Native Cobalt in Deep Levels of the Kola Superdeep Borehole

Abstract

Native cobalt was found in amphibolite of the Kola superdeep borehole (SG-3) from a depth of 9630 m by a complex of local analytical methods (analytical scanning electron microscopy, electron backscatter diffraction). The studied amphibolite is a fine-grained melanocratic rock composed mainly of magnesioferri- hornblende and containing accessory native metal, telluride, sulphotelluride, and sulphide (Au, Ag, Pd, Bi, Cu, Pb, Zn, Sb, and Fe) mineralization.It also includes the clinopyroxene xenoclast carrying a fundamentally different set of accessories—poor in sulphides and including native cobalt. The absence of significant impurities, including iron, in native cobalt and its belonging to the hexagonal α-modification was determined. It is shown, that the metallic cobalt particles exposed during ion polishing of a compact pyroxene matrix cannot be a contaminant, introduced into the sample during drilling and sample preparation. Cobalt, obviously, was formed not only before the stage of retrograde metamorphism and subsequent hydrothermal processing of the amphibolite containing it, but probably long before the formation of the sedimentary protolith of this rock. That is, native cobalt belongs to the early high-temperature mineral phases inherited from the older Proterozoic—Archean main volcanism, while the time of formation of this layer of amphibolites dates back to the age of 2.4 billion years—the Early Karelian era of the Proterozoic.