The sources of mineralizing fluids of orogenic gold deposits of the Baikal-Patom and Muya areas, Siberia: Constraints from the C and N stable isotope compositions of fluid inclusions

The compositions of fluid inclusions in quartz and sphalerite from five gold deposits of the Baikal-Patom (Sukhoi Log, Veminsk and Dogaldyn) and Baikal-Muya (Uryakh and Irokinda) foldbelts in the northern margin of the Central Asian Orogenic Belt, along its boundary with the Siberian craton, indicate that the gold mineralization was formed predominantly from heterogeneous CO2-H2O orogenic fluid. All deposits are structurally controlled and composed of veinlet-disseminated and vein to stockwork types of ores that are hosted by rocks that underwent regional metamorphism to the greenschist facies in Baikal-Patom and up to the granulite fades in Baikal-Muya. The gold-quartz veins of all the Baikal-Patom deposits and the Uryakh were formed by fluids of similar composition with salinity of 1.4-9.5 wt% NaCl equiv. and CO2 content of 1.4-8.6 mol/kg of solution, under comparable physical conditions, at temperatures of 128-385 degrees C and pressures of 570-3290 bar. Fluids of the granulate-hosted Irokinda deposit are distinguished for the highest temperatures and pressures and involve relics of medium- to high-temperature (179-453 degrees C) brines with salinity of 4.6-46.3 wt% NaCl equiv. The temperature and pressure of the mineralizing fluid increase roughly from north to south, which correlates with the increase in the metamorphic grade of the host rocks. The increase is accompanied by the simultaneous systematic shift in the C isotope composition of CO2 (released from the quartz by the chrush-leach method) from -1.9 to - 5.5 parts per thousand delta C-13(CO2) in the Baikal-Patom and Uryakh from -0.6 to + 0.7 parts per thousand delta C-13(CO2) at Irokinda. The C and N isotope compositions of CO2 suggest that Irokinda fluids were partly derived from decarbonated marine limestone, whereas the orogenic gold-bearing fluids of the other deposits are interpreted to have been composed mostly of fluid from a crustal magmatic (granitic?) source.