Elsevier

Geochimica et Cosmochimica Acta

Volume 330, 1 August 2022, Pages 165-190
Geochimica et Cosmochimica Acta

A new mode of mineral replacement reactions involving the synergy between fluid-induced solid-state diffusion and dissolution-reprecipitation: A case study of the replacement of bornite by copper sulfides

https://doi.org/10.1016/j.gca.2021.04.017Get rights and content

Highlights

  • A new mode of hydrothermal mineral replacement reactions has been proposed.

  • Solid-state diffusion & coupled dissolution-reprecipitation (CDR) operate together.

  • Surface dissolution induce bulk solid-state diffusion, a mechanism called FI-SSD.

  • The synergy between FI-SSD & CDR result in complex reaction pathways and textures.

  • Correct interpretation of mineral texture needs process-specific textural criteria.

Abstract

Mineral replacement reactions are one of the most important phenomena controlling the geochemical cycle of elements on Earth. In the early years, solid-state diffusion was proposed as the main mechanism for mineral replacement reactions, but over the past 20 years the importance of the coupled dissolution-reprecipitation (CDR) mechanism has been recognized. In the presence of a fluid phase and at low temperatures (e.g., <300 °C), CDR is the predominant mineral replacement process compared to relatively slow solid-state diffusion. However, in the present case study, we show that the rate of solid-state diffusion is comparable to the rate of CDR processes during the replacement of bornite (Cu5FeS4) by copper sulfides at 160–200 °C. The experiments initially produced chalcopyrite lamellae homogeneously distributed in the entire bornite grain, and each lamella was enveloped by digenite. The lamellae were formed by removing Fe3+ from bornite via solid-state diffusion, since there was no evidence for fluid entering the bornite grains during lamellae formation. An interesting discovery is that solid-state exsolution of chalcopyrite lamellae was induced by the bulk hydrothermal fluids surrounding the mineral grains, because in the absence of fluids under otherwise identical conditions, no exsolution occurred, and because the exsolution rate and lamellae size were sensitive to the composition of hydrothermal fluids. We hypothesize that this fluid-induced solid-state diffusion (FI-SSD) mechanism is made possible by the similar topology of the crystal structure of these phases. The solid-state diffusion of Fe3+ within bornite and across the resultant chalcopyrite and digenite phase boundaries is facilitated by the near-identical S framework. Parallel to and after lamellae exsolution, CDR reactions proceeded from the surface to the interior of the grains or along fractures, replacing chalcopyrite by digenite, and digenite by covellite and/or chalcocite, depending on experimental conditions. The synergy between FI-SSD and CDR resulted in complex reaction pathways for reactions in five acidic hydrothermal fluids with or without added Cu2+, Cu+, Cl, SO42−, and SO32−. The outcomes of these experiments imply that (1) under conditions where cation diffusion rates are of the same order of magnitude as dissolution and precipitation rates, hydrothermal fluids can induce and control solid-state diffusion processes, e.g., exsolution; (2) mineral replacement can be a result of the synergy between FI-SSD and CDR mechanisms; this happens at low temperatures (≤200 °C) in chalcogenide systems, but could affect silicate and oxide systems at amphibolite to granulite to eclogitic metamorphic grade; and (3) the synergy between FI-SSD and CDR mechanisms can lead to complex reaction pathways that cannot be easily predicted empirically.

Introduction

Pseudomorphic mineral replacement reactions play important roles in numerous geochemical processes (e.g., metamorphism, metasomatism, chemical weathering, and reactive transport and ore deposition) and anthropogenic processes (e.g., reservoir acidification, CO2 sequestration, acid mine drainage, bioremediation, glass corrosion, materials syntheses, and mineral processing; Brugger et al., 2010, Geisler et al., 2019, Noiriel, 2015, Knorsch et al., 2020, Kartal et al., 2020, Nikkhou et al., 2020a, Nikkhou et al., 2020b, Nikkhou et al., 2021, Ram et al., 2021, Xia et al., 2009a). Hence, understanding the mechanism and kinetics of these reactions is important not only to the geosciences but also has industrial applications. In the early years, it was assumed that mineral replacement reactions are the result of elemental diffusion in the solids, from the interior of the solid to the solid–fluid interface where excessive elements are removed by the fluid phase and needed elements diffuse into the solid from the fluid phase (e.g., Cole, 1983, Thornber, 1975, Wilkin and Barnes, 1996). However, later extensive experimental studies have reached a consensus that many of these reactions are controlled by the coupled dissolution-reprecipitation (CDR) mechanism (Duan et al., 2021, González-Illanes et al., 2017, Harlov, 2015, Manecki et al., 2020, Tenailleau et al., 2006, Spruzeniece et al., 2017b, Xia et al., 2009b, Xing et al., 2019, Zhao et al., 2014a, Zhao et al., 2014b, Weber et al., 2019). In the CDR mechanism, the dissolution of the primary mineral and the precipitation of the products are coupled in time and space, at the nano- to micrometer scales, via a layer of interfacial fluid separating the two phases (Altree-Williams et al., 2015, Putnis, 2002, Putnis, 2009). Recently, it has been recognized that in some mineral replacement reactions, the rate of solid-state diffusion is comparable to the rate of dissolution-reprecipitation, and complex mineral textures can be produced as a result of both solid-state diffusion and dissolution-reprecipitation processes (Zhao et al., 2013, Hidalgo et al., 2020). The interplay between solid-state diffusion and dissolution-reprecipitation has profound implications on our interpretations of mineralogical and petrological observations, which can influence our understanding of geochemical processes. Yet the mechanism of such mineral replacement reactions has not been adequately studied.

We chose to use the replacement of bornite (Cu5FeS4) by chalcopyrite (CuFeS2) and copper sulfides as a model system to study the interplay between solid-state diffusion and dissolution-reprecipitation processes because cation diffusion in bornite is fast (Grguric and Putnis, 1999), and dissolution-reprecipitation mineral replacement reactions are commonly observed in copper-iron sulfides in many ore deposits (Ciobanu et al., 2017, Cook et al., 2011, Graham et al., 1988, Halbach et al., 1998, Haymon, 1983, Li et al., 2020, Liu et al., 2017, Oszczepalski, 1999, Ramdohr, 1980, Robb, 2005, Robb, 2013, Tivey, 1995). The complex ternary Cu-Fe-S system has extensive solid solutions such as the bornite-digenite solid solution (bdss), the high- and low- temperature phases, and the metastable phases, which convolutes textural and mineralogical evolution along the Cu-Fe-S and Cu-S joins (Barton, 1973, Cabri, 1973, Craig and Scott, 1974). Therefore, studying the replacement processes in Cu-Fe sulfides not only contributes to our understanding of mineral replacement reactions in general but will also contribute to a clearer understanding of the ternary Cu-Fe-S system and hence to the interpretation of mineral textures from various copper ore deposits. So far, only a few hydrothermal experimental studies have been reported for the Cu-Fe-S system.

The pioneering experimental study of Roberts (1963) describes the replacement of chalcopyrite by bornite, and bornite by chalcopyrite under mild hydrothermal conditions (<150 °C). The author ruled out the solid-state diffusion mechanism due to the fast reaction (6 days at 100 °C) and proposed an ionic recombination mechanism. Recently, Zhao et al. (2014a) reported the replacement of chalcopyrite by bornite at 200–320 °C and demonstrated that the reaction proceeded both via pseudomorphic replacement of chalcopyrite following the CDR mechanism, and via overgrowth on the initial grain surface. The same authors also studied the sequential replacement of hematite by chalcopyrite and then chalcopyrite by bornite in Cu(I) and H2S-bearing hydrothermal fluids at 200–300 °C, also following the CDR mechanism (Zhao et al., 2014b).

Exsolution lamellae are common textures resulting from the decomposition of a solid-solution via a solid-state diffusion-controlled mechanism. Yet, recent studies have shown that hydrothermal fluids can affect the formation and coarsening of lamellae. In the hydrothermal replacement of bdss, Zhao et al. (2017) demonstrated that at high pH (pH25°C ~10) at 125–250 °C, bornite lamellae exsolve from bdss. They found that the rate of exsolution is ~1000 times faster than the equivalent experiments conducted under dry conditions. Based on a series of annealing experiments under different conditions, Zhao et al. (2017) postulated that the development of exsolution textures was influenced by fluid inclusions within the parent phase. These fluid inclusions were unquenchable and resulted from the formation of the parent solid solution via a CDR process. The rapid coarsening of lamellae was attributed to recrystallization associated with the healing of an open porous microstructure in the presence of trapped hydrothermal fluids within the mineral grains. Under acidic conditions (pH25°C ~6), Li et al. (2018) observed chalcopyrite exsolution from bdss (formed by chalcopyrite replacement at 300 °C) upon annealing at 150 °C, also in the presence of fluids trapped in the porous microstructure. A two-step process was proposed: (i) breakdown of bdss and formation of a bornite-digenite assemblage; and (ii) exsolution of chalcopyrite lamellae from bornite. Most recently, Hidalgo et al. (2020) studied the replacement of bornite-chalcocite-tennantite-quartz assemblage in hydrochloric acid and methanesulfonic acid at 90 °C and observed the fast replacement of chalcocite by digenite as well as the slow replacement of bornite by digenite and the exsolution of Cu-deficient and Cu-enriched bornite lamellae in bornite. They proposed that the lamellae were formed by solid-state diffusion, a process influenced by the presence of fluids.

In this study, we conducted systematic experiments on the replacement of bornite under anoxic hydrothermal conditions at 160–200 °C, aiming to elucidate (1) the role of bulk hydrothermal fluids (rather than fluid inclusions) in the formation of the lamellae by a solid-state diffusion process, and (2) the mechanism of the interplay between solid-state diffusion and dissolution-reprecipitation during hydrothermal alteration of minerals. The reactions were conducted in five hydrothermal solutions, including a pH 1 additive-free solution as a control, and solutions with added Na2SO3, CuSO4, CuCl2, and CuCl. To avoid the effects of impurities on the reaction mechanism, a pure bornite sample was used in all experiments.

Section snippets

Materials

A natural bornite specimen from the Moonta mines, South Australia, was used as the starting material. The sample was crushed and sieved into a 150–355 µm size fraction for the experiments. The Rietveld refinement of the synchrotron powder X-ray diffraction (PXRD) pattern revealed that the starting bornite is a low bornite (orthorhombic; space group Pbca with unit cell parameters a = 10.971 Å, b = 21.881 Å, c = 10.962 Å) (Fig. EA1a in Electronic Annex). No other phases were detected by PXRD.

Dry reference experiments

The PXRD patterns of bornite from the six dry experiments carried out at three temperatures (160, 180, and 200 °C) for 24 and 612 h showed no evidence for the formation of new minerals. Similarly, no exsolution lamellae of chalcopyrite or any other phase were noted during microscopic examination of the grain cross-sections (Fig. EA2).

Control experiments

The mineralogical changes during the transformation of bornite under hydrothermal conditions at three temperatures are evident in the PXRD patterns (Fig. EA3 in

Formation of chalcopyrite lamellae and digenite via fluid-induced solid-state diffusion (FI-SSD)

This study highlights the profound effects of temperature and fluid composition on the stability of stoichiometric bornite. Hydrothermal fluids triggered the exsolution of chalcopyrite lamellae and digenite from bornite and controlled the rate of formation and the size of chalcopyrite lamellae. In the absence of a fluid phase, end-member bornite remained intact in the temperature range of 160–200 °C in the dry experiments (Fig. EA2 in Electronic Annex). The stability of the end-member bornite

Conclusions and implications

Mineral replacement reactions are dynamic and complex. The texture of the resultant mineral assemblage depends on many factors that affect the dissolution, nucleation, and mineral growth rate directly or indirectly, including but not limited to hydrostatic pressure (Xia et al., 2012), non-hydrostatic pressure (Spruzeniece et al., 2017a), texture and composition of the starting mineral (Qian et al., 2011, Xia et al., 2007), mass transfer (Centrella et al., 2016), the presence of ligands (Xing et

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the Microscopy Australia Facility (MA) at the Centre for Microscopy, Characterisation & Analysis (CMCA), The University of Western Australia, for using SEM and EPMA, and Australian Synchrotron (part of ANSTO) for accessing the powder diffraction beamline. We are grateful to Allan Pring for providing the bornite specimen, and Andrew Foreman, Juita Juita, Marc Hampton, Malgorzata Kowalczyk, Jacqueline Briggs, Saijel Jani, Kenneth Seymour, Stewart Kelly, Lyn Kirilak, and

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    Present address: Integrated Geological & Mining Services (IGMS), East Fremantle, WA 6158, Australia.

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