au deposits, volcanogenic massive sulfide deposits, epithermal, mesothermal, sedimentary-hosted systems, and Archean Au lodes. The “invisible” (or refractory) form of Au is present in all types of hydrothermal ores and often predominates. Knowledge of the chemical state of “invisible” Au (local atomic environment/structural position, electronic structure, and oxidation state) is crucial for understanding the conditions of ore formation and necessary for the physical-chemical modeling of hydrothermal Au mineralization. In addition, it will help to improve the technologies of ore processing and Au extraction. Here we report an investigation of the chemical state of “invisible” Au in synthetic analogs of natural minerals (As-free pyrite FeS2, arsenopyrite FeAsS, and löllingite FeAs2). The compounds were synthesized by means of hydrothermal (pyrite) and salt flux techniques (in each case) and studied by X-ray absorption fine structure (XAFS) spectroscopy in a high-energy resolution fluorescence detection (HERFD) mode in combination with first-principles quantum chemical calculations. The content of “invisible” Au in the synthesized löllingite (800 ± 300 ppm) was much higher than that in arsenopyrite (23 ± 14 ppm). The lowest Au content was observed in zonal pyrite crystals synthesized in a salt flux. High “invisible” Au contents were observed in hydrothermal pyrite (40–90 ppm), which implies that this mineral can efficiently scavenge Au even in As-free systems. The Au content of the hydrothermal pyrite is independent of sulfur fugacity and probably corresponds to the maximum Au solubility at the experimental P-T parameters (450 °C, 1 kbar). It is shown that Au replaces Fe in the structures of löllingite, arsenopyrite, and hydrothermal pyrite. The Au-ligand distance increases by 0.14 Å (pyrite), 0.16 Å (löllingite), and 0.23 Å (As), 0.13 Å (S) (arsenopyrite) relative to the Fe-ligand distance in pure compounds. Distortions of the atomic structures are localized around Au atoms and disappear at R > ∼4 Å. Chemically bound Au occurs only in hydrothermal pyrite, whereas pyrite synthesized without hydrothermal fluid contains only Au°. The heating (metamorphism) of hydrothermal pyrite results in the decomposition of chemically bound Au and formation of Au° nuggets, which coarsen with increasing temperature. Depending on the chemical composition of the host mineral, Au can play a role of either a cation or an anion: the Bader atomic partial charge of Au decreases in the order pyrite (+0.4 e) > arsenopyrite (0) > löllingite (−0.4 e). Our results suggest that other noble metals (platinum group elements, Ag) can form a chemically bound refractory admixture in base metal sulfides/chalcogenides. The content of chemically bound noble metals can vary depending on the composition of the host mineral and ore history.

Acknowledgments

The authors acknowledge ESRF for allocating beamtime under proposals ES-184 and ES-360. The help and support of Sara Lafuerza and Pieter Glatzel during the beamtime is greatly appreciated. B.R.T. and K.O.K. thank Hugo Vitoux for outstanding technical support during the in situ experiment with micro-furnace at the ID26 beamline. We are grateful to Sara-J. Barnes for organizing the LA-ICP-MS measurements at the University of Chicoutimi, V. Abramova and E. Minervina for the LA-ICP-MS analysis of synthesized minerals. We are grateful to Andrey Girnis and Anastasia Plyasunova for correction of English grammar. We thank Barbara Etschmann, Martin Reich, and an anonymous reviewer for helpful comments and suggestions. The results of this work were obtained using the computational resources of MCC NRC “Kurchatov Institute” (http://computing.kiae.ru/). This study was supported by the Russian Scientific Foundation, grant no. 14-17-00693; RFBR grant no. 16-05-00938 supported salt flux synthesis experiments. D.Ch. was involved in Act 211 Government of the Russian Federation, agreement no. 02.A03.21.0006 (synthesis of sulfides via salt flux technique).

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Received: 2016-4-27
Accepted: 2016-12-23
Published Online: 2017-5-6
Published in Print: 2017-5-24

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