Abstract:
The paper contains results of geological, structural, mineralogical, isotopic, and thermobarogeochemical studies of one of the few known gold deposits in the Early Proterozoic greenstone belt of North Karelia. The Maisk deposit is localized in the Kuolajarvi structure, 80% of which is occupied by volcanic and intrusive basic and ultrabasic rocks. The shape of orebodies at the deposit is related to the deformations within the ore-bearing faults. The formation of veins at the deposit was accompanied by rock disintegration and its replacement by quartz. Three stages of mineral formation are distinguished: (1) formation of quartz bodies; (2) deposition of the gold-sulfide assemblage in the quartz bodies synchronously with the biotite + albite + amphibole + chlorite metasomatic rocks; and (3) deposition of the quartz + carbonate + chlorite + talc assemblage. The deposition of the vein quartz began at 470-300°C. The ore fluid was heterogenic, saturated with gas of low density, and of low fluid pressure. Sulfides were deposited at a temperature of around 200°C from fluid with 22-26 wt % salinity. The main salt in the solution was CaCl 2, and the methane content varied from 16-25 to 70-100 mol %. Pressure and temperature decreased during the sulfide deposition from 940 to 500 bar and from 270 to 190°C, respectively. The gold was deposited at 200-140°C from solution with 22-28 wt % salinity and with CaCl 2 as the predominant salt. According to the stable isotope ( 18O, 13C, and 34S) data, the hydrothermal fluids that formed the quartz veins and ore mineralization were composed mainly of magmatic water. The isotope composition of Pb corresponds to the crustal source of the ore metals. The K-Ar age data of metasomatic and vein minerals fall into two groups: 1800-1700 and 1610 ± 30 Ma.
