Národní geovědní bibliografie - GeoLib: Podrobnosti záznamu

139 samples of iron-formations from all the five continents belonging to 16 countries and 45 localities were investigated. The investigation of the rocks is based on ore-microscopic studies, electron-microprobe and XRF analyses. The results of the whole-rock analyses reveal that the SiO2 + iron (expressed as Fe2O3) contents are mostly higher than 90 wt per cent. This indicates that iron-formations are enriched in magnetite, martite and hematite rarely goethite and SiO2 mainly in the form of quartz. These mineral assemblages represent iron-formations that belong to the oxide facies comprising the magnetite and hematite subfacies. In the magnetite-free silicate facies, the magnetite-silicate facies and the carbonate facies, the (Fe2O3 + SiO2) concentrations are lower (75.5 to 86.1 wt per cent) and the Al2O3 or the CO2 concentrations are correspondingly higher.

In general, the iron-formations are characterized by their low concentrations of Na2O, K2O and P2O5 (mainly below 0.1 wt per cent). The first Fe-oxide occurring in iron-formations is magnetite in the form of porphyroblasts, whereas hematite is mainly of secondary origin due to the formation of martite (= pseudomorphic oxidation of magnetite) and its subsequent recrystallization. Goethite is the youngest Fe-mineral which originated either from the replacement of magnetite (not of hematite) or from Fe-rich descending solutions. Some of the investigated iron deposits, known as iron-formations contain minerals or relics of minerals within later and replacing quartz which cast doubt upon their classification as iron-formations. However, these deposits cannot be differentiated from true iron-formations within the [(FeO + MnO) - Fe2O3 - SiO2 = 100 per cent ]-diagram.

Their doubtful origin concerns in particular the occurrence of amphiboles (magnesiohasting-site, magnesiohornblende, actinolite, tschermakite, wincheite and richterite) with Mg-numbers distinctly higher than 0.5. In iron-formations amphiboles (ferrohornblende, ferroactinolite and grunerite) have Mg-numbers markedly lower than 0.5. Other minerals of the deposits of doubtful origin are unknown in iron-formations (e.g. fluorite, allanite) or have compositions that are not known in iron-formations. These are: magnetite (with TiO2-contents up to 6.6 wt per cent); pyroxenes (diopside and aegirine-augite); chlorite (32.2 to 59.2 mol per cent clinochlore, 24.1 to 43.4 mol procent chamosite and 14.7 to 26.6 mol per cent pennantite); garnet (andradite and grossularite); and mica (mainly phlogopite).

In iron-formations these minerals are: pure magnetite; Fe-rich augite and clinoferrosilite; chlorite (59.9 to 63.1 mol per cent chamosite, 21.1 to 24.4 mol per cent pennantite and 14.9 to 16.5 mol per cent clinochlore); almandine-spessartite solid solutions; and annite. Additionally, the deposits of doubtful origin contain high concentations of P2O5 (up to 0.7 wt per cent in the form of microscopic apatite). These deposits are classi

Přispěvatel

Česká geologická služba

Kód přispěvatele

ČGS (UNM)

Zdrojový formát

Datum vložení

11. 7. 2007

Datum importu

8. 8. 2012