Podrobnosti záznamu

Název
    General and comparative considerations of whole-rock and mineral compositions of Precambrian iron-formations and their implications
Údaj o odpovědnosti
    Arno Mücke
Autor
    Mücke, Arno
Jazyk
    anglicky
Zdrojový dokument - seriál
    Neues Jahrbuch für Mineralogie. Abhandlungen
Svazek/č.
    Vol. 179, no. 2
Strany
    p. 175-219
Rok
    2003
Poznámky
    7 obr., 6 pl., 7 tab., 4 s. bibl.
    il.
    Zkr. název ser.: Neu. Jb. Mineral., Abh.
Předmětová skupina
    amfiboly
    chemismus hornin
    chlority
    facie
    formace železitá
    geneze ložiska
    goethit
    granáty
    hematit
    magnetit
    prekambrium
    pyroxeny
    rekrystalizace
    rudy Fe
    silesikum
Geografické jméno
    Afrika
    Amerika severní
    Austrálie
    Brazílie
    Břidličná (Bruntál)
    Čína
    ČR-Morava
    Indie
    Norsko
    Švagrov (Šumperk)
    Švédsko
    Ukrajina
    Venezuela
    Vernířovice (Šumperk)
Klíčové slovo
    Comparative
    Compositions
    Considerations
    General
    Implications
    Iron-formations
    Mineral
    Precambrian
    Whole-rock
Abstrakt (anglicky)
   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
    U
Datum vložení
    11. 7. 2007
Datum importu
    8. 8. 2012