Fluid types and their genetic meaning for the BIF-hosted iron ores, Krivoy Rog, Ukraine

The metasomatic stage post-dated and partially overlapped the metamorphic stage and led to the upgrade of iron quartzites to the high-grade iron ores. The genesis of these ores, which are located in the Saksaganskiy tectonic block (Saksaganskiy ore field), and the factors controlling iron ore-forming processes are highly controversial. According to the study of quartz-hosted fluid inclusions from the thrust zone the metasomatic stage involved at least three different episodes of the fluid flow, simultaneous with thrusting and deformation. During the 1st episode three types of fluids were introduced: CO2-CH4-N2(± C), CO2(± N2-CH4) and low salinity H2O-N2-CH4-NaCl (6.38-7.1 mass% NaCl equiv.). The 2nd episode included expulsion of the aqueous fluids H2O-N2-CH4-NaCl(± CO2, ± C) of moderate salinities (15.22-16.76 mass% NaCl equiv.), whereas the 3rd event involved high salinity fluids H2O-NaCl(± C) (20-35 mass% NaCl equiv.). The fluids most probably interacted with country rocks (e.g. schists) supplying them with CH4 and N2. The high salinity fluids were most likely either magmatic-hydrothermal fluids derived from the Saksaganskiy igneous body or heated basinal brines, and they may have caused pervasive leaching of Fe from metavolcanic and/or the BIF rocks. The baro-acoustic decrepitation analyses of magnetite comprising the high-grade iron ore showed formation T = ~ 430-500 °C. The fluid inclusion data suggest that the upgrade to high-grade Fe ores might be a result of the Krivoy Rog BIF alteration by multiple flows of structurally controlled, metamorphic and magmatic-hydrothermal fluids or heated basinal brines.