Exceptionally Au-rich mid-crustal fluids from the Kola super deep borehole

In the drillcore of the Kola super-deep borehole(SG-3, 12262m depth) gold bearing rocks of Archaean age have been located at depths of 9500 to 11000 m. Quartz veins, between 9052 and 10744 m, within this gold zone, contain fluid inclusions with exceptional concentrations of gold, which are present as nanoparticles. There are 4 types of fluid inclusions (1) gas inclusions of dense CO2, (2) liquid-vapour two-phase aqueous inclusions, (3) three-phase inclusions with NaCl daughter crystals, and(4)CO2-aqueous inclusions. In all inclusion types, there are extremely high concentrations of Au. In the high salinity FIs the average concentration isc. 750 ppm and may be as high as 6000 ppm. We suggest these fluids could be a precursor of “Orogenic gold fluids” which, at the Au concentrations determined, would reduce the requirements for large volumes of metamorphic fluids to form orogenic ore deposits. The gold is currently present as colloids and indicates that in this form it is possible to transport greater amounts of gold than in true solution