Andradite and magnetite in contact-metamorphosed Chalk near Portmuck, Islandmagee, County Antrim

Skarn mineralisation was first reported 70 years ago by the mineralogist G. P. L. Walker at several coastal localities near Portmuck on the Islandmagee peninsula in eastern County Antrim. Described here is mineralisation associated with a 4 m wide Palaeogene dolerite dyke intruded through Cretaceous chalk. Along the dyke margin is a decimetre thick, dull-grey band of massive magnetite-calcite rock which, in thin section, has a clustered granular texture. In nearby marble, 2–3 mm diameter brown euhedral crystals of andradite garnet occur in close association with nodules of flint that have been metamorphosed to micro-crystalline quartz. Analyses by EDX indicate that the andradite is close to the ideal end-member, Ca3Fe2(SiO4)3, with minor substitution of Mg and Al. Flint is likely to be the source of silica for the crystallisation of andradite, whereas iron was probably introduced by hydrothermal fluids derived from the basaltic magma that formed the nearby sill-dyke complex.