Stratabound scheelite in altered Archaean komatiites, West Greenland

Abstract

Extensive scheelite-bearing calc-silicate zones with elevated contents of chromium, nickel, copper, zinc, yttrium and barium are found in Archaean supracrustal rocks of the Nuuk area, West Greenland. The calc-silicate zones are largely stratabound and occur in mafic and ultramafic metavolcanic rocks. The calc-silicates are interpreted as amphibolite facies metamorphosed alteration zones in komatiitic intrusive and extrusive rocks as well as in tholeiitic pillow lavas. The alteration which antedates the amphibolite facies metamorphism and the main deformation of the supracrustal rocks, presumably took place at the sea-floor or immediately below the sea-floor. The brines which caused the alteration were enriched in elements such as tungsten, boron, sulphur, iron, copper and zinc. The boron was precipitated as thin tourmalinites and the iron in massive to semi-massive sulphide bodies. The scheelite in the calc-silicates is mainly found as disseminated grains, as stringers and as veinlets. Rare veins up to 15 cm wide with massive scheelite are encountered. Channel sampling revealed grades up to 0.35% W over a width of 2.5 m. Highly anomalous bromine contents up to 418 ppm and equally anomalous chlorine/bromine ratios down to 3 have been found in scheelite-bearing calc-silicates. Somewhat similar anomalous bromine contents and Cl/Br ratios have been reported from serpentinites in the Outokumpu ore body, Finland. At Mount Isa, northern Australia and in the Antimony line in the Murchinson greenstone belt, southern Africa anomalous Cl/Br ratios have been observed. The possible role of bromine in the ore-forming processes is as yet poorly understood.