Characterization and identification of Au pathfinder minerals from an artisanal mine site using X-ray diffraction

At mineralogical mining sites, the fast location of ore bodies is paramounted in order to reduce exploration costs. For this purpose, pathfinding minerals are important. These minerals act as an aid in the original ore body discovery. Au that can be traced from the presence of pathfinding minerals mostly originates as anhedral crystal assemblies (i.e., without well-defined crystal facets) that naturally exist as single or polycrystalline mineral aggregates that are usually found in situ in hydrothermal quartz veins and other kinds of key deposits in metamorphic and igneous rocks [1, 2].

The most common mineral at most Au mining sites is pyrite (FeS₂) that can also be found in oil shales and coal [3]. Other common minerals at Au mining sites are arsenopyrite, different forms of silicate minerals (garnet) and magnetite (Fe₃O₄). Both mineralogical and geochemical information are indispensable to provide an initial valuation of the potential ore zone of an exploration area.

Quantitative interpretation of X-ray diffraction (XRD) data [4] has long been applied to distinguish between mineral assemblages, and to define chemical and mineralogical compositions [5]. Previous XRD studies of Au and associated minerals have mostly been performed to determine the grain size morphology and crystallinity [6]. XRD has been used to conclude that highly hydrated and water-saturated environments contribute to the migration of Au within alluvial regimes and on hydrothermal mineral assemblages [7‐9]. Multivariate statistical analysis and geostatistical methods have been applied to identify pathfinding elements [10, 11]. Bayari et al. [4] found that mineralized regolith profiles and mobility of elements (minerals) in the soil at the Bole–Nangoli gold belt in the north-eastern Ghana could mainly be attributed to amorphous mineral phases. Furthermore, Zhao and Pring (2019) [12] studied the mineral transformation in Au and silver (Ag) in fluids using the telluride group of minerals associated with Au and focused on the texture, reaction mechanism and the kinetics of the oxidation leaching of the tellurides.

Cairns et al. [13] identified topsoil minerals and pathfinders of Au by considering the fine grain size and amorphous nature of the minerals. Furthermore, XRD studies on the influence of thermal stability of magnetite investigate the effect of temperature on the phase transitions [14‐21]. This information is of importance for the investigation of magnetite as a pathfinder mineral of Au. As follows from this background, there is still a need for the characterization of pathfinding Au-associated minerals by XRD on residual samples to establish their relationship and to preserve information about the physiochemical situations of their origin.

In this work, we investigate the crystal structure of Au in relation to the corresponding pathfinding minerals, such as quartz (SiO₂), Fe₃O₄, Fe₂O₃, FeS₂ and Fe_1-xS, collected from an artisanal mining site, i.e., a small-scale hand-mining site, in the central region of Ghana. XRD was used for phase identification and to obtain structural information including Rietveld refinement. In addition to the known minerals, we also identified hematite (Fe₂O₃) as an important pathfinding mineral. The present study can be used to enable future identification of pathfinding minerals for Au exploration.

Figure 4 shows an X-ray diffractogram of the Au nugget sample in Fig. 2b with the result of Rietveld refinement assuming pure Au together with the residual of the fit [28]. The six pronounced peaks in the diffractogram are indexed as a cubic fcc Au structure (Fm-3 m space group) with lattice parameter of a = 4.079 Å. Table 1ists the full assigned observed peak list as well as the resulting crystallographic parameters from the refinement. These are in agreement with literature assignments for Au [28, 29].

Figure 5 shows an X-ray diffractogram of the impure powder Au sample shown in Fig. 3a. Table 2 lists the refined crystallographic parameters of Au and the pathfinder minerals identified from powder Au samples. These data are in agreement with reference data [27‐32]. The diffractogram from the unrefined powder sample shows the presence of other minerals, that is, pathfinder minerals for Au. These are dominated by SiO₂ (quartz) with some Fe₃O₄ (magnetite).

The lattice parameter of the SiO₂ in the impure Au were found to be a = 4.91 Å and c = 5.43 Å (space group P3221), consistent with reference data [30]. This sample also contains Fe₃O₄ (cubic, space group Fd-3 m) with a lattice parameter of 8.36 Å, consistent with literature data [27].

Figure 6 shows an X-ray diffractogram from the residual black sand after Au panning. The diffraction peaks of this sample were identified as the crystalline structure of Fe₂O₃ (hematite). Table 3 lists the diffraction peaks and crystallographic parameters determined from the Rietveld refinement of Fe₂O₃. This is in accordance with literature and reference data [32, 33]. The crystal structure of Fe₂O₃ is rhombohedral with a space group R-3c and lattice constant of 5.0991 Å [34, 35].

Comparing Figs. 4 and 5, it can be seen that the latter sample contains Au together with pathfinder minerals in the form of magnetite and quartz. The most abundant mineral observed in the diffraction pattern of the impure powder Au sample is SiO₂ (quartz) having three distinct peaks at 2θ = 40.284°, 67.957° and 90.818° corresponding to {111}, {212} and {312} crystalline planes of the SiO₂ phase, respectively. The refined pattern of SiO₂ shown in Fig. 5 is in agreement with the literature data in refs [36‐39], which also holds true for the moderate amount of magnetite present [27]. This shows that impure Au or final concentrate (non-pure Au) have a high quantity (percentage) of pathfinder minerals as impurities. Note that Au atoms easily substitute with Ag atoms forming an alloy with the same fcc crystal structure and that it is impossible to distinguish pure Au from an Au–Ag alloy with XRD.

The diffractogram in Fig. 6 contains major peaks at 2θ = 32.609°, 34.915°, 38.658°, 40.196°, 48.618°, 53.966°, 62.990°, 69.975°, 80.837°, 83.475° and 91.533° identified as {101}, {110}, {006}, {113}, {024}, {116}, {214}, {208}, {128}, {134} and {042} crystalline planes of Fe₂O₃ (hematite), respectively. These refined peaks are in good agreement with the rhombohedral structure of Fe₂O₃ [32, 33].

Generally, including possible microstrain in the Rietveld refinement has a negligible effect on the convergence of the fit (residual), indicating that the samples are essentially strain-free. The results from the impure Au powder sample indicate that SiO₂ (quartz) is the dominant impurity mineral serving as the host rock containing all the pathfinder minerals at the mining site. It is known that SiO₂ is a so-called gangue mineral (i.e., a commercially nonvaluable mineral that surrounds or is mixed with a valuable mineral) in hydrothermal ore veins [40], to preserve information about the physiochemical situations of the origin of the veins and to understand the formation of mineral deposits. These dominant SiO₂ species contain structural defects that favor mineral infusion due to underlying conditions and geological processes, such as crystallization, metamorphism, alterations, changes in crystallization temperatures and precipitation [41‐43].

Au associated with Fe₃O₄ is mostly formed in skarns of granular magnetite usually found in contact with metamorphosed areas with magma intrusion into carbonate or silico-carbonate rocks that also consist of garnet and silicate minerals, among others. The residual black sand together with other dense minerals is considered to be ore that is left over during Au refinement and washing at riverbanks when recovering its Au content [44]. This shows that two of the three most common iron ore minerals; Fe₃O₄ and Fe₂O₃ are widely spread within the mining site and contribute to the Au host minerals alongside SiO₂. In a near-surface environment (oxide area) Fe₂O₃ act as the gangue mineral and can be transformed to Fe₃O₄ depending on the environmental conditions such as high temperature, oxidation, and pH [45]. The same color of Fe₂O₃ in comparison with black Fe₃O₄ makes it difficult to distinguish between the two in branded iron formations and standing water [44, 46]. It is likely that during the formation of Fe-oxides in the alluvial regime at the Dunkwa–Kubi geological site, Au is internally captured within structures associated with Fe₂O₃ (hematite) that acts as crusts in saprolite and laterite environments. These minerals reveal information about the physiochemical conditions of the origin of structures (structural defects) useful for the understanding of mineral deposit formations.

This study has revealed that sediments and black sands containing Au are associated with pathfinding minerals in impure compositions. This is indicative that Au and pathfinding minerals are all deposited in nature during hydrothermal activation. The XRD analysis identified Au, SiO₂ (quartz), Fe₃O₄ (magnetite) and Fe₂O₃ (hematite). From the XRD patterns, the impure Au and Fe₂O₃ samples can be attributed to the decomposition and transformation of these indicator minerals. Also, the surface (oxide zones) mineralization is altered by Fe₂O₃ as one of the indicator minerals apart from the garnet and the gangue mineral SiO₂ to host Au with other pathfinder minerals beneath the surface.

These results are of importance for the mining industry to underscore the usefulness of XRD in studying soil and sand sediments from mining sites by identifying pathfinder minerals of Au in potential geological sites.