The origin and hydrothermal mobilization of carbonaceous matter associated with Paleoproterozoic orogenic-type gold deposits of West Africa

Precambrian Research 270 (2015) 300–317

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The origin and hydrothermal mobilization of carbonaceous matter associated with Paleoproterozoic orogenic-type gold deposits of West Africa b ´ Bohdan Kˇríbek a,∗ , Ivana Sykorová , Vladimír Machoviˇc c , Ilja Knésl a , Frantiˇsek Laufek a , d Jiˇrí Zachariáˇs a

Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republic Institute of Rock Structure and Mechanics, Czech Academy of Sciences, V Holeˇsoviˇckách 41, 182 09 Prague 8, Czech Republic c Institute of Chemical Technology, Technická 5, 166 28 Prague 6, Czech Republic d Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Prague 2, Czech Republic b

a r t i c l e

i n f o

Article history: Received 9 June 2015 Received in revised form 2 September 2015 Accepted 14 September 2015 Available online 26 September 2015 Keywords: Carbonaceous matter Graphite Gold deposits Paleoproterozoic West Africa

a b s t r a c t The chemical and physical properties of carbonaceous matter (CM) were studied in Paleoproterozoic metasediment-hosted, orogenic-type gold deposits in Burkina Faso (the Inata deposit), in Mali (the Syama deposit), and in Ghana (the Obuashi and Bogoso deposits). Two types of CM occur in all the studied deposits: metamorphosed and hydrothermal. Metamorphosed CM prevails in all the deposits. Hydrothermal CM occurs in small veinlets in hydrothermally altered rocks and in quartz veins or forms irregular accumulations parallel or sub-parallel to C-type cleavage within the shear zones. The origin of hydrothermal CM, which occurs in paragenesis with Au-bearing arsenopyrite or pyrite, seems to have been due to supersaturation of hydrothermal fluids with carbon at the deeper or middle crustal levels. The isotopic composition of carbon in bulk CM (−33.1 to −26.2‰, VPDB) indicates its biogenic origin. The isotopic composition of carbon in hydrothermal carbonates ranges from −14.5 to −4.4‰ (VPDB), which suggest mixing of carbon derived from a deep-seated source with carbon derived from an organic source. The interaction of hydrothermal fluids with metamorphosed CM could be one of the causes of the reduction of hydrothermal fluids and formation of the respective mineralization. The optical properties and Raman spectra of the metamorphosed CM particles in the individual studied mineral deposits differ considerably. The temperatures calculated on the basis of the Raman spectra of metamorphosed CM vary between 280 and 440 ◦ C, depending on the thermometer used, and correspond to temperatures of metamorphism of upper sub-grenschist and greenschist facies. The temperatures calculated for hydrothermal CM at the individual deposits, are only slightly lower compared to the metamorphosed CM at the same deposits, which indicates approximately the same temperature of the metamorphic and hydrothermal processes. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Many mesozonal orogenic gold deposits in metasediments (Groves et al., 2003; Hronsky et al., 2012) occur mainly as auriferous quartz veins or disseminations bound to fault- or shear zones, which contain a significant amount of carbonaceous matter (CM; i.e., a solid, black, heterogeneous substance containing mostly solvent-insoluble high-weight macromolecular structures

∗ Corresponding author. Tel.: +420 724071813; fax: +420 251818748. E-mail address: [email protected] (B. Kˇríbek). http://dx.doi.org/10.1016/j.precamres.2015.09.017 0301-9268/© 2015 Elsevier B.V. All rights reserved.

of carbon, variable amount of organic sulfur, oxygen and some hydrocarbons). At the Hoyle Pond deposit, Abitibi Greenstone Belt, Ontario, Canada, for example, the CM occurs in 5–20 m thick alteration zones (called gray zones) accompanying, and therefore, coeval with the auriferous quartz veins (Dinel et al., 2005). Gas chromatographic analyses (Downes et al., 1984), stable carbon isotope analyses (Hodges, 1982) and ultraviolet absorption investigations suggested that much of the CM is derived from carbonaceous rocks (black schists) and was deposited from reduced hydrothermal fluids. In the Western Lachlan Orogen, Victoria, Southeastern Australia, Bierlein et al. (2001) describe several types of carbonaceous material associated with gold: (1) in fault-fill veins associated with

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micaceous laminae and stylolites; (2) as fine-grained dissemination in cataclased carbonaceous slate in faults; and (3) in high-grade gold-bearing quartz veins mainly in intersections with carbonaceous slates. Whereas Cox et al. (1995) quote examples where gold deposition in Australia may have been facilitated by the presence of carbonaceous matter, Bierlein et al. (2001) conclude that CM is not critical for ore genesis at a deposit scale. The Otago Schist Belt, South Island, New Zealand, hosts a single world-class Macraes gold deposit. Craw et al. (1999) and Craw (2002) conclude that the CM of the mineralized zone is postmetamorphic and was deposited coeval with the mineralization. Similarly, Petrie et al. (2005) assume that the CM enrichment in “black shears” resulted during the late brittle deformation together with extensive fracturing and cataclastic deformation of pyrite and arsenopyrite and with remobilization of older gold mineralization. Craw (2002) and Petrie et al. (2005) hypothetize that the accumulation of CM may have occurred as a result of mixing of two fluids, water + methane, and water + carbon dioxide. During their study of the barren part of the Hyde-Macraes shear zones Henne and Craw (2012) identified several stages of the CM remobilization and metamorphic processes. The hydrothermal CM was also recorded in Late Archean gold deposits of the Hammersley Basin of Western Australia (Ventura et al., 2007). The Paleoproterozoic gold deposits of the Ashanti Belt, Southern Ghana, occur mainly as auriferous veins in CM-rich shears located parallel or oblique to the regional schistosity of slightly metamorphosed sediments with numerous intercalations of black schists and tholeiitic volcanics. Allibone et al. (2002a,b) showed, using detailed maps and profiles, that CM-rich shear zones occur in direct contact with carbonaceous schists. Koch (1991) considered the CM in shear zones to be of sedimentary origin, deposited as a constituent of shales and later mechanically reconcentrated along shear planes. He did not detect any indications of hydrothermally or pneumatolytically formed carbon modifications. Leube et al. (1990), however, argued that significant amounts of CM in hosting auriferous quartz veins of the Ashanti belt gold deposits may have been remobilized in, and accumulated from reduced hydrothermal fluids together with gold. Therefore, the origin and the mode of remobilization and deposition of CM into auriferous fault- and shear zones are still uncertain. Similarly, the role of the CM in the genesis of gold mineralization is also not clear. Some authors believe that carbonaceous matter plays a crucial role in the deposition of gold (Leube et al., 1990; Lotz, 1994; Upton and Craw, 2008; MacKenzie et al., 2010), whereas other authors conclude that carbonaceous matter is not critical for ore genesis at a deposit scale (Oberthür et al., 1994; Bierlein et al. (2001)). The aim of this paper is to assess the possible origin and modes of carbonaceous matter remobilization and accumulation in auriferous orogenic-type deposits in Ghana (the Obuashi and Bogoso deposits), Burkina Faso (the Inata deposit) and Mali (the Syama deposit) using methods of optical microscopy, Raman spectroscopy, XRD measurements, elemental analyses of carbonaceous matter, microprobe and isotopic studies.

2. Regional geology The Paleoproterozoic Birimian volcano-sedimentary belts with associated granitoids belonging to the Baoule-Moss domain of the West African Craton formed between 2250 and 1980 Ma (Feybesse et al., 2006). The dominant structural nucleus was formed during the Paleoproterozoic Eburnean orogeny. The greenstone belts (Fig. 1) consist of the Birimian sedimentary basins and volcanics, sometimes considered as separate units (Bessoles, 1977; Leube et al., 1990; Pouclet et al., 1996; Vidal et al., 1996).

301

Fig. 1. A simplified geological map of the Leo-Man Craton (modified after Milési et al., 2004; A), with indication of the studied gold deposits (B).

The Tarkwaian sedimentary rocks are considered by most authors to be the youngest unit of the Paleoproterozoic greenstone sequence (Leube et al., 1990; Davis et al., 1994; Castaing et al., 2003; Feybesse et al., 2006). Most of the volcanic and sedimentary suites were metamorphosed from prehnit-pumpellyite (Kˇríbek et al., 2008) to upper greenschist facies (cf. John et al., 1999; Feybesse et al., 2006). Regional amphibolite facies metamorphism was reported from Ghana (John et al., 1999; Klemd et al., 2002; Galipp et al., 2003). Radiometric dating (Oberthür et al., 1998; Leube et al., 1990; Taylor et al., 1992; Davis et al., 1994) indicates that Birimian volcano-sedimentary belts and associated granitoids developed during two separate orogenic cycles during the Eburnean orogenesis.

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The first Eburnean stage, which took place between ca 2200–2130 and 2150–2110 Ma (U–Pb and Pb–Pb zircon ages on rhyolites and granitoids) is called Eburnean I (Allibone et al., 2002a,b), D1 (Milési et al., 1989; Ledru et al., 1991, 1994; Feybesse et al., 2006) or Stage I (Vidal et al., 2009). The second stage, Eburnean II, D2–3 or Stage II, took place between ca 2130–2110 and 2090–1980 Ma (Pouclet et al., 1996; Allibone et al., 2002a,b; Feybesse et al., 2006; Vidal et al., 2009; Baratoux et al., 2011). It is characterized on a regional scale by N to NE-trending transcurrent faults. Locally, up to five successive deformation episodes were recognized (D1–D5; Allibone et al., 2002a,b). Gold mineralization is concentrated along these lateorogenic shear zones (Milési et al., 1989, 1992; Blenkinsop et al., 1994; Oberthür et al., 1994, 1998; Allibone et al., 2002a,b; Feybesse et al., 2006;). The Paleoproterozoic Birimian basement is unconformably overlain by Neoproterozoic sedimentary rocks of the Taoudeni and Volta basins (Castaing et al., 2003). 3. Gold mineralization All the studied gold deposits are confined to faults or shear zones developed in carbonaceous metasediments with intercalations of metavolcanites and magmatic rocks. Two dominant types of fault- or shear zone-related mineralization were described for deposits hosted by the Birimian sedimentary rocks in West Africa: (1) Higher grade mineralization commonly containing >10 g/t of free gold is generally hosted by quartz veins in brittle pinnate fractures adjacent to major faults (Appiah, 1991; Allibone et al., 2002a,b), and (2) generally lower grade sulfide mineralization, commonly containing 2–10 g/t Au, associated with disseminated arsenopyrite and/or pyrite in CM-rich shear zones and adjacent relatively undeformed wall rocks (Leube et al., 1990; Milési et al., 1992; Olson et al., 1992; Oberthür et al., 1994; Mumin and Fleet, 1995; Mumin et al., 1996). Trace amounts of galena, sphalerite, stibnite, bornite, chalcopyrite, and tellurides are locally associated with both types of mineralization, but the principal association of gold is with arsenopyrite and arsenian pyrite. Hydrothermal alterations are very weak and restricted to small amounts of ankeritic or sideritic carbonate, white mica, and quartz within the faults and shear zones, and usually do not overprint the adjacent wall rocks. Fluid inclusion and stable isotope data are consistent with mineralization developing from fluids released during metamorphism and decarbonization of the deep level Birimian rocks (Oberthür et al., 1994; Mumin et al., 1996). Generally, the gold mineralization originated late in the tectonic history of the Birimian sediments and volcanites, toward the end of the latest stage of ductile shearing on the various faults (Oberthür et al., 1994; Allibone et al., 2002a,b). Locally, the mineralization was partly affected by younger brittle deformation. Water with low salinities (below 10 wt.% NaCl equivalent; Leube, 1990) is the predominant component in fluid inclusions in Au-bearing quartz veins. Occasionally, CO2 with up to 10 mol.% dissolved CH4 may accompany water-rich inclusions. 4. Methods Thirty representative samples of metamorphosed clastic or volcaniclastic metasedimentary rocks rich in organic carbon (TOCCM > 0.5 wt.%) were collected and analyzed. Samples were taken mostly from exploration boreholes and open pits. For the purpose of chemical, X-ray diffraction (XRD), and isotopic analyses, about 20 g of rocks were ground and pulverized under ethanol for 3 min. It was assumed that such a short period of mechanical grinding would not affect the microstructure of the carbonaceous matter. The chemical separation of bulk

carbonaceous matter from rock samples was performed using a standard procedure involving hydrochloric and hydrofluoric acid treatment followed by heavy-liquid separation (Lewan, 1986). The resulting concentrate contained a total of 3.1–94.8 wt.% non-carbonate carbon. The amount of carbon and hydrogen in concentrates of CM was established using a CHNS microanalyzer (Thermo Finnigan Flash. FA 1112, Milan, Italy). Because large amounts of residual fluorosilicates can substantially affect the determination of carbon and hydrogen (Wang, 1980) the hydrogen-to-carbon atomic ratios (H/Cat ) were calculated only in concentrates with carbonaceous matter carbon (TOCCM ) > 25 wt.%. X-ray powder diffraction (XRD) analyses of the CM were performed in Bragg-Brentano geometry on a Philips XPert diffractometer using CuK␣ radiation and a graphite secondary monochromator. The studied samples were mounted on the surface of a low-background spinning specimen holder. In order to identify residual or newly formed mineral phases in the concentrate after HCl-HF treatment, first XRD analyses in the range of 10–70◦ of 2 were carried out. In addition to carbonaceous matter, pyrite, arsenopyrite, and rutil were identified in almost all the studied samples. Small amounts of pentlandite, gypsum, loellingite, gersdorffite, anatase, pyrite, arsenopyrite, and gypsum were also detected in a few samples. Detailed power patterns were collected in a range of 2 from 23 to 29◦ in steps of 0.05◦ and an exposure time of 6 s per step. The step-scanned powder diffraction data were processed by the XFIT program (Coelho and Cheary, 1997), applying the split asymmetric Pearson profile shape function to yield the peak positions and full width at half maximum peak height (FWHM). For profile fitting of broad peaks merging partially into the background, a pseudo Voigt function was used. Vein graphite from Sri Lanka was used as a standard. For determination of the stable isotope composition of carbon, aliquots of the carbonaceous concentrates were combusted in tin capsules in Fisons CHNS Analyzer (Carlo Erba Instruments EA1108) at a temperature of 1040 ◦ C. The CO2 from the generated gases was separated in a chromatographic column filled with Porapak Q and analyzed in a Delta V isotope ratio mass spectrometer. The results are reported using standard delta notation relative to the Vienna Pee Dee belemnite (VPDB) standard. The reproducibility on international standard NBS 22 was ±0.15‰. For carbonate (dolomite) carbon and oxygen determination, the samples were dissolved in 100% phosphoric acid in a vacuum according to a method described by McCrea (1950). The released CO2 was analyzed using the Delta V (Thermo Scientific) isotope ratio mass spectrometer. The results are reported using standard delta notation relative to VPDB or SMOW (Vienna Mean Ocean Water) standards. The reproducibility assessed by replicate analyses of international standards was ±0.1‰. The distribution, morphology and reflectance of organic matter was studied and measured on polished X-Y sections of rock slabs, i.e. perpendicular to banding or foliation of the rocks studied. Measurement of random reflectance (Rr ; in %), minimum (Rmin ; in %) and maximum reflectance (Rmax; in %) of carbonaceous particles were made in normal light at  = 546 nm and polarized light using Microscope Carl Zeiss Axio Imager M2 equpped with Spectrometer with oil objectives (magnification of 50× and 100×). Yttrium-aluminum-garnet (R = 0.900%), gadolinium-gallium-garnet (R = 1.717%), cubic zirconia (R = 3.06%), and stroncium-titanate (R = 5.34%) reflectance standards were used for the calibration of these measurements. For micro-Raman analyses, slabs of rock (5–7 mm thick) were cut parallel to the X–Z plane of the strain ellipsoid. As the Raman spectra of carbonaceous materials are strongly affected by disorder caused by friction (Pasteris, 1989; Wopenka and Pasteris, 1993), the slabs were only slightly polished under a stream of water with cerium oxide (Prolabo) powder to allow microscopic recognition

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of the different types of organic particles. Micro-Raman analyses were performed using a Thermo-Scientific model DXR microscope dispersion spectrometer with a spatial resolution of about 1 ␮m. A laser diode operated at 532 nm with an input power of 10 mW. The analytical conditions were as follows: laser power about 2 mW, a spectrograph with holographic grating (400 gr./mm) and pinhole width 25 mm, acquisition time 10 s, and 10 accumulations added together to obtain the spectrum. A liquid nitrogen-cooled CCD camera was used as the detector. Graphitized carbonaceous matter and graphite have two major vibrons in the first-order Raman spectra region attributed to the crystalline and disordered phases of the material at ∼1580 cm−1 (G) and ∼1350 cm−1 (D), respectively (Pasteris and Chou, 1998; Beyssac et al., 2002a and references therein). For example, in poorly crystallized CM, the D and G bands attain broad FWHH values and the D/G ratio of the integrated peak areas is elevated. The D/G intensity ratio also depends on the grain orientation with respect to the incident laser beam; however, the contribution of this effect to the D/G ratio has been shown to be less than 0.15% (Wang et al., 1989; Wopenka and Pasteris, 1993 and references therein). There is also a second-order Raman band of the D, G overtone in the ∼2450–2950 cm−1 frequency range. A weak peak at 2450 cm−1 is observed mainly in well-ordered CM or graphite, whereas that at 2950 cm−1 is found in a poorly crystallized structure due to the possible presence of C H groups. In our study, we focused on the first-order D and G Raman bands in the 1100–1800 cm−1 area. The relative distribution of the D and G bands is associated with the degree of crystallinity and the thermal regime of mineral formation (Pasteris and Wopenka, 1991; Wopenka and Pasteris, 1993). Progressive structural evolution (graphitization) of the CM with increasing temperature forms the basis of a metamorphic thermometer for metasedimentary rocks (Beyssac et al., 2002a,b; Rantisch et al., 2004; Rahl et al., 2005). The degree of structural order in the carbonaceous matter is expressed by two ratios (Beyssac et al., 2002a): R1 = and, R2 =

 D1  G



H

D1 G + D1 + D2

(1)



A

303

linear relationship between the metamorphic temperature and parameter R2: T (◦ C) = −445R2 + 641

(3)

with a correlation coefficient of R2 = 0.98. The thermometer was empirically calibrated employing samples having known “peak temperatures”, estimated using metamorphic index minerals. Because the R2 ratio shows little variability in the low-temperature interval (100–300 ◦ C), a temperature of 330 ◦ C marks the lower limit on the thermometer. Rahl et al. (2005) introduced a modified thermometer based on parameter R1 (the ratio of the heights of disordered peaks to ordered peaks) and parameter R2 (the areas of disordered peaks relative to ordered peaks), which is applicable in the temperature range of 100–700 ◦ C with the fit parameter R2 = 0.94: T (◦ C) = 737.3 + 320.9R1 − 106R2 − 80.3638R12

(4)

Electron microprobe analyses of carbonaceous rock samples were performed at the Institute of Geological Sciences of Masaryk University and the Czech Geological Survey in Brno using a Cameca SX100 electron microprobe. An accelerating voltage of 25 keV and a current of 20 nA were used for chemical analyses of the individual minerals. The width of the electron beam was 2 ␮m. 5. Results 5.1. Reflectance of bulk carbonaceous matter The random reflectance (Rr ) and the maximum and minimum reflectance (Rmax and Rmin ) values of carbonaceous particles for individual bulk samples are given in Table 1. The values of Rmin and Rmax are plotted in the diagram after Teichmüller et al. (1979; Fig. 2) in which the outlined fields correspond to the reflectance of vitrinite in the course of coalification and graphitization. The figure shows that the higher values of Rmax corresponding to temperature conditions of the upper greenschist facies are typical of the Obuashi and Inata deposits. Conversely, the lowest Rmax value, typical of temperature conditions for sub-greenschist to lower greenschist facies, was recorded at the Bogoso deposit. The Rmax values at Syama lie between the two groups.

(2) 5.2. XRD patterns of bulk organic matter

where indices A and H show that the ratio is based on the peak areas and peak heights, respectively. Using the R2 ratio, Beyssac et al. (2002a,b) were the first to formulate a quantitative empirical metamorphic thermometer using Raman spectroscopy. They demonstrated that the crystallinity of CM is strongly correlated with the peak metamorphic temperature, but not with the metamorphic pressure. Their thermometer is based on the observed

The results of XRD analyses of the bulk carbonaceous matter correspond well to results of reflectance studies and reveal a very low degree of CM structural ordering (almost amorphous carbon) for the Bogoso deposit (Table 2, Fig. 3). The same parameters for the Obuashi deposit indicate a poorly ordered CM structure, while the low value of FWHM and the shift of the d(002) CM value to

Table 1 Values of the random reflectance (Rr ; %) of carbonaceous particles (normal light, in oil) and values of the maximum reflectance (Rmax ; %) and minimum reflectance (Rmin ; %; plane-polarized light, in oil) in the studied rock samples. Parameter deposit and sample code

Rr (%)



Number of measurements

Rmax (%)

Rmin (%)

Number of measurements

Syama, SYD044/4 Inata, INDD0064/2 Inata, INDD0064/1 Obuashi, SIBI GOS41B Obuashi, SIBI GOS42B Bogoso, BUVTDD127/4 Bogoso, BUVTDD037/10 Bogoso, BUVTDD037/11 Bogoso, BUVTDD037/12 Bogoso, BUVTDD127/6 Bogoso, BUVTDD127/13

4.96 5.87 5.79 5.72 4.71 4.39 4.56 4.51 4.60 4.52 4.55

0.53 0.69 0.62 0.60 0.57 0.51 0.58 0.54 0.42 0.35 0.32

85 38 88 70 90 58 80 135 32 30 30

6.23 7.72 7.46 7.52 5.74 5.40 5.66 5.50 5.47 5.42 5.49

2.60 2.04 2.55 2.54 2.43 2.24 2.19 2.66 2.74 2.80 2.82

55 32 68 70 75 55 75 120 25 22 20

 – standard deviation.

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Table 2 Content of carbon in the CM concentrate (TOCCM ) after HCl/HF digestion and separation in heavy liquids, and X-ray diffraction parameters of the (0 02) “graphite” peak of bulk carbonaceous matter from the studied gold deposits. Deposit

Sample code

CCM a concentrate (wt.%)

Position (◦ 2)

˚ d(002) (A)

FWHM (◦ 2)

Bogoso Bogoso Bogoso Bogoso Bogoso Bogoso Bogoso Syama Syama Obuashi Inata Graphite standard

BUUTDD 127/1 BUUTDD 127/4 BUUTDD 127/5 BUTDD 127/13 BUTDD 037/14 BUTDD 037/14 BUTDD 037/16 SYD-044/1 SYD-044/2 SIBI GOS 41B INDD 064/1

46.9 27.7 14.9 61.9 22.6 18.6 25.6 43.5 39.7 89.2 71.9 91.5

25.65 25.58 25.80 25.67 25.83 25.03 25.67 25.79 25.57 26.55 26.48 26.55

3.469 3.480 3.450 3.467 3.447 3.450 3.467 3.452 3.481 3.354 3.363 3.355

3.99 3.88 2.40 4.57 3.56 c.a. 8 3.61 5.30 4.68 0.70 0.48 0.26

a

Concentration of carbon of carbonaceous matter.

3.36 A˚ at Inata correspond to a more ordered CM structure. On no account does this data match the values for a well-ordered graphite structure.

while at the Obuashi and Inata deposits the carbon was found to be less depleted in the heavy isotope (−27.4 and −23.3‰; Table 3, Fig. 4).

5.3. Chemical and isotopic composition of bulk carbonaceous matter

5.4. Content and isotopic composition of carbonates

The contents of carbon in carbonaceous matter (TOCCM ; wt.%) in the studied samples range from 0.28 to 8.56 wt.% (Table 3). An exceptionally high content of TOCCM (25.1 wt.%) was found in one sample collected in a shear zone in black schists of the Inata gold deposit. The H/Cat values for CM from Bogoso (0.11–0.23) and Syama (0.19–0.23) deposits were higher than the same values from the Obuashi (0.06) and Inata (0.09–0.17; Table 3) deposits. This indicates that relatively large differences exist in the degree of thermal maturation of the bulk CM at individual deposits. The values of the isotopic composition of carbon in the bulk CM consisted of two quite separate groups. The isotopic composition at the Bogoso and Syama deposits ranged between −32.9 and −28.5‰,

The contents of carbonates (Ccarb ), ranged from <0.1 to 3.77 wt.%, and total sulfur contents (Stot ) varied from 0.02 to 3.15 wt.%. The chemical composition of the carbonates mostly corresponds to Fedolomite, sulfur is bound mainly in pyrite and arsenopyrite, and other sulfides occur as accessories. A small amount of gypsum was detected by microprobe analysis in hydrothermal veinlets in the Bogoso deposit. The isotopic composition of oxygen in carbonates in the individual studied deposits was not very different, ranging from 13.9 to 16.8‰ (SMOW); Table 3; Fig. 5). However, the isotopic composition of Ccarb was very variable and ranged widely from −14.5 to −4.4‰. The isotopic composition of carbon in fluids calculated from the isotopic data for the carbonates ranged from −12.0 to −1.95‰ ‰ for a temperature of 350 ◦ C, and between −12.4 and 2.25‰ for a temperature of 300 ◦ C.

Fig. 2. The maximum (Rmax ) and minimum (Rmin ) reflectance of bulk carbonaceous matter at the Bogosso, Obuashi, Syama, and Inata gold deposits. Data from barren Birimian schists from Burkina Faso (Kˇríbek et al., 2008) are given for comparison. The outlined fields correspond to the reflectance of vitrinite according to Teichmüller et al. (1979). The boundaries between sub-greenschist, greenschist, and amphibolite facies are indicated after Diessel et al. (1978), Mählmann (1995) and Potel et al. (2006).

Fig. 3. Examples of X-ray diffraction (XRD) patterns of the bulk carbonaceous matter from gold deposits in West Africa. (A) Poorly ordered carbonaceous matter (almost amorphous carbon) showing a very broad “graphite” peak centered at ∼25.65◦ from the Bogoso deposit, (B) better ordered carbonaceous matter exhibiting an asymmetric (002) “graphite” peak centered at 26.55◦ from the Obuashi deposits, and (C) relatively well-ordered asymmetric peak centered at 26.48◦ from the Inata gold deposit. FWHM: peak width in half maximum (in◦ 2).

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305

Table 3 Concentration of the carbon of carbonaceous matter (TOCCM ), the H/C atomic ratio of carbonaceous matter, the concentration of carbonate carbon (Ccarb ), total sulfur (Stot ), the isotopic composition of the carbonate carbon and oxygen (dolomite), and the isotopic composition of the CM carbon in the studied deposits. DEPOSIT

SAMPLE CODE

TOCCM a (wt.%)

H/Cat

Ccarb (wt.%)

Stot (wt.%)

␦13 Ccarb (‰, VPDB)

␦18 Ocarb (‰, SMOW)

␦13 CCM , (‰, VPDB)

(␦13 Ccarb ) − (␦13 CCM )

Bogoso Bogoso Bogoso Bogoso Bogoso Bogoso Bogoso Bogoso Bogoso Bogoso Bogoso Bogoso Bogosso Obuashi Obuashi Obuashi Obuashi Syama Syama Syama Syama Syama Inata Inata Inata Inata Inata

BUVTDD127/1 BUVTDD127/2 BUVTDD127/3 BUVTDD127/4 BUVTDD127/5 BUVTDD127/6 BUVTDD127/7 BUVTDD127/8 BUVTDD127/9 BUVTDD037/10 BUVTDD037/11 BUVTDD037/12 BUUTDD 127/13 SibiGOS41A SibiGOS41B SibiGOS42A SibiGOS42B SYD-044/1 SYD-044/2 SYD-044/3 SYD-044/4 SYD-MSP 09 INDD0064/1 INDD0064/2 INNN0064/3 INNDD0064/4 INNDD0064/5

4.4 4.10 3.90 3.02 0.88 1.20 1.22 1.23 4.07 1.05 0.66 2.86 3.2 0.50 2.71 0.50 2.71 1.97 3.56 2.89 0.98 3.15 8.56 25.04 5.36 9.36 4.23

0.181 0.150 0.232 0.126 0.227 0.172 0.298 0.257 0.142 0.228 0.160 0.112 0.224 0.102 0.063 0.090 0.059 0.230 0.200 0.190 0.201 0.204 0.166 0.129 0.131 0.126 0.095

1.52 1.71 1.65 1.72 1.39 0.11 0.07 0.60 1.67 1.29 1.10 0.87 0.87 1.54 3.77 1.54 3.77 1.94 1.22 1.98 2.20 2.71 <0.01 <0.01 0.65 <0.01 <0.01

2.12 1.51 2.34 2.92 2.25 0.77 1.15 0.02 0.47 0.65 0.50 1.89 1.15 1.70 0.02 1.79 0.06 1.48 1.85 1.69 0.15 2.68 3.00 2.81 2.86 3.15 1.99

−13.0 −12.8 −12.6 −13.1 −13.2 n.a n.a. −12.9 −12.4 −11.5 −12.6 −12.4 −12.8 −13.0 −14.5 −13.0 −14.5 −6.5 −7.2 −6.8 −4.4 −6.8 n.a. n.a −12.4 n.a n.a.

13.9 15.9 15.6 15.4 15.5 n.a. n.a. 15.2 15.4 16.8 15.5 15.8 15 14.8 14.9 14.8 14.9 16.3 15.8 15.8 17.0 16.8 n.a. n.a. 15.6 n.a. n.a.

−31.1 −29.8 −30.9 −30.2 −30.4 −30.7 −30.5 −32.1 −28.5 −29.3 −29.7 −29.6 −30.3 −24.5 −25.7 −23.5 −27.4 −31.4 −28.2 −30.5 −29.3 −32.9 −26.3 −24.0 −25.2 −23.3 −25.1

18.1 17.1 18.3 17.1 17.2 – – 19.1 16.2 17.8 17.1 17.2 17.5 11.5 11.3 10.5 12.9 25.0 21.0 23.6 25.0 26.2 – – 12.8 – –

a

Concentration of carbon of carbonaceous matter.

5.5. Types of carbonaceous particles, their morphology and Raman spectra Two types of carbonaceous particles can be distinguished by microscopic study at the individual gold deposits: metamorphosed and hydrothermal. 5.5.1. Morphology of metamorphosed carbonaceous particles 5.5.1.1. Type A1 particles. These particles are of the minute, elongate or filamentous types (width up to 3 ␮m, max. length 30 ␮m) organized in parallel with the penetrative foliation of metamorphic rocks (Fig. 6A and B). These particles are referred to in the following text and figures as Type A1. Particles of this type are

Fig. 4. The isotopic composition of carbon in bulk CM from the studied deposits. For comparison, the range of ␦13 C values is given for barren black schists in Burkina Faso that were metamorphosed in sub-greenschist and greenschist facies (Kˇríbek et al., 2008), and the range of ␦13 C values at Obuashi, Bogoso and Prestea gold deposits in Ghana given by Oberthür et al. (1994).

interpreted as graphitized relics of the original organic matter in the rocks. Type A1 particles very often agglomerate, and form CMrich laminae max. 2 mm thick, alternating with laminae poor in CM, mainly formed by quartz and albite. Continuous CM laminae are often deformed and remobilization of the CM along the planes of younger crenulation cleavage can also take place locally (Fig. 6C). During hydrothermal alteration of the rocks, relative enrichment with CM along the boundary between the altered and fresh rocks occurs due to the replacement of rock-forming minerals by light mica, calcite, and quartz (Fig. 6D).

Fig. 5. The isotopic composition of the carbonates in individual deposits and the calculated values of Cfluid and Ofluid for temperatures of 300 and 350 ◦ C. The coefficients of isotopic fractionation given by Horita (2014) for the dolomite–water–CO2 system were used for the calculations.

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Fig. 6. Photomicrographs of the metamorphosed CM from gold deposits of West Africa. (A) Elongate particles of metamorphosed CM (light gray, Type A1) arranged parallel to planes of penetrative schistosity in black schist at the Syama gold deposit in Mali (reflected plane-polarized light, oil immersion). (B) Filamentous particles of metamorphosed CM (light gray, Type A1) in folded quartz-rich carbonaceous schist at the Obuashi gold deposit, Ghana (reflected light, oil immersion). (C) Mechanical remobilization of metamorphosed CM (black) in black schist along the planes of younger crenulation cleavage (transmitted plane-polarized light). (D) Re-concentration of metamorphosed CM (black) during the hydrothermal alteration of quartz-rich black schist at the Inata gold deposit. During replacement of the original rock by hydrothermal carbonates and quartz, the metamorphosed CM becomes concentrated along the edges of the hydrothermally altered area. Pressure shadows behind the pyrite grains (Py) indicate the synmetamorphic nature of the mineralization (transmitted plane-polarized light). (E) Filamentous bodies of metamorphosed CM (light gray, Type A1) and irregular, porous, and anisotropic particles of metamorphosed CM resembling migrabitumen (light gray, Type A2) adjacent to the position rich in recrystallized framboidal pyrite (white) in black schist from the Bogoso gold deposit, Ghana (reflected plane-polarized light, oil immersion). (F) Clastic particles of metamorphosed CM (light gray, Type A2) resembling migrabitumens with a small inclusion of pyrite (white) in black shale from the Bogoso gold deposit (reflected light, oil immersion).

5.5.1.2. Type A2 particles. Less common are isometric particles, often porous and anisotropic, max. 50 ␮m in size, very frequently forming minute clasts in the rock matrix (Fig. 6E and F). These particles, hereinafter referred to as Type A2, are considered to be a product of migration of hydrocarbons (migrabitumens) in the premetamorphic stage of the development of the rock complex. 5.5.2. Morphology of hydrothermal carbonaceous particles (Type B particles) Hydrothermal particles formed together with fine hydrothermal white mica, chlorite, rutile, carbonates, arsenopyrite, and pyrite veinlets in gold-bearing quartz veins (Figs. 7A, B and 8A, B), in albitized host rocks (Fig. 8C and D), or irregular positions up to several mm thick, deposited parallel to the C-type band cleavage of shear zones (Fig. 8 E and F). The hydrothermal CM is composed of

very fine isometric particles <5 ␮m in size (Fig. 9A–C) or by larger, isometric or elongate bodies (nodules) up to 100 ␮m in size, often plastically deformed (Fig. 9D–F). These grains of hydrothermal CM frequently overgrow auriferous arsenopyrite and pyrite grains (Fig. 9A). Aggregates of hydrothermal CM grains in some samples enclose older CM clasts resembling migrabitumens (particles of Type A2; Fig. 9A). Exceptionally, the hydrothermal CM at the Inata deposit forms almost monomineralic veinlets up to 2 mm thick, with only a small amount of inorganic phases: hydrothermal chlorite, white mica, and carbonate (Fig. 9F). 5.5.3. Raman spectra of metamorphosed and hydrothermal carbonaceous particles The R1 and R2 Raman spectra ratios of hydrothermal and metamorphosed CM particles differ only slightly within a single mineral

B. Kˇríbek et al. / Precambrian Research 270 (2015) 300–317

Fig. 7. Macrophotographs of a nodular body and veinlets of the hydrothermal CM (black) in a quartz vein at the Obuashi gold deposit, Ghana (A), and a detail of the hydrothermal CM veinlets in a quartz vein (B). Bright points within the hydrothermal CM-rich veinlets are auriferous arsenopyrite and pyrite.

deposit (Table 4). Metamorphosed CM particles of Type A1 show a somewhat higher degree of structural ordering, i.e. lower R1 and R2 Raman ratios than the hydrothermal CM (Type B) and particles of Type A2 (Table 4. Fig. 10A). Significant differences were, however, found in the degree of structural ordering of CM particles between the individual studied deposits (Fig. 10A). The lowest R1 and R2 values and thus the highest intensity of heat stress are exhibited by both the metamorphosed and hydrothermal CM particles at the Inata and Obuashi deposits. The highest R1 and R2 ratios and thus the lowest temperature of “maturation” by contrast are exhibited by the CM particles at the Bogoso deposit. The formation temperatures of the CM particles calculated using the empirical thermometer introduced by Beyssac et al. (2002a,b, 2003) show very good correlation with the values of the maximum reflectance of the individual CM particles (Fig. 10B). The calculated temperatures of CM particles at the Inata and Obuashi deposits correspond to temperatures typical of the upper greenschist facies or slightly exceed it. Calculated temperatures at the Bogoso and Syama deposits then correspond to conditions typical of sub-greenschist or lower greenschist facies. When comparing the temperatures calculated by the thermometer of Beyssac et al. (2002a, 2003) with those calculated using the empirical thermometer by Rahl et al. (2005), the latter are lower and appear to be more realistic, especially for particles exhibiting a higher thermal stress value (the Inata and Obuashi deposits, Fig. 10C). 6. Discussion 6.1. Source of carbonaceous matter The isotopic composition of bulk CM at the studied deposits points to a biogenic origin of carbon. The data correspond to the values for the organic carbon of Paleoproterozoic sedimentary rocks

307

(␦13 C –27.4 ± 4.6‰; Strauss et al., 1992), and also to the values for the graphitized organic matter of the Birimien sediments in Burkina Faso reported by Kˇríbek et al. (2008; ␦13 C = −32.6 to −23.5‰). The ␦13 C values of bulk CM roughly correlate with the H/Cat values in the individual samples (Fig. 11). This confirms that the metamorphosed CM releases isotopically lighter CH4 and/or CO2 with increasing thermal stress (Watanabe et al., 1997). However, it should be noted that the bulk CM in the individual samples represents a mixture of metamorphosed and hydrothermal organic matter in varying proportions. Only in one sample from the Bogoso deposit was it possible to isolate pure hydrothermal CM (this was a tiny veilet in hydrothermal quartz). Nevertheless, the ␦13 C value of this sample was no different from similar values for the other samples in this deposit. Oberthür et al. (1994), reported much higher variability of ␦13 C for CM (−28.8 to −12.7‰; Fig. 4) in the gold deposits in Ghana. This indicates that isotopic exchange between the organic matter-derived carbon with deep-seated carbon, whose isotopic composition is around −5‰ (Deines and Wickman, 1973; Kerridge, 1985), or with carbonate carbon of the host rocks took place to varying extent during the hydrothermal processes. The reaction between the isotopically light carbon of organic origin and heavy carbon of an inorganic nature is also supported by the wide dispersion of the ␦13 C values of the carbonates (Fig. 5). Large dispersion of the ␦13 C values in carbonates (−17.4 to −9.9‰) has also been reported by Oberthür et al. (1994) for the deposits in Ghana. Temperatures calculated from isotopic difference (␦13 Ccarb ) − (␦13 TOCCM ; Table 3) using the coefficients of isotopic fractionation between the carbonate carbon and CM carbon (calibrated for the temperature interval of 270–650 ◦ C; Morikiyo, 1984) range from 265 (Obuashi deposit) to 437 ◦ C (Inata deposit).

6.2. Remobilization of carbonaceous matter in metamorphic and hydrothermal processes The occurrence of solid CM (composed mainly of organicsolvent insoluble high molecular weight macromolecular structures of C N S O and some hydrocarbons) in sediment-hosted hydrothermal deposits has been described by numerous authors (for reviews see, for example, Simoneit, 1993; Leventhal and Giordano, 2000; Greenwood et al., 2013). The presence of aliphatic and aromatic hydrocarbons and metalloporphyrins in these deposits shows that the origin of CM can be connected with migrating oil and metal bearing brines (Disnar and Sureau, 1990; Disnar, ˙ 1999, 2000; Hu et al., 2000; Distler et al., 2004). Migra1996; Gize, tion of oil in the pre-metamorphic or early metamorphic stage of the development of the Birrimien rock complexes in the studied rocks of West Africa can be demonstrated by the presence of isometric, porous and anisotropic particles of CM (Type 1B) randomly dispersed in the rock and mostly affected by subsequent processes of ductile and brittle deformation. Similar clastic particles were described by Henne and Craw (2012) at the Macraes gold deposit in New Zealand, by Kˇríbek et al. (1994) in greenschist facies metamorphosed Paleozoic rocks of the Bohemian Massif, Czech Republic, and by Bierlein et al. (2001) in black shales within the Cambro-Ordovician succession in central Victoria, Australia. Hydrothermal processes leading to the formation of orogenic gold deposits, however, are not tied to the initial, but to the final stage of the metamorphic process, when the original CM in sediments already occurs in an advanced stage of thermal maturity. Under these conditions, the remobilization of CM takes place either through mechanical or chemical processes.

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B. Kˇríbek et al. / Precambrian Research 270 (2015) 300–317

Fig. 8. Photomicrographs of hydrothermal CM (Type B) at the studied gold deposits. (A, B) Veinlets of hydrothermal CM (black) in association with a fine-grained mixture of white mica and dolomite, and numerous grains of auriferous pyrite and arsenopyrite replacing quartz at the Obuashi gold deposit (reflected plane-polarized light). (C, D) Hydrothermal CM in association with white mica, chlorite and pyrite in hydrothermally altered (albitized) metasediments at the Inata deposit (reflected, plane-polarized light). (E) Accumulations of hydrothermal CM forming an anestomosing network along the C-shear planes of the mylonitized black schist at the Bogoso deposit. Preserved original schistosity of the rock domains is accentuated by thin laminae of metamorphosed CM (Type 1A). Transmitted light. (F) Contact of a thick layer of hydrothermal CM with numerous grains of auriferous arsenopyrite (fault- gouge) at the contact with folded and sheared black schist rich in metamorphosed CM organized in parallel with the original rock foliation (transmitted light).

Mechanical remobilization of the metamorphosed CM or graphite, due to carbon (graphite) sheet (easy glide plane) slip, was described, for example, by Crespo et al. (2005) in graphiterich positions (“veins”) containing up to 20 wt.% graphite in rocks located in the internal part of the Iberian Hercynian Belt of Spain. Mechanical remobilization of CM along faults giving rise to positions containing up to 50% TOCCM was also described by Kadounová (1992) for graphite deposits in Southern Bohemia (Czech Republic). She shows that graphite easy-glide intracrystalline planes have orientations controlled by those of the dynamically recrystallized quartz grain boundaries and by the principal directions of rock ductile structures. Subsequent mylonitic deformation and progressive disintegration of metamorphosed CM or graphite particles often leads to the formation of streaks made of scattered small grains (often a few ␮m or <1 ␮m in diameter (Kretz, 1996; Krabbendan et al., 2003). The accumulation of the metamorphosed CM along the planes of crenulation cleavage was also observed in this study in all the investigated deposits in West Africa (Fig. 6C). During straininduced plastic softening and pressure solution of quartz in shear zones, the CM-rich streaks can be combined to form continuous

or discontinuous CM layers up to several mm thick, which can substantially affect the friction properties of shear or fault zones (Oohashi et al., 2011). During the late brittle deformation, the CM particles may form a matrix of cohesive fault breccia characterized by angular fragments or fault-gouge. Kˇríbek et al. (2009) described this mechanism in the Roˇzná uranium deposit that is confined to amphibolite facies metamorphosed rocks of the Bohemian Massif (Czech Republic). Some authors, however, believe that the mechanical processes of CM remobilization are of only local character, and moreover these processes cannot explain the occurence of CM in volcanic or igneous rocks (Binu et al., 2003; Luque et al., 2012). We agree that the main mechanisms of remobilization of carbon in hydrothermal processes are apparently chemical reactions and processes. Hydrothermal fluids that have a relatively low O/(O + H) ratio will react with CM in metasedimentary rocks according to the net redox reactions (Huizenga, 2011): C(rock) + O2(aq) → CO2(g)

(5)

B. Kˇríbek et al. / Precambrian Research 270 (2015) 300–317

309

Fig. 9. Types of hydrothermal CM particles in rocks of the studied gold deposits. (A) Hydrothermal veinlet at the Bogoso gold deposit composed of a mixture of isometric, anisotropic bodies of hydrothermal CM (Type B particles, light gray), white mica, dolomite, and pyrite (white) in black schist. In some places, hydrothermal CM particles overgrow grains of pyrite (arrow) and enclose clasts of the older Type A2 particles resembling migrabitumen. (B) Irregular, anisotropic nodules of the hydrothermal CM in association with white mica and pyrite in a shear zone of the Bogoso gold deposit. (C) Nodular, anisotropic particles of hydrothermal CM (light gray) and pyrite porphyroblast (white) in a shear zone of the Obuahi deposit. The CM particles are locally affected by plastic deformation. (D) Large nodular particles of hydrothermal CM in a hydrothermal quartz vein at the Obuashi deposit. (E) Nodular, elongate particles of hydrothermal CM (light gray) in sheared black schist of the Syama deposit. (F) Hydrothermal vein in altered (albitized) rock at the Inata gold deposit composed of particles of hydrothermal CM (light gray) with only a small admixture of white mica and quartz (dark gray). All photomicrographs in reflected polarized light, oil immersion.

C(rock) + 2H2 O → CO2(g) + 2H2(g)

(6)

C(rock) + 2H2 O → CO2(g) + CH4(g)

(7)

C + 2H2 O → CH4 + O2

(8)

Consumption of carbon may occur during high temperature metamorphic processes at depth as well as during the final stage of cooling of the C O H-fluid-carbonaceous matter hydrothermal system at temperatures over 300–400 ◦ C (Holloway, 1984; Pasteris and Chou, 1998; Huizenga, 2011). Thompson (1972) proposed a more complex CM oxidation reaction based on a study of the mineral assemblages at the Callie gold deposit (Northern Australia): CM (or graphite) + 2 magnetite + 2K-feldspar + 2H2 O → 2 (white mica) + CO2(g) .

(9)

This reaction corresponds to a gradual oxidation of metamorphosed CM in hydrothermally altered rocks that was observed

during our investigation at the Inata deposit (Fig. 12A), where a gradual replacement of the original metamorphic mineral assemblage in rocks by hydrothermal fine-grained white mica with a small admixture of carbonates took place. This resulted in a decrease of metamorphosed CM content in rocks and in the formation of “bleached zones”. The CM relics are preserved in these rocks only in the form of carbonaceous paths or clots (Fig. 12B and C). However, no such phenomenon was observed at the other studied deposits. Therefore, it could be assumed that the volatilization of carbon occurs primarily at deeper crustal levels in the source area of the hydrothermal system. Carbonaceous matter-consuming reactions, together with metamorphic decarbonization, have been suggested to explain CO2 -rich fluids in shear zones (Wawrzyniec et al., 1999; Axen et al., 2001; Ault and Selverstone, 2008). Although it is apparent that the main mechanism of mobilization of carbon in metamorphic and hydrothermal processes includes the oxidation and hydrolysis of CM and formation of CO2 and/or CH4 rich fluids, at a low metamorphic stage, i.e. under conditions of sub-greenschist and lower greenschist facies, the

310 Table 4 Parameters obtained from peak-fitting of the Raman spectra, temperature calculated from the Raman R2 ratio after Beyssac et al. (2002a,b; T Beysac), and from the Raman R1 and R2 ratios after Rahl et al. (2005; T Rahl), and Rmax , Rmin and Rmax − Rmin values (bireflectance) for different particle types from the Inata, Syama, Obuashi, and Bogoso gold deposits. Sample/analysis code

D1 position

Inata A1 A1 A1 A1 A1 A1 A2 B B B B

INDD0064/4a INDD0064/4b INDD0064/4 h INDD0064/g INDD0064 INDD0064/4i INDD0064/4f INDD0064/a INDD0064/4c INDD0064/4e INDD0064/4 m

1350 1349 1349 1351 1348 1349 1348 1348 1348 1348 1349

100 192 142 42 222 188 279 321 276 230 228

46 41 43 46 43 46 44 41 41 42 43

Syama A1 A1 A1 A1 A1 A1 A2 B B B B B B

SYD041/1A3 SYD041/1A4 SYD041/A6 SYD041/B2 SYD041/B4 SYD041/B5 SYD041/A2 SYD041/A1 SYD041/A5 SYD041/A7 SYD041/B1 SYD041/B3 SYD041/B6

1337 1337 1336 1331 1336 1334 1335 1336 1332 1333 1334 1332 1335

248 190 179 172 192 207 188 363 196 234 206 190 202

Obuasi A1 B B

SibiGOS41A/c SibiGOS41A/b SibiGOS41A/a

1350 1347 1349

Bogoso A1 A1 A1 A1 A1 A1 A1 A1 A1

BUVTDD127/4a BUVTDD127/4c BUVTDD127/4b BUVTDD127/4e BUVTDD127/4 h BUVTDD127/4 g BUVTDD127/4e BUVTDD127/4i BUVTDD127/11a

1343 1343 1343 1344 1346 1343 1346 1346 1343

D1 intensity

D1 FWHM

D1 area

D2 FWHM

D2 area

R1

R2

T (◦ C) Beysac

T (◦ C) Rahl

Rmax (%)

Rmin (%)

17 36 26 5 41 36 58 65 51 43 46

27 22 21 27 23 25 22 21 21 21 22

699 1221 848 221 1474 1392 2026 2191 1680 1439 1634

0.49 0.49 0.59 0.39 0.88 0.85 1.14 0.93 1.01 0.87 1.02

0.47 0.47 0.52 0.45 0.57 0.57 0.62 0.58 0.60 0.57 0.60

432 432 410 441 387 387 365 383 374 387 374

374 374 344 370 349 344 337 347 339 347 341

8.79 8.98 8.62 9.72 7.69 7.74 6.86 7.45 6.99 7.71 6.9

1.4 1.23 1.63 1.56 1.67 1.72 1.81 1.65 1.74 1.7 2.14

7.39 7.65 6.99 8.12 6.02 6.02 5.05 5.8 5.25 6.01 4.76

1611 1615 1614 1608 1612 1611 1611 1608 1608 1607 1609 1606 1610

104 69 69 80 82 88 83 143 86 102 83 78 84

27 23 24 30 27 27 27 30 29 31 29 31 28

4465 2515 2592 3760 3451 3792 3510 6673 3961 4870 3740 3780 3647

1.40 1.20 1.23 1.45 1.37 1.38 1.36 1.53 1.44 1.55 1.41 1.50 1.45

0.72 0.72 0.72 0.73 0.72 0.73 0.72 0.73 0.73 0.73 0.73 0.74 0.73

320 320 321 316 319 317 318 318 316 317 315 313 316

259 237 241 254 254 249 251 265 253 264 247 250 255

6.32 6.94 6.8 6.23 6.5 6.47 6.57 6.06 6.27 6.01 6.34 6.1 6.36

2.26 2.03 2.1 2.14 2.09 2.11 2.2 2.74 2.53 2.25 2.61 2.48 2.6

4.06 4.91 4.7 4.09 4.41 4.36 4.37 3.32 3.74 3.76 3.73 3.62 3.76

9631 16,370 14,980

1618 1616 1619

24 39 49

18 20 19

686 1203 1478

0.56 0.80 0.98

0.45 0.50 0.54

441 419 401

412 409 398

8.66 7.87 7.35

1.58 2.02 1.95

7.08 5.85 5.41

20,661 24,529 24,529 18,704 20,908 23,618 23,639 19,355 22294

1610 1610 1610 1611 1614 1610 1614 1613 1609

106 98 98 90 93 96 103 93 93

31 29 29 31 28 30 29 30 33

5170 4423 4423 4444 4086 4480 4673 4357 4757

2.14 1.73 1.73 2.10 2.05 1.93 2.02 2.19 1.98

0.70 0.70 0.70 0.69 0.69 0.67 0.68 0.70 0.67

330 330 330 334 334 343 338 330 343

308 304 304 319 320 341 331 306 342

5.6 5.95 5.98 5.53 5.56 5.84 5.86 5.53 5.88

2.19 2.01 2.07 2.22 2.03 1.96 2.15 1.9 2.02

3.41 3.94 3.91 3.31 3.53 3.88 3.71 3.63 3.86

G position

G intensity

G FWHM

G area

D2 position

7151 12,369 9515 2997 14,858 13,657 19,083 20,617 17,783 14,991 15,264

1579 1580 1580 1581 1581 1579 1581 1581 1580 1580 1581

205 388 239 107 252 221 245 346 274 264 223

23 21 21 21 25 25 26 23 24 23 24

7314 12,638 7989 3482 9691 8724 9919 12,502 10,353 9679 8355

1617 1618 1618 1618 1618 1617 1618 1619 1618 1618 1618

107 104 106 118 108 112 110 108 115 113 116 121 110

41,433 30,789 29,577 31,620 32,320 36,187 32,197 61,229 35302 41420 37288 35922 34652

1595 1599 1598 1591 1596 1595 1595 1591 1592 1590 1592 1589 1593

178 158 146 119 139 150 138 237 136 151 147 126 139

41 38 39 43 41 41 40 44 43 45 43 45 42

11,526 9385 8898 7927 8892 9663 8705 16,391 9062 10569 9831 9017 9214

115 229 268

48 48 46

8545 17,346 19,294

1580 1579 1582

206 287 274

30 36 35

568 494 494 494 534 566 598 518 516

67 88 88 68 68 64 65 67 66

59,938 67,491 67,491 52,346 56,732 57,000 60,505 54,479 53751

1583 1583 1583 1584 1587 1581 1587 1587 1581

265 285 285 235 261 294 295 236 260

50 55 55 51 51 51 51 52 55

D2 intensity

Rmax − Rmin

B. Kˇríbek et al. / Precambrian Research 270 (2015) 300–317

Deposit/particle type

BUVTDD127/4j BUVTDD127/4k BUVTDD127/4l BUVTDD127/11b BUVTDD127/11e BUVTDD127/11f BUVTDD127/6a BUVTDD127/6b BUVTDD127/6c BUVTDD127/6d BUVTDD127/7a BUVTDD127/6e BUVTDD127/6f BUVTDD127/6 g BUVTDD127/6 h BUVTDD127/6i BUVTDD127/7j BUVTDD127/7k BUVTDD127/7p BUVTDD127/4a BUVTDD127/4b BUVTDD127/4c BUVTDD127/4d BUVTDD127/4f BUVTDD127/4 g BUVTDD127/11l BUVTDD127/4 g BUVTDD127/12b BUVTDD127/12c BUVTDD127/6 m BUVTDD127/6n BUVTDD127/13a BUVTDD127/13b BUVTDD127/13c BUVTDD127/13d BUVTDD127/13e BUVTDD127/7b BUVTDD127/7c BUVTDD127/7d BUVTDD127/7e

1351 1345 1346 1344 1342 1351 1348 1350 1351 1349 1350 1351 1352 1352 1353 1354 1356 1356 1357 1345 1345 1344 1342 1344 1347 1345 1345 1344 1342 1349 1349 1349 1349 1337 1350 1350 1351 1353 1354 1355

188 544 550 777 943 299 103 75 46 73 76 102 45 88 70 83 94 86 103 960 650 1057 557 1188 551 525 662 943 976 94 86 91 132 169 84 138 102 80 92 78

61 71 64 65 68 53 66 64 63 66 65 63 65 63 64 72 65 64 65 59 68 63 69 61 63 65 66 66 67 63 65 66 67 208 68 64 63 70 64 65

18021 60900 55497 78949 10107 24,728 10,582 7460 4551 7453 7824 10,149 4596 8795 7023 9414 9587 8635 10,412 88,662 68953 105001 59856 113,567 54,350 53,415 68,029 97,679 103,334 9347 8788 9372 13,818 54,654 8883 13,783 10,057 8742 9314 7884

1590 1588 1588 1583 1581 1590 1590 1591 1591 1592 1592 1592 1593 1592 1592 1593 1592 1592 1592 1589 1587 1586 1580 1587 1588 1586 1587 1587 1582 1591 1592 1591 1593 1579 1592 1592 1591 1596 1593 1591

111 256 264 369 439 150 47 36 25 34 37 47 22 41 31 39 43 41 52 352 306 411 287 457 244 250 298 390 437 42 38 43 57 70 39 62 48 34 43 39

48 52 52 53 55 48 48 48 48 46 47 46 47 46 45 47 47 47 49 48 52 50 54 51 49 52 52 52 53 47 46 46 46 62 48 47 47 46 47 50

8293 20829 21634 30492 37718 11,375 3543 2749 1892 2500 2742 3415 1643 2964 2207 2877 3171 3060 4049 26410 24847 31977 24278 36,649 18,738 20,558 24,191 32,115 36,559 3105 2759 3120 4078 6825 2941 4498 3562 2449 3128 3079

1619 1614 1615 1612 1610 1620 1617 1618 1620 1618 1619 1619 1619 1619 1618 1618 1619 1620 1619 1615 1614 1613 1608 1614 1615 1613 1613 1614 1610 1618 1618 1618 1618 1606 1619 1618 1618 1620 1619 1619

33 96 92 123 149 47 19 15 10 14 14 19 9 17 15 17 17 16 18 143 116 174 96 181 96 95 115 148 156 17 17 18 27 104 17 27 20 15 16 14

26 31 29 29 30 24 28 27 29 29 29 27 29 28 28 29 29 27 27 27 28 29 32 28 28 29 29 28 30 27 28 29 28 52 29 27 27 29 25 26

1350 4649 4145 5630 7016 1777 828 642 448 641 656 817 399 714 628 767 762 685 790 6053 5160 7881 4862 7853 4277 4356 5303 6590 7382 747 765 822 1189 8512 771 1161 824 664 656 561

1.70 2.13 2.08 2.11 2.15 1.95 2.17 2.06 1.86 2.10 2.04 2.17 2.04 2.17 2.24 2.15 2.20 2.09 1.96 2.73 2.12 2.57 1.94 2.60 2.26 2.10 2.22 2.42 2.24 2.24 2.26 2.09 2.32 2.40 2.13 2.24 2.11 2.36 2.17 1.97

0.65 0.71 0.68 0.69 0.69 0.65 0.71 0.69 0.66 0.70 0.70 0.71 0.69 0.71 0.71 0.72 0.71 0.70 0.68 0.73 0.70 0.72 0.67 0.72 0.70 0.68 0.70 0.72 0.70 0.71 0.71 0.70 0.72 0.78 0.71 0.71 0.70 0.74 0.71 0.68

352 325 338 334 334 352 326 335 347 328 331 327 333 327 324 320 325 331 337 316 330 321 343 321 330 338 330 321 330 326 323 328 319 294 327 326 331 313 325 337

356 297 330 319 318 363 299 323 350 305 312 301 318 302 291 285 296 312 328 233 308 261 341 258 304 330 305 273 305 296 289 305 275 210 303 295 312 259 295 327

6.05 5.59 5.83 5.81 5.77 5.93 5.57 5.8 5.98 5.65 5.7 5.9 5.74 5.62 5.82 5.58 5.59 5.65 5.91 4.73 5.52 4.94 5.51 4.89 5.37 5.74 5.47 5.11 5.43 5.42 5.35 5.67 5.12 5.11 5.61 5.4 5.55 5.54 5.26 5.38

1.88 2.07 1.8 2.43 2.27 2.78 2.56 2.73 2.14 2.4 2.46 2.04 2.74 2.71 2.42 2.26 2.56 2.53 2.12 2.41 2.24 2.54 2.18 2.3 1.98 2.66 2.52 2.85 2.5 2.68 2.83 2.58 2.89 2.95 2.76 2.81 2.85 2.83 2.9 2.83

5.93 3.52 4.03 3.38 3.51 3.15 3.01 3.07 3.85 3.25 3.24 3.86 301 2.91 3.41 3.32 3.03 3.12 3.79 2.32 3.28 2.41 3.33 2.59 3.39 3.08 2.95 2.26 2.93 2.74 2.52 3.09 2.23 2.16 2.84 2.59 2.71 2.71 2.36 2.55

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A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A2 B B B B B B B B B B B B B B B B B B B B

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Fig. 10. (A) Correlation between the Raman R1 and R2 ratios for metamorphosed (Types A1 and A2) and hydrothermal CM particles (Type B particles) in the studied gold deposits. The R1 and R2 ratios of the CM particles from barren Birimian black schists in Burkina Faso (Kˇríbek et al., 2008) are given for comparison. (B) Correlation between the maximum reflectance (Rmax ) of the A1, A2, and B-type CM particles and their formation temperature calculated from the Raman R2 ratio after Beyssac et al. (2002a,b). Fields of sub-greenschist, greenschist, and amphibolite facies according to the Rmax values are plotted according to Diessel et al. (1978), Mählmann (1995) and Potel et al. (2006). (C) Correlation between the temperature of formation of CM particles calculated from the Raman R2 ratio after Beyssac et al. (2002a,b) and temperature calculated from the Raman R1 and R2 ratios after Rahl et al. (2005).

metamorphosed CM can still produce, besides CO2 and CH4 , a trace amount of low-molecular-weight organic compounds (e.g., hydrocarbons, carboxylic acids). Lewan and Kotarba (2010), for example, showed that some overmature coals with initial vitri-

nite reflectance values of 4–6%, indicative of metamorphic regimes, can produce up to 5 mg CO2 and methane per gram of total organic carbon and trace amounts of higher hydrocarbons after exposure to hydrous pyrolysis conditions. The precise chemical mechanism of generation remains unclear, but it has been hypothesized to include the simple release of methane sorbed onto the carbonaceous matrix, or continued pyrolytic degradation of high molecular weight carbon structures (kerogen). Preliminary experiments carried out by Robert et al. (2014) revealed that the hydrous pyrolysis of the lower-greenschist metamorphosed CM produces trace amounts of hydrocarbons including a homologous series of n-C15 –n-C36 hydrocarbons and polyaromatic hydrocarbons including pyrene and its hydrogenated products. Kˇríbek et al. (1999) also assumed the possibility of the formation of solid metallic bitumens and the presence of free hydrocarbons at a hydrothermal vein-type uranium deposit of Pˇríbram (Czech Republic) through the process of hydrous pyrolysis of CM in host black schists metamorphosed under greenschist facies conditions.

6.3. The formation of hydrothermal carbonaceous matter

Fig. 11. The values of the H/Cat ratio in the bulk carbonaceous matter vs. the ␦13 C values of the bulk carbonaceous matter at the studied gold deposits. The enrichment of the carbonaceous matter in a heavy isotope with decreasing H/Cat ratio is interpreted to represent an increase in the thermal maturity of the carbonaceous matter.

The amount of hydrothermal CM varies considerably even within individual deposits studied. Based on the study of polished thin sections its amount can be estimated to fluctuate between 5 and 40 vol.% of the total amount of CM in rocks. Hydrothermal CM as such was found to occur only exlusively (in quartz veins of the Obuashi gold deposit in Ghana, Fig. 7).

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313

temperature, during which an increase in the stability field of CM or graphite in the C O H system occurs (French, 1966; Ohmoto & Kerrick, 1977; Frost, 1979; Dubessy, 1984; Holloway, 1984; Lamb and Valley, 1985) according to the reaction: CH4 + CO2 → 2C + 2H2 O

(10)

(2) If the chemical composition of the C O H fluids changes, either due to water removal as a result of retrogressive hydration of the host rocks or during fluid-fluid mixing in which a relatively oxidized (H2 O CO2 ) fluid mixes with a relatively reduced fluid (H2 O CH4 ) the solid CM or graphite may be formed. (Holloway, 1984; Luque et al., 1998; Pasteris and Chou, 1998; Craw, 2002). (3) The reduction of CO2 -bearing fluids in rocks with low fO2 (e.g. by carbonaceous rocks) has been suggested as the mechanism behind the precipitation of vein-type graphite in Sri Lanka and elsewhere (Baker, 1988; Connolly and Cesare, 1993; Santosh and Wada, 1993; Dissanayake, 1994). Studies of fluids from the gold deposits of West Africa revealed that the gaseous inclusions mostly consist of CO2 (80–95 mol.%), with small amount of N2 (2–20 mol.%) and CH4 (0–10 mol.%; Mumin et al., 1996; Yao et al., 2001). It is well recognized that redox reactions in metamorphic systems are largely influenced by the local diffusion of hydrogen, because of its higher fugacity and diffusivity. The hydrogen fugacity in the CM-rich metamorphic rocks is low (perhaps 1 bar), but it is certainly very much higher than the oxygen fugacity (about 10−34 bar) at 350 ◦ C; Eugster, 1959; Frost, 1979). Therefore, it can be concluded that the hydrothermal CM in gold deposits of West Africa originated at least partially by the reduction of CO2 -rich high-temperature fluids and their interaction with rocks relatively poor in oxygen according to the reaction: 2CO2 + 6H2 → C + CH4 + 4H2 O,

(11)

and the CH4 formation may contribute to a further shift in the chemical composition of the fluids into the stability field of solid CM or graphite simply by mixing with the initial fluids rich in CO2 . This mechanism was monitored by Pasteris and Chou (1998), who experimentally diffused H2 into natural pure CO2 inclusions in quartz, causing the formation of graphitized CM. However, we would not exclude the thermal decomposition of metamorphosed CM by interaction with hot hydrothermal fluids inducing the formation of hydrothermal petroleum-related products by reaction with H2 O with the CM and loss of CO2 . 6.4. Dependence of structural ordering and maximum reflectance values of carbonaceous matter on temperature

Fig. 12. Photomicrographs of various stages of hydrothermal alteration of metamorphosed CM at the Inata deposit. (A) Initial stage: A replacement of CM in black schist along a hydrothermal veinlet composed of fine-grained white mica and carbonate and with a few laths of biotite (Bt). (B) Advanced stage: Pervasive hydrothermal alteration of black schist. Only relics of the original CM-rich positions (“black knots”) are preserved. (C) Final stage of the alteration: the original rock was completely replaced with a mixture of white mica (Ms), carbonate (Carb), and albite (Ab). Only an accumulation of relic grains of metamorphosed CM indicates the original foliation of the rock.

Generally, the CM formation from C O H fluids has been attributed to carbon supersaturation as a result of several principal processes: (1) cooling of the hydrothermal fluids, (2) fluid compositional changes and (3) the infiltration of oxidized C O H fluids into a reduced environment. (1) With hydrothermal fluids of a given composition, the most frequent cause for formation of CM is ascribed to the drop in

The optical properties and structural ordering of CM mostly depend on the temperature. The effect of pressure is much smaller being manifested mainly in metamorphic rocks or faults when, due to oriented pressure, an increase in reflectivity and better structural ordering of CM particles may occur (Wada et al., 1994; Oohashi et al., 2011). Many authors (French, 1964; Landis, 1971; Grew, 1974; Diessel et al., 1978; Itaya, 1981; Wintsch et al., 1981; Okuyama-Kusunose and Itaya, 1987; Itaya et al., 1997) documented that, with increasing temperature, the generally broad asymmetric powder diffraction peak of the 0 0 2 plane in incompletely graphitized carbonaceous matter at the 2 values of 25.57–25.72 (d002 : ˚ becomes narrower and better defined (i.e. the peak 3.48–3.49 A) width in half maximum (FWHM) decreases), while shifting toward ˚ takes place. This can the higher 2 value (26.54◦ ; d(002) = 3.355 A) also be observed in samples of bulk CM at various gold deposits studied in this work (Table 2). However, in comparison with the values of d(002) and of FWHM for Birimien black schists in Burkina Faso (3.34–3.36 and 0.3–0.7, respectively), reported by Kˇríbek et al. (2008), the degree of structural ordering of the CM is lower in most

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This interpretation is supported by the results of studies of carbonaceous particles using Raman spectroscopy. The R1 and R2 ratios of metamorphosed CM particles (Type A1) in all the studied deposits are slightly lower (and therefore the degree of thermal maturity is higher) than the particles of hydrothermal CM (type B; Fig. 10A). Foustoukos (2012), for example, demonstrated experimentally that, even at high temperature and pressure (600 ◦ C and 1000 MPa), the structural ordering of fluid-derived CM particles may be very low, which is explained by their relatively rapid formation. As already stated, differences between individual studied deposits are far more significant than the difference between the degree of structural ordering of metamorphosed and hydrothermal CM, established by Raman spectroscopy within individual deposits (Fig. 10A). This is manifested by different values of R1 and R2 for individual deposits and also by different temperatures, as calculated using both thermometers according to Beyssac et al. (2002a, 2003) and by Rahl et al. (2005). The calculated temperatures correspond to the temperatures reported for the individual deposits by other authors. André-Mayer et al. (2012) suggest relatively high temperatures (380–450 ◦ C) of ore-bearing fluids in the Inata deposit, while the Syama deposit in Mali exhibited a temperature of 250–300 ◦ C. Fluid inclusion studies from the Ashanti gold belt in Ghana indicate temperatures ranging from 280 to 390 ◦ C (Mumin et al., 1996). In order to assess at least indicatively the differences in temperature of hydrothermal fluids at individual deposits, an arsenopyrite thermometer was used (Kretschmar and Scott, 1976) to verify temperatures calculated using the Raman spectra-based thermometres. Analyses were carried out on arsenopyrite grains associated with hydrothermal CM (Fig. 13). Calculated temperatures of arsenopyrite crystallization ranged from 271 ± 42 ◦ C at the Bogoso deposit to 317 ± 52 ◦ C at the Obuashi deposit and up to 358 ± 58 ◦ C at the Inata deposit, and therefore roughly correspond to the temperatures calculated using the Raman thermometer of Rahl et al. (2005). The composition of arsenopyrite at the Syama deposit was not studied due to its trace amounts in hydrothermally altered rocks.

7. Conclusions

Fig. 13. Atomic % of As in arsenopyrite in paragenesis with hydrothermal carbonaceous matter at the studied gold deposits and temperature of the hydrothermal fluid calculated for the pyrite-arsenopyrite equilibrium after Kretschmar and Scott (1976).

of the studied gold deposits (Table 2). This can be explained by the fact that bulk samples from gold deposits studied by XRD are a mixture of better structurally ordered metamorphosed CM with somewhat less structurally ordered hydrothermal CM. This is also indicated by the markedly asymmetric character of the graphite peak 0 0 2 in most samples, which points to a mixture of carbonaceous particles of different crystallinity.

The major part of the metamorphosed CM in the studied orogenic-type gold deposits of West Africa can be categorized as products of organic matter maturation. This also corresponds to the ␦13 C value of the bulk CM (−33.1 to −26.2‰), which falls within the range of values common for Paleoproterozoic organic carbon. The results of the Raman analyses of metamorphosed CM particles, their values of optical reflectance and the results of XRD studies have shown that the degree of thermal maturation of metamorphosed CM in the individual deposits differs significantly and varies in a wide range which corresponds to the sub-greenschist up to the upper greenschist facies conditions. In the final, ductile and brittle-ductile stage of metamorphic processes, to which the gold mineralization is bound, the metamorphosed CM contained in the host metasediments reacted to various extents with the hydrothermal fluids to form a CO2 –CH4 -rich gaseous phase, as indicated by the variable isotopic composition of the carbon in the carbonates, which varies widely from −14.5 to −4.4‰. Compared to the metamorphosed CM particles, the optical properties and the degree of structural ordering of hydrothermal CM particles are only slightly lower in the individual deposits. This means that the temperatures of the hydrothermal fluids – or more accurately the temperatures of Au-bearing pyrite and arsenopyrite formation – were not much different from the temperatures of the metamorphic processes at the individual studied deposits. This point to the hydrothermal-metamorphic origin of

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gold mineralization. This hypothesis is also supported by the very limited extent of hydrothermal alterations at the studied deposits. It should be noted that the temporal relationship between the precipitation of auriferous pyrite or arsenopyrite and hydrothermal CM do not reveal whether the hydrothermal CM played a genetic role in the mineralization processes, because it is very difficult to demonstrate that hydrothermal CM acted as a pattern for reduction, chemisorption or other processes during which the gold-bearing sulfides precipitated. The occasional evidence that gold and hydrothermal CM were precipitated at the same time provides only information that sulfides originated in the field of stability of solid carbon. However, the accumulation of hydrothermal CM in mineralized structures could have affected the frictional properties of the shear zones and faults and the reduction of friction might have led to the opening of deep crustal zones of structural weakness, and thus to the facilitation of hydrothermal fluids flow.

Acknowledgements This study was conducted within the P934A – West Africa exploration initiative – Stage 2 (@ AMIRA International) program. Dr. Mark Jessell (IRD Toulouse) was in charge of the project. We would like to thank Mr. Robert Seed and Mr. Thomas Amoah (Avocet Mining) for providing us with details on the Inata deposit in Burkina Faso. The authors are also grateful to Mr. Dean Bertram and Mr. Cedric Gineste (Resolute Mining Ltd.) for accompanying us at the Syama deposit in Mali and to Mr. Clement Asamoah Owusu and Mr. Iduapriem Tebogo Mushi (Anglogold Ashanti) for helping us with field work at the Obuashi deposit. It is also like to mention Mr. Yan Bourassa (Golden Star Resources) who introduced us to the geology and metallogeny of the Bogoso deposit in Ghana. Our thanks are specifically directed to Jorge E. Spangenberg (Institute of Mineralogy and Geochemistry, University of Lausanne, Switzerland), and to John Parnell (School of Geosciences, University of Aberdeen, United Kingdom), who must have spent a lot of their precious time on reading and commenting the MS.

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