Introduction

Lode gold and silver mineralization is often the footprint of a metal-laden hydrothermal fluid, circulating through a system of faults and shear zones at some point in Earth’s history. Herrington et al. (1999) placed gold deposits into seven major categories; Carlin-type gold deposits, epithermal high and low sulfidation deposits, porphyry copper–gold deposits, iron-oxide copper–gold deposits, gold-rich massive sulfide deposits, orogenic gold deposits, and secondary gold deposits. Constraining the physico-chemical conditions of formation, and subsequently classifying a given gold deposit, is of paramount importance in developing effective exploration plans and efficient mining operations (Zhou et al. 2022). Different gold deposit types show different defining mineralogical associations and these are usually reflected in the mineral inclusion suites in gold particles from these deposit types (Chapman et al. 2021b, 2023). For example, Chapman and Mortensen (2006) showed that the mineral inclusion suite of the Eureka Creek gold grains closely matches the mineral associations (pyrite, sphalerite, galena, chalcopyrite, hessite, tetrahedrite, hessite, argentite, and petzite) of the high-grade epithermal Au–Ag gold deposit at Pajingo, Australia. Also, predictive models have shown that the Au/Ag ratios of native gold and electrum are a direct indication of the [(Au/Ag)aq] ratios and concentrations of other minor trace elements (e.g., Cu, Hg, Cu, Pd, Bi, and Te) in the hydrothermal fluid from which the gold was precipitated (Gammons and Williams 1995). These findings have provided a solid scientific foundation for the use of mineral chemistry and mineral inclusion suites in native gold and electrum as a powerful tool to discriminating among Au deposits of different types in recent studies (Chapman et al. 2018, 2022a, 2021b, 2022b, 2023; Liu and Beaudoin 2021). For example, Chapman et al. (2023) demonstrated this quite clearly using composition and inclusion suites of 11,840 detrital gold particles from 160 localities of known deposit types in British Columbia (Canada) to generate templates which were then used to fingerprint the source deposit types of detrital gold from 41 locations where gold genesis was previously unknown. The microchemical signatures of detrital gold and electrum have also been shown to be particularly useful in exploring for primary deposits in regions of poor rock exposures such as the Eureka Creek in Yukon territory in Canada and Borland Glen in Scotland (Chapman and Mortensen 2006). Similar studies have also been carried in Cameroon, to establish the source of alluvial gold from the Lom and Nyong basins (Omang et al. 2015) and the Ako’ozam and Njabilobe creeks (Fuanya et al. 2019).

The observed sulfide mineral assemblages in a deposit can be linked to a particular style of mineralization in magmatic-hydrothermal systems. For example, low sulfidation epithermal deposits are typically associated with pyrite–pyrrhotite–arsenopyrite and Fe-rich sphalerite, while the high-sulfidation deposits are usually characterized by enargite-luzonite-covellite + pyrite assemblages (Hedenquist 2000). Where there is sufficient paragenetic evidence to show that sphalerite precipitated from a fluid in equilibrium with pyrite or pyrrhotite, it is possible to use the Fe contents of sphalerite to deduce the sulfidation state of a deposit (Toulmin and Barton 1964; Shikazono 1985; Einaudi et al. 2003).

In Cameroon, most known primary gold occurrences are confined to regional scale NE–SW faults and shear zones which are aligned to the Central Cameroon Shear Zone system (e.g., Suh et al. 2006; Ateh et al. 2017; Vishiti et al. 2017). These deposits have largely been described as orogenic and are thought to have been emplaced during the Pan-African orogenic event (Suh et al. 2006; Ateh et al. 2017; Ndonfack et al. 2021a). Suh et al. (2006) and Asaah (2010), respectively, observed that gold mineralization in the Dimako-Mboscoro and Batouri districts is hosted in steeply dipping quartz veins that display zoned alteration envelops consisting of quartz, sericite, hematite, and pyrite with an ore mineral association of gold, hematite, sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, chalcocite, and covellite, prompting the conclusion that gold mineralization in the Precambrian mineral belt of Cameroon bears the hallmarks of orogenic gold systems, as described by Goldfarb et al. (2005). Furthermore, fluid inclusion associations of CO2–NaCl–H2O ± N2 ± CH4 with disproportionate H2O–CO2 ratios, homogenization temperatures of 300 to 320 °C, and fluid salinities between 0.5 and 10.8wt% NaCl eq. have also been reported in quartz veins from the Betaré Oya and Colomine gold districts in East Cameroon (Ndonfack et al. 2021a, 2021b). These data permitted the estimation of trapping temperatures of 310 °C, apparent trapping pressures of 1.4–3.5 kbar at average depths of ~ 6 to 9 km for the Au events at the Betaré Oya and the Colomine districts, further supporting previous conclusions of the orogenic origin of gold mineralization in the Precambrian mineral belt of Cameroon. Examples of these orogenic gold occurrences in northern Cameroon with active exploration are the Wapouze and Bibemi districts (exploration licenses wholly owned by Oriole Resources PLC). Maiden resource estimates at the Bibemi project stand at 4.3 million tons grading 2.19 g/t Au for 305,000 oz Au.

The Galim-Legalgorou district is part of an emerging metallogenic province in north Cameroon with active gold exploration as well as extensive hard-rock artisanal mining by local communities. Despite the ongoing exploration and artisanal mining, there is no published scientific literature on the nature, occurrence, and classification of the gold mineralization in this district. However, preliminary field data and the recently updated geological map of Cameroon show that the district is largely underlain by volcanic rocks of the Cameroon Volcanic Line, and that the mineralization is hosted by these volcanic rocks. In 2015/2016, the exploration company, Daewoo International SA completed 15 vertically oriented diamond drill holes to test the extent of the mineralization. The drill hole data show a pretty simple stratigraphy with brecciated tuff, intercalating with hydrothermal breccia units of rhyolitic-to-trachytic protoliths (Daewoo log sheets 2015/2016). Fresh mineralized core samples for ore mineral studies in this study were obtained from drill holes LH15-08 and LH15-10, which are the deepest holes with depths of 202 and 196-m, respectively. The close spatial association of this mineralization with rocks of the CVL only raises the important question of whether or not it represents an epithermal style, because the CVL is not a typical hotspot or rift magmatic system.

In this contribution, we present key field observations, in conjunction with a full characterization of the ore mineral and alteration assemblages and compositional data for Au alloys and sphalerite from the Galim-Legalgorou district to constrain the origin, classification, and evolution of the mineralization.

Geological setting

Regional geological setting

The Galim-Legalgorou Ag–Au mineralization is located in one of the major blocks of the north-western branch of the Cameroon Volcanic Line (CVL) on latitude 7° 16′ 20″ N and longitude 12° 18′ 32″ E (Fig. 1). The CVL is an NE–SW-trending chain of Cenozoic volcanoes running from the Atlantic coast of Cameroon into the Central African Republic, Chad, and Nigeria, and whose origins remains debated due to lack of age progression in the volcanic centers (Adams 2022). Most authors have attributed the formation of the CVL either to a mantle plume (e.g., French and Romanowicz 2014; Njome and Wits 2014) or the upward limb of a small-scale magma convection cell (e.g., Gallacher and Bastow 2012; Saeidi et al. 2023). The plutonic rocks of this line, consisting of gabbros, syenites, and granites (Moreau et al. 1987), are predominantly alkaline, with some acidic volcanic equivalents which range from rhyolite to phonolite. Ngako (2007) noted that these plutonic complexes contain anomalous concentrations of lead, zinc, tin, niobium, tantalum, uranium, and thorium. Volcanism along the CVL began at ~ 52 Ma (Moundi et al. 2007), and Mt Cameroon (4100 m above sea level) is the only presently active member of the CVL with seven eruptions recorded in 1909, 1922, 1954, 1959, 1983, 1999, and 2000 (Aka et al. 2004). The ages of igneous rocks from the CVL indicate that the earliest igneous activity mainly consisted of granitic and syenitic intrusive ring complexes that range from 66 to 30 Main age without any evidence for age progression (Fitton 1987). Petrological studies by Fitton (1987) and Suh et al. (2010) show that the CVL’s mostly basaltic volcanoes have erupted poorly fractionated alkaline (silica poor) basalts in relatively low volume. This raises the question if the crust beneath the CVL is characterized by cooled mafic intrusions and present-day melt, as is normally the case in hot spots and rifts worldwide. Gallacher and Bastow (2012) noted that the velocity ratios (Vp/Vs) of seismic waves observed along the CVL were lower compared to the global average for hotspots and rifts, and therefore concluded that the bulk crustal composition below the CVL was rather of granodiorite and granite-gneiss.

Fig. 1
figure 1

(Modified from Njeudjang et al. 2022)

a Geologic map of Cameroon showing the Cameroon Volcanic Line (CVL), alongside the Tchiolire-Banyo Fault (TBF), the Adamwa fault (AF), and the Sanaga fault (SF). Together, the TBF and the AF represent the central Cameroon shear zone system. The yellow star represents the study area. The map is modified from Adams (2022). The inset (not to scale) shows the major African cratons and their respective flanking fold belts. b Geological map of the North central Cameroon (Adamawa) showing the location of the study area and the relationship between the CVL and older geologic formations.

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The CVL has a combined length of about 1800 km and cuts across Pan-African Neoproterozoic rocks of the north-western Cameroon domain) and the Adamawa-Yade domain (Ngako et al. 2008; Adams 2022). Together with the Yaoundé domain, these Precambrian rocks collectively form what is known as the North Equatorial Fold belt of Cameroon (Van-Schmus et al. 2008). The Adamawa-Yade domain extends eastwards from central Cameroon into the Central African Republic as the Yade massifs and is consisted of widespread Pan-African syn-tectonic plutonic rocks of high-potassium calc-alkaline affinity (Tchameni et al. 2006; Toteu et al. 2006; Tagne-Kamga 2003). The central African shear zone system is the main structural feature cross-cutting the Adamawa-Yade domain and the north-western Cameroon domain rocks. According to Ngako et al. (2008), this shear zone is a Pre-Mesozoic crustal strike-slip fault system, which extends from central Africa, across the Atlantic, into NE Brazil and forms two branches in Cameroon. These are the Central Cameroon Shear Zone to the north and the Sanaga fault to the south. Much of the southern part of Cameroon is covered by the Archaean Ntem complex which represents the Congo craton in Cameroon. According to Tchameni et al. (2010) and Akame et al. (2020), the rocks of the Ntem complex include charnockites and rocks of the tonalite–trondhjemite–granodiorite (TTG) association occurring in conjunction with banded iron formations and greenstones and constitute the oldest known rocks in Cameroon (3155 to 2850 Ma).

Deposit geology and field relations

The main rock types observed in the Galim-Legalgorou area are felsic (rhyolite) breccia, mafic and intermediate volcanics (basalts and trachyte), pyroclastics (tuff), quartz veins, gneisses, and granites (Fig. 2). Drill hole logs (LH15-08 and LH15-10) show that the tuff breccia intercalates with hydrothermal breccia from the surface to about 200-m, where is it underlain by biotite gneiss, and is variably mineralized with fine-grained pyrite, which occurs alongside sphalerite and galena, within numerous fractures in the fresh hydrothermal breccia, although the sphalerite and galena are only visible in thin section. The volcanic units and quartz veins overlay or intrude biotite gneisses, which form the basement in the area and belong to the Pan-African North western Cameroon rock domain. The main breccia body measures about 1.0 km at its thickest point and extends over a strike length of about 2.5 km. In some places, the rhyolite is tectonically undisturbed with characteristic flow banding texture clearly observed on outcrops (Fig. 3a). It occurs as large boulders and outcrops and varies in color from white, yellowish brown to grey. Rhyolite is composed principally of K-feldspars which are clearly visible as phenocrysts in hand specimen. Basalt occurs as outcrops on mountain chains with high rises and expanses of steep slopes that go all around the felsic and intermediate units. In hand specimens, the basalt is dark in color and shows varied textures from fine to medium grain, with most samples having large pyroxene phenocrysts in a finer groundmass (Fig. 3d). Trachyte is much less common and occurs as outcrops and boulders in contact with the rhyolitic unit. In hand specimen, it has a fine-grained to porphyritic texture, a pink coloration with the presence of phenocrysts embedded within a fine groundmass (Fig. 3e).

Fig. 2
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Deposit geology of Galim-Legalgorou based on field mapping data and location of active artisanal mining sites

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Fig. 3
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Hand specimen samples from the study site. a Undisturbed rhyolite showing flow banding texture. b Centimetric scale ore jogs in brecciated rhyolite. c Mineralized hydrothermal breccia showing the close link between mineralization, hematite, and dickite alteration. d Basalt showing plagioclase phenocrysts within a finer groundmass. e Trachyte with phenocrysts of potassium feldspar. f Volcanic breccia with large, zoned angular clast, indicating a thermal alteration event. g Core sample of fresh ore with pyrite bearing micro-veins, some of which also contain dickite. h Outcrop of main quartz vein showing two generations, Qz-1 (early) and Qz-2 (late). i Amethyst crystals from Akouri pit

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The volcanic breccia is light grey in color with large angular fragments of rhyolite or trachyte (up to 20 cm) caught in an ash cement of a mafic-to-intermediate composition (Fig. 3f). The clasts show rim to core zonation, probably indicative of thermal alteration during eruption.

The biotite gneiss is strongly folded and occurs as extensive outcrops predominantly in eroded valleys and shows a strong foliation that is fairly in accordance with the regional structures, with NE–SW strike and dipping to the NW. It is characterized by quartz, plagioclase, and mica (biotite), and the color is marked by alternating white and dark bands. The granite flanks the gneiss to the south and occurs as outcrops on massive hills. In hand specimen, the granite shows a granoblastic texture, with large grains of quartz and k-feldspars making up the bulk of the sample.

Artisanal mining is currently concentrated on three main sites; Morgue, Koutourou, and Akouri (Fig. 2). Each mining site is an agglomeration of small interconnected mining pits with a complex network of hand-dug adits and shafts, some of which are up to 70 m deep. The gold mineralization is hosted in discontinuous ore jogs within a large body of the felsic-to-intermediate volcanic rocks that are highly brecciated and commonly show intense hydrothermal alteration (Fig. 3b). Individual ore jogs commonly have lengths of up to 3 m and thicknesses ranging between 2 and 20 cm. The ore jogs occur close to one another and only separated by large altered clasts of the altered host rock. Magnetite and hematite commonly occur with electrum in the weathered zone. In the fresh ore zone, pyrite is the most abundant ore mineral distinguishable in drill core (Fig. 3h). Quartz is the most important gangue mineral and occurs both as quartz veins and within mineralized ore jogs where it shows a characteristic dog-tooth and honey-comb texture. Three types of quartz (Qz-1, Qz-2 and Q-3) can be identified in outcrop and hand specimens (Fig. 3h–i). Qz-1 constitutes the major quartz vein in the area and is oriented fairly east–west (N070) and steeply dipping at 80° to the south. It has a smoky brown appearance, which is texturally massive, exhibiting local silicification.

The quartz vein measures about 15–20 m in width and can be traced on the surface for more than 70 m along the local river bank. The vein is hosted within the gneiss which is overlain by the rhyolite breccia zone. An argillic alteration zone of about 10 m wide can be traced on either side of the exposed vein. Qz-2 is mainly chalcedony showing a weak colloform banding and open-space filling textures, a milky white appearance and is hosted within locally healed fractures in Qz-1 (Fig. 3h). Qz-3 mainly amethyst exhibits a characteristic dog-tooth texture and is exclusively associated with the mineralized brecciated zone in the felsic–intermediate volcanic unit (Fig. 3i). From field observations, Qz-1 clearly predates Qz-2, but the cross-cutting relationship between Qz-1 and Qz-3 or Qz-2 and Qz-3 are not exactly clear, as Qz-3 (amethyst) does not outcrop on the surface. It is only observed in hand specimens that are dugged out with ore from the underground workings at about 40–60 m depths. Pyrite (pyritization), chlorite/smectite, kaolinite and dickite (argillic alteration), and hematite are the major alteration minerals associated with the mineralization (Figs. 4a–h and 5a). Dickite is intimately associated with both the weathered and fresh ore zones, and typically is present as open-space filling within hydrothermal breccia bodies (Fig. 4a). Kaolinite is pervasive and commonly varies in color from grey/white in the altered felsic rocks to reddish where intermediate rocks (Fig. 4b–c). Specular hematite commonly occurs with goethite in ore jogs in the felsic breccia zone (Fig. 4d–e). Chlorite/smectite alteration zone are also very common around the intermediate brecciated unit, and this alteration is typically observed between the gneiss wall-rock and the trachytic breccias (Fig. 4f).

Fig. 4
figure 4

Wall rock alteration categories observed in Galim-Legalgorou. a Dickite vein network within weathered rhyolitic rocks. b,c Pervasive kaolinite observed on walls of mining pits d,e Hematite and goethite in mineralized ore jog. f Chlorite/smectite alteration in weathered intermediate (trachyte) rocks

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Fig. 5
figure 5

a Full slide-scan of polished section of fresh ore sample showing pyrite alteration/mineralization in rhyolite. The mineralization occurs both as tiny vein filling and dissemination within host rock. b Back-scattered electron (BSE) image showing veinlet-hosted pyrite, sealed on one side by quartz and by dickite on the other side. c BSE images of pyrite hosting inclusions of sphalerite and quartz. d Sphalerite filling voids in rhyolitic host rock matrix. e Section of polished rock showing pyrite and sphalerite occurring together, indicating coeval precipitation. f BSE image of polished section of fresh ore, focusing Au-bearing rutile phase occurring within open micro-fractures and healed by quartz

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Sampling and analytical methods

Rock sampling and electrum grain collection

Samples of different rock units in the study area were collected along field traverses in and around the main artisanal mining pits. Rock samples were collected by chipping with a geologic hammer on outcrops and boulders. The rock sample was then transferred into a polythene bag and appropriately labelled with a permanent marker. Great care was taken to only collect samples that are as fresh as possible, and from outcrops or drill core. However, in line with our study objectives, we also collected some mineralized samples, which have invariably been affected, to some degree, by hydrothermal alteration. Hydrothermal alteration exposed in mining pits was also mapped and recorded. For the purpose of this study, two mineralized half-core samples representing the un-weathered hypogene ore stage were provided by Daewoo International Cameroon SA. The samples were respectively from hydrothermal breccia zone from two different vertically oriented drill holes (LH15-08 and LH15-10) on the mineralized breccia zone. The samples were collected respectively from depths of 130 and 65 m, with significant visible sulfide mineralization. In all, 20 rock samples representing the different rock types in the area were collected.

Gold grains were obtained by crushing and panning about 70 kg of ore from the Morgue, Akouri and Koutouri artisanal mining pits in the weathered zone of the mineralization. The bulk samples were crushed by means of a diesel-powered mini ore crusher adapted for small-scale mining activities, down to ca. 1-mm size fraction. The crushed sample was then carefully washed with water over a retention mat to remove all clay. The residue was concentrated and the remaining clay panned to obtain a heavy mineral fraction. The heavy mineral concentrate was then sun-dried for 12 h to remove all moisture. To eliminate contamination of the sample, the ore crusher was cleaned with a high pressured air from a compressor, before the sample was crushed.

Mineral chemistry

This study makes use of 51 electrum grains and 2 mineralized core samples from Galim-Legalgorou. Mineralized zones of core samples were cut, embedded in epoxy, polished and observed in reflected light and scanning electron microscopy. The dried heavy mineral concentrate obtained from panning of the crushed sample was passed through an L-1 Franz Isodynamic Magnetic separator to remove the very abundant magnetic component. All magnetic components were removed from sample stream when the accelerating current reached 1.5 A. The rest of the concentrate, including electrum was then embedded in epoxy and polished. Mineral phases were identified by combination of optical microscopy using a LEICA DM4500P polarization microscope, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS) using a JXA-iSP100 JEOL Superprobe electron-microprobe and Raman spectroscopy using a confocal Raman microscope equipped with a 488 nm solid-state laser. The chemical composition of electrum and sphalerite was determined by wavelength-dispersive electron-probe microanalysis (EPMA) using a JXA-iSP100 JEOL Superprobe instrument. Analyses were performed at an acceleration voltage of 25 kV and a beam current of 2.8 × 10–8 A was used. The beam size was set to 1–2 µm size. Electrum was analyzed for the elements Ag, As, Au, Bi, Cu, Fe, Hg, Pb, S, Sb, Te, Tl, and Zn, whereas sphalerite was analyzed for Ag, As, Au, Mo, Bi, S, Sb, Te, Fe, Mn, Cu, and Zn. In addition, electrum compositional maps were obtained by wavelength-dispersive X-ray mapping using a counting time of 10 ms/pixel and a resolution of 1100 × 1300 pixels. The matrix correction was performed with the PRZ-XPP and the ZAF methods for electrum and sphalerite, respectively. Only elements with concentrations above the detection limits have been reported in this study. The detection limits for the reported elements were set at Ag (0.01 wt%), Au (0.01 wt%), Bi (0.02 wt%), and Hg (0.1 wt%). The peak counting time ranged between 10 and 30 s, while background counting time ranged from 5 to 10 s. The complete detection limits, standards used for instrument calibration, radiations, and respective counting times for all other elements are listed in Table 1.

Table 1 EMPA measurement conditions for electrum and sphalerite
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Results

Ore microscopy

Electrum, pyrite, sphalerite, galena, Ti-bearing magnetite acanthite, goethite, specular hematite, and rutile phases constitute the hydrothermal ore mineral phases in the mineralization. It should be noted that pyrite, rutile, sphalerite, and galena were only observed in the fresh ore while electrum, acanthite, Ti-bearing magnetite, goethite, and specular hematite were only observed in the weathered ore samples (heavy mineral concentrate). The term ‘electrum’ is used here to denote any naturally occurring Au–Ag alloy. In the fresh ore, pyrite is disseminated within the hydrothermally altered host rock and also occupies healed micro-fractures, being closely associated with quartz and dickite (Fig. 5a–c). Sphalerite occurs both as inclusions in pyrite and also fills open cavities and fractures within the rock matrix, where it is sometimes intergrown with pyrite (Fig. 5c–e). Galena is observed exclusively as tiny inclusions (< 5 µm) in pyrite. Fine-grained intergrowths of Au particles sometimes occur with rutile phases in fresh ore samples (Fig. 5f).

Acanthite occurs as inclusions in electrum and frequently contains significant Se (up to 8.0 wt.%) and Tl (up to 2.2%) concentrations. The mineral has also been observed as a separate phase in the heavy mineral concentrate. Ti-bearing magnetite is abundant in the concentrate but could not be detected in the fresh ore.

Goethite is abundant in the weathered zone and is thought to be a product of low-temperature dissolution of pyrite and sphalerite under oxidizing conditions. Tiny hematite inclusions were observed in dickite isolated from the weathered zone. A single grain of lead oxide (~ 90 wt% PbO) bearing numerous inclusions of quartz was observed in the heavy mineral concentrate section.

Electrum morphology and chemistry

A total of 51 electrum grains from the concentrate were analyzed in this study. Most grains show irregular, elongated-to-oval shapes with curved margins in reflected light and a size range from 20 to 514 µm (Fig. 6a–d; f–g). Many grains are physically heterogeneous and show simple primary zonation in BSE imagery (Fig. 7). Primary zonation is characterized by a bright Au-rich core that is usually surrounded by a darker Ag-rich intermediate zone, with both zones roughly of equal thickness. The boundary between the Ag-rich and Au-rich zones is gradational, and the compositions in the area in between both zones contain Au and Ag in approximately equal proportions. The rims are preferentially depleted in Ag and characterized in many cases by the dispersion of tiny, (< 1 µm) high fineness gold particles, forming a ‘belt’ around the electrum particle (Figs. 6c; 7a–f). A majority of the non-zoned electrum grains are either relatively Au-rich or Ag-rich.

Fig. 6
figure 6

a Selected electrum grains seen under a binocular microscope. b Electrum particle showing association with Fe-S-Mn phases, which are also common trace elements in in the sphalerite mineral structure. c Au-rich rims and pits resulting from preferential Ag-leaching. Also notice the Al–Si–O inclusions in electrum which may be pre-existing feldspars before fluid injection. d Se- and Tl-bearing acanthite inclusion in electrum. e Acanthite grain with quartz inclusions. f,g Au-rich fissures (> 90% Au) are common on the surfaces of some electrum particles and represent Ag-leaching and electrowinning processes in low PH and low-temperature supergene environment

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Fig. 7
figure 7

EMPA element maps of selected zoned electrum grains. a and d Two selected electrum particles shown in reflected light. Both grains are apparently homogeneous when viewed in reflected light. b and e Inner-core to outer-core gradational variation of Au. c and f Inner-core to outer-core gradational variation of Ag

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Some electrum grains also show random Au-rich zones occurring as completely healed micro-fractures with Au concentrations as high as 96.74 wt% (Fig. 6f–g). Zoned grains commonly show a concentration difference of about 10–15 wt% of Ag and Au between the core and the intermediate zone, with the concentrations of Au and Ag showing a continuous inverse correlation. The most significant mineral inclusions in electrum are quartz, acanthite, and Al–Si–O (± Na-Ca-Fe) oxides thought to be remnants of feldspars (Fig. 6c–d). Acanthite also occurs freely in the heavy mineral concentrate (Fig. 6e).

The electronic Appendix (EA-1) summarizes the electron-microprobe data and shows that electrum contains a few minor elements in addition to Au (36.85–70.59 wt%) and Ag (27.31–60.67 wt%). Bi (0.08–0.54wt%), Hg (0.39–1.05 wt%), and Te (0.06–0.19 wt%) are the most significant trace elements. In summary, the average elemental concentrations and respective standard deviations (SD) in electrum are as follows; Ag (53.26 wt%; 6.65 SD), Au (44.38; 6.64 SD), Bi (0.29 wt%; 0.07 SD), Hg (0.67 wt%; 0.14 SD), and Te (0.12 wt%; 0.02 SD). A cumulative percentile plot of Ag contents of the core compositions of electrum shows two distinct gold populations (Fig. 8a). This bimodality of the electrum grains is also observed in the bivariate plots of Au vs Te, Bi, and Hg (Fig. 8b–d). The first population (Pop 1) ranges from 39.66 to 46.58 wt% Ag, while the second (Pop 2) ranges from 54.0 to 60.67 wt% Ag, after a brief transition (48.26–53.87 wt% Ag). Au is observed to show positive correlations with Bi (fair) and Hg (weak) but no correlation at all with Te. The Hg and Bi values appear to be slightly higher in high Ag population compared to the lower Ag population.

Fig. 8
figure 8

a Cumulative Ag percentile plot showing two distinct sub-populations of electrum. b–d Bivariate plots of Au against Te, Bi and Hg, respectively. All data are presented in weight percent, except otherwise indicated

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Sphalerite chemistry

Representative sphalerite compositions alongside respective FeS mole percentages are shown in Table 2. On average, the composition of sphalerite is mainly Zn (51.2wt%; 3.38 SD), Fe (13.21wt%; 2.4 SD), and S (34.0wt%; 1.28 SD). Mn is essentially the only minor element in sphalerite, with an average concentration of 0.24 wt% at 0.18 SD. The calculated average FeS content of sphalerite is 23.13 mol% (4.38 SD), with minimum and maximum values of 13.99 and 28.91 mol% respectively. The stoichiometry of sphalerite in this study approximates (Fe, Zn)S. Zn shows a strong positive correlation with Fe in sphalerite but varies negatively with Mn (Fig. 9).

Table 2 Representative sphalerite compositions (wt%) from Galim-Legalgorou and corresponding FeS contents (mol%)
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Fig. 9
figure 9

Bivariate plots of Zn against Fe, Mn for sphalerite from Galim-Legalgorou

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Discussion

Paragenetic evolution of the mineralization

Macro- and microfabrics developed in minerals during primary mineralization or during subsequent modification by changing fluid composition or residence in the surficial environment can play an important role in the relative timing of a mineralizing event (Chapman et al. 2021b; Ndonfack et al. 2021a). Information obtained from physical and chemical heterogeneities in electrum grains and mineralized rock samples, coupled with field observations in Galim-Legalgorou, suggests that Ag and Au were deposited from chemically evolving hydrothermal fluids in two distinct hypogene stages (EA-2). Stage I is characterized by the precipitation of quartz, dickite, chlorite/smectite, Au-rich electrum, pyrite, sphalerite ± galena, hematite, Ti-bearing magnetite, and rutile at relatively higher temperatures and near-neutral pH (5–7) conditions. This stage is represented by the relatively Au-rich cores of zoned electrum grains. The Al–Si–O (± Ca ± N ± Fe) mineral phases observed as inclusions in electrum are hereby interpreted as remnants of pre-existing feldspars and pyroxenes that served as nucleation sites for electrum. The Ag-rich mantle of zoned electrum grains represents stage II, which is characterized by the deposition of Ag-rich electrum, acanthite, pyrite, sphalerite ± galena, and kaolinite at relatively lower temperatures and more acidic conditions (pH 2–4) (Hedenquist and Arribas 2022). Acanthite and electrum constitute the dominant minerals of the Ag–Au–S system in many Ag–Au deposits (Barton 1980). A rapid inversion is known to occur at about 180 °C during disequilibrium cooling of the system, allowing only the low-temperature phases likely to be found in nature, thus explaining why the ternary phases (e.g. Ag3AuS2, AgAuS) and the high-temperature polymorph of Ag2S (argentite) are rare or restricted to environments with unusually high sulfur concentrations, high gold-to-silver ratios, and the absence of such elements as Sb, As, Bi, Se, or Te, which tend to form competing phases with Ag or Au (Barton 1980). The presence of Se- and Tl-bearing acanthite in the system, therefore, suggests a low-temperature epithermal environment (< 177 °C) in which acanthite is more stable than argentite, and according to Simmons et al. (2000), provides the first indication that pressure drop and boiling may have driven ore deposition at Galim-Legalgorou. Similar Se-bearing Ag mineral inclusions (notably naumanite–Ag2Se) have been reported in gold particles from the Blackdome and Brucejack low sulfidation epithermal deposits in British Columbia, Canada (Chapman et al. 2022b, 2023), further supporting a low sulfidation origin for the Galim-Legalgorou deposit. Colloform banded chalcedonian quartz and hydrothermal breccia is also a common feature of epithermal Ag–Au systems (Simmons et al. 2000).

Magnetite and hematite are common accessory minerals in crustal rocks and many hydrothermal mineral deposits. Under reducing conditions, magnetite is more stable than hematite, given than all other species and minerals in solution are in equilibrium (Otake et al. 2007). Also, both redox and non-redox transformations can occur between magnetite and hematite in hydrothermal solutions under reducing conditions and temperatures above 100 °C at pH 2–6 (Otake et al. 2010). The coexistence of Ti-bearing magnetite and specular hematite in the ore at Galim-Legalgorou could therefore reflect the composition of a moderately reducing fluid that evolved over time to become more oxidizing as it ascended and cooled. Argillic alteration represented by dickite + kaolinite + smectite/chlorite assemblage may have occurred in both stages, with dickite dominating in stage I, while kaolinite and sericite/chlorite in stage II. According to Hedenquist and Arribas (2022), dickite can serve as a pH-temperature marker in magmatic-hydrothermal systems, as it is only stable between 200 and 260 °C under acidic conditions. Taking into account the stability limits of the minerals acanthite and dickite, we therefore propose a temperature range from below the upper stability temperature of acanthite (< 177 °C) to 300 °C (upper stability limit of dickite) for the gold event at Galim-Legalgorou. Finally, goethite constitutes the main ore mineral in the supergene stage and is basically the alteration product of pyrite and sphalerite.

Electrum morphology and fluid evolution

Electrum from Galim-Legalgorou shows primary growth zoning, with a Au-rich core, surrounded by a Ag-rich mantle and a transition zone in between (Fig. 7). This microfabric has been reported in some epithermal and volcanogenic massive sulfide deposits globally (e.g., Shikazono 1985; Liu and Beaudoin 2021). Primary zonation of gold alloys of this sort is often interpreted as the result of an abrupt change in the fluid regime in response to processes, such as fluid pressure drop, boiling, cooling, and mixing with meteoric water, or due to a chemical modification of an existing Au-rich electrum grain by a distinct, late Ag-rich fluid at lower temperatures (Chapman et al. 2006; Liu and Beaudoin 2021). The transition zone between the Au-rich core and Ag-rich mantle suggests that Ag may have been mobilized from Au-rich electrum via a solid-state diffusion (SSD) process at low pH and reducing hydrothermal fluid conditions, facilitated by a new pulse of Ag-rich fluids (Liu and Beaudoin 2021). Primary zoning observed in electrum from Galim-Legalgorou is comparable to the previous findings by Liu and Beaudoin (2021) for the Ken Snyder low sulfidation epithermal deposit, in the northern Nevada rift, USA. However, the step-change zonation from a core to a more Ag-rich mantle constitutes a relatively uncommon textural feature in the literature, as Chapman et al. (2022a) did not report any such textures in their study of over 40,000 gold particles from different deposit types around the world.

Gold-rich rims observed on some electrum grains from Galim-Legalgorou are thought to be the product of post-depositional modification (notably by preferential Ag-depletion) in the supergene environment (Vishiti et al. 2015; Fon et al. 2021; Chapman et al. 2021b, 2022b). However, the random Au-rich tracks (Fig. 6f–g) are considered to be the result of overprinting by a late Au-rich fluid in the hypogene environment (Liu and Beaudoin 2021). This enrichment process is thought to proceed by replacement of Ag on the pre-existing electrum grains driven by the injection of a relatively gold-rich fluid, although many authors recognize that this process requires much more time than the actual age of the mineralization, to account for the observed thicknesses of these tracks (Liu and Beaudoin 2021).

Electrum chemistry and fluid evolution

The chemical composition of native gold or electrum has been shown to be a sensitive indicator of the fluid chemistry, physical parameters, and environment of ore deposition (Gammons and Williams-Jones 1995; 2021; Liu and Beaudoin 2021). Furthermore, the trace-element chemistry of gold particles can have important genetic implications for a given mineral deposit (Chapman et al. 2021b). In addition to Au and Ag, electrum in this study also contents significant amounts of the trace elements Bi, Hg and Te. This element suite closely conforms to the trace-element inventory of gold alloys reported for low sulfidation epithermal deposits globally (Liu and Beaudoin 2021; Chapman et al. 2022a). According to Liu and Beaudoin (2021), these elements could have been incorporated into electrum a result of the loss of H2S from the hydrothermal fluid triggered by pressure drops, boiling, and sustained temperature reduction. Cumulative percentile plots for Ag and bivariate plots for Au vs Te, Bi, and Hg (Fig. 8a–d) show two distinct populations for electrum from Galim-Legalgorou. These two populations (Pop 1 and Pop 2) are strongly and respectively correlated to the compositions of the Au-rich cores and Ag-rich mantles of the zoned electrum grains and represent two distinct gold deposition events (stages). This conclusion is consistent with field relations (Fig. 3h) where one generation of chalcedonian quartz (Q2) is observed to cross-cut another (Q1), suggesting that at least two separate fluid influxes were responsible for the mineralization.

In this study, Bi is present with consistent concentrations of about 0.29 wt% in all electrum analyses and shows a positive correlation (r2 = 0.40) with Au. This suggests that deposition of Au alloys resulted in co-precipitation of Bi. The weak correlation between Au (or Ag) and Te (or Hg) may mean that these elements exist as sub-micrometer or nano-particle inclusions or lattice impurities (Liu et al. 2021). However, Hg is also known to show strong affinity for Au and to partition strongly into the vapor phase of hydrothermal fluids (Chapman et al. 2021b) and may therefore have been co-precipitated and incorporated into electrum during boiling at Galim-Legalgoro.

Electrum from Galim-Legalgorou has rather high Ag concentrations that vary within a comparatively wide range (27.3 wt% to 60.7 wt%; average 53.1 wt%). This high values and wide range of Ag in electrum is common for most low sulfidation epithermal deposits, although some bonanza deposits have reported significantly lower Ag concentrations (Chapman and Mortensen 2006). Bisulfide complexes (Au(HS)2−, AuHS0) in these environments represent the dominant form of gold in solution, while Ag transport is dominated by chloride complexes (AgCl2−) in hydrothermal fluids which are reducing and have near-neutral pH and moderate H2S concentrations (Morrison et al. 1991; Chapman et al. 2021b). Furthermore, predictive modelling calculations have shown that increasing Ag contents of electrum are directly linked with falling fluid temperatures, decreasing Au/Ag ratios in solution, decreasing acidity, and increasing H2S activity relative to that of chloride in hydrothermal solutions (Gammons and Williams-Jones 1995).

The concentrations of Au and Ag in naturally occurring Au alloys are often expressed in terms of the Au fineness, which is calculated as (Au·1000)/(Au + Ag) (Morrison et al. 1991). Gold fineness in the Galim-Legalgorou ranges from 379 to 721 with a deposit average of 455 (EA-1). This relatively low fineness is close to the values reported for many known low sulfidation epithermal systems (Morison et al. 1991; Liu and Beaudoin 2021; Chapman et al. 2021a), although gold fineness is generally not diagnostic for environment of gold deposition. Higher Ag concentrations in electrum have been attributed to either high Ag/Au ratios in the fluid, a steeper precipitation gradient for Ag relative to Au in response to changes in fluid conditions (Gammons and Williams-Jones 1995). The low Cu concentrations and the absence of Cu-bearing minerals in this study are in line with low sulfidation epithermal environments (Townley et al. 2003).

Sphalerite composition and the sulfidation state of mineralization

The results of EPMA analysis of sphalerite show a strong negative correlation between Zn and Fe, which suggests a temperature-dependent substitution of Fe for Zn in sphalerite. Sphalerite observed in this study occurs both in tiny pores (10–20 µm) within the rock matrix and as tiny inclusions (5–10 µm) with not very sharp grain boundaries in pyrite. These observations suggest that sphalerite has co-precipitated with pyrite from the same fluid and that sphalerite formed in equilibrium with pyrite. Therefore, it is possible to use the Fe contents in sphalerite for estimation of physico-chemical conditions of ore precipitation; because this is a function of the temperature, pressure, and sulfur fugacity of the hydrothermal fluid (Toulmin and Barton 1964; Shikazono 1985; Einaudi et al. 2003; Tămas et al. 2021). Einaudi et al. (2003) reported that the sulfidation state of an epithermal system can be evaluated from the FeS content in sphalerite coexisting with pyrite. Sphalerites with FeS contents between 20 and 40 mol% are classified as low sulfidation states, 1 and 20 mol% as intermediate-sulfidation states, and less than 0.05 to 1.0 mol% as high-sulfidation states. Sphalerite from the Galim-Legalgorou deposit contains on average 23.1 mol% (4.38 SD), with minimum and maximum values of 13.99 and 28.92 FeS mol%, respectively, suggesting that the mineralization would represent a low sulfidation system. Many epithermal Au deposits contain Mn minerals such as Mn carbonates (e.g. rhodochrosite, Mn-bearing calcite) and Mn silicates (e.g. rhodonite) in their assemblage, and Mn is also common minor element in sphalerite (Wang et al. 2019). However, Cooke et al. (2009) noted that the Mn content in sphalerite was on average below 2 wt% in the Lihir epithermal deposit of Papua New Guinea. This is in line with sphalerite in this study which contains only low Mn concentrations (0.00 to 0.59 wt%; average 0.24 wt%), suggesting limited competition with Fe, and being in line with the absence of Mn-rich minerals such as alabandite or rhodonite.

Conclusion

By establishing the host rock characteristics, ore mineral assemblages, wall-rock alteration categories, and microchemical compositions of electrum and sphalerite, this study has demonstrated for the first time, the presence of low sulfidation epithermal Ag–Au mineralization (Galim-Legalgorou), in a region only previously known for orogenic gold mineralization. The deposit characteristics can be summarized in the following points;

  1. 1.

    Ag–Au deposition at Galim-Legalgorou can be linked to at least two hydrothermal fluid events, with a rapid, but complex evolutionary cooling path as evident in the primary zonation pattern in zoned electrum, the Ag-cumulative percentile plot, and bivariate plots of Au vs Bi, Te, and Hg. The almost sharp transition from the early to late fluid stages is accompanied by decreasing temperatures and pH conditions and explains the high concentrations of Ag in electrum. This is supported by the absence of high sulfur minerals such as chalcopyrite, enargite, and covellite, and a corresponding absence of Cu in electrum.

  2. 2.

    The presence of Se- and Tl-bearing acanthite as inclusions in electrum provides further supporting evidence for a low sulfidation epithermal environment similar to the Blackdome and Brucejack deposits in British Columbia, Canada.

  3. 3.

    The FeS content of sphalerite (23.13 mol percent) provides additional evidence that the mineralization at Galim-Legalgorou is of a low sulfidation fluid origin.

The discovery of epithermal Ag–Au system on the CVL (a non-hotspot, intra-plate volcanic system) is a major offset in current ore geology models, and opens a whole new frontier for research and exploration of epithermal/porphyry mineralization in similar volcanic units around the world.