Mineralogy, petrology and geochemistry of Caradocian phosphorites, N. Wales, U.K.

Abstract

Caradocian phosphorite nodules sampled from the Powys county of N. Wales, U.K. have been investigated in order to characterize the deposit geochemically, mineralogically and petrographically and to evaluate their mode of formation. Nodules are the main form of phosphorite although phosphatized organic fragments and oolitic grains are also present. On the basis of petrographio characteristics, nodules are grouped into two types; Type I nodules (with abundant organic material) and Type II nodules (with abundant clay minerals). Mineralogical compositions of the phosphorite nodules reflect varying degrees of dilution of the phosphate material, francolite, by authigenic and detrital minerals. Examination with the scanning electron microscope of freshly fractured surfaces of nodules suggests that the apatite formed authigenically as a direct chemical precipitate. Surfaces of abundant siliceous spicules and other organic fragments as well as some minerals appear to be favoured sites for apatite nucleation. Geochemical studies showed significant impoverishment of lattice elements and enrichment of non-lattice elements in both types of nodules. Chemically the phosphorite nodules may be described in terms of four major components: SiO2, CaO, P2O5 and F. The average concentrations of trace elements present in these nodular phosphorites were compared with an average concentration in marine shales. The enrichment and/or depletion largely suggested precipitation from sea water. The proposed model of phosphorite formation involves inorganic (or biochemical) precipitation of apatite within pore waters of anoxic sediments and subsequent concentration of the apatite by physical processes. Oxidation of organic material during sulphate reduction is the main source of phosphate. This is supported by the very light delta13c isotopic composition of structural carbonate present in the francolites. These studies have also shown that these phosphorites have undergone differential leaching during weathering processes with the development of secondary phosphate: minerals, decarbonation of francolite and removal of many major and minor elements.