Pathfinder elements and indicator minerals of Au from the Kubi Gold ore deposits in Ghana

Au mineralization in the Kubi mining area (denoted the “513 zone”) was discovered in 2010 during geochemistry (auger drilling, soil, and stream sampling), geophysics, geology, and diamond core drill exploration activities as part of extended exploration programmes over several decades (Smith and Amaor 2014). The findings suggest that within the Kubi area, the highest gold concentrations occur in deformed garnet-hornfelses that are part of the carbonaceous schist. The rocks contain a maximum of about 15 vol.% sulfide minerals comprising pyrite, pyrrhotite, and some arsenopyrite.

Elemental and mineral distributions in geological settings are important tools for identifying gold and other precious metals. In mineral exploration, indicator minerals that are resistant to weathering usually remain in the parent material. Examples of such minerals include zircon, apatite, quartz, titanite, rutile magnetite, chromite, ilmenite, and garnet of gemstone quality. Indicator minerals can also be found in till, soil, and stream sediments and are usually involved in specific mineralization, alteration, and can possibly be used for rock identification (McClenaghan 2005). The weakest resistant minerals to weathering, such as talc (McClenaghan 2005) and most rock salts (halites), are easily eroded or transported from one place to another. Pathfinder elements of gold include As, Mo, Ag, Sn, Sb, Te, W, Bi, Ni, and Cu (Bayari et al. 2019; Plouffe 2001; Somarin et al. 2021). The elements Sb, Nb, Ta, and Sn are often found in Au-bearing minerals or pegmatite rocks and can easily be distinguished from their rutile mineral signatures (Porter et al. 2020).

Previously, studies on pathfinder elements and indicator minerals in Ghana have mainly involved statistical methods with a multivariate numerical approach to identify trace elements such as Fe, Mn, Ag, As, Cu, Zn, Ni, and Pb associated with Au (Nude et al. 2012). As for progression, Bayari et al. (2019) applied X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS) to identify pathfinder elements and their indicator minerals in the Bole-Nangoli gold belt in the north-eastern part of Ghana. Recently, Nzulu et al. (2021a) applied X-ray photoelectron spectroscopy and energy dispersive X-ray spectrometry (EDX) to identify the pathfinder elements Ag, Hg and Ti that act as pathfinder elements in the host minerals of Au ore in the alluvial regime collected at a small-scale mining site at Kubi. These elements were attributed to the indicator minerals quartz, hematite, pyrite (marcasite), garnet, and other occasional silicate minerals, such as biotite and hornblende. Recent studies have focused on understanding the pathfinders of Au mineralisation in the western margin of the Ashanti Belt, the characterisation of Au indicator minerals (Nzulu et al. 2021a), and the elucidation of the pathfinder elements (Nzulu et al. 2021b) in the Kubi concession. As previous samples were collected in rather shallow deposits, at depths of 0–20 m in an artisanal mine, the above conditions call for further investigations to greater depths of ~ 100 m in the same concession area.

The identified minerals from the XRD are consistent with the mineralogy/lithological logging codes, EDX elemental distributions are attributed to alteration/mineralogical codes, and the SEM findings are assigned to structural/geotechnical logging codes in agreement with Table 1 and the assay result in Table 2. The elemental distributions for EDX in Table 4 that show Si, O, K, Cl, and Na for quartz; Fe and S for pyrite; and Fe, Si, O, Al, Mn, and Ca for garnet indicate the presence of these minerals as Au indicators as well as other impurities (Lignie et al. 2012; Bayliss 1977; Hazen 1976; Brigatti et al. 2000; Götte et al. 2011; Ali et al. 2019). These elements are products/components of the minerals identified in the analysis: that is garnet–[Mg, Fe, Mn]₃Al₂(SiO4)₃, pyrite–FeS₂, goethite–Fe₃⁺O(OH), arsenopyrite–(FeAsS), and quartz-SiO₂. The Na, K. Mn could be from the feldspars in the host rocks.

These results agree with the geochemical analysis of Table 2 that shows stringers–vuggy quartz, pyrite, and garnets in gabbro rocks and occasional phyllite and greywacke. The presence of C in all the samples is attributed to graphitic or carbonate alterations (Dochartaigh et al. 2011). The SEM image of the garnet in gabbroic rock in Fig. 6c shows some occurrences of surface layer euhedral shapes consistent with expectations for garnet that show a variety of fractured textures and shapes containing quartz, periclase, and biotite. The periclase is believed to have originated from the gabbroic rock, which mainly consists of olivine. Olivine can undergo a reversible reaction to form a mixture of perovskite and ferropericlase (Lin et al. 2007) according to the equation \(\gamma - \left( {{\text{Mg}},{\text{Fe}}} \right)_{2} \left[ {{\text{SiO}}_{4} } \right] \leftrightarrow \left( {{\text{Mg}},{\text{Fe}}} \right)\left[ {{\text{SiO}}_{3} } \right] + \left( {{\text{Mg}},{\text{Fe}}} \right){\text{O}}\) within the transition zone. Therefore, the periclase in the garnet–gabbro sample is a confirmation of a breakdown of olivine mineral within the hydrothermal formation of the rock/mineral assemblage.

The periclase (MgO) in the garnet gabbroic rock can undergo a chemical reaction and break up the oxide. This increases the amount of Mg elements in the mineral assemblage, and the Fe component can be replaced by the dominant Mg under certain conditions of temperature, pressure, and pH that promote the mineral to transform into pyrope-rich garnet (Mg₃Al₂(SiO₄)₃) (Geiger et al. 1992). Although the XRD refinement excludes the pyrope-type garnet as a component in the sample, it explains why the breakup reaction of MgO has not occurred. Periclase (MgO) causes hematite mineral alteration of magnesium hydroxide in most hydrothermal vein rocks in the Kubi area as well as in the soil sediments. Apart from performing a vital role in the ferruginous–garnet–hematite enrichment, the hematite alteration mineral derived from periclase also acts as a useful source of nutrients for the Kubi soils, balancing the soil acidity and pH level.

As shown in the cross section of the drill hole of the Kubi concession in Fig. 1, the garnet minerals containing periclase in the orebody at > 75 m below the soil surface, alter into hematite and protrude from greater depths through the transition zone (59–75 m depth) and finally into the oxide zone (0–59 m) to form the rich alluvial Au ore deposit found in the Kubi saprolite. It can be anticipated that during the chemical weathering and transport process, the almandine garnet can undergo a weathering process to cause a mineralogical change and transform into hematite. Hematite transforms into goethite under a hydration process (Graham et al. 1989a, b), since the Fe molar volume in goethite is greater than that of hematite (Campbell and Schwertmann 1984). Therefore, the dominant hematite alterations in the Kubi concession are associated with garnet–periclase-rich indicator minerals. Thus, the ferrous iron end member of the garnet mineral class represents an important silicate group of rock-forming minerals that constitutes the main component of the Kubi terrain and the rock transition zone (Geiger et al. 1992). The results indicate that the almandine-type-garnet can be considered as part of the main indicator minerals in Table 1 apart from the pyrite, arsenopyrite, chalcopyrite, and carbonates that are known to geologists in the concession area. These mineral compositions that trend down the soil can be ascribed to the presence of plutonic or orogenic activities considering the types of metasediments and metavolcanic terrain rocks such as amphibolite, hornblende gneiss, biotite gneiss, quartzite, iron formation, metaconglomerate, marble, pelitic schist, etc. (Kesse and Foster 1984) that form part of the Birimian–Tarkwaian supergroup (Kesse 1985).

The natural quartz fragments show the extent to which the sediments have been transported in high-energy subaqueous environments (Helland and Holmes 1997); predominantly by the Offin and Gyimi rivers, and their weathering profile due to the spotty flat–angular–conchoidal fracture and crystal overgrowth nature of the material. Therefore, we anticipate that the mineralogy of this surface terrain has an impact on the chemical weathering reaction of garnet (almandine), such that different products of indicator minerals occur in fractions within sediments from the top to the orebody. Thus, the disseminated minerals (periclase, biotite, pyrite, other sulfates, and oxides) are influenced by the flow mechanism of garnet and other indicator minerals depending on the density, mineralogy of by-products, and environmental conditions (Price et al. 2013), such that the reaction mechanism of the weathering products of garnet within the study area depends on the composition of almandine experiencing weathering profiling.

From the results, we attribute the identified elements as constituents of the indicator minerals as well as pathfinder elements of Au mineralization. The identified elements Fe, Ag, Al, N, Ca, and Mn, K, Cl, O, Si, and Ti form either alloys with gold or with innate elements in the soil sediments and can be ascribed to primary geochemical distribution from the crystallization of magma and hydrothermal fluids (Nzulu et al. 2021b). Some of these identified elements, such as Si and Ag, are strongly associated with Au due to their eutectic characteristics, while N, C, and O have affinity for S or are associated with sulfide minerals. In addition, the migration of metasomatic elements and the rapid rate of chemical weathering of minerals from garnet zone lateralization in secondary processes might be contributing factors in the alteration and mineral distribution in the area.

There are four major gold deposits in the Ashanti belt, of which Kubi forms a part. The steeply dipping quartz veins in shear zones in the Birimian sedimentary rocks show mesothermal deposits, where Au mineralization occurs at moderate temperature and pressure along openings or fractures in rocks from hydrothermal fluids at intermediate depths. Thus, the hydrothermal fluids from the orebody at the Kubi concession deposit gold along openings of quartz and other silicate groups of minerals. The sulfide ores pyrite and arsenopyrite that are closely associated with quartz veins, gold, rounded pebbles, and cobbles in the Kubi concession are formed by auriferous deposits. This type of deposit usually results from the solidification of molten magma to form igneous rocks such as the gabbroic rock in the Kubi concession area, which contains the host Au garnet and periclase minerals. In addition, in the Ashanti belt, there are deposits associated with sulfide disseminations and stockworks in granitoids, where the sulfide is destroyed at pressure oxidation and the gold is converted into metallic form as free or visible gold in the rocks. These stockworks veins consist of quartz, calcite, pyrite, and arsenopyrite, which are important indicator minerals in the Kubi concession. Finally, the gold that occasionally occurs, particularly within conglomerates and in a smaller amount within the interbedded quartzite, reveals a palaeoplacer gold mineralization of the Tarkwaian Group caused by hydrothermal alterations that deposit the gold as free gold in the milieu of the conglomerates and as inclusions in other mineral grains. From Table 2, the Au assay result shows that the Au grades downward, indicating that these hydrothermal activities resulted in a low-grade distribution of gold mineralization horizons in the Kubi concession above the garnet zone.

We anticipate that the indicator minerals at the Kubi concession area, from the oxide zone to greater depths (the ore zone), have experienced similar physical, chemical, and geological conditions and protruded from a hydrothermal system, where the minerals have been precipitated by hot fluids and altered by veins of surrounding rocks.

X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy were applied for identifying and comparing pathfinder elements and indicator minerals at different depths in the Kubi mining area in Ghana. With these combined analytical techniques, we identified pathfinder elements and indicator minerals of Au in a diamond drill hole; especially, the secondary mineral phases periclase and hematite that are present in the almandine-type garnet found in the gabbroic rock located at the mining area. Pathfinding elements Fe, Si, O, C, Al, Ca, Mg, Na, Mn, Cl, S, and K in the samples act as the main contributions to the indicator minerals; pyrite, quartz, periclase, and almandine-type garnet. Grains within sediments in the surface oxides are controlled by quartz and hematite, the bedrock contains pyrite and pyrrhotite, and the orebody contains garnet, periclase, and biotite as pathfinder minerals within the garnet zone. The analysis also indicates a mineralized oxide zone attributed to the orebody of several sulfide minerals and the segregation of alteration from garnet–periclase minerals to hematite. We attribute these indicator minerals to the four major types of primary Au mineralization that form part of the Ashanti belt: (1) sulfide mineral disseminations and stockworks in granitoids; (2) mesothermal, which comprises steeply dipping quartz veins in shear zones in Birimian sedimentary rocks; (3) sulfide ores containing auriferous arsenopyrite and pyrite that are closely associated with the quartz veins; and (4) paleoplacer deposits of the Tarkwaian Group, where gold occurs as free gold in the conglomerates. We conclude that the mineralogy of the alluvial zone is the chemical weathering reaction of almandine-type-garnet mineral from the orebody. This weathering process produces by-products such as periclase and hematite that act as pathfinders in the Kubi concession area.