CONDITIONS OF FORMATION AND DIAGENETIC EVOLUTION OF UPPER PROTEROZOICPHOSPHATE NODULES FROM SOUTHERN SWEDEN - EVIDENCE FROM PETROLOGY, MINERAL CHEMISTRY AND ISOTOPES

Dark-gray to black cryptocrystalline francolite occurs as nodules (sim ilar to 2-8 cm in diameter) which are embedded in organic-matter-rich mudstones of the Visingso Group (Upper Proterozoic, southern Sweden). The delta(13)C(PDB) values (-18.4 to +0.98 parts per thousand) of stru ctural-CO2, complemented by studies of the petrographic character and strontium isotopic composition of francolite, indicate precipitation i n the suboxic and sulfate-reduction zones. Francolite is intergrown wi th berthierine that has formed in the suboxic Fe-reduction zone. Calci te enriched in Mn and, to a lesser extent Fe, occurs as fracture- and void-fillings, and as replacement of francolite. Mn- and Mg-rich calci an siderites have precipitated as pseudomorphs after bacteria in the z one of microbial methanogenesis. The carbon (delta(13)C(PDB) = -0.75 t o +7.04 parts per thousand) and oxygen (delta(18)O(PDB) = -14.21 to -1 0.58 parts per thousand) isotopes of calcite suggest derivation of car bonate ions from microbial methanogenesis and from thermal decarboxyla tion of organic matter at temperatures of 60 degrees to 80 degrees C. Some of the void- and fracture-fining calcites (delta(13)C = -20.8 to -18.63 parts per thousand; delta(18)O = -5.46 to -5.21 parts per thous and) have, however, precipitated at lower temperatures (similar to 25- 30 degrees C) in the sulfate-reduction zone.