1 Introduction

Research by various authors such as Goldschmidt and Peters (1933), Noddack (1936), Burnet (1986), Finkelman and Brown (1991), Meitov and Rodionov (1993), Seredin (2008), and Seredin and Finkelman (2008) has shed light on gold mineralization associated with organic matter (OM)-rich rocks. These rocks, often found in sedimentary basins alongside various compositions of granites, volcanic rocks, shales, and limestones, exhibit significantly higher concentrations of precious metals than typical values, indicating the economic potential for primary extraction or by-product recovery. Bowell et al. (1999) found OM in the Getchell mine (Nevada, USA) with 2–3 ppm Au. Emsbo and Koenig (2007) reported veins of reworked organic matter in the El Rodeo deposit (Dominican Republic) containing up to 100 ppm Au. Hallbauer (1986) described gold intimately associated with hydrocarbons in the Witwatersrand (South Africa), with an average whole rock grade of 5–7 ppm Au.

The widespread enrichment of gold within organic-rich sedimentary formations is related to the Carboniferous Variscan cycle, during which gold was cycled into organic-rich reservoirs globally (Parnell 2019). This mineralization, occurring between 410 and 310 Ma, extended over an impressive distance of 10000 km from the Appalachians to China. The Variscan orogenic gold cycle also includes two distinct regions of Central Europe (Hungary) and Central Asia (Kazakhstan), which are the current interest of this study. Parnell (2019) regarded these regions as a significant gold occurrence associated with the global coal depositions of Variscan gold formation in the Late Palaeozoic age, around 300 million years ago. In Central Europe, gold mineralization takes the form of palaeoplacers spread across numerous basins, enriching coalified plant material. This gold presence is often accompanied by a diverse mix of silver, mercury, platinoids, and selenium (Malec et al. 2012; Franus et al. 2015). While, in Central Asia, gold mineralization is associated with sedimentation within coal-bearing sandstone and mudstone sequences. Here, gold is situated within carbonaceous sediments and cross-cutting quartz veins, exhibiting heightened concentrations adjacent to thrust surfaces (Levitan 2008).

Two distinct ore deposits linked with organic matter (OM) have been selected from specific regions: the Bakyrchik gold deposit in Kazakhstan and the Western (W) Mecsek uranium ore deposit in Hungary.

According to authors (Rafaylovich et al. 2011; Marchenko and Komashko 2011; and Azerbaev and Zhautikov 2013), gold within the Bakyrchik gold deposit is enriched within OM in the form of nanostructured particles, along with arsenic sulfide minerals that are embedded in organic-rich sedimentary rocks. The presence of gold in OM within the Bakyrchik deposit emphasizes the importance of conducting further research.

Numerous publications concerning the organic-rich uranium ore deposit of W-Mecsek, such as those by Szalay (1954), Benkő & Szadeczky-Kardoss (1957), Szalay and Almassy (1956), Kiss (1958, 1960), Ódor (1969), Varga et al. (1972), Kádas (1983), Selmecziné and Vincze (1986), Barabás and Konrád (2000), and Barabás (2010), have highlighted the association of gold within organic-rich uranium ore deposits in Hungary. Kádas (1983) specifically noted gold concentrations of up to 2 ppm in the Mecsek area.

Despite the wealth of both ore deposits within OM, the deposits remain relatively unexplored regarding international publications, with a limited focus primarily on petrology, geochemistry, and gold extractions concerning OM.

The primary objective of this study is to offer new insights and concepts concerning OM characterization, gold geochemical extraction from OM, and identifying potential gold exploration within OM in these ore deposits. This will be achieved through 1) determination of the total organic carbon content, organic components, and maturation of the OM; 2) optical and electron microscopic observations of OM; 3) geochemical analysis of gold concentration in OM and 4) comparing gold values present in OM between the two ore deposits.

2 Geological settings and sampling locations

The Bakyrchik deposit is one of the largest in the West Qalba gold province in Eastern Kazakhstan, including over 450 gold deposits in organic-rich sedimentary formations from the Carboniferous period (Shcherba 2000; Goldfarb et al. 2014). The gold reserve of the deposit is 410 t, on average 8–9 g/t (Goldfarb et al. 2014; Dyachkov et al. 2017). The bulk (90%) of gold in the deposit occurs as an “invisible gold’’ type, bound to the lattice of arsenian pyrite and arsenopyrite (Umarbekova and Dyusembaeva 2019) and gold concentrates in a structure of amorphous OM (Marchenko and Komashko 2011). Gold mineralization occurs as a stratiform type, within zones containing finely disseminated Au-containing pyrite–arsenopyrite ores in carbonaceous–carbonate terrigenous rocks and gold-polysulfide ores formed in the Middle-Late Carboniferous (C2–3) of the Bukon suite (Zhautikov and Maulenov 1985; Dyachkov et al. 2011), see Fig. 1a.

Fig. 1
figure 1

Geological maps showing the Bakyrchik gold deposit in Kazakhstan (a) and the Western Mecsek uranium ore deposit in Hungary (b) are presented alongside stratigraphic columns (modified after Trubnikov 1976; Umarbekova et al. 2017; Konrád and Sebe 2010; Emese et al. 2022); and showcase six selected samples (each with a scale of 1 cm): three samples collected from (1) the Bukon suite and three from (2) the Kővágószőlős Sandstone Formation

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As Lubecky et al. (2008) and Wong et al. (2017) have explained that the Bakyrchik gold deposit was formed in an ophiolite Irtysh-Zaisan suture zone relating to post-tectonic plate collisions, which includes the Late Palaeozoic Variscan orogenic tectono-magmatic event (Parnell 2019). During the tectono-magmatic event, the collision played a role in the creation of the Kunush granitic batholith, a process occurring in the Upper Carboniferous to Lower Permian periods (C3-P1) (Rafaylovich 2009).

The recent structure of the Bakyrchik deposit shows a terrigenous basin, including the Carboniferous sequence from the marine in the Late Devonian—Early Carboniferous (D3-C1), coastal-marine in the Middle Carboniferous (C2), and continental carbonaceous molasses in the Middle-Late Carboniferous (C2–3) (Lubecky et al. 2008). The lower part of the stratigraphic sequence comprises a 1500 m thick carbonate and volcanogenic shales and volcanoclastic sandstones containing lenses and interlayers of siliceous sandstone and carbonaceous-cherty siltstones. The upper part of the sequence consists of 600–800 m thick sandstone with layers of carbonaceous siltstones with fossil remains. Carbonaceous-terrigenous sequences are cut through by a magmatic formation (depth 3–3.5 km), which is paragenetically associated with ore mineralization (Rafaylovich et al. 2011; Umarbekova et al. 2017). Previous research on fluid inclusions within quartz in the ore zone revealed the primary gold mineralization ranges 200–340 °C and 110–130 MPa pressure (Novojilov and Gavrilov 1999).

The W-Mecsek uranium ore deposit is located in the Southern Central part of Hungary. The recent resource estimate reevaluation indicates a quantity of 17946 t of uranium ore, with an average uranium concentration of 0.117% (IAEA Report 2014). The ore deposit occurs in the western part of the Mecsek Mountains. The Mecsek Mountains occur in the southwestern part of the Tisza Mega-unit (Haas and Hámor 1998). The Tisza Mega-unit forms the basement of the Eastern Pannonian Basin, the northern part of the Mecsekalja strike-slip fault zone (Szederkényi et al. 2012), shown in Fig. 1b. The unit belongs to the Variscan orogenic collage, accreted during the Carboniferous–Permian (C-P) (Szederkényi et al. 2012). The European Variscan orogeny occurred when crystalline rock associations were formed by mega, macro, and microfolding. The Late Variscan low-pressure and high-temperature regime (late orogenic heating in the 330–270 Ma period) undoubtedly contributed to the granitization (Haas 2012). This unit includes crystalline rocks that only crop out in the Mecsek Mountains (Haas 2012), mainly granitic formations of the Carboniferous Mórágy complex. Variscan granitoids characterize the Mórágy Complex (Buda 1996; Klötzli et al. 1999).

A 2500–3200 m-thick Permian sequence has various types of Late Carboniferous crystalline rock (non-metamorphic rocks) that occur in the entire area of the Mecsek Mountains (Szederkényi et al. 2012). Permian sequence consisting of uranium ores mainly in the Kővágószőlős Sandstone Formation (KSF) (Barabás and Konrád 2000) follows the Boda Claystone Formation, occurring in the Permian-Triassic anticline of the Mecsek Mountains (René 2014). The KSF represents a fluvial depositional environment, where the thickness of the formation reaches 600 m (René 2014). The uranium ores in the KSF are of polygenic origin (Virágh and Vincze 1967) as they show stratiform pennaccordance, disseminated and transversal enrichments (Barabás 2013). Barabás and Konrád (2000); and Barabás (2013) indicated that the uranium mineralization forms in the deposit up to 200 ℃ hydrothermal temperatures. The occurrence of gold is poorly known in the sequence. Some sporadic analysis indicated gold might be enriched at a maximum of 6.5 g/t which relates to the reductive carbonaceous matter-rich intercalations (Földessy 1998).

A total of six samples were selected from the ore mineralization zones of two deposits, representing organic-rich sedimentary rocks. The Bakyrchik samples are characterized by dark-colored, dull, and hard carbonaceous-siltstone rocks containing arsenic sulfide mineral inclusions. Conversely, the W-Mecsek samples show brittle, blackish-grey, medium-grained dolomitic silty sandstone with carbonaceous shale lenses and sulfide inclusions.

In Bakyrchik, sampling was conducted across the organic-rich sedimentary bedrocks within the mineralization zones, situated near the shear zone within the open pit mine of the ore deposit (Fig. 1), labeled as Bak 1, Bak 2, and Bak 2.1. Sampling in W-Mecsek involved drill core samples from the borehole WHEI/2 (Figs. 1, 2), located in the mining area of the uranium ore deposit, labeled as WH2-057, WH2-073, and WH2-077. These samples of the drill core were extracted from depths ranging between 831.4 m (WH2-057) to 907.5 m (WH2-077).

Fig 2
figure 2

Photomicrographs in reflected normal light (a and c) and fluorescence UV light excitation (b and d) under oil immersion, showing solid bitumen (B) and reworked vitrinite (V) of samples Bakyrchik (a and b) and W-Mecsek (a and d)

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3 Analytical and experimental methods

Six rock samples were chosen for analytical and experimental observations. Specimens of OM-rich host rocks, embedded in epoxy resin, were polished for various investigations: reflected-light organic petrography (A Zeiss AxioImager, A2m) was conducted using 50 × oil immersion lenses (R.I. of the oil = 1.5180) using incident normal and fluorescence light for maceral analysis (a photography camera (AxioCam MRc5) was used with the computer program AxioVision Rel. 4.8 by Carl Zeiss MicroImaging, GmbH); and vitrinite reflectance analysis was made on Axioplan Zeiss microscope extended with J&M spectrometer system of the MOL Ltd; electron microprobe (EMPA, JEOL SXA 8600 Superprobe (20 keV and 20 mA)) with backscattered electron (BSE) imaging, energy dispersive spectroscopy (EDS) for elemental analysis and minor grain imaging of organic matter and minerals, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS, a New Wave UP213ss Laser Ablation System with Perkin Elmer Sciex Elan DRC II ICP–MS) for gold measurement on individual organic grains.

The powder samples underwent various analytical procedures: crushed into fine powder (10∼50 μm) using a tungsten carbide ring mill and agate mortar, then homogenized and portioned for subsequent tests. Each powder sample was subjected to X-ray fluorescence (XRF, WDX-XRF (RIGAKU Supermini Pd source, 50 kV-4 mA), X-ray powder diffraction (XRPD, BRUKER D8 ADVANCE Cu-Kα 40 kV, 40 mA), and Soxhlet extraction. Soxhlet extraction was employed, 100 g of powdered samples underwent extraction with dichloromethane and methanol at 46 °C for 3 days, but extracts weren't fractionated due to low organic carbon content

After eliminating inorganic carbonate minerals with 10% chloric acid, carbonate-free powders were employed for Fourier-transform infrared spectroscopy (FTIR, FTS-14) with an attenuated total reflectance (ATR) attachment for identifying organic structures and organic elemental analysis (OEA, ELTRA's CS-2000) for CHNS determination.

The gold concentration in both bulk and sequentially separated samples was determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES, 720 ES by VARIAN Inc., Arial Plasma-view, a simultaneous multi-element ICP spectrometer) throughout the experiment.

The experiment involved a two-step sequential extraction method to extract gold from soluble fractions in six samples. This method, previously utilized by various authors (Liang et al. 2016; Longinos et al. 2021a, 2021b; Junussov et al. 2021), aimed to evaluate heavy metal associations with organic fractions. The materials used included a test tube rack, funnel, filter paper, and chemical reagents provided by the Chemical Department of Miskolc University.

The experiment focused on samples with significant amounts of organic matter. These samples were reduced to less than 63 µm size using an agate mortar and pestle.

The selection of hydrogen peroxide for the Bakyrchik samples, as referenced by many authors (Dold and Fontbote 2001; Usmanova et al. 2017; Longinos et al. 2021c; Junussov et al. 2021), was preferred due to their composition with the low-grade organic sulfur. Additionally, potassium hydroxide was chosen for the W-Mecsek sample based on studies by Lakatos et al. 1997, Varshal et al. 2000, and Henrique-Pinto et al. 2015. Potassium hydroxide demonstrates higher reactivity with samples containing high-sulfur organic matter, as highlighted in research by Mukherjee and Borthakur (2003).

In the first stage, for the Bakyrchik samples: 2 g of powder was mixed with 18 mL of 30% and 35% hydrogen peroxide in a glass beaker and placed in a water bath at 85 °C for an hour; for the W-Mecsek samples: 5 g of powder was mixed manually with 45 mL of 1M KOH in a 100 mL glass beaker. The beaker was then placed in a 70 °C water bath for 2 hours. Following this, it underwent 15 minutes of ultrasonic bath treatment and subsequent centrifugation at 2500 rpm for 10 minutes. The resulting mixture was decanted for both samples, filtrated, and separated into liquid extracts and solid residues.

For the second stage, involving residues, most likely sulfide minerals, 1 g of residue material from the first step was mixed with 9 mL of aqua regia and heated in a water bath at 100 °C for an hour. The liquid extract obtained was filtered, and the gold concentration from arsenic sulfide minerals was measured using ICP-OES.

This method facilitated the extraction and assessment of gold associations with both organic matter and sulfide minerals in the samples. These procedures were conducted at the University of Miskolc, Hungary.

4 Results

4.1 XRPD and XRF analysis

The three rock samples of Bakyrchik are of the non-metamorphosed origin of clay minerals and contain typical sulfide and organic-rich sedimentary rock minerals: XRPD values show muscovite (average 10.5 wt%), albite (average 5 wt%), and illite (with low-grade smectite, general average 17 wt%), hydrothermal carbonate minerals (ankerite and siderite, general average 16 wt%), and quartz. Quartz is found as vein and veinlets that penetrate diagonally into laminated clay minerals as secondary minerals, indicating a hydrothermal origin. Quartz is found in an average of 25.1 wt% in bulk samples and paragenetically associated with the arsenic sulfide ore minerals (pyrite up to 2.9 wt% and arsenopyrite 2.2 wt%) (see Table 1).

Table 1 The XRPD and XRF analyses show quantitative results of the mineralogical and chemical compositions from samples obtained from Bakyrchik and W-Mecsek
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The other three sandstone samples of the W-Mecsek are composed mainly of quartz, feldspar, carbonates and clay; average values based on XRPD results show that quartz is up to 25 wt%; feldspar minerals consist total of 20.2 wt% of microcline (12%) and albite (8.2%); carbonate minerals range between 7.7% ankerite (low-grade Mg-Mn siderite) and 19.5% of dolomite; clay minerals have total average 13.7% of illite (13%), smectite (0.7%), traces of muscovite, and pyrite mineral content varies between 2.2 wt% and 2.6 wt%.

Chemical compositions of the whole rock samples from two ore deposits confirm that major felsic rock-forming minerals consisting of elements Si, Al, Na, K, Mg, Mn, and Ca vary from 0.25 wt% to 57.8 wt%. For quartz, potassium and sodium feldspar minerals, and carbonate minerals while minor elements Ti (average 0.28%) and P (average 0.09%) form compounds of titanium-oxide and phosphate minerals. The amorphous matter content of two ore deposits ranges from 4 wt% to 14 wt%.

4.2 Organic petrology and electron microprobe analysis

The OM found in all six samples from the two ore deposits presents in two primary forms: dark brown to grayish veined solid bitumen (referred to as B, Fig. 2), and reworked vitrinite (V), appearing as trapped, cracked, angular-shaped macerals within the fracture zone. The solid bitumen fills fractures and voids, lacking fluorescence in all ore deposit samples, typically occupying larger fractures and voids within quartz present in every sample. This non-fluorescent solid bitumen forms a discontinuous membrane, filling veins with diameters smaller than 0.5 µm, adhering to the walls.

The reworked vitrinite manifests as cracked, smaller particles, often aligned linearly and parallel to the fracture orientation. It's found infrequently, situated within larger voids subsequently filled by spheroidal quartz growth. In Bakyrchik samples, the reworked vitrinite particle sizes are typically less than 0.4 µm, while in W-Mecsek samples, they tend to exceed 10 µm in size. Further elemental content analysis of the reworked vitrinite, obtained through EMPA.

Microprobe analysis is carried out for the reworked vitrinite particles associated with diagenetic pyrite crystal growth, and other veined bitumen types. These latter solid bitumens were also planned to test. Still, individual elements were not applicable due to the detection limit, because the vein fillings and filaments have very thin microtextures, which were also mentioned by other authors before (Belin 1994; and Cardott et al. 2007). EMPA-BSE image with EDS shows that the reworked vitrinite in the two deposits samples contains organic carbon content 87 wt% – 89 wt%, comparatively low sulfur content 0.8 wt% and arsenic content 0.17 wt% in the Bakyrchik samples; no Arsenic content and high sulfur content 6.83 wt% found in the W-Mecsek samples. The diagenetic pyrite growths in the vitrinite particles have a form of globular framboids, sometimes polyframboids in the samples. The framboidal pyrite is a sedimentary-diagenetic origin with a diameter of about 7 µm in the samples of two deposits, containing relatively lower amounts of arsenic content (up to 0.46 wt%). The reworked vitrinite is closely associated with clay minerals, namely illite (Fig. 3a and b).

Fig. 3
figure 3

EMPA-BSE images (a and c) and EDS X-Ray maps (b and d) with quantitative results (middle side displaying rock matrices and right-side showing elemental composition) showing fractured vitrinite particles (black color) associated with pyrites (white color) in the rock matrix on the BSE images from samples of a Bakyrchik and c W-Mecsek; and the elemental mapping of EMPA-EDS of b Bakyrchik samples present fractured vitrinite (marked in red) within the rock matrices (identified as illite in green, albite in pink, quartz in purple, and pyrite in yellow); and d W-Mecsek samples displays fractured vitrinite (highlighted in blue) among the rock matrices (identified as illite in red and quartz in green)

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We could not find any free grains of precious metals in the bitumen and vitrinite particles, early diagenetic pyrite, and recrystallized pyrites.

5 Organic components, functional groups, thermal maturity, and soluble fraction analysis

The total organic carbon (TOC %) in the samples was measured after the removal of carbonates using an organic elemental analyzer. This was preceded by treatment with 10% chloric acid at both room temperature and 60 °C for 20–30 minutes in a water bath. The TOC values varied, ranging from 0.32 to 0.34 wt% in the Bakyrchik samples and from 0.15 to 0.25% in the W-Mecsek samples. The lowest TOC content (0.15%) was observed in the W-Mecsek sample (refer to Table 2). Across the Bakyrchik samples, hydrogen content ranged from 0.19 wt% to 0.43 wt%, nitrogen from 0.10% to 0.21%, and sulfur up to 0.17%. Conversely, the W-Mecsek samples exhibited the highest nitrogen (0.52 %) and sulfur (5.81 %) contents, alongside the lowest nitrogen value at 0.15%.

Table 2 Total organic carbon (TOC %), H, N, and S contents of samples
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The organic matter composition in samples from both deposits was determined through FTIR spectra, revealing similarities. The spectra exhibited distinctive characteristics: a broad peak at 3375 cm−1 signifying stretching vibrations of hydroxyl (O–H) groups, albeit in very small amounts. The appearance of methylene groups was observed in the antisymmetric stretching peak at 2932 cm−1 and the stretching peak at 2854 cm−1, respectively. Notably, peaks at 1632, 1525, 1497, and 1282 cm−1 indicated characteristic vibrations from aromatic rings, characteristic of polycyclic aromatic hydrocarbons. However, the stretching vibration of C-O for carbonyl groups was absent in the samples.

The presence of characteristic vibrations of the aromatic rings and phenolic hydroxyl groups at 1632, 1523, 1499, and 1282 cm−1, along with similar vibrations at 2854 and 2931 cm−1, indicated a resemblance to the phenolic hydroxyl group. Surprisingly, the carbonyl area of the phenolic hydroxyl group peak at 1733 cm−1 was absent in the organic matter composition studied, possibly due to a decarboxylation process. The composition and structure of the studied organic matter also displayed functional groups corresponding to the phenolic hydroxyl group characteristics found in lignin material.

Raman reflectance (RmcRo%) has been identified as 3.76% in the Bakyrchik samples (referenced the result of the same samples, Junussov et al. 2021), while the vitrinite reflectance (Ro%) measurement ranges between 1.7% (Min) and 2.9% (Max), with a random vitrinite reflectance of 2.25% observes in the W-Mecsek samples. Raman reflectance (RmcRo%) proved to be a more dependable analysis method for the Bakyrchik samples. This was primarily due to the absence of suitable vitrinite grains for vitrinite reflectance analysis. This limitation stemmed from the challenges posed by the extremely small grain size and considerable thermal alteration in those samples.

The extractable (soluble) fraction and the volume of the soluble organic matter were minimal and did not undergo further fractionation.

5.1 ICP-OES and LA-ICP-MS analysis results

The optical and electron microscope techniques failed to confirm the presence of free gold grains in samples from both ore deposits. However, the experimental extraction of gold was successful from the previously separated organic fraction. Besides gold, the organic fraction also contains Ag in the samples from both deposits, indicating that gold and other precious metals are finely dispersed within the organic matrix.

Gold content, along with Ag, extracted via sequential extraction from soluble organic fractions of solid pyrobitumen and arsenic sulfide minerals in six samples, was measured using ICP-OES (refer to Table 3).

Table 3 ICP-OES results depict metal extraction from samples using three digestion stages, measured in units of parts per million (ppm)
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In the first stage, the solvent effectively oxidized organic materials, removing significant amounts of precious metals from all samples across both deposits. The Bakyrchik samples exhibited higher concentrations of gold (ranging between 0.76 and 3 ppm) and silver (ranging from 0.32 to 1.7 ppm) compared to the W-Mecsek samples, which showed concentrations of gold (1.89–3.28 ppm) and silver (0.01–0.11 ppm) (Fig. 4).

Fig. 4
figure 4

FTIR analysis and comparison of organic fractions include: a the molecular structure of lignin with silica nanocomposites (modified from Weizhen et al. 2017), and the molecular structure of OM with rock matrices of carbonate-free samples of b Bakyrchik and c W-Mecsek

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In the second stage, the dissolved precious metal quantities in the six samples exceeded those of the first stage. The potent aqua regia solvent completely dissolved all remaining precious metals from arsenopyrite and some pyrite remnants, even retrieving metals that were not recoverable in the first stage. The Bakyrchik sample displayed the highest values for Au (4 ppm) and Ag (27 ppm), while in the W-Mecsek sample, there was no Au but Ag (up to 1.19 ppm) was dissolved by the aqua regia solvent, likely originating from the arsenopyrite and arsenic pyrite minerals.

In total, 127 spots were analyzed using LA-ICP-MS focusing on OM particles from six samples across both mineral deposits (refer to Fig. 5). Specifically, the Bakyrchik samples underwent 32 spot analyses using LA-ICP-MS, with 32 spots targeting solid bitumen particles. On the other hand, the W-Mecsek samples were subjected to 95 spot analyses using LA-ICP-MS, encompassing 50 spots for all bitumen types and 45 spots for vitrinite.

Fig. 5
figure 5

Representative time-resolved LA-ICP-MS depth profiles for Au and Ag along a linear track through OM grains are depicted. The profiles showcase a a laser spot on the OM grain in the Bakyrchik sample and b laser measurements on the OM grain surrounding quartz matrices (qtz) in the W-Mecsek sample. (Each with a scale of 100 μm in the photomicrographs)

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The results of the LA-ICP-MS spot analyses on the grains of organic matter in all samples of both mineral deposits show the presence of Au and Ag elements, Au ranges from 3.25 to 23 ppm and Ag contents 8–49.9 ppm in the samples of Bakyrchik, while samples of W-Mecsek show Au content ranges between 0.01 and 5.06 ppm and Ag content 1 ppm and 90 ppm.

Au and Ag determinations were not performed on sulfide minerals associated with OM due to limitations in reliable detection size.

6 Discussion

6.1 Characterization of OM in Bakyrchik and W-Mecsek

The organic petrology analysis of six samples derived from two distinct ore deposits of Bakyrchik and W-Mecsek revealed two primary forms of solid bitumen and reworked vitrinite in the samples. These forms exhibited distinct properties and distributions within the rock matrices. The solid bitumen filled fractures and voids, lacking fluorescence and forming a discontinuous membrane within veins. Reworked vitrinite, although less frequent, displayed cracked particles often aligned linearly within larger voids filled by quartz growth. Notably, the reworked vitrinite particles occurred with clay minerals (mainly, illite) and differed in size between the Bakyrchik and W-Mecsek samples. Moreover, geochemical results of OM in both ore deposits, such as the presence of polycyclic (hetero) aromatic hydrocarbon and humic acid, indicate most likely the terrestrial original (TOC, 0.32% and 0.34%) coal source, mainly from vascular plants (lignin), which is likely derived from lacustrine and fluvial environments (Azerbaev and Zhautikov 2013; Barabás 2013). The organic composition was assessed using FTIR spectra, showcasing characteristic vibrations associated with aromatic rings and phenolic hydroxyl groups. However, the absence of certain vibrations, such as C-O for carbonyl groups, indicated potential alterations or degradation processes during diagenesis. This analysis provides insights into the nature and structural features of the organic matter present in these deposits. The elemental analysis of OM from both ore deposits revealed significant sulfur content, notably measuring the highest concentration of sulfur (5.81%) in the W-Mecsek sample. This high S content characterizes it as high-sulfur OM, a finding further supported by EMPA-EDS analysis. Tseng et al. (1986); Harrison (1991); and Ward and Gurba (1998) have successfully quantified organic sulfur in individual maceral grains of OM. According to Ward and Gurba (1998), EMPA can detect a broader range of organic sulfur forms compared to chemical combination analyses limited to pyritic sulfur and sulfate, offering a more comprehensive understanding of sulfur content in OM.

The thermal maturity analysis shows that Bakyrchik and W-Mecsek samples highlight significant challenges in assessing thermal alteration within these deposits. While the Raman reflectance (RmcRo%) measurement in Bakyrchik revealed a value of 3.76%, the vitrinite reflectance (Ro%) values in W-Mecsek ranged from 1.7% to 2.9%, with an observed value of 2.25%. A notable obstacle encountered during the vitrinite reflectance measurement process in Bakyrchik samples was the difficulty in accurately distinguishing solid bitumen from vitrinite. This challenge is primarily attributed to the inherent complexities associated with OM characterization. The presence of OM in low abundance, typically below 1% of total organic carbon (TOC), and its finely disseminated nature exacerbate the difficulty in differentiation. Additionally, when the solid bitumen component is notably abundant, as discussed by Ronald et al. (2018), it further complicates the precise delineation between solid bitumen and vitrinite.

6.2 ICP-OES and LA-ICP-MS: Au enrichment within OM

The optical and electron microscopy analyses reveal the absence of precious metals within the samples obtained from both ore deposits. Various researchers (Varshal et al. 2000; Dold and Fontboté 2001; Henrique-Pinto et al. 2015; Usmanova et al. 2017) explained that optical or electron microscopy alone may not suffice for accurate determination, due to gold often exists within solid matrices or as inclusions smaller than 100 nm, prompting the use of sophisticated methods like LA-ICP-MS and digestion methods with ICP-OES. Studies have highlighted the effectiveness of LA-ICP-MS in detecting trace elements, providing sensitivity, precision, and accuracy similar to total digestion-based techniques (Butler et al. 2007; Deol et al. 2012; Zheng et al. 2013; Steadman et al. 2021). Au and Ag content associated with the organic matrix in the samples of both ore deposits was evaluated through sequential extraction, confirming the successful extraction of these precious metals from the organic fraction in both Bakyrchik and W-Mecsek samples. The first stage of extraction using a solvent significantly removed precious metals from all samples. The Bakyrchik samples exhibited almost a similar concentration of Au (3 ppm) to the W-Mecsek samples (3.28 ppm of Au) in this stage. In the second stage, aqua regia dissolved even more precious metals, particularly from sulfide minerals like arsenopyrite and pyrite remnants of the first stage. Notably, the Bakyrchik sample yielded the highest concentrations of Au (4 ppm), whereas W-Mecsek showed higher Ag concentrations (27 ppm) but no detectable Au after the aqua regia treatment. Extracting gold from organic matter is challenging due to its close association with organic matter and its entrapment within sulfide minerals, where it forms a solid solution (Osseo-Asare et al. 1984). In our case, however, we found that using hydrogen peroxide and potassium hydroxide proved to be reliable methods for gold extraction from organic matter. The presence of Au and Ag within the OM was not only confirmed through the sequential extraction method but also verified by LA-ICP-MS analysis. The LA-ICP-MS spot analyses identified elemental signatures of Au and Ag within the organic particles, indicating concentrations and distribution patterns across both mineral deposits. The Bakyrchik samples exhibited higher concentrations of Au (up to 23 ppm) in the OM particles than the W-Mecsek samples, but higher concentrations of Ag (up to 90 ppm) were found in W-Mecsek than in Bakyrchik.

6.3 OM association with Au

The Au concentrations obtained after sequential extraction were utilized for correlation analysis. The correlation between gold and the organic phase (total organic carbon, TOC) shows a positive correlation in samples from both ore deposits. Specifically, samples from the W-Mecsek deposit exhibit a strong positive correlation (r = 0.8) between Au and OM, whereas samples from the Bakyrchik deposit demonstrate a weaker positive correlation (r = 0.3) (refer to Fig. 6a, b). The weaker correlation observed in Bakyrchik samples may be attributed to the relatively weak thermal stability of certain organic fractions within the groundwater or hydrothermal system during polymerization. Despite the weaker correlation, the results of the gold-to-organic phase ratio suggest that the samples overall hold promise and relevance for further investigation.

Fig 6
figure 6

General perspective of migration (a) and enrichment (b) of gold within organic matter. Detailed view at the micron size (a) and nano-size (b)

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The findings from ICP-OES and LA-ICP-MS, along with the high-grade gold and Ag concentrations observed in OM, as well as the positive correlation between Au and OM, explain that the metals are intimately associated with OM, suggesting its involvement in the migration and precipitation of metals within OM in the studied deposits (Fig. 7). The presence of a network of veins and veinlets filled with solid bitumen within reworked vitrinite grains alongside the rock matrix, provides compelling evidence supporting this evidence. This evidence implies that solid bitumen, originating from a mobile liquid hydrocarbon precursor, facilitated the movement of Au and Ag, leading to their enrichment compared to concentrations in organic-rich sediments (Fig. 7a). The migration of these metals is likely facilitated by a hydrothermal solution abundant in gold, where compounds are mainly in a sulfate state and transported by permeating through the pore spaces of the rock matrix. Gold is likely enriched in both ionic and solid states within the lattice structures of organic matter, forming metal-organic compounds. These compounds may precipitate as final products with solid bitumen and reworked vitrinite (Fig. 7b).

Fig. 7
figure 7

General perspective of migration (a) and precipitation (b) of gold within organic matter. Detailed view at the micron size (a) and nano-size (b)

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7 Conclusion

Based on our research data, the presence of organic matter (OM) associated with Au in the samples from the Bakyrchik and W-Mecsek deposits, and its role in gold mineralization, is supported by the following observational evidence: (1) The research findings from the Bakyrchik gold-sulfide deposit and W-Mecsek uranium ore deposit offer valuable insights into the nature of OM and gold occurrence; (2) Organic petrology analysis of samples from Bakyrchik and W-Mecsek ore deposits revealed distinct forms of solid bitumen filled fractures and voids, lacking fluorescence, and forming a discontinuous membrane within veins. Reworked vitrinite particles were often aligned linearly within larger voids filled by quartz growth, indicating variations in depositional environments; (3) Geochemical results suggest a terrestrial origin for organic matter, with significant sulfur content characterizing high-sulfur organic matter in the ore deposits; (4) Examination of gold enrichment within organic matter using optical and electron microscopy analyses showed no presence of precious metals. However, employing advanced methods such as LA–ICP–MS and sequential extraction method with ICP-OES revealed the occurrence of gold and silver associated with the organic matrix in the samples of both ore deposits; (5) The comprehensive analyses provide valuable insights into the complex nature of these ore deposits, highlighting the significant role of organic matter in hosting and distributing precious metals; (6) The findings carry substantial implications for further exploration and understanding of similar mineralization environments.