Carbonates and pyroxenoids from the manganese deposit near Bald Knob, North Carolina'

Studies of minslals from a metamorphosed manganese deposit at Bald Knob, North Caro-lina, yield estimates of temperatures and local fluid compositions at the peak of metamorph-ism. Electron microprobe analyses of Mn-Ca carbonates demonstrate that they span the rho-dochrosite-kutnahorite solvus without a break, suggesting metamorphic temperature>550oC. The adjacent lid-nmphibolite facies metamorphism implies temperatures not greatly exceeding this value. Pressure is estimated at 5tl kbar based on nearby occurrences of aluminosilicates. Compositional data on coexisting rhodonite and pyroxmangite sriggests that a narrow miscibility gap separates the two phases at these P-Tconditions. The assem-blage rhodonite/pyroxmangite-rhodochrosite-quartz requires water-rich fluids during meta-, morphism. The common occurrences of the accessory mins16fu cattierite (CoSr), alabandite (MnS), and pyrophanite (MnTiOr) at Bald Krob require a relatively reduced and sulfidizing local metamorphic environment. Occasional finds of rhodonitg,/pyroxmangite-alabandite-quartz and rhodonite/pyroxmangite-tephoite-alabandite restrict fO2/fS2 to a narrow band between l0-'e/10-3 and l0-t6/100. Conditions were probably butrered throughout meta-morphism by reaction of a sulfur-rich, oxygen-poor protolith