ИСТИНА

In the core of the Kola super-deep borehole (SG-3, 12262m depth) gold-enriched rocks of Archaean age were located at depths between 9.5-11.0 km. These rocks were overprinted by Proterozoic regional epidote-amphibolite facies metamorphism at 500-650 °C and 3.5-6 kbar. We have studied fluid inclusions (FI) in quartz from a vein, located at a depth of 9907.5m, within the gold enriched interval of the stratigraphy. The quartz contains 3 types of FIs: 1) gas inclusions of dense CO2, 2) vapour-liquid two-phase aqueous inclusions, and 3) three-phase inclusions with NaCl daughter crystals. CO2 inclusions homogenized into the liquid phase at temperatures ranging from +21.2 to -6.1 °C with TmCO2 from -57.1 to -58.9 °C and density of 0.76 to 0.96 g/cm3. The salinity of the water phase of vapour FI is 3.4-4.1 wt. %-eq. NaCl. Microthermometry showed that the two-phase inclusions are brines containing chlorides of Ca and Na with Te from -55 to -74 °C and Tmice from -33 to -63 °C, corresponding to salinities of 25.9 to 30.2 wt. % -eq. CaCl2, Th was between 137-185 °C. Three-phase inclusions have Thhalite between 231-123 °C and Thvapour 107-185 ºC. They also contain Na and Ca chlorides with Te of c. -64 °C and salinities of 28.7-33.5 wt. %- eq. NaCl. Individual FI compositions were analysed by LA-ICP MS. Elemental ratios of the 3-FI types are reasonably consistent and confirm the major cations are Na, K and Ca, with several 100’s to a few thousand ppm of Fe, Cu, Zn, Pb in the higher salinity fluids. There are extremely high concentrations of Au in all FI types. In the high salinity FI’s the average concentration is c. 300 ppm and as high as 1500 ppm. Thermodynamic simulations indicate such high Au concentrations correspond to a saturated solution of Au in the chloride complexes at temperatures above 500 ºC. We suggest these fluids could be a precursor of “Orogenic gold fluids” which at these Au concentrations would reduce the requirements for large volumes of metamorphic fluids to form such ore deposits.