FORMATION OF THE MUZO HYDROTHERMAL EMERALD DEPOSIT IN COLOMBIA

FOR over 1,000 years, the emerald deposits of Colombia have been the p rincipal source of the world's largest and finest gem-quality emeralds -a variety of beryl containing chromium and vanadium(1). Whereas most emerald deposits are found in association with igneous host rocks(1), the Colombian deposits occur in organic-rich black shales, and their o rigin in the absence of any evidence of igneous activity has been a pe rsistent enigma. Here we present evidence from the Muzo mine (located about 100 km from Bogota) that hydrothermal brines transported evapori tic sulphate to structurally favourable sites, where it was thermochem ically reduced. We suggest that the sulphur generated by this process reacted with organic matter in the shales to release trapped chromium, vanadium and beryllium, which in turn enabled emerald formation.