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Precipitation and alteration of late Cretaceous sedimentary apatites and siderites (Leonie Trough, Bavaria, Germany)
Abstract
Late Cretaceous sedimentary siderites and fluorapatites of the iron ore deposit 'Leonie' (Bavaria, Germany) have been investigated by geochemical and mineralogical methods to define their origin. The siderites consist to more than 90 mol% of FeCO3. This elemental composition relates to an early diagenetic fresh water depositional environment. The stable isotope geochemistry of carbon and oxygen (δ18O: +24‰ SMOW; δ13C: -12‰ PDB) also supports a siderite genesis in meteoric waters, with carbon originating from oxidation of organic matter. The chemical composition of the fluorapatites is relatively pure and shows a very low elemental substitution for calcium and phosphate. This is the result of an intense epigenetic alteration of the primary carbonate fluorapatite and, thus, cannot be related to specific source aspects. Microscopic investigations and thermodynamic calculations reveal a precursory apatite precipitation before siderite was formed. This process is thought to have removed calcium from karst waters to a level which enables siderite to be precipitated. Because of the data and observations a siderite formation in a stagnant fresh water basin is postulated, while the apatite formation probably was initiated during a connection to the open ocean (Tethys) with temporary marine ingressions. The input of iron and partly of phosphorus and fluorine into the karst basins resulted from the draining of the uplifted easterly mountains of the igneous Bohemian Massif under the influence of a humid warm climate. During the postulated marine ingressions especially phosphorus and fluorine were brought into the system whereas most of the calcium and carbonate reached the karst troughs by dissolution of the Late Jurassic Malm limestones.
Vibrational spectroscopy has been used to study the rare earth mineral churchite of formula (REE)(PO4)⋅2H2O. The mineral contains a range of rare earth metals including yttrium depending on the locality. The Raman spectra of churchite-(REE) are characterized by an intense sharp band at 984cm(-1) assigned to the ν1(PO4(3-)) symmetric stretching mode. A lower intensity band observed at around 1067cm(-1) is attributed to the ν3(PO4(3-)) antisymmetric stretching mode. The (PO4(3-)) bending modes are observed at 497cm(-1) (ν2) and 565cm(-1)(ν4). Raman bands at 649 and 681cm(-1) are assigned to water librational modes. Vibrational spectroscopy enables aspects of the structure of churchite to be ascertained.
The limnic ferromanganese ore concretions in some Finnish lakes are described. Their chemical and mineral compositions have been measured as have their natural surroundings — the latter by means of physico-chemical in-situ analysis. The sources of the nodules' contents are discussed, and a theory based on the calculated precipitation fields of the important ore minerals is presented for the ore formation.
The major advantage of the oxygen in phosphate isotope paleothermometry is that it is a system which records temperatures with great sensitivity while bone (and teeth) building organisms are alive, and the record is nearly perfectly preserved after death. Fish from seven water bodies of different temperatures (3–23°C) and different δ18O (values −16 to +3) of the water were analysed. The δ18O values of the analysed PO4 vary from 6 to 25. The system passed the following tests: (a) the temperatures deduced from isotopic analyses of the sequence of fish from Lake Baikal are in good agreement with the temperatures measured in the thermally stratified lake; (b) the isotopic composition of fish bone phosphate is not influenced by the isotopic composition of the phosphate which is fed to the fish, but only by temperature and water composition.Isotopic analysis of fossil fish in combination with analysis of mammal bones should be a useful tool in deciphering continental paleoclimates.
The Cretaceous iron-ore deposits in NE Bavaria, which have been mined for approx 1000 yr, have been the subject of geological and economic investigation for approx 100 yr. The interdependence of these deposits on their location and the development of flexure and fracture structures has been clarified during the last several decades. In contrast, their age and environment are still controversial. The age of these deposits can still only be classified somewhere within the period 'upper Lower Cretaceous to Lower Cenomanian'. The environment of their deposition can be proved to be non-marine. These are weathering deposits, caused by preceding flexure tectonics, which developed in a terrestrial, limnofluvial environment. Subsequent fracture tectonics prevented their being eroded away. Palaeogeographical, climatic and hydrographic conditions are described, and the probable physicochemical development of the ore is discussed.-P.Br.
Several examples of phosphorite weathering (Morocco, Senegal, Israel), are studied. Weathering of phosphorites is shown to begin by loss of CO2 from the carbonate-fluor-apatite, which tends toward a fluor-apatite; dolomite loses its Mg, then calcite is leached. When calcium carbonates are eliminated, calcium apatite transforms to aluminum- or iron-phosphates.
Four cases are studied, two of slight weathering by J. L. and L. P., one of intense weathering in an aluminous environment by R. F. and one of intense weathering in an environment rich in iron by Y. N. and Y. S.; the discussion is by all authors.
The physicochemical processes which may have formed the Cretaceous iron-ore deposits near Amberg, Sulzbach-Rosenberg and Auerbach in NE Bavaria are described, using thermodynamic calculations. Iron was selectively mobilized under slightly acid conditions (pH approx 4-6) by the action of decomposition products from luxuriant sub-tropical vegetation on Fe-bearing minerals. As a result, transportable complexes of Fe with organic ligands were formed in solution and in suspension. At the same time, P and Mn oxides were dissolved, the latter reduced to Mn2+ at these low pH values. There are three possible causes for the low concentration of Al relative to Fe in oxide ores: 1) Al was fixed as kaolinite in weathering residues in the presence of quartz and was therefore less mobile; 2) the weaker affinity of organic ligands for the Al ion than for Fe(II); 3) the parent rocks contained more Fe than Al. The oxidation of organic material continued during its transport in surface waters and during entry of these waters into trough-like lakes, which had slightly alkaline, CaCO3 conditions, and were surrounded by Malm limestones. At the same time, the presence of Ca ions in the lakes produced a decrease in organic Fe complexes in favour of inorganic Fe hydroxide complexes. These are soluble to approx 2 ppb in the neutral pH region. The excess Fe was precipitated, and phosphate was coprecipitated. Organic compounds were also sorbed by Fe-oxyhydroxides, except in stagnant basins. Iron sulphides developed locally under strongly reducing conditions. The causes of the authigenic development of chamosite rather than glauconite in the siderite ore are discussed.-P.Br.
The known natural phosphate minerals exceed 250 in number (Fisher 1973). However, the more common ones are small in number and belong to the apatite family. This family constitutes the main group of primary phosphates of the lithosphere. The majority of the other minerals result from a supergene alteration by groundwaters, or bat- and bird-guanos. This alteration has been well described in detail by Altschuler (1973), in a fundamental synthesis based on three main topics: the weathering of marine phosphorites, the weathering of carbonatites and the weathering caused by acid guanos. The weathering of phosphatic igneous and metamorphic rocks is not mentioned, probably because it is little known, except in the northeast of Brazil (Lima Da Costa, verbal comm.). Since Altschuler’s synthesis, many studies have been published, some related to the weathering of phosphorites, others to the genesis of new minerals during the weathering process. Most of the mineralogical results obtained from these works are shown in Table 1. Consideration of Table 1 permits the findings of Altschuler (1973, p 78–80) to be extended and allows some general conclusions on the conditions of phosphate minerals weathering and on their behavior during weathering.
The concentrations of the major substitutents Na, Sr, CO3 and SO4 in the carbonate—fluorapatite phase of marine sedimentary phosphorites show large systematic variations between, and within, deposits now on land, whilst variations are minimal between, and within, present sea-floor deposits. These variations reflect the influence of weathering and suggest that all marine carbonate—fluorapatite originally has approximately the same chemical composition.
Early diagenetic siderites from marine and fresh-water depositional environments are characterized by distinctive compositional trends. Siderite from fresh-water environments is often relatively pure (i.e., greater than 90 mol% FeCO3) and commonly attains end-member composition. Siderite from marine environments, however, is always extremely impure and has extensive substitution of Mg (up to 41 mol%) and, to a lesser extent, Ca (up to 15 mol%) for Fe in the siderite lattice. In addition, marine siderite generally contains less Mn and has a higher Mg/Ca ratio than fresh-water siderite. This compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters, inasmuch as early marine pore waters generally have a higher Mg2+/Ca2+ ratio and contain less Mn2+ and Fe2+ and more Ca2+ and Mg2+ than meteoric waters.
Siderite, dolomite and ankerite were reacted with ">103°" phosphoric acid at temperatures up to 150°C with >99° yields achieved in less than two hours, using a modification of the McCrea (1950) technique. The oxygen fractionation factors, , between the 18 O of the carbonate and that of the acid-extracted CO 2 are: An empirically derived relationship between the chemical composition of a carbonate in the CaCO 3 -(Ca, Mg)(CO 3 ) 2 -FeCO 3 system and the function 10 3 In at 100°C is 10 3 In = 8.94 X CaCO 3 + 9.29 X MgCO 3 + 8.77 X FeCO 3 where X i is the mole percent of component i in the carbonate.
THE investigation of the first phosphatic nodules found on the sea bottom in the Agulhas Bank region led Murray and Renard1 to conclude that the formation of phosphorites is more complex than a simple one stage process. During the later study of the phosphorites from the sea off California a number of phosphatization stages were observed and found to be of Miocene and Plio-Pleistocene ages2,3. But during the third and fourth cruises of r.v. Akademik Kurchatov (1968) material from the South-West Africa and Chile shelves was recovered so that the process of phosphorite formation could be traced in a more detailed manner.
The temperature variation of the fractionation of oxygen in exchange reactions between dissolved carbonate and water and between calcite and water and calculated on theoretical grounds, and checked experimentally. In the course of the experiments it was necessary to investigate several methods of decomposing calcium carbonate to carbon dioxide for mass spectrometer analysis. A method was developed for growing calcium carbonate from solution with the same isotopic composition as the carbonate shells of organisms produced at the same temperature from water of the same isotopic composition, and the results of these experiments at various temperatures are expressed in an equation relating the temperature of formation with the isotopic composition of the calcium carbonate and of the water.
Phosphorites from sedimentary sequences ranging in age from Archaean to Recent were analysed for δ18O in both the PO4 (δ18Op) and CO3 (δ18Oc) in the apatite lattice. The oxygen isotope record is considerably better preserved in phosphates than in either carbonates or cherts. The use of the Longinelli and Nuti [8] temperature equation yields temperatures for Recent phosphorites that are in good agreement with those measured in the field. The δ18Op values of ancient phosphorites decrease with increasing age. These changes with time are not likely to be due to post-depositional exchange. Changes in δ18O values of seawater and variations of temperature with time can account for the δ18Op time trend, but the latter explanation is preferred. In Ancient phosphorites δ18Oc in structurally bound carbonate in apatite is not a reliable geochemical indicator.
The fluorine and P2O6 content has been determined in more than 100 samples of magmatic and sedicm]Oslo rocks (single rocks and average mixtures). In numerous computations the fluorine is allotted to the minerals of the rocks and in this way regularities of distribution and migration recognized. From the single fluorine values the average content of eruptive and sedimentary rocks, and of the upper lithosphere, is calculated. Finally an attempt is made to draft the fluorine metabolism and circulation in the earth.
Oxygen isotope ratios in phosphate (δ18Op) were determined in 219 samples of phosphoritic rocks from all major phosphogenic provinces of the world, ranging in age from Lower Proterozoic to Recent. In 58 samples, oxygen was analysed in the two oxygen lattice sites of francolite—both in phosphate and in structural carbonate (δ18Oc), δ13C was also analysed in the latter, and in coexisting calcite for 10 of them. Both δ18Op and δ18Oc decrease sharply from the Recent to the Cretaceous, and decrease with a gentler slope to the Precambrian. The nature of the time variation is elucidated by means of a δ18Opvs. δ18Oc plot, which suggests that: 1.(a) the main cause of the time trends is the increase in temperature between Recent and the Cretaceous;2.(b) the Pre-Cretaceous trend is overprinted by both low-temperature nonequilibrium exchange between 18O-depleted water and the rocks, and higher temperature exchange between phosphate and carbonate;3.(c) the observed trend cannot be explained by 18O-depleted Cretaceous and Paleozoic oceans. The calcite coexisting with apatite in phosphatic rocks is similar in both δ18O and δ13C to the structural carbonate of the apatite. Even though phosphate may exchange its oxygen with environmental water to a greater extent than we originally claimed (Kolodnyet al., 1983), it is probably still a very good isotopic recorder of sedimentary environments.