Chemical reactivity and alteration of pyrite mineral in the Kubi gold concession in Ghana

This study aims to identify individual elements and to gain an understanding of the surface reaction mechanisms, as well as the properties of precipitated pyrite particles observed during the hydrothermal formation of an ore deposit. A combination of X-ray diffraction (XRD), air annealing in a furnace, scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX), and X-ray photoelectron spectroscopy (XPS) was applied to investigate the crystal structure and identify the individual elements, permanence, transformation and chemical/electronic properties of such pyrite. Background Pyrite (FeS2) is the most common among the group of sulfide minerals on Earth and abundant in most geological settings. This gangue mineral, in association with garnet, hematite, magnetite and other sulfide minerals, acts as an indicator mineral in the Kubi concession of Asante Gold Corp. in Ghana. Pyrite, along with other sulfide minerals linked with gold such as arsenopyrite, chalcopyrite, pyrrhotite, covellite, sphalerite and galena, gives rise to minuscule gold particles bonded to the grains of sulfide compounds. Fig. 1 (a) Pyrite nugget with a chipped-off piece, (b) pristine pyrite nugget, (c) crushed powder pyrite sample before annealing, and (d) fused pyrite sample after annealing. Methods The mineralogical compositions of both pristine and annealed samples were first examined by XRD using a PANAnalytical X’pert powder diffractometer configured in a θ-2θ, Cu-Kα radiation with a wavelength of 1.5406 Å. The pyrite powder sample then underwent annealing in an OTF-1200X-SDVD compact 2-in. tube furnace from MTI Corp. in room atmospheric conditions for two hours at a furnace temperature of 1,100 °C. XRD analysis was then repeated on the annealed sample. The pyrite sample was further analyzed using SEM/EDX operated at 20 kV using a Zeiss Supra 35VP field emission SEM equipped with an EDX detector-facilitated elemental mapping to comprehend the elemental distribution. Furthermore, XPS was employed to scrutinize the elemental distribution and chemistry of the pyrite sample. Using an Axis Ultra DLD instrument from Kratos Analytical (UK) with monochromatic Al Kα radiation (hν = 1486.6 eV), measurements were conducted under a base pressure lower than 1.1 × 10–9 Torr (1.5 × 10–7 Pa) during data acquisition. XPS depth profiles were obtained by sputter etching with 0.5-keV Ar+ ions incident at a 70° angle relative to the sample surface normal to minimize sputtering damage. Recorded spectra included core levels (Fe 2p, S 2p, O 1 s and C 1 s) and valence band, with a total acquisition time of 348 s with the charge neutralizer off. Results The XRD analysis showed that both pristine and an- nealed samples contain some hematite and quartz besides pyrite. Results from air annealing indicate that the relationship between pyrite and hematite-magnetite is controlled by temperature. The EDX analysis revealed that the sample has oxygen (O) and carbon (C) as contaminants, while the XPS analysis additionally revealed barium (Ba), gold (Au), phosphorus (P), aluminum (Al) and nitrogen (N). These elements are attributed to pyrite that bonds metallically or covalently to neighboring ligands/impurity minerals such as oxides, chalcogenide sulfides, as well as the gangue alteration minerals of magnetite and hematite in the pyrite sample. A notable chemical shift of 0.28 eV for metal iron (Fe) toward higher energies, attributed to screening effects and charge transfer, indicates bonding between Fe atoms and neighboring ligands in indicator or rock-forming minerals within the Kubi concession. This charge transfer enhances surface reactivity and conductivity of pyrite, which bonds both metallically to metallic oxides and covalently to nonmetallic components through hybridization, σ and π interactions in metal complexes. Air annealing results suggest that the interplay between pyrite, hematite and magnetite is influenced by temperature and pressure, with magnetite formation occurring at high temperatures around 1,100 °C. These transformation processes (pyrite-hematite-magnetite) are governed by oxidation- reduction conditions and specific pH levels. Moreover, the conversion of pyrite-hematite with intermediate phases within gangue quartz minerals in the Kubi gold concession may result from increased temperature and pressure related to regional metamorphism, facilitated by oxidizing and reducing conditions. Conclusions By combining EDX with XRD, air annealing in a furnace, core-level and valence-band XPS electronic structure analyses, we explored the electronic structure and chemical bonding of pyrite. The combined findings reveal that polished pyrite samples undergo transformation to hematite upon annealing, accompanied by the presence of contaminants such as O, C, Ba, P, Al, N and Au. These contaminants are attributed to impurity minerals like barium and aluminum oxides, chalcogenide sulfides and phosphorus as well as gangue alteration minerals such as magnetite and hematite in the sample. The findings suggest that during the hydrothermal flow regime, pyrite, pathfinder elements and impurity minerals/metals were in contact with quartz minerals before undergoing hematite transformation, and pyrite thus becomes an indicator mineral in the Kubi gold concession. The combined techniques offer valuable insights into surface chemistry and the characterization of chemical alterations on pyrite mineral surfaces. Selected references Craig JR, Scott SD (2018) Sulfide phase equilibria. In: Ribbe PH (ed) Sulfide mineralogy, 1976. De Gruyter, Berlin, Boston, pp. 124–233 Nzulu G, Eklund P, Magnuson M (2021) Characterization and identification of Au pathfinder minerals from an artisanal mine site using X-ray diffraction. J Mater Sci 56:7659–7669 Nzulu GK, Högberg H, Eklund P, Hultman L, Nude PM, Yaya A, Magnuson M (2023) Pathfinder elements and indicator minerals of Au from the Kubi Gold ore deposits in Ghana. Environ Earth Sci 82(386):1–11

Chemical reactivity and alteration of pyrite mineral in the Kubi gold concession in Ghana

Authored by:
Per Eklund, Hans Högberg, Lars Hultman, Martin Magnuson, Prosper M. Nude, Gabriel K. Nzulu, Abu Yaya

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