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

摘要

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_2T 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, fermginized and redeposited. Transitional subtypes which are of minor significance are the slightly oxidized, the magnetite-bearing and the moderately fermginized 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 Fetot (ranging from 73.9 to 42.2 atom percent), Mg (16.2 to 5.2 atom percent) and A1VI (19.9 to 41.6 atom percent) and may be replaced by carbonate. Carbonate consists of the end members siderite (69.5-90.6 mol percent), magnesite (6.7-17.4 mol percent), calcite (1.6- 12.2 mol percent) and rhodochgjsite (0:2 to 2.3 mol percent). 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 (la) 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_2Tfe_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 kaolinife and "unaltered kaolinite subtypes. The latter subtype in which the ooids and the groundmass consist of kaolinite, senso stricfo 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-comer. 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 ferrugin