Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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Fluid types and their genetic meaning for the BIF-hosted iron ores, Krivoy Rog, Ukraine.

Marta Sośnicka, Ronald J. Bakker, Curt Broman, Iain Pitcairn, Ihor Paranko & Kingsley Burlinson


This paper contributes to the understanding of the genesis of epigenetic, hypogene BIF-hosted iron deposits situated in the eastern part of Ukrainian Shield. It presents new data from the Krivoy Rog iron mining district (Skelevatske–Magnetitove deposit, Frunze underground mine and Balka Severnaya Krasnaya outcrop) and focuses on the investigation of ore genesis through application of fluid inclusion petrography, micro- thermometry, Raman spectroscopy and baro-acoustic decrepitation of fluid inclusions. The study investigates
inclusions preserved in quartz and magnetite associated with the low-grade iron ores (31–37% Fe) and iron-rich quartzites (38–45% Fe) of the Saksaganskaya Suite, as well as magnetite from the locally named high-grade iron ores (52–56% Fe). These high-grade ores resulted from alteration of iron quartzites in the Saksaganskiy thrust footwall (Saksaganskiy tectonic block) and were a precursor to supergene martite, high-grade ores (60– 70% Fe). Based on the new data two stages of iron ore formation (metamorphic and metasomatic) are proposed. The metamorphic stage, resulting in formation of quartz veins within the low-grade iron ore and iron-rich quartzites, involved fluids of four different compositions: CO2-rich, H2O, H2O–CO2(±N2–CH4)–NaCl(±NaHCO3) and H2O–CO2(±N2–CH4)–NaCl. The salinities of these fluids were relatively low (up to 7 mass% NaCl equiv.) as these fluids were derived from dehydration and decarbonation of the BIF rocks, however the origin of the nahcolite (NaHCO3) remains unresolved. The minimum P–T conditions for the formation of these veins, inferred from microthermometry are Tmin = 219–246 °C and Pmin = 130–158 MPa. The baro-acoustic decrepitation analyses of magnetite bands indicated that the low-grade iron ore from the Skelevatske–Magnetitove deposit was metamorphosed at T = ~530 °C.

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.

Published in:    Ore Geology Reviews 68 (2015) 171–194          Extract (PDF file)(3.5 Mbyte) here

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