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Experimental studies on hydrothermal mineral replacement of bornite by copper sulphides

Adegoke, Idowu Abiodun (2021) Experimental studies on hydrothermal mineral replacement of bornite by copper sulphides. PhD thesis, Murdoch University.

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Pseudomorphic mineral replacement reactions occur in numerous geological processes (e.g. metamorphism, metasomatism, ore deposition, and chemical weathering) and anthropogenic processes (e.g., reservoir acidification, CO2 sequestration, acid mine drainage, materials syntheses, and minerals processing). Therefore, the understanding of the mechanism and kinetics of these reactions is important not only to geosciences but also to industrial applications. These reactions involve the replacement of a primary mineral by a product mineral in the presence of a fluid phase, with the product mineral preserving the external dimension of the primary mineral. In the early years, solid-state diffusion (SSD) was proposed as the main mechanism for such reactions, while over the past 20 years the importance of coupled dissolution-reprecipitation (CDR) mechanism has been recognized, especially at low temperatures when SSD is assumed to be a very slow process. However, it has recently been recognized that in some mineral replacement reactions the rate of solid-state diffusion can be comparable to the rate of dissolution-reprecipitation, and hence complex mineral textures can be produced from a synergy between SSD and fluid-mediated CDR processes. The interplay between SSD and CDR mechanisms has profound implications in our interpretations of petrological and geological observations, yet it has not been adequately studied so far. Also, the role of the bulk hydrothermal fluids in the formation of lamellae textures by solid-state diffusion process in parent phases that are free of fluid inclusions is still not well elucidated. These and other issues were addressed in this present study.

Thus, to better understand the interaction between SSD and CDR reactions, the replacement of bornite (Cu5FeS4) by copper sulphides was used as a model system. In the present thesis, a series of experimental studies have been conducted into the mineral replacement reactions of bornite by copper sulphides to obtain an insight into the kinetics and mechanisms of the replacement reactions, as well as the formation and evolution of porosity during the replacement reactions. The outcomes of these sets of experiments are summarised below:

In Chapter 2, the hydrothermal dissolution and replacement of bornite by copper sulphides was studied experimentally in pH 1 solutions at three temperatures (160, 180 and 200 °C) under anoxic conditions, and the effects of background additives (Na2SO3, CuSO4, CuCl2 and CuCl) on the replacement pathway were established.

The results revealed that the reaction firstly involved the decomposition of bornite to form chalcopyrite (CuFeS2) lamellae and digenite-I (Cu9S5) via fluid-induced solid-state diffusion (FI-SSD) of Fe3+ from bornite structure. As the reaction progresses, chalcopyrite was later replaced by the second generation of digenite (digenite-II). Subsequently, both digenite-I and -II were replaced by covellite (CuS) and/or chalcocite (Cu2S) via the CDR reaction mechanism, depending on the experimental conditions. In addition, nantokite (CuCl) and atacamite (Cu2Cl[OH]3) formed due to the reaction between the dissolved copper with Cl-rich hydrothermal fluids. The observed replacement reactions at the rim and along fractures, the preservation of the external shape of the original bornite grain, and the presence of porosity in the product phase are characteristic features of the CDR mechanism.

The first critical finding from this set of experiments is the fluid-induced exsolution of chalcopyrite lamellae and digenite from bornite. This is because the exsolution rate and lamellae size were sensitive to the composition of the fluids, and no exsolution was observed in the heating experiment in the absence of fluid. This FI-SSD mechanism is made possible by the near-identical topology of the crystal structure of bornite, chalcopyrite and digenite. Secondly, the replacement of bornite is multi-step but proceeds via different pathways under the various conditions studied. The interplay between the FI-SSD and CDR mechanisms resulted in complex reactions which cannot be easily predicted empirically.

In Chapter 3, a combined kinetic, textural and mineralogical study of the replacement of bornite by copper sulphides under oxic conditions is presented, in order to obtain insights into the competing reactions between FI-SSD and CDR reactions under oxic conditions. The mechanism and kinetic behaviour of the reactions were described by exploring the effects of key variables including pH (1-6), temperature (160, 180 and 200 °C) and time on the reaction kinetics, and on the evolution of mineralogy and sample textures.

Like the anoxic experiments, the resulting textures under oxic conditions also revealed the interplay between the FI-SSD and CDR mechanisms during the replacement of bornite. However, the most important distinctive difference from the anoxic conditions is that the ICP-OES results suggests both Cu and Fe removal into the solution at pH 1, and predominantly Cu-removal at pH 2, 3 and 5 during the FI-SSD under oxic conditons as against predominantly Fe-removal under anoxic condtions.

The kinetic behaviour shows a complex dependence on various physical and chemical parameters including temperature, pH and contact time. For example, the result revealed that the reaction rate increases with temperature at pH 1 to 3, and decreases with temperature at pH 4 to 6. This change in reaction trend is linked to a change in the rate-limiting step of the reaction. For the effect of pH, the rate decreases with increasing pH from 1 to 6 at 180 and 200 °C, while the results show a complex effect of pH at 160 °C.

There are several important findings from Chapter 3: (1) the dissolution and the replacement of bornite under oxic conditions not only changed with pH but also with temperature, (2) bornite is relatively kinetically stable at higher pH 4-6, while pervasive fast replacement/dissolution of bornite was observed at lower pH (1-3), and (3) the exsolution rate and lamellae distribution is dependent on pH, indicating a FI-SSD mechanism.

In Chapter 4, the formation and evolution of porosity during the replacement of bornite by copper sulphides under anoxic and oxic conditions were monitored using combined (ultra) small-angle neutron scattering (USANS/SANS) measurements and microscopic textural examinations. The USANS/SANS measurements were carried out under both dry conditions and after filling the open pores with contrast matching D2O-H2O fluid, which made it possible to differentiate open pores from closed pores.

The reactions created pores in the product phases, and all samples contained pores with very broad size distributions from nano to micrometre. Nearly all small pores (<20 nm) were closed while larger pores were mostly open. The textures resulted from the FI-SSD process are largely non-porous, while the products from CDR reactions presents high porosity. Porosity generation in the product phase(s) is due to molar volume and relative solubility differences between the parent and the replacing phase. Porosity dropped at the initial stage of the reaction, but increased again with the progressive replacement, reaching a maximum at complete replacement, and then porosity slowly dropped again, showing porosity creation during the replacement and evolution after the replacement reaction. Therefore, this study provides another experimental evidence demonstrating the transient and dynamic nature of porosity during mineral replacement reactions. Porosity coarsening also occurred during and after replacement reactions, likely driven by surface energy minimization via Ostwald ripening.

Overall, this experimental study provides new insights into the physical chemistry of bornite replacement in nature. The documented mineralogical and textural differences could be used as one of the paleoproxies of the chemistry (including pH) and temperature of the fluids responsible for the alteration of copper minerals and remobilization of metals in ore deposits. Particularly, this study shows that mineral replacement can proceed via the synergy between SSD and CDR mechanisms; this happens at geologically low temperatures (≤200˚C) in the chalcogenide systems under both anoxic and oxic conditions, and may be used to interpret observations in other mineral systems such as the silicate systems at amphibolite or eclogitic metamorphic grades. This study also provides insights about porosity formation and evolution in copper-iron sulphides interacting with hydrothermal fluids, and how different reaction mechanisms contributes to porosity evolution, suggesting that mineral porosity can evolve and may be annealed out over geological timescale.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Chemistry and Physics
Supervisor(s): Xia, Fang and Deditius, Artur
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