PRECIPITATION AND ALTERATION OF LATE CRETACEOUS SEDIMENTARY APATITES AND SIDERITES (LEONIE TROUGH, BAVARIA, GERMANY)

Late Cretaceous sedimentary siderites and fluorapatites of the iron or e deposit ''Leonie'' (Bavaria, Germany) have been investigated by geoc hemical and mineralogical methods to define their origin. The siderite s consist to more than 90 mol% of FeCO3. This elemental composition re lates to an early diagenetic fresh water depositional environment. The stable isotope geochemistry of carbon and oxygen (delta(18)O: +24 par ts per thousand SMOW; delta(13)C: -12 parts per thousand PDB) also sup ports a siderite genesis in meteoric waters, with carbon originating f rom oxidation of organic matter. The chemical composition of the fluor apatites is relatively pure and shows a very low elemental substitutio n for calcium and phosphate. This is the result of an intense epigenet ic 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 b efore siderite was formed. This process is thought to have removed cal cium from karst waters to a level which enables siderite to be precipi tated. 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 pho sphorus and fluorine into the karst basins resulted from the draining of the uplifted easterly mountains of the igneous Bohemian Massif unde r the influence of a humid warm climate. During the postulated marine ingressions especially phosphorus and fluorine were brought into the s ystem whereas most of the calcium and carbonate reached the karst trou ghs by dissolution of the Late Jurassic Maim limestones.