Whole-Rock and Mineralogical Composition of Phanerozoic Ooidal Ironstones: Comparison and Differentiation of Types and Subtypes

Ore Geology Reviews 26 (2005) 227–262 www.elsevier.com/locate/oregeorev

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

Arno Mu¨ckea,T, Farhad Farshadb

aExperimentelle und Angewandte Mineralogie, Go¨ttinger Zentrum Geowissenschaften der Georg-August-Universita¨t, Goldschmidtstrasse 1, 37077 Go¨ttingen, Germany bDepartment of Geology, Sharood Technical University, Tehran, Iran Received 27 June 2003; accepted 22 August 2004 Available online 13 May 2005 Dedicated to Jan Petranek, Praha

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 À 2+ 3+ which were augmented by the determination of the H2O , LOI, Fe and Fe contents. The ironstones were classified on the basis of the (FeO+MnO)–Fe2O3–SiO2 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):Fe2O3-ratios. Within the (FeO+MnO)–Fe2O3–SiO2- 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%), Mg (16.2 to 5.2 atom%) and AlVI (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:Fe2O3-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:Fe2O3-ratio is marked higher than 60:40); (Ic) magnetite-bearing chamosite subtype (the FeO+MnO:Fe2O3-ratio, which mainly depends on the quantity of magnetite 2+ 3+ Fe Fe 2O4 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:Fe2O3-ratios vary from about 20:80 to 5:95. The redeposited chamosite subtype consists of ferruginized ooids

T Corresponding author. Fax: +49 551 393851. E-mail address: [email protected] (A. Mu¨cke).

0169-1368/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.oregeorev.2004.08.001 228 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 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:Fe2O3-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)–Fe2O3–SiO2 diagram, the unaltered kaolinite type is located close to the SiO2-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 SiO2–Fe2O3 join the direction of the Fe2O3-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 P2O5, indicating the occurrence of apatite. Some ironstones may also be enriched in CaCO3 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. D 2005 Elsevier B.V. All rights reserved.

Keywords: Ooidal ironstones; Whole-rock analyses; Mineral assemblages; Mineral compositions; (FeO+MnO)–Fe2O3–SiO2 diagram; Types and subtypes of ironstones

1. Introduction earlier Precambrian banded iron formations. More than 500 deposits are known, summarized as Phaner- Ooidal ironstones are sedimentary rocks with N5 ozoic ooidal ironstones. They were especially com- vol.% ooids and N15 wt.% iron, corresponding to 21.4 mon in the Ordovician and the latter part of the wt.% Fe2O3 (Young, 1989; Petrańek and Van Houten, Silurian, Devonian and again in the Jurassic and 1997). By definition, ooids are spherical or ellipsoidal Cretaceous. In contrast, only a few occurred in the grains b2 mm in diameter displaying regular concentric Cambrian, Permian, Triassic, or in the late Cenozoic laminae. Grains similar to ooids, but N2 mm are known (Petrańek and Van Houten, 1997). as pisoids. Peloids are grains of fine-grained material James (1992) showed that Phanerozoic ironstones with diameters in the range of ooids to pisoids, but can be distinguished from Precambrian iron formations without recognizable internal structure (Young, 1989). within the (FeO–Fe2O3–SiO2 =100%) diagram. Generally, the ooids and their groundmass consist of According to this author, iron formations have SiO2- chamositic clay. This term is used in cases when this concentrations higher than 50 wt.%, whereas those of mineral was investigated by chemical analysis only the ironstones are below about 35 wt.%. However, such (Mu¨cke, 2000). The usage of this name is based on the a clear separation of these two types cannot be fact that chamosite (belonging to the chlorite group) observed. This is illustrated in Fig. 1, based on 131 and berthierine (serpentine group) have the same analyses of ironstones and 129 analytical points of iron chemical composition, but can be differentiated by X- formations (Mu¨cke, 2003), distributed over all con- ray analysis. Berthierine was not detected in inves- tinents and originating from 17 countries and 43 tigations of nearly all ironstones confirmed by X-ray localities. Additionally, Fig. 1 contains the proposed study. Therefore, in the following chamositic clay is lines of James (1992) separating the two types of considered to belong to the chlorite group. deposits. Ooidal ironstones accumulated throughout the In textbooks, ironstones are classified as the Clinton Phanerozoic Eon, from the Cambrian to the Recent and the Minette types (e.g., Bottke, 1981; Barnes, (Petrańek and Van Houten, 1997), following the 1989, p. 33; Evans, 1993, p. 257). The first type is A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 229

Fig. 1. Analytical points of 131 samples of iron formations and 129 of ironstones expressed as (FeO+MnO)–Fe2O3–SiO2 diagram including the lines separating the fields of iron formations from those of ironstones inferred by James (1992). common in rocks of Cambrian to Devonian age and (Siehl and Thein, 1989, following the model of Nahon contains the Silurian type deposits occurring along a et al., 1980, and Tardy and Nahon, 1985). NNE trending zone of about 1800 km in the U.S.A., but For a better appreciation of this paper, the genesis also the Ordovician Wabana ironstone of Newfound- of ooidal ironstones will be briefly summarized land. The Minette type, on the other hand, is according to the proposal of Mu¨cke (2000). The particularly widespread in the Mesozoic of Europe. initial material of ooidal ironstones is derived from the Type deposits are the Jurrasic ironstones of the weathering of various rocks in the hinterland under Lorrainian Minette Basin of France. Ironstones of the lateritic conditions and subsequently transported into Salzgitter district of Germany and the eastern part of the the marine basins via fluvial drainage systems. In English Midlands (Southern North Sea Basin) are also these basins, unlithified sediments were formed considered to belong to the Minette type deposits. The consisting of three constituents: Clinton type is dominated by a carbonate-rich iron- oxide facies mainly consisting of hematite, whereas the 1. Ooids, pisoids and peloids. They were formed by Minette type shows a stronger facies differentiation. rolling on the sea floor and accretion of iron- The facies consist of three mineral assemblages. These bearing platy kaolinite crystals leading to the are the iron-oxide, the iron-carbonate and the iron- formation of tangentially arranged laminae. silicate facies (Bottke, 1981). Apart from these differ- 2. Detritus, comprising quartz, zircon, and heavy ences and their different ages, the Silurian Clinton and minerals (ilmenite, magnetite, rutile, and chromite). Jurassic Minette types appear to be similar. This led In addition, fossils, brecciated shelly material, and Young (1989) to the conclusion that the use of the two organic material may be present. terms is unnecessary. However, he distinguishes 3. Fine-grained iron-bearing kaolinitic material. This between two other classes of Phanerozoic ironstones. is suspension-derived and forms the matrix of the The majority of deposits belong to marine ooidal ooids, pisoids, peloids and detritus. ironstones. These were differentiated from a few deposits inferred to be formed under non-marine Subsequent diagenesis of the sediment composed of conditions, e.g., the Minette type of Lorraine, France the constituents mentioned under 1 to 3 affects 230 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 mineralogy and texture rather than the whole-rock chemistry and the structure of the deposits. At an early stage, the iron-bearing clay mineral kaolinite of the ooids and matrix is recrystallized or transformed into chamosite under reducing conditions (caused by decomposing organic material due to bacterial oxidation). The formation of chamosite is an indication of the availability of Mg, which was extracted from the sea water. The formation of recrystallized kaolinite (indicating the absence of Mg) or chamosite is followed by the formation of pyrite and siderite (predominantly in the form of framboids) being formed together with the bacterial oxidation of organic matter. The diagenetic mineral assemblage forms the protore of the deposits. Due to the mineral association having developed under reducing conditions and to the porosity, the protore is unstable in an oxidizing environment and is easily permeated by aqueous solutions under continental conditions which cause ferruginization of the protore (Mu¨cke, 1994, 2000). Ferruginization is the consequence of descending meteoric waters that cause dissolution, migration Fig. 2. Geological map of the Welsh Basin, U.K., including the locations of the Ordovician ooidal ironstones (modified after and precipitation of various elements depending on Trythall, 1989). the prevailing Eh/pH conditions. Ferruginization appeared to be an important phenomenon in the total quantity of twelve samples was selected. They Cretaceous, especially in Late Cretaceous deposits, originated from the following localities: Betws but is insignificant or of only minor significance in Garmon (3 samples), Cross Foxes (1), Ffordd Ddu Ordovician ironstones. (2), Nant Ffrancon (2), Pen-y-Gear (3) and Tremadog (1). According to Trythall (1989), the ooidal ironstones occur at two main stratigraphic 2. Origin and description of the samples horizons (upper Arenig and at the border between upper Llandvirn/lower Caradoc), whereas Young The investigated ironstones (137 rock samples) (1990) is of the opinion that the ooidal ironstones were collected over a long period from 38 localities in occur in three stratigraphically separated horizons. 8 countries. They range in age from Ordovician to The first accumulated during the lower Arenig, the Late Cretaceous. second at the border between Llandvirn/Caradoc and the third in the lower Caradoc. The ironstone 2.1. United Kingdom beds of Betws Garmon and Pen-y-Gear belong to the second horizon, and the others to the third. The In the U.K., two areas where ooidal ironstone occurs geological position of Cross Foxes is controversial, were investigated. These are the Welsh Basin where the but may also belong to the third horizon. Accord- ironstones occur at several levels within the Ordovician ing to the petrographic description of Farshad succession, and the Cleveland and Frodingham Iron- (2001), the number of ooids consisting of chamo- stones located in the eastern part of the Midlands site (confirmed by X-ray analysis) varies strongly (Southern North Sea Basin) in settings of Jurassic age. within the six investigated deposits. Oooids form 0 to 10 vol.% at Cross Foxes and Nant Ffrancon, 1. The Ordovician ironstones belong to the northern between 40 and 60 vol.% at Pen-y-Gear, Betws part of the Welsh Basin (Fig. 2). From this area, a Garmon and Tremadog and about 80 vol.% at A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 231

Ffordd Ddu (Fig. 3A). The ooids are associated chamosite with the same or similar compositions to with pisoids (Betws Garmon and Ffordd Ddu), those of the associated ooids. At the localities of peloids (Betws Garmon, Cross Foxes, Nant Ffran- Pen-y-Gear (one sample) and Tremadog (where con and Pen-y-Gaer) and spastolites (deformed newly formed goethite occurs, mainly replacing ooids or peloids; Betws Garmon). Ooids, pisoids siderite), chamosite of the groundmass and the and peloids are embedded in a groundmass of ooids is almost completely replaced by carbonate

Fig. 3. (A) Closely packed chamosite ooids in a chamositic groundmass. Transmitted light, longer edge 4 mm. Ffordd Ddu, Welsh Basin. (B) Ooids and groundmass originally consisting of chamositic clay are nearly completely replaced by siderite in which relics of chamositic clay are preserved (arrows). Reflected light, length of the longer edge 3.2 mm. Tremadog, Welsh Basin. (C) Replacement of chamosite by siderite parallel to the original laminated structure of the ooid. Note the chamosite relics within the ooid (arrows). The chamositic groundmass is also partially replaced by siderite. Reflected light, crossed polars, longer edge 3.2 mm. Tremadog, Welsh Basin. (D) Slightly corroded chamositic ooid embedded in siderite which is pseudomorphously replaced by goethite and contains relics of chamositic clay (arrows) and some framboidal pyrite inclusions (highest reflectance). Reflected light, oil immersion, longer edge 450 Am. Pen-y-Gaer, Welsh Basin. (E) Chamositic ooids enclosed in a chamositic groundmass which contain abundant authigenic siderite (highest reflectance). Reflected light, 1.6 mm. Two Foot, Cleveland ironstone. (F) Detrital quartz grains (arrows) and chamosite relics of the ooids and the groundmass within replacing siderite. The elongated particles represent shelly material. Reflected light, 1.6 mm. Avicula, Cleveland ironstone. 232 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

(dominated by the siderite end-member; Fig. 3B). The replacement of the ooids occurs along the margin and follows the original laminated structure developed by the arrangement of chamosite crystals (Fig. 3C). At the localities of Cross Foxes and Ffordd Ddu, siderite was not observed, whereas in Betws Garmon and Nant Ffrancon, siderite is disseminated in the form of authigenic crystals within the groundmass. Detrital quartz grains may also be an important rock constituent, ranging from about 60 vol.% (Pen-y-Gaer, one sample) and 15 vol.% (Ffordd Ddu) to 0 vol.% (Tremadog). At two localities (Betws Garmon and Cross Foxes) magnetite is also present (up to 30 vol.%). Sulphides may locally constitute up to 50 vol.% of the rock. These are pyrrhotite in association with arsenopyrite (Betws Garmon and Nant Ffrancon) and pyrite (Pen-y-Gear). Of minor abundance is illite, occur- Fig. 4. Geographical map of the eastern part of the British Midlands ring locally in the form of ooids and groundmass (=Southern North Sea Basin) showing the Middle Lias Cleveland (one sample of Betws Garmon). In one sample of Ironstone and the Lower Lias Frodingham Ironstone including the Pen-y-Gear, the ironstone is moderately ferrugi- sample locations, modified after Howard (1985) and Young et al. nized and many of the above-mentioned minerals (1990a). are partially replaced by goethite and hematite (Fig. 3D). In all localities, organic material and pyrite Young et al., 1990b). Two samples are from the (mainly framboidal) are present. locality of Scunthorpe (Yarborough Pit). 2. The Early Jurassic ironstones of the U.K. are The CIF is characterized by the preponderance of located in the Southern North Sea Basin and occur siderite. Siderite replaced both the groundmass and within the Cleveland Ironstone Formation (CIF) the ooids, which occur in the form of relics consisting and the Frodingham Ironstone Formation (FIF) of chamosite (Fig. 3E). Ooids and groundmass may (Fig. 4). According to Young et al. (1990a,b), the also consist of kaolinite (Osmotherley and Avicula). CIF, which belongs to the Middle Lias, is Quartz grains (especially in Raisdale), shelly material sandwiched between the Staithes Sandstone For- (N40 vol.% in Avicula, Fig. 3F; about 20 vol.% in mation at the base and the Whitby Formation on Two Foot), calcite (N50 vol.% in one sample of top. The CIF consists of two members; the Penny Pecten) and in subordinate proportions pyrite and Nab overlain by the Kettleness Member. The organic material are also part of the ironstones. The Penny Nab Member contains four ironstone hori- Frodingham Ironstone (Scunthorpe) comprises two zons: Osmotherley, Avicula, Raisdale and Two samples: one contains chamositic ooids, about 20 Foot from bottom to top. The Ketteless Member vol.% detrital quartz grains and some small authigenic consists of the Pecten Seam at the base and the siderite crystals within a chamositic groundmass. Main Seam. The samples (7) are from the above- Constituents of the second sample are framboidal mentioned seams and are from the two localities pyrite and shelly material; groundmass and ooids are Staithes [Osmotherley (1 sample), Avicula (1) and locally replaced by goethite. The second sample Raisdale (1)] and Grosmont [Two Foot (2) and contains about 50 vol.% ooids (0.3 to 0.4 mm), Pecton (2)]. consisting of goethite (core) and hematite (rim) (Fig. 5A), and about 5 vol.% of quartz grains. These grains The FIF is the most important of the several are embedded in a slightly altered, brownish chamo- ironstone beds within the lower part of the Lias in the sitic groundmass which may contain rice-grain- eastern part of the Midlands (Davies and Dixie, 1952; shaped siderite crystals (b1 vol.%). A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 233

Fig. 5. (A) Ferruginized ooids which consist of a goethite core. Along the rim, goethite is replaced by hematite (due to dehydration). The ooids are embedded in a chamositic groundmass. Reflected light, oil immersion, longer edge 450 Am. Scunthorpe, Frodingham ironstone. (B) Chamositic clay of the ooids, but especially that of the groundmass, are replaced by coarse-grained siderite. The sideritization of the ooids predominantly takes place along the original laminated structure. Reflected light, longer edge 1.6 mm. Nucˇice, Prague Basin. (C) The original chamositic groundmass which contains abundant quartz grains is completely replaced by siderite. The ooids, consisting of chamosite, are incompletely replaced by siderite. Reflected light, longer edge 3.2 mm. Zdice, Prague Basin. (D) Zoned ooid embedded in a fine-grained sideritic groundmass. The inner part of the ooid (badly polished) consists of chamosite followed by two well-polished layers consisting of mixtures of apatite+chamosite. The outer and brighter zone (arrows) contains the highest apatite-content (close to 50 vol.%). Reflected light, longer edge 3.2 mm. Nucˇice, Prague Basin. (E) Fine-grained magnetite crystals arranged parallel to the original laminated structure of the ooids, which contain also chamosite (small arrow) and siderite (large arrow). Coarse-grained magnetite porphyroblasts occur in the siderite groundmass. Reflected light, oil immersion, longer edge 450 Am. Glaserz, Nucˇice, Prague Basin. (F) Goethite replacing groundmass and ooids. The ooids are ferruginized parallel to the laminated structure. Some zones of the ooids which originally consisted of mixtures of chamositic clay and apatite were not affected by ferruginization (arrows). Reflected light, longer edge 3.2 mm. Nucˇice, Prague Basin. 234 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

2.2. Czech Republic ironstone horizons occur within the sedimentary rock sequence. These are the Klabava-Osek ore horizon The most important ironstones of the Czech (Arenigian), the Nucˇice ore horizon (at the Llandei- Republic are of Ordovician age and occur within the lian/Caradocian border), the Karlı´k ore horizons Prague Basin. These ironstones were first investigated (upper Caradocian) and finally the Podolı´ ore horizon by Vala´ and Helmhacker (1877) and later by Petranek (at the Caradocian/Ashgillian border). The investi- (1964), Petranek et al. (1988) and Farshad (2001). The gated ironstones are exclusively from the Nucˇice Prague Basin, located in the center of Bohemia, trends horizon, from the localities of Nucˇice (4 samples), SW of Prague up to the town of Beroun (Fig. 6). Chrustinice (1) and Zdice (1) (Fig. 6). Along a profile of locally varying thicknesses, a The ironstones of the three deposits are grey-green variety of sedimentary rocks (sandstones and quartz- in colour and contain ooids in varying proportions. ites) occur, reaching from Tremadocian at the base up Groundmass and the ooids (pisoids and peloids) are to Ashgillian at the top (Petranek, 1964). Four composed of chamosite (confirmed by X-ray analy-

Fig. 6. Geological map of the Ordovician Prague Basin, Czech Republic including the locations of the ooidal ironstones. A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 235 sis). The ooids (peloids), but especially the ground- 2.3. Germany mass, are replaced by siderite (Fig. 5B), at Nucˇice (1 sample) and Chrustinice nearly completely. In these Among the various ooidal ironstones of Germany, cases, relics of chamosite may be admixed with high the two most important districts are included in this proportions of newly formed kaolinite. Quartz grains paper. These are the ironstones of east-central are also constituents of the ironstones especially in Germany (Thuringian Basin) and of north-central Zdice (up to 50 vol.%; Fig. 5C). Apatite occurs in the Germany (Salzgitter district). ooids and the groundmass in the form of fine-grained 1. The ooidal ironstones of east-central Germany mixtures consisting of chamosite and apatite. In were mined in Wittmannsgereuth and Schmiedefeld contrast to pure chamosite, which reveals a poor SW of the Thuringian town of Saalfeld lying about 35 polishing behaviour, chamosite–apatite mixtures have km SSW of the town of Jena (Fig. 7). Three relatively excellently polished surfaces (Fig. 5D). Other rock stable ironstone horizons belonging to the Ordovician constituents are pyrite (mainly framboids; strongly Gra¨fenthaler Formation are known in the area (Hetzer, enriched in Nucˇice), fossil fragments, graphite, 1958; Petra´nek and Van Houten, 1997). The lower organic material, zircon, anatase and sulphides ironstone horizon, also known as third seam (up to 11 (galena, sphalerite, marcasite and chalcopyrite). In m thick), occurring at the Tremadocian/Arenigian Nucˇice (1 sample), magnetite occurs (about 10 to 15 boundary, overlies the Phycodes Quartzite Formation vol.%) particularly within ooids. It was formed at the and is overlain by the so-called Griffel Shale which expense of siderite in the form of small idiomorphic has a thickness of 60 to 200 m. The middle ironstone grains (30 Am, but mainly below 5 Am; Fig. 5E). In horizon of Arenigian age, which never has had any one sample of Nucice goethite and hematite are the economic significance, occurs in the upper part of the dominating minerals which both replaced, at least Griffel Shale and has a thickness of up to 5 m. The partially, chamosite and siderite without destroying upper ironstone horizon, which is subdived into the the original rock structure (Fig. 5F). lower or second seam and the hanging or first seam,

Fig. 7. Geological map including the Ordovician ironstones of Wittmannsgereuth and Schmiedefeld, Thuringia, Germany (after Hetzer, 1958). 236 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 occurs at the Caradocian/Ashgillian border and is kutnahorite=51 to 67:43.8 to 23.4:5.3 to 9.4 mol%; overlain by the so-called banded Leder Shale (Fig. 7). calculated from electron microprobe analyses). The analyzed samples are from the third seam of 2. In the Salzgitter district of north-central Ger- Schmiedefeld (1), the second seam of Wittmannsger- many, close to Braunschweig, ooidal ironstones were euth (2) and the first seam of Schmiedefeld (1) and accumulated from Jurassic up to Cretaceous time Wittmannsgereuth (5). Two samples of Wittmanns- (Kolbe, 1957, 1958, 1970; Petra´nek and Van Houten, gereuth are from the so-called hanging marker beds 1997). The ironstones of Late Jurassic (late Oxfor- (Hangende Leitschichten) which overlie the first dian) are known as Gifhorn ooidal ironstone (=Kor- seam. allen-Oolit) and occur in a 60 km long, The ironstones of the first and second seam do not synsedimentary trough trending NS between the show significant petrographic-mineralogical differ- villages of Vorhop and Lebenstedt (Fig. 9). Four ences (Farshad, 2001) and will be described together. ironstone layers are known, occurring at depths of The only difference between these two seams is that about 1000 to 1200 m with thicknesses in the range of the second seam is mainly pisolitic and subordinately 2 to 18 m. The ironstone sequence is underlain by ooidal (grain sizes lying in the range between 3.4 and limestone and overlain by marlstone (Simon, 1965). 0.4 mm), whereas the overlying first seam is The samples of this area (2) are from the Konrad shaft. dominated by ooids (grain sizes vary between 0.8 The mixed ooidal and detrital ironstones (so-called and 1.2 mm). Peloids may occur in both seams. Ooids Tru¨mmererz) of Early Cretaceous age (Valanginian to and pisoids as well as the groundmass consist of Aptian) lie around the town of Salzgitter (Fig. 9) chamosite (confirmed by X-ray analysis) which is within the so-called Salzgitter Ho¨henzug (e.g., Kolbe, replaced by siderite. The latter affects especially the 1957, 1958, 1970; Petra´nek and Van Houten, 1997). groundmass (Fig. 8A), whereas the ooids/peloids are The ironstones have thicknesses of 5 to 30 m, locally replaced along the margins or parallel to the original up to 100 m. The ironstones were economically laminated structure (Fig. 8B). Sideritization is more important deposits and were exploited for about 2000 pronounced in the first seam where the groundmass years (open pit and underground). Mining activities may locally exclusively consist of siderite. Chamosite, stopped in the late 1970s. Within the area, a variety of especially that in the groundmass, is often admixed mines are located including Finkenkuhle (2 samples) with varying proportions of apatite, particularly in the and Haverlahwiese (2 samples). second seam (in maximum about 50 wt.%). Quartz The ironstones consist of goethite-rich ooids [up to grains may occupy up to 20 vol.% of the ironstones. 80 vol.% in Finkenkuhle; about 60 vol.% in Haver- Other rock constituents are ilmenite, zircon, graphite, lahwiese (Fig. 8C) and Konrad shaft]. In the first two organic material, and sulphides (pyrite, marcasite, mines, the ooids are often broken and associated with galena, sphalerite, arsenopyrite and chalcopyrite). In clasts. The clasts may consist of goethitic ooids contrast to the first and second seams, the third seam embedded in a goethitic groundmass (Fig. 8D). In which may contain up to 50 vol.% quartz grains is the ironstones of the Konrad shaft and Haverlahwiese, dominated by goethite and/or hematite. These two the groundmass is of brownish colour whereas that of minerals occur in the form of replacements which the Finkenkuhle ironstone is reddish. However, the mainly affects the original chamositic groundmass. groundmass of all these locations may contain Therefore, the latter may consist exclusively of irregularly distributed idiomorphic siderite crystals goethite and hematite, whereas the ooids, mainly (Fig. 8E), framboidal pyrite (Fig. 8C), organic composed of the same minerals, may contain some material and detrital quartz grains. The groundmass relics of chamosite. In rocks of the hanging marker is often composed of illite, particularly in the deposits beds, ooids (up to 50 vol.%) may also occur. They are of Finkenkuhle and Haverlahwiese. The ironstone of embedded in a groundmass of carbonate which may the Konrad shaft may also contain pseudomorphic contain some relics of chamosite and quartz grains. replacements of goethite after fossils, late calcite and The carbonates consist either of fine-grained siderite small magnetite crystals of diagenetic origin (Fig. 8F). or of younger, coarse-grained and zoned carbonate of The latter was not observed in any other ironstone the dolomite group (consisting of dolomite:ankerite: investigated in this study. A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 237

Fig. 8. (A) Chamosite relics of the groundmass in replacing siderite which occurs as xenomorphic authigenic crystals. Reflected light, longer edge 3.2 mm. Second seam of Schmiedefeld, Thuringian Basin. (B) Siderite replacing tangential arranged chamositic clay of a pisolite. Reflected light, oil immersion, longer edge 450 Am. Second seam of Wittmannsgereuth, Thuringian Basin. (C) Ferruginized ooids consisting of goethite in a slightly ferruginized groundmass which contains rice-grain-shaped siderite (small arrows) and framboidal pyrite (big arrow). Reflected light, longer edge 3.2 mm. Hawerlahwiese (Tru¨mmererz), Salzgitter Ho¨henzug. (D) Goethitic clast which contains ferruginized ooids and small quartz grains (arrows). The clast is embedded in a fine-grained groundmass which contains ooids, clasts and small quartz grains (arrows). Reflected light, longer edge 3.2 mm. Hawerlahwiese (Tru¨mmererz), Salzgitter Ho¨henzug. (E) Ooids showing the original laminated structure of primary chamositic clay embedded in a chamositic groundmass which contains abundant rice-grain-shaped siderite porphyroblasts. Reflected light, crossed polars, longer edge 3.2 mm. Konrad shaft, Gifhorn ooidal ironstone. (F) Aggregate consisting of fine-grained idiomorphic magnetite crystals within the groundmass in which ooids are embedded. Groundmass and ooids are slightly ferruginized and contain hematite (small arrows) and goethite (large arrows). Reflected light, oil immersion, longer edge 450 um. Konrad shaft, Gifhorn ooidal ironstone.

2.4. France and Luxembourg the south with an average width of 25 km. This area is cut by a series of NE to SW trending faults. In the In the north-eastern margin of the Paris Basin, south of the town of Metz, the Nancy Basin occurs ooidal ironstones extend over a distance of about 140 and in the north the Minette Basin of Lorraine and km from Luxembourg in the north to near Nancy in Luxembourg (Fig. 10). The latter basin is divided into 238 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

megasequences, whereas the Middle Jurassic ironstones are represented by at least twelve ironstone layers. In the lower part, these ironstones are siliceous and are designated by their colour into grey, black and brown ironstone layers. These are overlain by calcareous beds known as gray, yellow and red ironstone layers. The analyzed samples are exclusively only from the Lorrainian Minette Basin (total quantity 8) and are from Bazaille (Mine de Bazaille) (2 samples), Piennes (MinedelaMourie`re), Droitaumont, Hussigny, Ottange, Montrouge (close to the village of Audun- le-Tiche) and Tressange (Mine Ferdinand). Twenty samples came from the open pit of Rumelange, Luxembourg. The latter deposit lies just at the Luxembourg–France border, ca. 2 km north of French deposit of Ottange (Fig. 10).

Fig. 9. Ironstone deposits and their distribution (Late Jurassic Gifhorn Ooidal ironstone and Early Cretaceous detrital ironstone= Salzgitter Ho¨henzug) within the Salzgitter district and the location of the ironstones (modified after Kolbe, 1958). three subbasins (Orne, Landres-Ottange and Longwy Subbasins) caused by three faults, the Metz Fault in the south, the Avril Fault and the Crusnes-Audun-Le Tiche Fault in the north (Bouladon, 1989). The ooidal ironstones were investigated over a long period by, for instance, Lucius (1954), Bubeni- cek (1961, 1971), Siehl and Thein (1978, 1989), Teyssen (1984) and Petra´nek and Van Houten (1997). The ironstones, reaching a maximum thickness of 60 m, were deposited in a near-shore shallow marine environment. The deposition of the ironstones [with ages from latest Early Jurassic (Toarcian) to early Middle Jurassic (Aalenian)] had been preceded by that of bituminous shales, calcareous marls and finally, by Fig. 10. Minette Basin of France and Luxembourg and the distribution of the Early to Middle Jurassic ironstone deposits sandstone. The ironstone is overlain by conglomerate, (dotted line parallel to the river Mosel) including the locations of sandstone and, finally, marl. The Early Jurassic ironstone deposits. Dashed line is the frontier of ooidal ironstone ironstones occur at the top of four coarsening-upward occurrences (modified after Einicke, 1950). A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 239

The ironstones of France contain ooids (ranging may occur in association with rounded goethitic clasts from 0.2 to 0.6 mm) in varying proportions (from 40 (up to 10 vol.% in Montrouge), are embedded in a to 50 vol.% in Tressange and Hussigny up to 80 vol.% groundmass of either green (Droitaumont, Bazaille in Bazaille and Piennes). The ooids showing the and Piennes) or brownish to reddish colour (Hussigny, original texture of the primary chamosite consist Ottange, Tressange and Montrouge). The groundmass mainly of goethite (Fig. 11A). The ooids, which may contain siderite crystals (Droitaumont has highest

Fig. 11. (A) Ferruginized ooids consisting of goethite embedded in a chamositic groundmass. The latter contains newly formed, xenomorphic and coarse-grained siderite (arrows). Reflected light, longer edge 1.6 mm. Bazaille, Minette Basin. (B) Ooids consisting of goethite embedded in a groundmass consisting of late calcite. Reflected light, oil immersion, 450 Am. Rumelange, Minette Basin. (C) Pseudomorphic replacement of goethite after fossiliferous material and idiomorphic pyrite (arrow). Reflected light, oil immersion, longer edge 450 Am. Rumelange, Minette Basin. (D) Slightly ferruginized ooids consisting of goethite predominantly arranged parallel to the laminated structure. The ooids are embedded in an altered chamositic groundmass which is partially replaced by hematite (arrows) and contains abundant detrital quartz grains. Reflected light, longer edge 3.2 mm. Aswan, Egypt. (E) Strongly altered groundmass consisting of goethite patches which are surrounded by hematite (highest reflectance). Reflected light, oil immersion, longer edge 450 Am. Aswan, Egypt. (F) Kaolinite pisoids and kaolinite clasts embedded in a kaolinite groundmass. Reflected light, longer edge 3.2 mm. Kalabscha (rock type 1), Egypt. 240 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 siderite-content, followed by Montrouge, Bazaille and Piennes). With the exception of Montrouge, siderite is developed in the form of xenomorphic coarse-grained crystals (Fig. 11A). In Montrouge, the siderite crystals occur in the form of corroded relics in the weathered, reddish groundmass. Other constituents of the ironstones are well-rounded quartz grains (up to 40 vol.% in Ottange), shelly material (up to 25 vol.% in Hussigny and Tressange, and 10 vol.% in Droitau- mont), late calcite (up to 25 vol.% in Hussigny and in minor quantities in Tressange), framboidal pyrite and organic material. The ironstone of Rumelange is similar to that of Tressange and is characterized by the occurrence of goethitic ooids (sizes mainly between 0.3 and 0.6 mm) embedded in a groundmass of calcite (Fig. 11B). The latter is the youngest mineral which predominantly replaced the original chamositic groundmass which now only occurs in the form of small relics. Other rock constituents are quartz grains, clasts (mainly consisting of goethite), pseudomorphic replacements of goethite after fossils (Fig. 11C) and shelly material. The amounts of calcite and the other rock constituents vary across wide ranges.

2.5. Egypt

Two ironstone districts, both lying in southern Egypt, were investigated. These are the ironstones of Fig. 12. Simplified geological map of southern Egypt including the location of the Late Cretaceous ironstones of Wadi Subeira and Aswan and Kalabscha (Fig. 12). Wadi Abu Agag close to Aswan and the Late Cretaceous ironstone 1. The largest ironstone district of Egypt, inves- and kaolinite deposits of Wadi Kalabscha (according to Fischer, tigated, for instance, by Bhattacharyya (1989), Doer- 1989). ing (1990) and Mu¨cke (2000), occurs NE of the town of Aswan in the Wadi Abu Agag and the Wadi Subeira. Fifteen localities are known within an area of inantly of mixtures of goethite and hematite, with about 50 km2. The Late Cretaceous sediments of the minor chamosite and kaolinite (Fig. 11D). Generally, area consist of the basal Abu Agag Formation, the the groundmass is more strongly altered than the Timsha Formation and the uppermost Um Barmil ooids (Fig. 11E). Other constituents are detrital quartz Formation. The Timsha Formation has a thickness of grains (Fig. 11D), apatite, organic material, pyrite and between 10 and 35 m and contains at least four siderite. The latter two minerals occur pseudomorph- horizons of ooidal ironstone which are of Coniacian to ically replaced by goethite. Santonian age. The samples (total quantity 32) are 2. The deposits of Wadi Kalabscha, investigated by from the Wadi Abu Agag [Central South (21), Timsha Said and Mansour (1971), Fischer (1989) and Mu¨cke (5) and the Um Hugban area (3)] and the Wadi (2000), occur within the Timsha Formation. They are Subeira (3). situated in southern Egypt about 100 km SW of The coarse-grained and brownish- or reddish- Aswan and are of Coniacian to Santonian age. Three brown-coloured ironstones consist of ooids and rock types can be observed in the district of which the groundmass, both of which are composed predom- first two types are located in Central Wadi Kalabscha, A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 241 whereas the third type lies about 20 km SW of the contains rice-grained-shaped autigenic siderite pro- Central Wadi Kalabscha (Fig. 12): phyroblasts, framboidal pyrite, apatite and rarely newly formed goethite. The youngest mineral is Rock type 1 (2 samples) contains white to whitish- calcite which replaced all the above minerals. yellow pisoids/peloids, rounded clasts and minor 2. The ironstones of the Agbaja Plateau lying close to ooids. These constituents are embedded in a white the town of Lokoja (Fig. 14) was investigated by groundmass (Fig. 11F). The pisolites, clasts and Jones (1955, 1965), Adeleye (1973), Haase (1993), groundmass are made up of kaolinite which Abimbola (1997), Mu¨cke et al. (1999) and Mu¨cke contains fine-grained detrital quartz grains which (2000). The ironstones occur within the upper part are restricted to the groundmass. of the Cretaceous sedimentary sequence of the Rock type 2 (2 samples) consists of reddish- to NW–SE trending Benue Trough (also known as brown-coloured pisoids/ooids and peloids. They are Nupe Basin). The Agbaja Ironstone Formation embedded in a white kaolinite groundmass (Fig. averages about 15 m in thickness, is Campanian to 13A) which contains detrital quartz. The pisoids Maastrichtian in age, and contains ooidal and and peloids, representing about 10 vol.% of the rock pisoidal ironstones from which 13 samples were only, consist of mixtures of goethite and/or hematite analyzed. The groundmass of the oolites and and kaolinite in widely varying proportions. pisolites is submicroscopically fine and of yellow- Rock type 3 (2 samples) is made up of yellow to ish, brownish or reddish-brown colour. According brownish pisoids (ooids). They are embedded in a to Mu¨cke et al. (1999), both ooids (pisoids) and deep reddish-brown groundmass which contains groundmass are composed of fine-grained mixtures pseudomorphs of goethite after pyrite. The com- consisting of kaolinite, goethite and hematite in position of the pisoids/ooids is a mixture of varying proportions (Fig. 13C). These are com- kaolinite, bo¨hmite and minor goethite. The ground- monly associated with pseudomorphs of hematite mass is a mixture of goethite and/or hematite and and goethite after idiomorphic and framboidal kaolinite. Bo¨hmite is of subordinate abundance pyrite and siderite (Fig. 13D). The P2O5-content (Mu¨cke, 2000). of the ironstone is relatively high (2.6 wt.% P2O5; Table 10, column V), but apatite was not detected. 2.6. Nigeria 2.7. United States of America Two of the various other ironstones of Nigeria were investigated, namely the Leru-section and the The ironstones of North America, representing the Agbaja Plateau which both are located in the Benue Silurian Clinton type senso stricto, have been inves- Trough (Fig. 14). tigated for more than 150 years. The most important publications are those of Hall (1843), Alling (1947), 1. The Leru-section was investigated by Akande and Simpson and Gray (1968), Hunter (1970), Cooper Mu¨cke (1993), Gebhardt (1998) and Mu¨cke (2000) (1980), Chowns and McKinney (1980), Van Houten and lies close to the town of Okigwe just along the (1990) and Baarli et al. (1992). Enugu-Port Harcourt highway. The ooid-bearing The investigated ironstones are exclusively from Mamu Formation is of Late Cretaceous age the Red Mountain Formation close to the town of (Maastrichtian) and consists of a 60-m-thick Birmingham in the State of Alabama (Fig. 15). The carbonaceous shale–sandstone sequence, of which Red Mountain Formation is part of a NNE-trending the basal and middle parts of the section consist of zone of about 1800 km reaching from Alabama State a total of 9 carbonate-rich ooid-bearing horizons of via Tennessee up to the State New York (Fig. 15). The greenish colour. These beds, from which 4 samples sample location (10 samples were investigated) lies were analyzed, contain many chamositic peloids, directly at the so-called Red Mountain Expressway. rarely ooids (sometimes up to 60 vol.% of the Along the profile of about 70 m mainly fine-grained rock), embedded within a chamositic groundmass (in the basal part) and coarse-grained sandstone (in the which is partially replaced by siderite (Fig. 13B) or upper part) occur. According to Baarli et al. (1992), 242 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

Fig. 13. (A) Ferruginized and redeposited pisoids in a kaolinite groundmass. Field-photo. Scale bar 4 cm. Kalabscha (rock type 2), Egypt. (B) Chamositic peloids and ooids embedded in a groundmass consisting of chamositic clay which is partially replaced by siderite (highest reflectance). Reflected light, oil immersion, longer edge 450 Am. Leru-section, Okigwe, Nigeria. (C) Inhomogeneous ferruginized kaolinite ooids and kaolinite groundmass. Reflected light, oil immersion, crossed polars (strongly decrossed), longer edge 1.6 mm. Agbaja Plateau, Nigeria. (D) Pseudomorphs of goethite after idiomorphic siderite in the ferruginized kaolinitic groundmass. Reflected light, oil immersion, 450 Am. Agbaja Plateau, Nigeria. (E) Ferruginized sandstone consisting of quartz grains which are embedded in a groundmass of late goethite which replaced chamositic clay. Reflected light, longer edge 3.2 mm. Red Mountain Formation, Alabama, U.S.A. (F) Inhomogenuously ferruginized ooids consisting of hematite (small arrows) and mixtures of goethite/hematite and altered chamositic clay (big arrows) embedded in late and replacing calcite. Note the rounded quartz nuclii in the ooids. Reflected light, longer edge 3.2 mm. Red Mountain Formation, Alabama, U.S.A. twelve ironstone horizons occur, known as the seams The ironstone horizons of all these seams are of (from base to top): Irondale, Kidney, Big, Ida (two similar from a petrographic-mineralogical point of horizons) and Hickory Nut (seven horizons). The view, consisting of reddish sandstones. Generally, the sequence described is underlain by the Ordovician quartz grains of the sandstones are well-sorted and Chickmauga Limestone and overlain by the Missisi- rounded with smooth grain boundaries which are pian Tuscumba Limestone. embedded in a goethitic groundmass (Fig. 13E). The A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 243

Fig. 14. Geological map of the Nigerian Benue Trough and its geological setting including the locations of the Late Cretaceous ironstones of the Agba Plateau and the Leru-section. sandstones are conglomeratic at the base and contain morphically replaced by goethite. Calcite is the ooids, pisoids, peloids and rarely spastolites. These youngest mineral of the ironstone (up to 40 vol.%) grains, varying between 20 and 50 vol.%, are replacing all the above-mentioned minerals, especially composed of chamosite-relics, abundant hematite the fine-grained groundmass (Fig. 13F). and/or goethite. Ooids and detrital quartz grains are embedded in a fine-grained groundmass consisting of chamosite, mainly goethite and/or hematite. Other 3. Mineralogy constituents are goethitic fossils, organic material, rutile, zircon and framboidal pyrite and rarely altered The main mineral of the ironstones of the Welsh, siderite. The latter two minerals are mainly pseudo- Prague and Thuringian Basins, the Cleveland Iron- 244 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

solution is the highest at more than 73 atom%, followed by AlVI (19.9 – 21.7 mol%) and Mg (5.2 – 6.2 mol%; Table 1, columns I and II). Chlorites of the Prague Basin (Table 1, columns III and IV), Witt- mannsgereuth, Thuringian Basin (Table 1, columns V and VI), France (Bazaille; Table 1, column X), the Leru-section (Table 1, column IX) and the Cleveland Ironstone (Table 1, columns VII and VIII) contain VI 55.3 – 67.4 atom% Fetot, 23.5 – 32.0 atom% Al and 7.8 – 13.1 atom% Mg. In the ironstones of Aswan, Fetot of the chlorite solid solutions is distinctly lower (42.2 – 46.2 atom%) and those of Mg (16.2 atom%) and in particular of AlVI are higher (37.6 – 41.6 mol%) (Table 1, columns XI and XII). In some deposits, chamosite occurs together with kaolinite (Avicula, Table 3, column IV; Osmotherley, Table 3, column V) or illite (Betws Garmon and Hawerlahwiese) which may form mixtures with apatite and goethite (Hawerlahwiese, Table 3, column III). In the ironstones of the Welsh and Prague Basins and Wittmannsgereuth, Thuringia, chamosite may also be admixed with apatite (up to 45 wt.%; Table 3, columns I and II; Fig. 5D). Contrary to the above- mentioned ironstones, those of Kalabscha and Agbaja do not contain chamosite. According to Mu¨cke et al. (1999), in these locations kaolinite (Table 2, column Fig. 15. Distribution of the Silurian Clinton type within the Eastern part of the U.S.A., including the sample location close to XII) or kaolinite admixed with goethite (Table 3, Birmingham, Alabama (modified after Ka¨stner et al., 1979). columns IX and X) forms the groundmass and the ooids (Fig. 13C). Siderite-rich carbonate is a characteristic mineral of stone and the Leru-section is chamosite. This mineral Phanerozoic ironstones. In the Welsh, Prague and has green internal reflections and occurs in the form of Thuringian Basins, and the Cleveland Ironstone, ooids and the groundmass (Figs. 3A, E, 5B, 8B and siderite replaces chamosite of both the ooids and the 13B). Altered chamosite having brownish to reddish groundmass (Figs. 3C, E and 8A, B). In the Salzgitter internal reflections occurs in the ironstones of Aswan district, the Minette Basin of France, the Frodingham in the form of relics in later and replacing goethite Ironstone and the Leru-section, siderite occurs exclu- and/or hematite (Fig. 11D). In the ironstones of the sively in the groundmass in the form of coarse-grained French Minette Basin (Droitaumont, Piennes and aggregates (Figs. 11A and 13B) or as rice-grain- Bazaille), the Salzgitter Ho¨henzug (Finkenkuhle and shaped authigenic crystals (Fig. 8E). The composition Haverlahwiese) and one sample from Scunthorpe of the carbonate solid solutions is always dominated (Frodingham Ironstone), green chamosite is restricted by the siderite end-member (69.5 to 90.6 mol%), to the groundmass in which goethite-rich ooids are followed by that of magnesite (6.7 to 17.4 mol%) and embedded (Figs. 8C and 11A). calcite (1.6 to 12.2 mol%; Table 4, columns I to VII). Chamosite belongs to the chlorite group and has The rhodochrosite end-member is the lowest, and lies 2+ 3+ the general formula (Fe ,Mg,Al,Fe )6[(OH)8/ in the range 0.21 to 2.3 mol%. In the deposits of (Si,Al)4O10]. Following this formula, the end-mem- Agbaja, Aswan and the Red Mountain Formation, VI bers are as follows: Fetot, Mg and Al . In the Welsh siderite is pseudomorphically replaced by goethite or Basin, the quantity of Fetot in the chlorite-solid hematite (Fig. 13D). A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 245

Table 1 Selected electron microprobe analyses of chlorite in weight percent (upper part) Welsh Basin Prague Basin Thuringian Basin Cleveland Leru Bazaille Aswan I II III IV V VI VII VIII IX X XI XII

SiO2 24.47 22.60 24.40 23.49 23.03 22.94 25.94 26.73 26.54 23.65 30.01 32.79 TiO2 n.d n.d. 0.11 0.79 0.87 0.08 0.10 0.10 0.12 0.07 0.13 n.d. Al2O3 18.34 18.88 20.89 21.03 20.97 21.36 22.57 21.19 20.15 19.82 23.30 23.00 Cr2O3 n.d. n.d. n.a. n.a. 0.12 0.14 0.05 n.d. n.a. n.d. n.a. n.a. V2O3 n.d. 0.09 n.a. n.a. 0.16 0.13 0.10 0.08 n.a. n.d. n.a. n.a. NiO n.d n.d. n.a. n.a. 0.09 0.05 n.d. n.d. n.a. n.d. n.a. n.a. FeO 44.91 45.71 41.34 41.94 40.26 41.09 34.85 35.99 36.65 39.65 30.07 27.40 MnO 0.10 n.d. n.d. n.d. 0.07 0.07 n.d. n.d. n.d. 0.03 0.08 n.d. MgO 1.80 2.15 3.05 2.74 3.45 3.47 4.48 4.65 4.59 4.24 5.93 5.91 CaO n.d. 0.07 0.08 0.43 n.d. 0.06 0.24 0.41 0.60 0.32 0.57 0.80

Na2O n.d n.d. 0.10 n.d. n.d. n.d. 0.05 0.10 n.d. 0.15 n.d. 0.32 K2O n.d. n.d. 0.03 n.d. n.d. n.d. 0.26 0.14 0.32 0.02 0.19 0.16 P2O5 n.d. n.d. n.d. n.d. n.d. n.d. 0.06 n.d. 0.46 0.04 n.a. n.d. FeO 35.83 33.44 33.78 33.94 32.46 34.31 32.54 33.07 33.22 30.01 24.05 14.49

Fe2O3 10.09 13.63 8.40 8.00 8.67 7.54 2.57 3.24 3.82 10.70 6.69 14.35 H2O 9.12 9.06 9.29 9.28 9.18 9.24 9.35 9.41 9.25 9.15 8.31 8.39 99.75 99.92 100.13 99.68 99.07 99.39 98.17 99.12 99.07 98.20 98.69 100.21 Si 2.814 2.615 2.756 2.655 2.630 2.604 2.908 2.980 3.006 2.711 3.246 3.514 Al 1.186 1.385 1.244 1.345 1.370 1.396 1.092 1.020 0.994 1.289 0.754 0.485 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 Ti – – 0.010 0.067 0.300 0.007 0.009 0.009 0.010 0.006 0.010 – Al 1.300 1.190 1.536 1.455 1.452 1.462 1.891 1.765 1.696 1.389 2.216 2.419 Cr––––0.011 0.012 0.005 – – – – – V – 0.008 – – 0.014 0.012 0.009 0.007 – – – – Fe3+ 0.886 1.187 0.714 0.756 0.745 0.644 0.217 0.272 0.325 0.924 0.544 1.157 Ni––––0.008 0.005 –––– –– Fe2+ 3.495 3.236 3.190 3.209 3.100 3.257 3.052 3.084 3.148 2.878 2.175 1.299 Mg 0.309 0.371 0.514 0.461 0.587 0.587 0.749 0.773 0.774 0.725 0.956 0.944 Mn 0.010 –––– 0.007 –––– –– Ca – 0.008 0.010 0.052 – 0.007 0.020 0.048 – 0.039 0.066 0.092 Na – – 0.022 – – – 0.011 0.022 – 0.033 – 0.066 K – – 0.004 – – – 0.037 0.020 0.047 0.003 0.026 0.022 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 O 10.000 10.000 10.000 10.000 10.000 10.000 10.000 10.000 10.000 10.000 10.000 10.000 OH 7.000 7.000 7.000 7.000 7.000 7.000 7.000 7.000 7.000 7.000 6.000 6.000 O 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 2.000 2.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

Fetot 73.13 73.85 65.57 67.42 65.35 65.56 55.32 56.93 58.44 64.26 46.16 42.21 Mg 5.16 6.19 8.63 7.84 9.98 9.87 12.68 13.12 13.02 12.26 16.23 16.22 AlIV 21.71 19.87 25.80 24.74 24.67 24.57 32.00 29.95 28.54 23.48 37.61 41.57 The lower part contains the composition of chlorites and the end-members of the solid solutions (in atom percent).—(I) Ooid (Tremadog), (II) groundmass (Betws Garmon); (III) ooid, (IV) groundmass (Nucice); (V) ooid, (VI) groundmass (Wittmannsgereuth); (VII) ooid (Pecten), (VIII) groundmass (Avicula); (IX) ooid (Leru-section); (X) groundmass (Bazaille); (XI) groundmass, (XII) ooid (Aswan).

A second carbonate generation was observed in youngest minerals (Figs. 11B and 13F). They are some of the investigated ironstones. This concerns the composed of nearly pure calcite (CaCO3 end-member: ironstones of Luxembourg and France, the Cleveland 94.6 to 97.09 mol%; Table 4, columns VIII and XI) and Ironstone (Pecten Seam), the Red Mountain Formation, occur in widely varying modal abundance. The calcite the Salzgitter Ho¨henzug (Konrad shaft) and, at least content within the ironstones is indicated by the CaO- partially, the Leru-section. These carbonates are the values of the whole-rock compositions (Red Mountain 246 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

Table 2 Selected electron microprobe analyses of ooids and groundmass Thuringian Basin Salzgitter France Rumelange Aswan Agbaja I II III IV V VI VII VIII IX X XI XII

SiO2 3.96 14.93 5.26 6.32 6.97 5.87 10.03 8.56 15.99 6.92 3.20 45.57 TiO2 0.74 0.47 0.57 0.43 0.17 0.27 0.15 0.37 n.d. 0.38 n.d. 0.15 Al2O3 1.05 14.95 5.94 6.00 6.75 5.07 6.45 6.32 13.90 5.92 2.62 38.43 V2O3 0.05 0.07 0.06 n.d. n.d. 0.12 0.10 n.a. n.a. n.a. n.a. n.a. FeO 77.79 56.87 77.56 66.15 66.42 66.89 59.31 66.61 51.61 72.72 81.61 1.71 MnO 0.82 n.d. n.d. 0.20 0.35 0.50 0.12 1.71 0.22 0.47 0.95 n.d. MgO 0.82 2.48 1.44 0.98 0.53 1.00 1.72 1.05 2.80 1.32 0.24 0.13 CaO 0.05 n.d. n.d. 0.37 0.43 0.89 1.95 1.29 0.77 0.21 0.64 n.d.

Na2O n.d. n.d. n.d. 0.36 n.d. 0.13 0.14 n.d. n.d. n.d. n.d. n.d. K2O n.d. 0.10 0.07 0.48 0.51 0.28 0.36 n.d. n.d. n.d. n.d. 0.49 P2O5 0.07 n.d. n.d. n.d. 0.53 0.81 2.05 0.68 n.a. n.d. 0.18 0.38 84.48 89.87 90.90 81.29 82.66 81.83 82.38 86.59 85.29 87.94 89.44 86.86 FeOOH 64.34 – 4.60 81.80 82.13 82.72 73.34 37.30 – 20.60 90.70 2.11

Fe2O3 28.55 63.21 82.06 – – – – 40.46 57.36 62.28 – – 92.89 63.21 86.66 81.80 82.13 82.72 73.34 77.76 57.36 82.88 90.70 2.11

Kaolinite

SiO2 1.24 14.93 5.26 6.32 6.97 5.87 7.60 7.45 15.99 6.92 3.20 45.11 Al2O3 1.05 12.66 4.46 5.36 5.91 5.07 6.45 6.32 13.90 5.92 2.62 38.43 H2O 0.37 4.47 1.57 1.89 2.08 1.79 2.27 2.23 4.78 2.07 0.92 13.54 2.66 32.06 11.29 13.57 14.96 12.73 16.32 16.00 40.07 14.91 6.74 97.28

Apatite CaO 0.05 – – – 0.43 0.89 1.95 0.90 – – 0.24 –

P2O5 0.04 – – – 0.33 0.68 1.48 0.68 – – 0.18 – H2O – 0.01 0.03 0.06 0.03 – 0.09 0.77 1.60 3.49 1.61 0.42

Contamination

SiO2 2.72 – – – – – 2.43 1.11 – – – 0.26 TiO2 0.74 0.47 0.57 0.43 0.17 0.27 0.15 0.37 – 0.38 – 0.15 Al2O3 – 2.29 1.48 0.64 0.84 – – ––––– V2O3 0.05 0.07 0.06 – – 0.12 0.10 ––––– MnO – – – 0.20 0.35 0.50 0.12 1.71 0.22 0.47 0.95 – MgO 0.82 2.48 1.44 0.98 0.53 1.00 1.72 1.05 2.80 1.32 0.24 0.13 CaO – – – 0.37 – – – 0.39 0.77 0.21 0.40 –

Na2O – – – 0.36 – 0.13 0.14 ––––– K2O – 0.10 0.07 0.48 0.51 0.28 0.36 ––––0.49 P2O5 0.03 – – 0.20 0.13 0.57 ––––0.38 4.36 5.41 3.62 3.46 2.60 2.43 5.59 4.63 3.79 2.21 1.59 1.41 Total 100.00 100.68 100.00 98.83 100.46 99.48 98.74 100.00 101.22 100.00 99.45 100.80 Upper part—analytical data and the subtotal (wt.%); Lower part—calculated into minerals (goethite, hematite, kaolinite and apatite) and their total (wt.%).—(I) groundmass, (II) and (III) ooids (Schmiedefeld); (IV) ooid (Hawerlahwiese), (V) ooid (Finkenkuhle); (VI) ooid (Droitaument), (VII) ooid (Bazaille); (VIII) ooid (Rumelange); (IX) groundmass, (X) and (XI) ooids (Aswan); (XII) groundmass (Agbaja Plateau). Element oxides summarized as contamination are admixed in goethite, hematite and kaolinite.

Formation 11.8 wt.%, Table 8, column VIII; Rume- quantities of shelly material; Konrad shaft 7.4 wt.%, lange 27.2, 40.4 and 20 wt.%, Table 9, columns II, V Table 7, column I; Pecten 39.3 wt.%, Table 6, column and VI; France: Droitaument 10 wt.%; Table 7, column XII). The high CaO-concentrations of some samples of VIII, Hussigny and Tressange 32.45 wt.%, Table 7, the Cleveland and Frodingham Ironstones are due to column X; the latter two deposits contain also high the high content of detrital shelly material only A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 247

Table 3 Selected electron microprobe analyses of ooids and groundmass in weight percent (upper part) Prague Welsh Salzg. Cleveland Red Mountain Formation Agbaja I II III IV V VI VII VIII IX X

SiO2 12.54 17.13 36.09 45.12 43.54 2.96 7.10 0.97 2.65 14.33 TiO2 n.d. 0.06 0.33 n.d. n.d. 0.25 0.29 0.45 n.d. 0.47 Al2O3 11.21 14.70 20.02 38.03 36.30 1.66 5.40 0.82 5.01 16.54 V2O3 0.06 0.10 0.05 n.d. n.d. 0.06 0.09 0.09 n.d n.d. FeO 23.59 34.42 19.27 1.39 3.45 83.10 74.49 84.12 72.65 49.97 MnO n.d. n.d. 0.05 0.03 0.03 n.d. 0.08 n.d. n.d. n.d. MgO 1.45 1.57 1.76 0.09 0.43 0.02 0.30 n.d. n.d. n.d. CaO 25.11 12.69 6.98 0.05 0.65 0.08 0.23 0.39 0.10 n.d.

Na2O 0.18 n.d. 0.71 n.d. 0.08 0.05 n.d. n.d. n.d. n.d. K2O n.d. n.d. 3.25 n.d. 0.16 0.12 1.00 n.d. n.d. n.d. P2O5 19.46 10.32 2.54 n.d. 0.36 n.d. n.d. n.d. 1.28 1.50 FeO 17.72 25.59 – – – –––––

Fe2O3 6.53 9.81 – – – ––––– FeOOH – – 23.83 – – –––––

H2O 5.84 6.89 2.89 – – ––––– Total 100.10 99.27 98.50 Subtotal 84.71 85.00 88.30 88.98 86.84 81.69 82.81 Apatite Kaolinite Kaolinite

CaO 25.11 12.69 3.35 SiO2 44.84 42.80 1.96 6.37 0.97 2.65 14.33 P2O5 19.06 10.32 2.54 Al2O3 38.03 36.30 1.66 5.40 0.82 2.25 12.15 H2O 0.80 0.41 0.11 H2O 13.42 12.80 0.59 1.90 0.29 0.79 4.29 44.97 23.42 6.00 96.29 91.90 4.21 13.67 2.08 5.69 30.77 Si 2.611 2.607 3.628 Apatite FeOOH 20.83 6.28 34.99 88.86 61.80

Al 1.389 1.393 0.372 CaO – 0.47 Fe2O3 73.61 77.33 62.00 – – 4.000 4.000 4.000 P2O5 – 0.36 94.44 83.61 96.99 88.86 61.80 Ti – 0.007 – H2O 0.02 Contam. Al 1.361 1.243 – 0.85 SiO2 1.00 0.73 – – – V 0.010 0.012 – Carbonate TiO2 0.25 0.29 0.45 – 0.47 3+ Fe 1.022 1.124 – FeO 1.39 3.45 Al2O3 – – – 2.76 4.39 2+ Fe 3.084 3.258 – MnO 0.03 0.03 V2O3 0.06 0.09 0.09 – – Mg 0.450 0.356 – MgO 0.09 0.43 MgO 0.02 0.08 – – – Ca – – 0.391 CaO 0.05 0.18 CaO 0.08 0.30 – 0.10 –

Na 0.073 – 0.138 CO2 1.01 2.74 Na2O 0.05 0.23 0.39 – – K – – 0.417 2.57 6.83 K2O 0.12 1.00 – – – H2O – – 0.054 Contam.P2O5 – – – 1.28 1.50 6.000 6.000 1.000 SiO2 0.28 0.74 1.58 2.72 0.93 4.14 6.36 O 10.000 10.000 10.000 Na2O – 0.08 OH 7.000 7.000 2.000 K2O – 0.16 O 1.000 1.000 – 0.28 0.98 8.000 8.000 2.000 Total 99.14 100.56 100.23 100.00 100.00 98.69 98.93

The last line of the upper part represents (1) the total (including the amount of FeO, FeOOH, Fe2O3 calculated from analytical FeO) and H2O (according to the compositions of the minerals; columns I to III) and (2) the subtotal (sum of the analytical data; columns IV to X). The lower part contains (1) the amount of apatite (wt.%) and the composition of the chlorites (columns I and II) and of illite (III) in atomic percent and (2) the calculated amount of kaolinite, goethite, hematite (analyzed in the form of mixtures) and contaminations (wt.%) including the calculated total (columns IV to X) in the last line.—(I) Apatite-rich ooid (Nucˇice); (II) apatite-rich ooid (Betws Garmon); (III) illite-rich groundmass (Haverlahwiese); (IV) kaolinite-rich ooid (Avicula), (V) kaolinite-rich groundmass (Osmotherley); (VI) ooid, (VII) groundmass, (VIII) fossil (Red Mountain Formation); (IX) ooid, (X) groundmass (Agbaja Plateau). Element oxides summarized as contamination are admixed in goethite, hematite and kaolinite.

(Avicula 28.1 wt.%, Table 6, column VIII; and Raisdale In the deposits of Aswan, the Salzgitter Ho¨henzug, 7.2 wt.%, Pecten 7.4 wt.%, Table 6, columns IX and XI; the Minette Basin of France and Luxembourg, the and Scunthorpe 7.7 wt.%; Table 5, column XII). Nigerian Agbaja Plateau and the Frodingham Iron- 248 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

Table 4 Average of carbonate compositions (determined by electron microprobe analyses) in weight percent (upper part; including the calculated amount of CO2) and mole percent (lower part) N =14 N =39 N =17 N =37 N =12 N =17 N =28 N =15 N =6 N =3 N =3 I II III IV V VI VII VIII IX X XI FeO 57.32 56.06 55.41 47.81 51.37 45.74 48.52 1.14 1.23 2.34 2.40 MnO 0.68 0.13 1.34 0.82 0.55 0.61 1.48 0.49 0.59 0.22 0.27 MgO 2.37 3.27 3.01 5.69 4.84 6.42 4.61 0.25 0.48 0.51 0.67 CaO 0.79 1.57 1.19 4.57 3.53 6.27 5.28 54.25 53.98 52.88 53.20

CO2calc 38.72 39.18 38.96 39.57 39.83 40.59 40.22 43.85 44.01 43.63 44.12 99.88 100.21 99.91 99.05 100.12 99.34 100.28 99.98 100.29 99.66 100.66

FeCO3 90.63 87.70 87.14 73.81 77.96 69.50 71.83 1.60 1.71 3.29 3.33 MnCO3 1.09 0.21 2.12 1.30 0.83 0.90 2.30 0.69 0.83 0.31 0.38 MgCO3 6.69 9.04 8.34 15.59 14.41 17.40 12.94 0.62 1.19 1.27 1.66 CaCO3 1.59 3.05 2.40 9.30 6.81 12.20 10.55 97.09 96.27 95.13 94.63 (I) Welsh Basin; (II) Prague Basin; (III) Thuringian Basin; (IV) Cleveland Ironstone; (V) Leru-section; (VI) Salzgitter district; (VII) France; (VIII) Red Mountain Formation (carbonate of the groundmass); (IX) Rumelange (carbonate of the groundmass); (X) Two Foot (shelly material), (XI) Pecten (carbonate of the groundmass). stone (1 sample), the ooids are highly enriched in iron. Minette Basin) are pseudomorphically replaced by Additionally, these ooids contain not only high goethite–hematite mixtures (Table 3, column VIII; concentrations of Al2O3, but also of SiO2 (Table 2, Fig. 11C). columns IV to VIII and XI). The ooids of Aswan In some Ordovician deposits, hematite and goe- consist of goethite and/or hematite that are admixed thite-bearing ironstones were also observed. Goethite with kaolinite as confirmed by X-ray analyses. and hematite occur in the ironstones of the third seam According to the analysis of Table 2, column XI, of Schmiedefeld, in one sample of Nucˇice (Fig. 5F) the ooid consists of mixtures of 6.7 wt.% kaolinite and and one sample of Pen-y-Gaer (Fig. 3D). As an 90.7 wt.% goethite, FeOOH. The other ooid analyses example, the analytical data of ferruginized ooids and of the above-mentioned deposits show that the the ferruginized groundmass of Schmiedefeld are SiO2:Al2O3-ratios also correspond, as at Aswan, to presented (Table 2, columns II and III). The analyses kaolinite and thus these analyses were calculated in represent mixtures of hematite+goethite or hematite the same manner. After calculation, the analyses show only that are admixed with kaolinite ranging from 2.7 that the ooids of the Salzgitter Ho¨henzug and of the to 32.1 wt.%. French Minette Basin, apart from their kaolinite In Phanerozoic ironstones, magnetite occurs only content (Salzgitter: 13.6 to 15.0 wt.%; Minette Basin: rarely. Magnetite was observed at the localities of 12.7 to 16.3 wt.%), are composed only of goethite Betws Garmon and Pen-y-Gaer, Nucˇice and Konrad (Salzgitter: 81.1 to 82.7 wt.%; Minette Basin: 73.3 to shaft. In the latter deposit, magnetite (V1 vol.%) 82.7 wt.%; Table 2, columns IV to VII; Figs. 8C and formed under diagenetic conditions (Fig. 8F). In the 11A), whereas those of Rumelange (Table 2, column other ironstones, magnetite (10 to 30 vol.%) is post- VIII), the Red Mountain Formation (Table 3, column diagenetic and was formed at the expense of siderite VI, Fig. 13F) and Scunthorpe (Fig. 5A) contain not (Fig. 5E) under conditions of the prehnite-pumpellyite only goethite, but also hematite. facies (Farshad, 2001). As with the composition of the ooids presented above, the groundmass of the Aswan ironstones (Table 2, columns IX and X; Fig. 11E) and the Red 4. Analytical methods and calculation of the Mountain Formation (Table 3, column VII) also mineral compositions consists of mixtures of hematite and/or goethite+kaolinite. Fossils of the Red Mountain Formation (similar Electron microprobe analyses were carried out with to those of the Konrad shaft, Salzgitter district and the a CAMECA SX 100 (correction program PHIRHO- A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 249

Table 5 Whole-rock XRF analyses of ironstones from Thuringia (Germany), the Prague Basins (Czech Republic), the Leru-section, Okigwe (Nigeria) and the Frodingham Ironstone (U.K.) Thuringian Basin Prague Basin Leru-section Scrunthorpe N =6 N =2 N =2 N =1 N =3 N =1 N =1 N =1 N =3 N =1 N =1 N =1 I II III IV V VI VII VIII IX X XI XII

SiO2 15.95 21.50 19.05 43.60 11.75 8.36 43.00 10.7 8.58 9.93 30.00 22.30 TiO2 0.75 0.32 0.72 0.53 0.46 0.29 0.22 0.34 0.19 0.19 0.55 0.41 Al2O3 8.75 10.29 8.14 13.60 7.10 6.17 3.69 7.84 5.18 6.59 10.10 8.41 Fe2O3 5.36 4.14 2.40 26.30 4.27 19.80 2.47 58.00 10.77 17.90 18.00 30.80 FeO 41.37 28.85 25.60 6.62 40.51 36.50 26.40 4.06 36.64 30.00 16.90 9.68 MnO 1.21 0.76 1.72 0.01 0.11 0.02 0.05 0.30 0.37 0.44 0.53 0.41 MgO 2.91 3.30 6.21 1.02 2.63 0.82 1.38 1.78 5.05 4.69 3.12 2.26 CaO 2.40 10.73 11.47 1.64 4.28 7.28 3.64 2.81 4.54 3.91 3.15 7.68

P2O5 1.02 7.11 1.06 1.25 2.53 5.55 2.32 1.98 1.95 1.86 0.38 0.50 Na2O 0.20 0.16 0.14 0.08 0.23 0.20 0.12 0.03 0.06 0.07 0.33 0.17 K2O 0.07 0.08 0.12 1.02 0.09 0.22 0.05 0.30 0.18 0.21 1.27 0.99 À H2O 0.28 0.26 0.14 0.32 0.54 0.49 0.38 2.35 1.10 1.67 1.41 1.09 LOI 19.88 12.94 22.70 3.67 25.77 13.10 15.50 8.68 24.93 21.60 13.40 16.20 100.15 100.44 99.47 99.66 100.27 98.80 99.22 99.17 99.54 99.06 99.44 100.63 Nb 42 33 42 20 19 16 10 16 21 20 16 15 Zr 198 97 164 204 116 98 121 131 149 176 249 171 Y 53 218 63 73 69 172 60 104 93 106 41 45 Sr 79 336 114 177 345 1192 326 548 169 149 96 138 Rb 10 6 12 53 10 5 5 13 9 9 57 48 Pb 17 11 12 14 36 68 19 76 14 21 61 79 Ga 19 17 17 14 13 16 6 17 7 10 13 10 Cu 51 179 245 12 81 48 5 21 b5 b5 b5 b5 Zn 201 157 175 16 109 110 32 170 177 270 108 163 Ni 208 200 299 93 77 60 62 11 13 24 102 100 Co 43 39 44 40 25 12 18 20 8 b53749 Cr 440 74 470 76 141 178 62 213 79 131 142 134 V 745 289 463 155 405 610 209 638 459 530 514 559 Ba 31 33 53 14 124 792 121 123 66 75 160 110 Sc 24 21 40 12 29 44 12 35 19 20 18 16 Thuringian Basin—(I) first seam of Wittmanngereuth (5 analyses) and Schmiedefeld (1); (II) second seam of Wittmanngereuth (2); (III) hanging marker bed in contact to the first seam of Wittmanngereuth (2); (IV) third seam of Schmiedefeld. Prague Basin—(V) siderite-rich ironstones of Nucˇice (2) and Chrustinice; (VI) magnetite-rich ironstone of Nucice; (VII) quartz-grain rich ironstone of Zdice; and (VIII) hematitic/goethitic ironstone of Nucˇice. Leru-section—(IX) and (X); Frodingham Ironstone (locality Scunthorpe)—(XI) and (XII).

Z). The analyzed areas were selected under the OHÀ were successively replaced by (7 OHÀ+1 microscope and the analytical data are summarized O2À), (6 OHÀ+2 O2À) etc. This replacement occurs in the upper part of Tables 1–4. until the valancies of the anions became greater than Chamosite (Table 1) was calculated on the basis of those of the cations (including iron in the divalent 10 cations including traces of CaO, Na2O and K2O. state). Finally, the excess of negative charges is Due to the fact that the valancies of the cations compensated by the introduction of Fe3+ proportions including iron in the form of Fe2+ are nearly always at the expense of Fe2+. Based on these calculations, higher than those of the anions (8 OHÀ and 10 equivalent proportions of water were calculated, O2À =28) and that the oxidation state of iron as well transformed into weight percent, and added to the as the H2O-content cannot be determined by electron analytical sum. microprobe, the following calculation procedure was The analytical data in some tables represent applied: To achieve electron neutrality for the chlorite analyses of mineral mixtures. In Tables 2 and 3 2+ 3+ formulae (Fe ,Mg,Fe ,Al)6 [OH)8/(Si,Al)4O10], 8 (columns VI to X), these mixtures are dominated by 250 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 hematite and/or goethite. They are admixed with to the contaminations. Hematite and goethite of these kaolinite, apatite and subordinated concentrations of analyses were calculated from analyzed FeO by other oxides (=characterized as contaminations). multiplying by the factors 1.1114 and 1.237, respec- Based on the formula of kaolinite Al2[(OH)4/Si2O5], tively. If after calculating FeO into FeOOH by equivalent proportions of SiO2,Al2O3, and H2O (not multiplying by 1.237, the total was distinctly higher determined by analyses) were calculated (in weight than 100, then recalculation of the analysis was percent). The total of the equivalent proportions of carried out in such a way that the FeO value was these three components is identical with the quantity distributed between goethite and hematite with an of kaolinite. Excess of either SiO2 or Al2O3 was assumed total of 100. summarized under contaminations. Like kaolinite, In Table 2, the analyses represent mixtures of equivalent proportions of CaO, P2O5 and H2O (not apatite and chamosite (columns I to III). The quantity analyzed) were calculated (in weight percent) as of apatite (in weight percent) was calculated as shown apatite. An excess of either CaO or P2O5 was added above and the remaining oxides were calculated as

Table 6 Whole-rock XRF analyses of ironstones from the Welsh Basin and the Cleveland Ironstone Welsh Basin Cleveland Ironstone N =2 N =4 N =3 N =1 N =1 N =1 N =1 N =1 N =1 N =2 N =1 N =1 I II III IV V VI VII VIII IX X XI XII

SiO2 21.80 38.70 15.03 4.72 52.20 30.00 12.10 3.95 20.70 8.98 8.67 8.57 TiO2 0.46 0.63 0.28 0.13 0.75 0.41 0.23 0.08 0.39 0.21 0.21 0.19 Al2O3 9.74 12.65 5.91 3.28 13.70 9.36 4.28 1.62 12.30 5.46 4.18 4.49 Fe2O3 6.17 5.63 34.90 16.2 6.70 31.10 8.50 4.75 1.40 8.53 4.59 2.56 FeO 35.40 25.15 27.83 42.40 14.80 11.30 34.30 23.50 28.40 34.95 36.60 7.42 MnO 1.72 0.13 0.08 0.38 0.07 0.05 0.80 0.76 0.42 0.45 0.49 0.32 MgO 2.48 1.65 1.37 0.98 1.21 1.04 4.57 3.11 3.47 4.23 4.97 1.80 CaO 5.19 3.62 3.64 3.83 1.24 3.66 4.58 28.11 7.16 6.77 7.42 39.30

P2O5 2.70 2.61 2.60 2.17 0.95 2.75 0.79 1.30 0.10 2.09 1.39 0.15 Na2O 0.14 0.27 0.08 0.10 0.14 0.07 0.46 0.27 0.73 0.44 0.17 0.22 K2O 0.25 0.28 0.05 0.02 0.93 0.02 0.50 0.17 0.72 0.49 0.43 0.26 À H2O 0.33 0.55 0.87 0.67 1.02 2.45 0.55 0.30 0.95 0.84 0.38 0.31 LOI 12.40 6.47 7.06 24.00 5.67 8.95 28.30 31.00 22.10 26.85 29.50 33.70 99.78 98.84 99.70 98.88 99.38 101.16 100.26 98.92 98.84 100.29 99.00 99.29 Nb 17 18 16 11 16 13 7 7 12 15 9 8 Zr 115 265 78 51 159 100 68 25 98 75 79 53 Y 8582887447764046371206149 Sr 101 307 229 89 94 136 118 543 173 174 144 1012 Rb 11 23 10 9 42 5 16 6 31 20 19 8 Pb 20 34 18 12 22 33 10 10 54 18 b10 10 Ga 18 22 13 9 19 18 5 b5208 5 5 Cu 17 18 19 12 34 41 b5 b5 b511b5 b5 Zn 105 77 49 104 114 100 27 1113 7861 154 92 213 Ni5311225186334243161685139 Co 49 29 10 9 11 6 10 14 12 31 62 11 Cr 103 180 146 111 99 115 39 33 230 72 78 96 V 392 373 553 447 235 416 196 122 839 335 279 179 Ba 14 305 36 10 777 53 83 47 117 114 93 67 Sc 20 18 14 6 15 15 19 16 24 24 20 29 Welsh Basin—(I) siderite-rich ironstones of Nant Ffrancon and Pen-y-Gear; (II) chamosite ironstones of Ffordd Ddu (2), Betws Garmon and Nant Ffrancon (pyrrhotite-bearing); (III) magnetite-bearing ironstones of Betws Garmon (2) and Cross Foxes; (IV) siderite-rich ironstone of Tremadog; (V) quartz-grain rich ironstone of Pen-y-Gear; (VI) hematite/goethite ironstone of Pen-y-Gear. Cleveland ironstone—(VII) Osmotherley, (VIII) Avicula, (IX) Raisdale, (X) Two Foot (2); (XI) and (XII) Pecten. A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 251 chamosite (in atomic percent) on the basis of 10 Letters, Vol. XIII, Special Issue, 1989). The XRF cations as shown above. In the columns IV and V, the analyses were augmented by the determination of the À analyzed mixtures are dominated by kaolinite which is H2O (heating at 110 8C) and the LOI contents admixed with apatite and carbonate. Similar to the (heating at 1000 8C). Fe2+ was determined by titration calculation procedure summarized above, the quantity in sulphuric acid in the presence of KMnO4. The of these minerals was calculated (in weight percent) in analyzed rock samples (total number 137) are which the water and the CO2 contents are calculated presented in Fig. 16 and are summarized in the Tables and added to the analytical sum. 5–10 which contain 50 analyses, often in an averaged Table 4 contains carbonate analyses in which the version. The calculations of averaged analyses were CO2 content (in weight percent) and the end-members carried out based on two assumptions: (1) the analyses of the solid solutions (in mole percent) are calculated. are from the same locality; and (2) the weight percent Whole-rock compositions were determined by of the components occur in comparable quantities. If XRF analyses (PHILIPS PW 1480). For calibration, the latter assumption was not observed, the variation international standards were used (published in News of the averaged oxides are presented (Table 8,

Table 7 Whole-rock XRF analyses of ironstones of the Salzgitter district and of France Salzgitter France N =2 N =2 N =2 N =1 N =1 N =1 N =1 N =1 N =1 N =2 I II III IV V VI VII VIII IX X

SiO2 11.25 19.30 19.55 5.34 21.50 12.70 36.40 10.40 15.40 5.63 TiO2 0.18 0.19 0.37 0.24 0.21 0.26 0.21 0.20 0.25 0.14 Al2O3 6.14 5.89 8.44 5.24 4.35 5.73 5.45 4.17 6.15 2.59 Fe2O3 54.05 55.15 44.75 63.70 53.50 51.00 43.80 42.20 45.50 26.70 FeO 3.77 1.23 5.68 5.05 4.25 9.03 0.70 10.40 11.10 0.99 MnO 0.19 0.11 0.14 0.31 0.39 0.48 0.13 0.39 0.29 0.24 MgO 1.21 1.86 1.42 1.61 1.51 2.24 0.77 1.81 2.05 0.66 CaO 7.44 2.55 3.78 2.79 1.97 2.82 1.88 9.99 3.40 32.45

P2O5 1.08 0.75 1.61 1.72 1.18 1.93 1.14 1.50 2.04 1.02 Na2O 0.19 0.09 0.27 0.09 0.07 0.08 0.10 0.14 0.09 0.10 K2O 0.25 1.61 1.10 0.14 0.12 0.26 0.50 0.16 0.24 0.18 À H2O 0.97 1.14 1.33 1.14 1.06 1.26 0.95 0.77 0.83 0.52 LOI 14.20 10.55 11.65 12.70 10.50 12.20 7.93 17.8 12.30 29.35 100.92 100.42 100.09 100.07 100.61 100.80 99.96 99.93 99.64 100.57 Nb 16 10 18 17 17 18 16 13 20 9 Zr 123 148 159 126 124 140 133 111 118 63 Y 86286062538462658347 Sr 232 72 172 131 132 227 81 242 146 310 Rb752576615179108 Pb 17 86 146 60 49 47 44 139 60 30 Ga6 713685867b5 Cu b5 b5 b5 b5 b5 b57b5 b5 b5 Zn 218 190 195 337 277 290 242 208 271 135 Ni 115 76 71 143 132 133 84 92 129 48 Co 52 36 38 82 51 58 47 52 52 28 Cr 143 465 282 196 125 154 206 144 176 102 V 683 1542 1286 665 854 812 392 495 532 272 Ba 25 54 126 29 135 381 87 26 49 24 Sc 25 15 15 26 21 25 20 21 26 27 Salzgitter district—(I) Early Jurassic ironstone: Korallen Oolite, Konrad shaft (2); Middle Jurassic ironstones: (II) Finkenkuhle (2) and (III) Haverlahwiese (2). France—(IV) Piennes, (V) Bazaille (wuche noir), (VI) Bazaille (wuche gris), (VII) Ottange, (VIII) Droitaument, (IX) Montrouge and (X) Hussigny and Tressange. 252 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

Table 8 Whole-rock XRF analyses of ironstones of Aswan (Egypt) and the Red Mountain Formation of Birmingham, Alabama (U.S.A.) Aswan Red Mountain Formation N =21 N =5 N =3 N =3 N =5 N =5 I II III IV V VI VII VIII IX X

SiO2 9.10–38.3 22.4 11.5–28.2 18.8 12.5 15.2 22.3–47.1 35.14 52.9–81.6 68.64 TiO2 0.12–0.82 0.31 0.26–0.37 0.32 0.28 0.27 0.11–0.21 0.15 0.12–0.52 0.35 Al2O3 2.98–7.39 4.66 6.04–11.80 9.42 7.67 4.97 1.80–2.99 2.31 2.10–4.29 3.33 Fe2O3 47.1–79.5 65.11 48.2–61.8 53.04 69.80 65.5 28.2–48.5 38.02 7.78–19.7 13.88 FeO 0.01–0.30 0.11 0.17–0.37 0.24 0.02 0.27 0.55–1.66 1.04 1.03–1.84 1.55 MnO 0.02–1.30 0.21 0.52–2.03 1.48 0.41 n.d. 0.06–0.64 0.29 0.04–0.19 0.12 MgO 0.02–1.31 0.29 0.99–2.46 1.76 0.86 0.61 0.50–0.66 0.60 0.53–0.89 0.69 CaO 0.43–5.76 2.23 1.01–4.95 3.60 2.33 5.64 5.62–19.3 11.76 0.32–9.36 4.97

P2O5 0.21–4.16 1.65 0.31–3.15 2.19 1.46 3.38 0.19–0.39 0.31 0.06–1.69 0.41 Na2O 0.09–0.15 0.11 0.31–0.72 0.51 n.d. n.d. b0.1 b0.1 b0.1–0.36 0.11 K2O 0.01–0.40 0.07 0.06–0.11 0.08 n.d. n.d. 0.30–0.77 0.52 0.56–1.01 0.83 – H2O 0.22–1.25 0.53 1.53–2.44 1.85 0.87 0.93 0.24–0.33 0.28 0.20–0.35 0.26 LOI 1.06–3.46 2.18 3.96–8.62 6.62 3.32 2.12 4.49–16.0 9.88 1.16–6.86 4.61 100.36 99.91 99.52 98.89 100.30 99.75 Nb n.a. n.a. n.a. n.a. n.a. b5–8 3 5–15 11 Zr 30–337 100 57–114 76 34 60 68–236 105 154–388 245 Y 0–248 72 70–225 166 100 140 26–40 36 16–94 38 Sr 52–1863 430 271–318 277 78 94 89–132 70 51–107 66 Rb n.a. n.a. n.a. n.a. n.a. 17–24 20 18–32 27 Pb n.a. n.a. n.a. n.a. n.a. 19–54 33 14–27 14 Ga n.a. n.a. n.a. n.a. n.a. b5–10 5 b5 b5 Cu 0–77 4 10–792 391 n.a. n.a. n.a. n.a. n.a. n.a. Zn 0–48 5 42–198 122 7 10 b5–28 10 b5–51 30 Ni 0–139 64 70–248 176 143 14 74–177 110 54–104 71 Co n.a. n.a. n.a. n.a. 26–32 29 16–47 35 Cr 58–142 105 0–230 129 69 21 19–76 52 31–48 40 V 0–578 330 451–866 364 1043 307 116–366 234 97–120 109 Ba n.a. n.a. n.a. n.a. b10–82 44 38–617 198 Sc n.a. n.a. n.a. n.a. 6–24 15 b5–11 7 Aswan—(I) variation and (II): average of 21 analyses of Central South (profile 10), (III) variation and (IV) average of 5 analyses of the Timsha area (profile 7); (V) average of 3 analyses of Um Hugban (profile 21); (VI) average of 3 analyses of Wadi Subeira (profile 2). Red Mountain

Formation—(VII) variation and (VIII) average of 5 analyses of ironstones with high Fe2O3-concentrations; (IX) variation and (X) average of 5 analyses of ironstones with high SiO2-concentrations. columns I, III, VII and IX; Table 9, columns I and III; Phanerozoic ironstones, Fig. 16 forms a basis for and Table 10, columns IV and VI). distinguishing ironstones of different compositions on a more detailed scale. Fig. 16 contains three fields (I to III) which are subdivided. The subdivision of the 5. Discussion three fields is illustrated by the horizontal curve which is included in Fig. 16. This curve separates ironstones 5.1. Mineralogical aspects of whole-rock composi- with (above the curve) and without (below the curve) tions and their classification detrital quartz grains. Field I belongs to the chamosite type. In contrast to the unaltered subtype (Ia), the The analytical data will be discussed in combina- second is slightly oxidized (Ib). A third subtype (Ic) 2+ 3+ tion with mineralogical aspects deduced from the contains newly formed magnetite Fe Fe2 O4. Field II petrographic description and the electron microprobe comprises moderately ferruginized ironstones, but in analyses of the minerals shown in Tables 1–4. particular those of the redeposited chamosite subtype. Following Mu¨cke’s (2000) classification of ooidal Apart from the unaltered kaolinite subtype, arranged A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 253

Table 9 Whole-rock wet-chemical analyses of ironstones from Rumelange, Luxembourg Rumelange N =11 N =6 N =1 N =1 N =1 I II III IV V VI VII

SiO2 5.01–25.0 14.64 11.0–22.8 16.53 9.22 12.5 72.7 TiO2 0.11–0.29 0.19 0.22–0.49 0.31 0.10 0.22 0.16 Al2O3 2.41–6.97 4.37 4.80–8.89 6.89 2.29 4.80 4.39 Fe2O3 14.6–33.0 23.15 40.0–63.3 54.25 12.9 40.0 9.56 FeO 0.16–5.17 0.96 0.08–0.39 0.21 0.45 0.13 0.19 MnO 0.13–.29 0.18 0.14–0.27 0.18 0.18 0.21 0.14 MgO 0.42–1.21 0.71 0.68–1.19 0.99 0.42 0.68 0.36 CaO 17.3–37.0 27.2 2.17–12.2 5.41 40.4 20.0 1.49

P2O5 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 0.12 Na2O 0.04–0.37 0.10 0.06–0.08 0.07 0.03 0.07 0.24 K2O 0.09–0.55 0.31 0.05–0.47 0.24 0.12 0.28 0.83 – H2O 0.65–2.03 1.16 1.87–3.83 2.91 0.80 1.61 1.39 LOI 19.2–32.9 25.25 9.78–13.9 11.60 32.9 20.4 5.39 98.22 99.59 101.81 100.90 100.96 (I) Variation and (II) average of 11 calcite-bearing ironstones, (III) variation and (IV) average of 6 iron-rich ironstones, (V) calcite-richest ironstone, (VI) iron-richest ironstone, and (VII) SiO2-richest ironstone. close to the SiO2-corner of Fig. 16 (note that the 6, the analytical data are included in column X; Al2O3-content of kaolinite is not considered in this Pecten, Table 6, column XI), and Leru section (1; diagram), field III contains exclusively strongly Table 5, column X). ferruginized ironstones not only of the chamosite, Field Ia (with detrital quartz grains) contains but also of the kaolinite type. ironstone samples from Welsh Basin [5; Pen-y-Gaer Field Ia (unaltered chamosite subtype) occupies with the highest quantity of quartz grains, arrow 2 in a field parallel to the (FeO+MnO)–SiO2 join of Fig. Fig. 16, Table 6, column V; Betws Garmon; and Nant 16 and represents analytical points of ironstone in Ffrancon which contains about 20 vol.% of pyrrhotite which the ratio of (FeO+MnO):Fe2O3 is greater than instead of siderite, arrow 3 in Fig. 16; and Ffordd Ddu four. These rocks do not show any indication of (2), Table 6, column II, and the ooid-richest ironstone secondary alteration (in reflected light and crossed with about 90 vol.% ooids, arrow 4 in Fig. 16. polars, chamosite has green and siderite has white Analytical data are contained in the averaged analysis internal reflections). Close to the (FeO+MnO)-corner, of Table 6, column II]; Prague Basin [1; Zdice, Table the rocks are siderite-rich (up to 50 vol.%) and contain 5, column VII]. The sample consists of detrital quartz chamosite in the form of relics. With increasing grains (N50 vol.%) which are embedded in siderite. chamosite content (and decreasing siderite content), The latter contains some relics of chamositic ooids]; the analytical points shift in the direction of the SiO2- Thuringian Basin [2; Schmiedefeld (third seam), corner. Above the horizontal curve, detrital quartz is Table 5, column IV; Wittmannsgereuth (dolomite- also a constituent of the chamosite-rich ironstones. bearing marker bed), Table 5, column III]; and North Field Ia (without detrital quartz grains) contains Sea Basin, Cleveland Ironstone (2; Raisdale, Table 6, ironstones samples from: Welsh Basin (2 samples; column IX; and Pecten, Table 6, column XII). Pen-y-Gear; and Nant Ffrancon containing pyrrhotite Ib (sligthly oxidized chamosite subtype): This instead of siderite, arrow 1 in Fig. 16, Table 6, column subtype summarizes ironstones in which chamosite I), Prague Basin (3; Nucˇice (2) and Chrustinice, Table and siderite are slightly oxidized (in reflected light 5, column V), Thuringian Basin (8; Wittmannsger- and crossed polars, chamosite and siderite have euth, Table 5, columns I and II), North Sea Basin, brownish internal reflections). Additionally, siderite Cleveland Ironstone (4; Osmotherley, Table 6, column may be slightly replaced by goethite predominantly VII ; Avicula, Table 6, column VIII; Two Foot, Table along the rim of the crystals. The (FeO+MnO):Fe2O3- 254 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

Table 10 Whole-rock XRF analyses of ironstones of Kalabscha (Egypt) and the Agbaja Plateau (Nigeria) Kalabsha, Egypt Agbaja, Nigeria N =2 N =2 N =2 N =8 N =5 I II III IV V VI VII

SiO2 44.95 41.50 14.35 2.24–11.41 5.71 17.94–49.11 29.54 TiO2 2.84 2.80 3.37 0.14–0.66 0.33 0.53–2.78 1.40 Al2O3 36.75 36.40 31.30 4.02–8.51 7.30 17.06–21.84 18.83 Fe2O3 0.79 4.34 35.40 54.77–76.91 71.24 15.55–47.34 36.48 FeO 0.21 0.45 0.21 n.d. n.d. n.d. n.d. MnO n.d. n.d. 0.04 0.00–0.57 0.09 0.01–0.06 0.04 MgO 0.10 0.11 0.42 0.14–0.26 0.21 0.14–0.23 0.18 CaO 0.28 0.05 1.21 0.02–0.54 0.18 0.03–0.17 0.03

P2O5 0.04 0.05 0.30 0.82–5.92 2.59 0.02–0.15 0.63 Na2O 0.34 0.26 0.23 n.d. n.d. n.d. n.d. K2O 0.03 0.04 0.05 0.00–0.09 0.03 0.02–0.15 0.10 À H2O 0.51 0.71 1.44 1.02–3.62 2.52 1.27–4.69 2.69 LOI 13.10 13.20 12.30 8.13–11.20 10.30 9.45–11.43 10.73 99.94 99.91 100.62 100.50 100.61 Nb 58 58 76 6–22 10 16–52 32 Zr 1091 1114 1614 68–386 133 222–1647 815 Y 37 35 44 19–172 57 32–128 60 Sr 125 68 1742 9–6514 923 41–648 174 Rb 5 5 5 0–16 6 0–10 7 Pb 14 12 15 30–118 72 34–117 54 Ga 49 45 42 12–30 17 28–40 32 Cu 21 66 335 n.a. n.a. n.a. n.a. Zn 40 46 119 67–400 266 28–251 112 Ni 30 43 70 8–38 23 10–37 23 Co 7 7 25 6–19 16 3–12 7 Cr 327 270 205 59–155 82 174–426 237 V 244 257 240 88–714 329 194–396 361 Ba 30 63 119 36–7315 1505 91–690 236 Sc 20 19 21 8–41 16 16–31 33 Kalabscha—(I) ooidal kaolinite, (II) kaolinite with goethitic/hematitic ooids, and (III) iron-rich ironstone. Agbaja Plateau—(IV) variation and (V) average of 8 iron-rich ironstones and (VI) variation and (VII) average of 5 kaolinite-rich ironstones.

ratio of subgroup Ib ranges from 4:1 to about 1:4. siderite. The (FeO+MnO):Fe2O3-ratio is in maximum However, many of the analytical points have ratios 1:4 and depends on the quantity of magnetite 2+ 3+ markedly higher than 3:2. Fe Fe2 O4. Subtype Ic has analytical points from Subtype Ib has analytical points from: Welsh Basin Prague Basin (1; Nucˇice, Table 5, column VI), and (1; Tremadog, Table 6, column IV); North Sea Basin, Welsh Basin (3; Betws Garmon (2); and Cross Foxes. Cleveland Ironstone (1; Two Foot, Table 6,the The analytical data of the samples of the Welsh Basin analytical data are included in column X); North Sea are summarized in Table 6, column III). Basin, Frodingham Ironstone (1; Scunthorpe, arrow 5 Field II comprises the redeposited chamosite in Fig. 16, Table 5, column XI), and Leru-section (3; subtype and the moderately ferruginized chamosite Table 5, column IX). subtype. Their (FeO+MnO):Fe2O3-ratio varies from Ic is a separately marked field within Ib represent- about 1:4 to 5:95. The ironstones of the redeposited ing the magnetite-bearing chamosite subtype. This chamosite subtype are characterized by the occurrence subtype contains ironstones which consist of chamo- of ferruginized ooids, consisting of goethite and/or site, moderate quantities of newly formed magnetite hematite which are admixed with Al2O3 and SiO2 (10 to 30 vol.%) which was formed at the expence of (Table 2, columns IV to VIII). These ooids are A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 255

Fig. 16. Analytical points of 137 ironstones from 38 localities of 8 countries expressed as (FeO+MnO)–Fe2O3–SiO2 diagram. Meaning of the arrows: (1) Pen-y-Gaer (ferruginized), (2) Nucice (ferruginized), (3) third seam of Schmiedefeld (ferruginized), (4) Scunthorpe (ferruginized ooids), (5) Kalabscha (ferruginized), (6) Nucice (magnetite ore), (7) Nant Ffrancon (FeS-bearing ironstone), (8) Nant Ffrancon (FeS-bearing ironstone), (9) Zdice (siderite+quartz grains), (10) Scunthorpe (moderately ferruginized), (11) Konrad shaft (moderately ferruginized), (12) Scunthorp (moderately ferruginized), (13) Kalabscha (ferruginized), (14) to (17) iron-rich redeposited chamosite subtype of Ottange (14), Tressange (15), Hussigny (16) and Finkenkuhle (17). redeposited and embedded in an unaltered ground- first seam of Schmiedefeld, arrow 10 in Fig. 16, Table mass consisting of siderite-bearing chamosite (Table 5, column IV); Salzgitter district, Gifhorn ooidal 1, column X). The analytical points of the following ironstone (2; Konrad shaft; arrow 11 in Fig. 16, Table ironstones belong to the redeposited chamosite 7, column I); and North Sea Basin, Frodingham subtype: Salzgitter district (2; Hawerlahwiese, arrows Ironstone (1; Scunthorpe, arrow 12 in Fig. 16, Table 5, 6and7inFig. 16, Table 7, column III), and column XII). Lorrainian Minette Basin, France (5; Bazaille, Table Field III, lying parallel and close to the 7, column V and VI, Droitaument, Table 7, column Fe2O3–SiO2 join containing negligible low VIII, Montrouge, Table 7, column IX and Piennes, (FeO+MnO)-concentrations, comprises ferruginized Table 7, column IV). ironstones which are dominated by goethite and/or Moderately ferruginized ironstones contain rem- hematite admixed with newly formed kaolinite nants of oxidized chamosite and siderite which both originating from decomposed chamosite (Table 2, have brownish to reddish internal reflections and may columns IX to XI and Table 3, columns VI to be partially replaced by goethite and hematite. The VIII). Restricted to rare cases, hematite and original rock structure is widely preserved. The goethite may contain relics of altered chamosite following analytical points belong to this subtype: (Table 1, columns XI and XII), but the original Welsh Basin (1; Pen-y-Gaer, arrow 8 in Fig. 16, Table rock structures are partially obliterated. The fol- 6, column VI); Prague Basin (1; Nucˇice, arrow 9 in lowing ironstones belong to this subtype (strongly Fig. 16, Table 5, column VIII); Thuringian Basin (1; ferruginized chamosite subtype): Aswan (32; 256 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

Table 8, columns II and IV to VI), and Red 5.2. Accessory constituents and trace-elements Mountain Formation, Birmingham (10; Table 8, columns VIII and X). The analytical data in Tables 5–10 show some Similar to chamosite type ironstones, those of the characteristic features which can be briefly summar- kaolinite type may also be secondarily ferruginized. ized as follows: In these ferruginized ironstones, primary kaolinite is In the unaltered and slightly oxidized chamosite replaced by goethite and/or hematite. The ferrugi- subtype ironstones of the Welsh and Prague Basins, nized kaolinite subtype contains the ironstones of Thuringian Basin and the Leru-section have not only Nigeria (14; Agbaja, Table 10, columns V and VII) the highest P2O5-values (e.g., 7.1 wt.% in the second and Egypt (2 samples of rock type 3, Kalabscha, seam of Wittmannsgereuth, Thuringia, Table 5, arrows 13 in Fig. 16, Table 10, column III). Bearing column II; and 5.5 wt.% in Zdice, Prague Basin, in mind that the Al2O3-content of the whole-rock Table 5, column VI), but also a positive correlation compositions is not considered in Fig. 16,the between the P2O5-values with those of CaO (Fig. 17). unaltered kaolinite subtype (2 sample of rock type This correlation was also observed in the ferruginized 1, Kalabscha) and the redeposited kaolinite subtype chamosite subtype of Aswan indicating that P2O5 (2 samples of rock type 2, Kalabscha) are also occurs in the form of apatite. Intimate intergrowths located within field III. Within the marked area of with chamosite (Table 3, columns I, II and III; Fig. the Kalabscha ironstones, the analytical points are 5D) show that apatite is of synsedimentary origin. either located close to the SiO2-corner of Fig. 16 Generally, the CaO-values are slightly higher than (unaltered kaolinite subtype, Table 10, column I) or required to balance the CaO:P2O5-ratio of apatite (Fig. slightly below the SiO2-corner (redeposited kaolinite 17). This is due to the fact that apatite is associated subtype, Table 10, column II). The low Fe2O3- with siderite which contains calcite in solid solution content of the redeposited subtype is caused by the (Table 4, columns I to III and V). low quantity of ferruginized ooids (about 10 vol.% In the other ironstones (ferruginized and redepo- only). sited chamosite and kaolinite subtypes), the relation- As exceptions, field III contains also ironstones ship between CaO and P2O5 is often obliterated (Fig. that belong to redeposited chamosite subtype. Their 18) or does not occur due to the absence of apatite. The high Fe2O3-content is explained by two reasons; (1) obliteration may be due to the enrichment of secondary the groundmass is weathered or replaced by late calcite [Rumelange: CaO 17.3 to 37.0 wt.%, P2O5- calcite and siderite does not occur and (2), the amount contents (b0.01 wt.%), Table 9; France, Droitaument: of ooids (which are ferruginized) is extraordinary CaO 9.99 wt.%, P2O5 1.50 wt.%, Table 7, column high. The following ironstones belong to these VIII, Hussigny and Tressange, CaO 32.45 wt.%, P2O5 exceptions: Lorrainian Minette Basin [France (3): 1.02 wt.%, Table 7, column X; Red Mountain Ottange, arrow 14 in Fig. 16, Tressange, arrow 15 in Formation: CaO 0.32 to 19.3 wt.%, P2O5 0.06 to Fig. 16 and Hussigny, arrow 16 in Fig. 16, Table 7, 1.69 wt.%, Table 8, columns VII to X; Salzgitter columns VII and X; Luxembourg: Rumelange (19): district, Konrad shaft: CaO 2.55 to 7.44 wt.%, P2O5 Table 9, columns II and IV to VII; one analytical point 0.75 to 1.61 wt.%, Table 7, columns I to III; Cleveland occurs in field II], and Salzgitter district (2; ooid- Ironstone, Pecten: CaO 39.30 wt.%, P2O5 0.15 wt.%, content about 90 vol.%: Finkenkuhle, arrow 17 in Fig. Table 6, column XII] or due to the enrichment of 16, Table 7, column II). detrital shelly material (Cleveland Ironstone, Avicula: The horizontal curve of Fig. 16 separates field III CaO 28.11 wt.%, P2O5 1.30 wt.%, Table 6, column into ironstones with and without detrital quartz grains. VIII). In the ironstone of Agbaja, P2O5 occurs in the Therefore, ferruginized ironstones lying above the form of florencite-(Ce) (Ce, Ca, Ba, Sr)1.00 Al3 [(OH)6/ curve and relatively close to the SiO2-corner (PO3OH)/PO4] and possibly also as strengite 3+ (SiO2 N50%) can be considered as ooid-bearing Fe [PO4]!2H2O(Mu¨cke et al., 1999). This is indi- sandstones which are embedded in a goethite and/or cated by the relatively high P2O5-concentrations (up to hematite-rich groundmass (7 analytical points of the 5.9 wt.%; Table 10, column IV) and the extremely low Red Mountain Formation). CaO-values, which are on average 0.18 wt.% only A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 257

Fig. 17. CaO–P2O5 diagram and 54 analytical points from 16 localities of 5 countries. The line represents the CaO:P2O5-ratio according to the formula of apatite.

(Table 10, column V). However, in some samples of The ironstones always contain vanadium lying France (Ottange, Bazaille, Montrouge and Piennes) mainly in the range between 150 and 850 ppm. The and the Salzgitter Ho¨henzug (Finkenkuhle and Haver- ironstones of Aswan (Um Hugban ironstone, Table lahwiese), the CaO:P2O5-ratio agrees with that of 8, column V) and Salzgitter (Hawerlahwiese, Table apatite (Fig. 18). 7, column III and Finkenkuhle, Table 7, column II)

Fig. 18. CaO–P2O5 diagram and 73 analytical points from 21 localities of 7 countries. The line represents the CaO:P2O5-ratio according to the formula of apatite. 258 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 have the highest V-values which exceed up to about During subsequent diagenesis, the matrix recrystal- 1600 ppm. lises into a groundmass of coarse-grained kaolinite or Chromium is also a characteristic constituent of the is transformed, in the presence of Mg, into chamo- ironstones, mainly lying in the range between 10 and site, whereas the goethitic ooids remain in a 400 ppm. Generally, the ironstones of Wittmannsger- metastable state. euth and Schmiedefeld, Thuringian Basin have higher The genetic development of the redeposited cha- Cr-contents (400 to ~600 ppm). Detrital chromite mosite subtype considered in this paper and briefly grains were only observed within the ironstone of summarized above, differs from the genetic model for Aswan containing Cr-concentrations between 0 to 230 the Minette-type of Siehl and Thein (1989). These ppm (Table 8, columns I and III). authors postulated that the ooids (pisoids) of the The element group (Zn+Pb+Cu+Ni+Co) has in Lorrainian Minette Basin were not formed under total a concentration which generally does not exceed marine conditions, but under pedogenic conditions. 650 ppm. The high values of the Cleveland Ironstone Therefore, the ooids were formed in situ during (Avicula and Raisdale) are due to the high Zn-content weathering of the rocks of the hinterland of the Paris (1113 ppm; Table 6, column VIII; and 7861 ppm, Basin (basement of the Ardennes, Eifel and Huns- Table 6, column IX), which occurs in the form of ru¨ck). These in situ products were subsequently sphalerite. mechanically reworked, transported via river systems and deposited in the Minette Basin (Siehl and Thein, 1989). 6. Conclusion The investigated deposits belong to the following types/subtypes classified in this paper. Ooidal Phanerozoic ironstones can be classified into two types, the chamosite and the kaolinite types. Chamosite type: Generally, these types are subdivided into the Unaltered, slightly oxidized and magnetite-bearing unaltered, the ferruginized and the redeposited chamosite subtypes: subtypes. Additionally, the chamosite type contains – Ordovician ironstone of the Welsh Basin three further subtypes, which are of local signifi- (Betws Garmon, Cross Foxes, Ffordd Ddu, cance only: the oxidized, the magnetite-bearing and Nant Ffrancon, Pen-y-Gaer and Tremadog), the moderately ferruginized chamosite subtypes – Ordovician ironstone of the Prague Basin (Table 11). (Nucˇice, Chrustinice and Zdice), In the unaltered chamosite and the kaolinite – Ordovician ironstones of Thuringia Basin subtypes, either chamosite or kaolinite occurs as (first and second seams of Wittmannsgereuth groundmass and in the form of ooids. Both types and Schmiedefeld). may contain siderite and pyrite. Due to post- – Early and Late Jurassic ironstones of the diagenetic weathering, these subtypes are often Southern North Sea Basin (=Eastern part of ferruginized and the pre-existing minerals are parti- the British Midlands: Cleveland Ironstone, ally or completely replaced by goethite and/or seams of Osmotherley, Avicula, Raisdale, hematite, leading to the ferruginized subtypes. The Two Foot and Pecten; and Frodingham unaltered kaolinite subtype sensu stricto does not Ironstone, Scunthorpe), and belong to ironstones, but is the protore of the – Late Cretaceous Leru-section. ferruginized kaolinite subtype. Redeposited subtype Moderately ferruginized chamosite subtype: deposits, in which goethitic and/or hematitic ooids – Welsh Basin (Pen-y-Gaer), are embedded either in a green chamositic and – Prague Basin (Nucˇice), and siderite-bearing groundmass or in kaolinite which – Thuringian Basin (third seam of Schmiedefeld). may also contain siderite and pyrite, originated from Strongly ferruginized chamosite subtype: ferruginized ooidal ironstones. The parental rocks – Late Jurassic Gifhorn ooidal ironstone (Kon- were mechanically reworked, redepositing and rad shaft), embedding the ferruginized ooids in a new matrix. – Late Cretaceous ironstones of Aswan, and A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262 259

Table 11

Types and subtypes of Phanerozoic ooidal ironstones, their characteristic features and FeO:Fe2O3-ratios within the (FeO+MnO)–Fe2O3–SiO2 diagram

Subtypes Main rock Rock fabricOther rock FeO: constituents constituents Fe2O3 Unaltered chamosite Chamosite (IR: green) Chamositic ooids embedded Organic material Pyrite < 4 : 1 subtype Siderite (IR: white) in chamositic groundmass (mainly framboidal) either replaced by siderite or Slightly oxidized Chamosite (IR: brown) containing authigenic Similar to above, but 4 : 1 chamosite subtype Siderite (IR: brownish) siderite crystals rarely newly formed to C goethite 3 : 2 H A Magnetite-bearing Chamosite (IR: green) Similar to above, but Organic material 3 : 1 M chamosite subtype Siderite (IR: white) siderite partially Pyrite (mainly to O Magnetite replaced by magnetite framboidal) 1 : 4 S Redeposited Goethite Ferruginized goethitic ooids Organic material I chamosite subtype Chamosite (IR: green) embedded in siderite- Pyrite (mainly T Siderite (IR: white) bearing chamosite framboidal) 1 : 4 E to Moderately ferruginized Chamosite (IR: brown to red) Newly formed goethite Organic material 5 : 95 T chamosite subtype Siderite (IR: brown) and hematite containing Pyrite and siderite Y Goethite abundant relics of oxidized partially replaced by P Hematite chamosite and rarely goethite E siderite Strongly ferruginized Goethite Newly formed goethite and Pseudomorphic chamosite subtype Hematite hematite rarely containing replacement of Chamosite (IR: brown to red) relics of oxidized chamosite goethite after siderite and pyrite K Ferruginized Goethite Fine-grained mixture Pseudomorphic A kaolinite subtype Hematite of goethite, hematite replacement of > 5 : 95 O Kaolinite and kaolinite rarely goethite after siderite L containing relics of and pyrite I kaolinite N Redeposited Goethite Ferruginized goethitic Organic material I kaolinite subtype Hematite ooids embedded in (pyrite and T Kaolinite kaolinite siderite may E also occur**)

T Unaltered Kaolinite Kaolinitic ooids Organic material Low Y kaolinite subtype*) Siderite (IR: white) embedded in kaolinitic Pyrite (mainly Fe2+- P groundmass containing framboidal) content E authigenic siderite crystals

T) this type does not belong to ironstones, but is the protore of the ferruginized kaolinite subtype. TT) not present in the investigated Kalabscha.

– Silurian ironstone of the Red Mountain – the Early Cretaceous ironstones (=Tru¨mmer- Formation. erz) of the Salzgitter Ho¨henzug (Finkenkuhle Redeposited chamosite subtype: and Haverlahwiese). – Early to Middle Jurassic ironstones of the Kaolinite type: Minette Basin (Bazaille, Piennes; Droitaument, Unaltered kaolinite subtype Hussigny, Montrouge, Ottange and Tressange, – Late Cretaceous (Coniagian to Santonian) France) and Rumelange, Luxemburg, and rock type 1 of Kalabscha, Egypt. 260 A. Mu¨cke, F. Farshad / Ore Geology Reviews 26 (2005) 227–262

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