Whole-rock and mineralogical composition of Phanerozoic ooidal ironstones: Comparison and differentiation of types and subtypes

Abstract

One hundred and thirty-seven samples of ooidal ironstones from 38 localities in 8 countries (U.K., Czech Republic, Germany, France, Luxembourg, Egypt, Nigeria, U.S.A.) were investigated. The ages of these ironstones vary from Ordovician up to Late Cretaceous. The investigation is based on ore-microscopic studies, electron microprobe analysis and XRF analysis which were augmented by the determination of the H 2O −, LOI, Fe 2+ and Fe 3+ contents. The ironstones were classified on the basis of the (FeO+MnO)–Fe 2O 3–SiO 2 diagram. Combining these data with mineralogical aspects derived from petrographic descriptions and electron microprobe analyses, two types of ironstones, namely, the chamosite and the kaolinite types, were identified. These types may be divided into the following subtypes: unaltered, ferruginized and redeposited. Transitional subtypes which are of minor significance are the slightly oxidized, the magnetite-bearing and the moderately ferruginized chamosite subtypes. The classification of the types and subtypes is based on varying (FeO+MnO):Fe 2O 3-ratios. Within the (FeO+MnO)–Fe 2O 3–SiO 2- diagram, the analytical points are distributed within three fields (I to III). Field I contains three chamosite subtypes. These subtypes consists of chamosite which occurs as groundmass and in the form of ooids (pisoids, peloids). Chamosite consists of the end-members Fe tot (ranging from 73.9 to 42.2 atom%), Mg (16.2 to 5.2 atom%) and Al VI (19.9 to 41.6 atom%) and may be replaced by carbonate. Carbonate consists of the end members siderite (69.5–90.6 mol%), magnesite (6.7–17.4 mol%), calcite (1.6–12.2 mol%) and rhodochrosite (0.2 to 2.3 mol%). Other rock constituents are pyrite (mainly framboidal) and magnetite (formed at the expence of siderite under conditions of the prehnite-pumpellyite-facies). The three subtypes are (Ia) unaltered chamosite subtype (chamosite has green and siderite white internal reflections; the FeO+MnO:Fe 2O 3-ratio is higher than 80:20); (Ib) slightly oxidized chamosite subtype (chamosite and siderite have yellowish to brownish or reddish internal reflections; the FeO+MnO:Fe 2O 3-ratio is marked higher than 60:40); (Ic) magnetite-bearing chamosite subtype (the FeO+MnO:Fe 2O 3-ratio, which mainly depends on the quantity of magnetite Fe 2+Fe 3+ 2O 4 is higher than 75:25 and lower than 20:80). Field II contains the analytical points of the moderately ferruginized chamosite subtype and the redeposited chamosite type. Their FeO+MnO:Fe 2O 3-ratios vary from about 20:80 to 5:95. The redeposited chamosite subtype consists of ferruginized ooids which are embedded in green chamosite in which authigenic siderite may occur. The moderately ferruginized chamosite subtype contains chamosite and siderite which are slightly altered and incompletely replaced by goethite and/or hematite. Field III having a FeO+MnO:Fe 2O 3-ratio smaller than 5:95 contains the analytical point of four ironstone subtypes: the strongly ferruginized chamosite subtype and the ferruginized kaolinite, the redeposited kaolinite and unaltered kaolinite subtypes. The latter subtype in which the ooids and the groundmass consist of kaolinite, senso stricto does not belong to ironstones, but is the proto-ore of the ferruginized kaolinite subtype. Within the (FeO+MnO)–Fe 2O 3–SiO 2 diagram, the unaltered kaolinite type is located close to the SiO 2-corner. The redeposited kaolinite subtype consists of ferruginized ooids that are embedded in kaolinite. With increasing quantity of the iron-rich ooids, the analytical points shift along the SiO 2–Fe 2O 3 join the direction of the Fe 2O 3-corner. Both, the strongly ferruginized chamosite subtype and the ferruginized kaolinite subtype, which cannot be differentiated within the diagram, consist of limonite and/or hematite which replaced either chamosite or kaolinite. Remnants of altered chamosite or kaolinite may be preserved in low quantities. In the ferruginized ironstones, diagenetic pyrite and siderite occur almost only in the form of pseudomorphic replacements that are composed of goethite. Additional constituents of all the ironstones types are detrital quartz grains, apatite, organic material, pyrite (mainly framboidal) and calcite. Quartz grains may be absent, but may also occur in high quantities leading to ironstones with and without detrital quartz grains. Ironstones with high quantities of quartz grains resemble ooid-bearing sandstones. Apatite does not occur in the form of visible grains, but whole-rock analyses generally show that there is a distinct correlation between the CaO and P 2O 5, indicating the occurrence of apatite. Some ironstones may also be enriched in CaCO 3 either in the form of detrital shelly material or as the youngest mineral in the form of calcite replacing many of the pre-existing minerals.