Original paper

General and comparative considerations of whole-rock and mineral compositions of Precambrian iron-formations and their implications

Mücke, Arno


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 90wt %. 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 86.1wt %) 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 %). 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. The analytical points of the iron-formations within the [(FeO + MnO) - Fe2O3 - SiO2 = 100%]-diagram occur in two fields (I = silicate and carbonate facies; II = magnetite-silicate and oxide facies; names according to the facies defined by James 1954 and 1992) which are clearly seperated from one another. The seperation of the two fields is based on two differences. 1. The chemical composition of the protoliths: It is inferred that the protolith of field I represents chemical precipitates of volcanic exhalations, whereas that of field II originated predominantly by the same source, but was contaminated from continental-derived material. 2. The environmental conditions during subsequent metamorphism: The silicate and carbonate facies of field I were formed under conditions of low oxygen fugacity due to the relatively high Fe2+-concentrations. The minerals occurring are aluminous Fe2+-silicates and siderite and rarely quartz. In the magnetite-silicate and the oxide facies of field II the minerals were formed under higher oxygen fugacity indicated by the occurrence of porphyroblastic magnetite Fe2+Fe3+2O4, minor aluminous Fe2+-silicates and varying quantities of quartz. Due to martitization (= oxidation of magnetite) and the recrystallization of martite into hematite, field IIa (magnetite-silicate facies) grades transitionally into that of field IIb (magnetite subfacies) consisting of magnetite, martite and quartz and finally into field IIc (hematite subfacies) which is dominated by hematite and quartz (of varying proportions). Hematite of field IIc may contain relics of martite and/or magnetite. 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%]-diagram. Their doubtful origin concerns in particular the occurrence of amphiboles (magnesiohastingsite, 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 %); pyroxenes (diopside and aegirine-augite); chlorite (32.2 to 59.2 mol % clinochlore, 24.1 to 43.4 mol % chamosite and 14.7 to 26.6 mol % pennantite); garnet (andradite and grossularite); and mica (mainly phlogopite). In iron-formations these minerals are: pure magnetite; Fe-rich augite and clino- ferrosilite; chlorite (59.9 to 63.1mol % chamosite, 21.1 to 24.4 mol % pennantite and 14.9 to 16.5mol% clinochlore); almandine-spessartite solid solutions; and annite. Additionally, the deposits of doubtful origin contain high concentations of P2O5 (up to 0.7wt % in the form of microscopic apatite). These deposits are classified as Itakpe type. The iron-rich rocks of Itakpe Hill, N