TOURMALINITES IN THE 3800-MA-OLD ISUA SUPRACRUSTAL BELT, WEST GREENLAND

Tourmalinites have been found in the early Archaean Isua supracrustal belt in mafic sediments representing metamorphosed mafic tuffs. The tourmalinites are spatially associated with sulphides and an iron-formation consisting of alternating magnetite and quartz bands. Tourmalines are also found as scattered grains in the mafic tuffs as well as in a conglomerate. The tourmalines are intermediate in composition between schorl and dravite, and have remarkably low calcium contents compared with younger stratabound tourmalines. It is suggested that the tourmaline in the tourmalinites is submarine exhalative in origin, derived from brines which were enriched in boron as well as iron and sulphur. The tourmaline was either precipitated on the seafloor or in altered seafloor sediments. The scattered tourmalines may have been formed by metamorphism of boron-rich clays, where the boron was absorbed from seawater.