At Kopylovsky Deposit, quartz was sampled in gold- bearing vein from a trench (sample 358-2). According to the atomic adsorption analysis, Au content in this vein is 0.73 ppm (after acid dissociation) and 1.40 ppm (after alkali dissociation). Quartz formed non-zoned anhedral grains up to 3 mm size. Fluid inclusions are bi-phase (VL) isolated, and groups unrelated to healed fractures. According to the classifications of Roedder (1984) and Van den Kerkhof and Hein (2001), these inclusions are primary and pseudosecondary confined to the fractures cracked during the quartz formation. Inclusions have irregular, rounded and sometimes 'negative crystal' shapes with 15-20 μm in size (Figs. 3a-3c).
Figure 3. Microphotographs showing fluid inclusions in quartz. (a)-(c) Kopylovsky Deposit: (a) auriferous quartz, (b) and (c) barren quartz, (b) Type 1, (c) Type 2; (d) auriferous quartz from Kavkaz Deposit; (e) CO2-rich fluid inclusion in quartz from Krasnoye Deposit. V. Gaseous; L. liquid.
The barren quartz of Kopylovsky Deposit was sampled from the core (sample 504-80.2), and two types of fluid inclusions have been analyzed. Type 1 is bi-phase (VL) inclusions with 15-20 μm in size and large vapor bubbles up to 50%-60% bulk. Sometimes vapor bubbles are moving but there is no liquid CO2. These inclusions are often located according to the growth zone of quartz grains why they are classified as primary inclusions. Type 2 is presented by isolated dark bi-phase (VL) fluid inclusions with size about 40 μm and large vapor bubbles (Figs. 3b, 3c). They are confined to fractures formed during the quartz formation and classified as pseudosecondary inclusions.
As for Kavkaz Deposit, we studied the quartz vein with 2 mm visible Au located in goethite cavities (sample 284-5). The bi-phase (VL) fluid inclusions are rounded, elongated, and rarely angular shapes. They formed zones or groups of 3-4 inclusions of 10-20 μm in size (Fig. 3d).
The investigated fluid inclusions in quartz from auriferous and barren quartz from Krasnoye Deposit were collected from different depths in carbonaceous schists and aleurolites confined to the folded black shales of Vacha and Aunakit suites of Patomsky Complex. In quartz from the auriferous galena-quartz vein (sample 141425-135.6), we observed bi-phase (VL) fluid inclusions of 5-25 μm in size and tri-phase CO2-rich inclusions (VLLCO2) with about 20%-50% liquid CO2-phase of the inclusion bulk (Fig. 3e). They are elongated, rounded and isometric, and sometimes 'negative crystal' shapes.
In all veins, in both auriferous and barren quartz, secondary fluid inclusions are small (not exceed 5 μm) and marked fractures in quartz. Also we observed small monophase dark (gas) and light (liquid) inclusions (not exceed 3-5 μm) which are co-genetic to primary bi-phase inclusions.
Deposit Veins n Teut (℃) (salts) Tfm (℃) C (wt.% NaCleqv) Thom (℃) Kopylovsky Auri* (P, VL)
Barren (P, VL)
Kavkaz Auri (P, VL) 45 -21…-23 (NaCl±KCl) -4…-5.8 6.5-8.8 212-280 Krasnoye Auri (P, VL)
Barren (P, VL)
Teut. first melting temperature; Tfm. final melting temperature; C. salinity; Thom. homogenization temperature; n. number of measurements. Auri*. auriferous quartz; fluid inclusion associations: P. primary; PS. pseudosecondary; CO2-rich. tri-phased inclusions; VL. bi-phase; VLL. tri-phase CO2 inclusions.
Table 1. Microthermometric data of quartz from the black shale-hosted gold deposits, Bodaybo region
Figure 4. Homogenization temperatures of fluid inclusions in quartz from auriferous (a) and barren (b) quartz veins. Deposits: 1. Kopylovsky; 2. Kavkaz; 3. Krasnoye.
Figure 5. Homogenization temperature vs. salinity plot of fluid inclusion in quartz. 1, 2. Kopylovsky Deposit: 1. auriferous quartz, 2. barren quartz; 3. auriferous quartz of Kavkaz Deposit; 4, 5. Krasnoye Deposit: 4. auriferous quartz, 5. barren quartz.
The eutectic temperatures measured in fluid inclusions from auriferous quartz vein of Kopylovsky Deposit range between -23.0 and -23.8 ℃ (n=11). These temperatures are marked by the NaCl-KCl-H2O fluid. For most inclusions, the final melting temperatures of ice range between -3.7 and -5.7 ℃ corresponding to salinities between 6.1 wt.% and 8.8 wt.% NaCleqv.
Fluid inclusions of Type 1 from barren quartz vein of Kopylovsky Deposit show the eutectic temperatures range between -21.7 and -23.9 ℃, suggesting a simple NaCl-H2O system with KCl presence. In most inclusions, the final ice melting temperatures range between -3.3 and -4.3 ℃, corresponding to a salinity of 5.5 wt.%-6.8 wt.% NaCleqv.
The eutectic temperatures of the Type 2 inclusions from barren quartz vein of Kopylovsky Deposit range between -36.7 and -37.0 ℃, suggesting the presence of FeCl2 (and possible MgCl2) in fluid. The final ice melting temperatures range between -4.5 and -5.7 ℃. Salinity is 7.5 wt.%-8.8 wt.% NaCleqv.
For inclusions in quartz from the auriferous vein of Kavkaz Deposit, eutectic temperatures range between -21.8 and -23.9 ℃, indicating NaCl-KCl-H2O fluid. The final ice melting temperatures range between -4.0 and -5.8 ℃ and correspond to a salinity of 6.5 wt.%-8.8 wt.% NaCleqv.
The eutectic temperatures and salinities of bi-phase fluid inclusions in auriferous and barren veins of Krasnoye Deposit are different. For auriferous veins, they range from -35 up to -36 ℃, which identifies MgCl2-NaCl-H2O fluid; and for barren veins, they range from -22 to -23 ℃, which specifies NaCl-KCl-H2O fluid. In auriferous quartz the final melting temperatures vary from -3.6 to -7.6 ℃ and salinity is 6 wt.%-10 wt.% NaCleqv with the peak 6.5 wt.%-8 wt.%. In barren quartz the final melting temperatures are -4.6 to -10 ℃, salinity is 7.3 wt.%-13.9 wt.%, and NaCleqv with peak 10 wt.%-12 wt.%.
All studied fluid inclusions are homogenized to a liquid phase. Fluid inclusions in quartz from gold-bearing quartz vein of Kopylovsky Deposit are homogenized between 300 and 350 ℃ with multimode frequency. Fluid inclusions in auriferous quartz of Kavkaz Deposit are homogenized between 212 and 280 ℃, with the peak between 260 and 270 ℃ on the distribution plot (Fig. 4).
The homogenization temperatures of Type 1 fluid inclusions in quartz from barren quartz of Kopylovsky Deposit range between 200 and 240 ℃, with peak of homogenization between 210 and 230 ℃. Homogenization temperatures of the Type 2 inclusions in barren quartz of Kopylovsky Deposit range between 260 and 280 ℃, with the peak between 270 and 280 ℃ on the distribution plot (see Fig. 4).
The temperatures of fluid inclusion homogenization of auriferous quartz from Krasnoye Deposit are 260-330 ℃ with the peak 300-320 ℃ on the distribution plot (see Fig. 4). The temperatures of homogenization show weakly positive correlation with salinity of fluid. The temperatures of fluid inclusions homogenization of barren quartz from Krasnoye Deposit range between 140 and 280 ℃ showing bimodal distribution with small peak 160-180 ℃ and high peak 240-260 ℃. The temperatures of homogenization show weakly negative correlation with salinity of fluid (see Fig. 4).
The secondary fluid inclusions are too small in size therefore we have determined only temperatures of homogenization ranging between 130 and 180 ℃.
Trapping pressures of fluid inclusions have been determined using CO2-rich inclusions in quartz from the galena-quartz vein of Krasnoye Deposit. The temperatures of CO2 melting range between -56.7 and -57.1 ℃. The temperatures of CO2 homogenization to liquid phase range between 16.1 and 21.3 ℃. The total homogenization temperatures of these inclusions range between 311 and 330 ℃. The CO2 densities range between 0.78 and 0.85 g/cm3; molar volumes are 57-58 cm3/mol (Thiery et al., 1994). The pressures at temperature of 300 ℃ are calculated in the range of 1.2-1.6 kbar (Brown, 1989).
According to gas chromatography analysis of 4 samples, fluids are dominated by H2O and CO2 (Table 2). The total contents of volatiles in auriferous quartz from Kopylovsky and Kavkaz deposits reach 400 ppm-500 ppm, and in barren quartz from Kopylovsky and Krasnoye deposits, they do not exceed 280 ppm. The CO2 contents in inclusions from auriferous quartz range between 196 ppm and 240 ppm. The N2 contents are between 0.56 ppm and 4.4 ppm; CH4 amount does not exceed 0.37 ppm and H2O content range between 45 ppm and 300 ppm. Inclusions in auriferous quartz of Kavkaz Deposit contain 0.28 ppm H2. The ratios of СО2/(СО2+Н2О) range between 0.39 and 0.61 and СО2/СН4 is from 643 to 2 051. The correlations of volatiles are CO2 > H2O > N2 > CН4 for Kopylovsky Deposit, and H2O > CO2 > N2 > CН4 for Kavkaz and Krasnoye quartz.
Point No. Sample No. H2O CO2 N2 CH4 H2 H2O CO2 N2 CH4 H2 (ppm) (mol%) 1 358/2 151 240.58 4.44 0.374 - 59.76 38.95 1.13 0.167 - 2 504a/80.2 45 69.42 0.56 0.062 - 60.95 38.47 0.49 0.094 - 3 284/5 308 196.95 1.54 0.096 0.28 78.53 20.54 0.25 0.028 0.643 4 141425/135.6 201 85.04 0.94 0.045 - 85.01 14.71 0.26 0.021 - 1, 2. Kopylovsky Deposit: 1. auriferous quartz; 2. barren quartz; 3. auriferous quartz from Kavkaz Deposit; 4. auriferous galena-quartz vein from Krasnoye Deposit.
Table 2. Bulk gas chromatography data of fluid inclusions in quartz
The primary fluid inclusions of different shapes reaching the size of 40 μm have been found in each of the quartz samples. Their Raman spectra are presented by the superposition of several narrow bands corresponding to CO2 and N2 molecule spectra. Using the expression from Burke (2001) it was determined that the variations of CO2 mole fraction in fluid inclusion in auriferous quartz are equal to 96.9%-98.6% mol and in barren quartz, 88.3%-98.0% mol. Thus, both Raman spectroscopy and gas chromatography are consistent excluding СН4 which has not been detectable upon Raman spectroscopy because the level is below the detection limit of Raman spectrometer.