Late rally continuous coticule horizons from Sulitjelma, Norway, related to the Cu-pyritic massive sulphide ores nearby contain zoned spessartine garnet within a matrix of quartz and Fe-Mn oxides. Electronprobe studies show that two compositionally distinct garnets exist, differentiated by Mn content. The high-Mn variety is interpreted as forming early during metamorphism, from chemical products of exhalite precipitation. The low-Mn type is attributed to regional metamorphism of a Fe-Mg silicate precursor mineral. Their coexistence is explained by lack of chemical equilibration allowed by the silicic matrix.
Coticules at Sulitjelma, Norway and their possible origin