High bromine contents and low Cl/Br ratios in hydrothermally altered Archaean komatiitic rocks, west Greenland

In Archaean supracrustal rocks in West Greenland komatiitic and tholeiitic rocks have undergone alteration by circulating hydrothermal solutions, whereby the rocks were partly transformed into calc-silicate assemblages. The alteration, which also formed extensive stratabound scheelite mineralizations, took place partly at the sea floor and partly below the sea floor prior to metamorphism and deformation of the volcano-sedimentary pile. The altered komatiitic and tholeiitic rocks contain up to 418 ppm bromine and Cl/Br ratios down to 1.6 compared with 60-1300 ppb bromine and Cl/Br ratios of 430 in mid-ocean ridge basalts. Cl and Br was not detected by electron microprobe in any of the minerals found. It is suggested that the volcano-sedimentary pile was altered by hot brines which had attained high halogen contents by reaction with mafic and ultramafic rocks. During brine-rock interaction the Cl/Br ratio of the brines decreased and the metal contents increased. The metalliferous bromine-enriched brines ascended towards the sea floor. The bromine and some of the metals precipitated in the upper part of the volcano-sedimentary pile and in the volcanic rocks at the sea floor. High bromine contents and low Cl/Br ratios may indicate presence of major orebodies. (C) 1997 Published by Elsevier Science B.V.