Chamosite, siderite and the environmental conditions of their formation in chamosite-type Phanerozoic ooidal ironstones

摘要

Two characteristic ironstone minerals occurring in chamosite-type ironstone deposits were investigated. These are the minerals of the chlorite and the carbonate series. The samples are from 31 localities and 8 countries (Czech Republic, France, Germany, United Kingdom, Egypt, Nigeria and USA); these deposits are identical to those presented by Mucke and Farshad [Mucke, A., Farshad, F. (in press). Whole-rock and mineralogical composition of Phanerozoic ooidal ironstones: Comparison and differentiation of subtypes. Ore Geology Reviews] with Ordovician to Late Cretaceous ages. Electron microprobe analyses of 197 carbonates and 64 chlorites were carried out. The analytical points of the carbonates in a [magnesite-calcite-(siderite+rhodochrosite)=100 mol. percent]-diagram are distributed within a limited area close to the siderite end-member. The area can be subdivided into two fields: 1. The carbonates of the Ordovician ironstones of the Welsh, Prague, and Thuringian basins have high siderite concentrations (87.1 to 90.6 mol. percent), magnesite contents from 6.7 to 8.3 mol. percent and calcite concentrations from 1.6 to 2.4 mol. percent. In the Ordovician ironstone of Krusna hora, Prague Basin, the siderite-concentrations are lower (78.3 mol. percent) and those of magnesite (15.6 mol. percent) and calcite (5.7 mol. percent) are higher. 2. The carbonates of the other ironstones (Jurassic to Late Cretaceous) have lower siderite (72.5 to 79 mol. percent) and higher magnesite (12.6 to 15.7 mol. percent) and calcite (6.9 to 12.2 mol. percent) contents. The average values of the rhodochrosite end-member lie in the range between 0.13 and 2.31 mol. percent and the siderite: rhodochrosite-ratio = 158 (on average). The analytical points of the chlorite analyses are presented in the (Fe_(tot)-Mg-Al~(VI)=100 atom. percent)-diagram. One field, containing exclusively chlorites with green internal reflections can be subdivided into two areas: one area contains the chlorites of the Ordovician ironstones (consisting of: 62.8 to 77.7 atom. percent Fe_(tot), 17.5 to 30.9 atom. percent Al~(VI), and 4.1 to 7.4 atom. percent Mg) and the other those of the Jurassic to Late Cretaceous ironstones (consisting of: 55.7 to 62.5 atom. percent Fe_(tot), 26 to 33.8 atom. percent Al~(VI), and 9 to 13 atom. percent Mg). The second field contains altered chlorites of ferruginized ironstones (Aswan and Red Mountain Formation). These chlorites present brown internal reflections, and have compositions (34.9 to 46.7 atom. percent Fe_(tot), 24.8 to 42.2 atom. percent Al~(VI), and 16.0 to 28.5 atom. percent Mg) that do not reflect the original diagenetic environment. Within the [Al/(Al+Fe+Mg)]-[Mg/(Mg+Fe)]-atom. percent diagram the chlorite analyses are concentrated in a limited field, which is distinctly separated from chlorites formed in other environments. The occurrence of carbonate and chlorite (both with slightly varying compositions), dominated by their iron-end members, which occur characteristically in association with framboidal pyrite and organic matter, are indications that three prerequisites for the formation of the ironstones are comparable in narrow limits. These are: 1. the composition of the protoliths, 2. the depositional environment of the protoliths and their rates of deposition, and 3. the environmental conditions that occurred during the lithification of the protoliths due to submarine diagenesis. The protoliths consist of mixtures of kaolinite, Fe~(3+)-oxide/hydroxide and organic matter; the depositional environments are marine basins in which fully marine conditions occur and thus the availability of Mg and Ca (not contained in the protoliths) exists. The rates of deposition control the amount of Mg (in the chlorite_(ss)) and that of Mg and Ca (in the carbonate_(ss)); and the environmental conditions during submarine diagenesis depend on the availability of organic matter creating reducing conditions due to bacterial oxidation. The resulting co