Geochemistry and mineralogy of banded iron-formation-hosted gold mineralization in the Gwanda greenstone belt, Zimbabwe

Abstract

The metamorphic rocks of the Archean Gwanda greenstone belt of southwestern Zimbabwe consist of mafic to felsic volcanics with intercalated sedimentary units among which banded iron-formations host a large number of gold occurrences. The studied Vubachikwe and Bar 20 mines are 8 km apart and occur in the northwestern end of the greenstone belt. The mineralization has been affected by regional greenschist facies metamorphism increasing in grade from southeast to northwest. Host rocks and ores were deformed by an early tensional phase and two fold phases. Arsenopyrite geothermometry yielded metamorphic recrystallization temperatures of 380 degrees to 500 degrees C for the ores.Two types of gold mineralization are distinguished in the studied area: (1) stratiform mineralization constituting a mixed carbonate-sulfide facies type of banded iron-formation, and (2) discordant vein-type mineralization. Generally both ore types are dominated by arsenopyrite. Gold occurs in particulate form and also as microscopic to submicroscopic inclusions in arsenopyrite, pyrrhotite, and other sulfides. Geochemical studies indicate a volcanic origin for the banded iron-formation and establish a close correlation between their gold, arsenic, and antimony contents.It is concluded that synsedimentary processes laid down the stratiform gold mineralization by hydrothermal fluids which discharged into restricted marine basins at periods of waning volcanic activity. The discordant ore types at Bar 20 are interpreted to represent a feeder zone of an epithermal system, which was obscured by metamorphic overprint. At Vubachikwe, discordant ore stringers are explained as metamorphic remobilizations which were emplaced into suitable tectonic structures.