Precious metals in ferromanganese crusts from the south-west Pacific

Abstract

Little is known about the abundance, distribution and origin of the precious metals in marine ferromanganese materials1–4, although it has long been recognized that elements such as silver and gold are frequently enriched in ferromanganese nodules when compared with average crustal abundance5. Relative to crustal abundance, silver is enriched in nodules5 by a factor of between 50 and 100, and for gold enrichment factors of between 100 and 1,000 have been found3. We know of no similar data in the literature for silver and gold in ferromanganese crusts, although Halbach et al.4 have described enrichment of platinum in crusts from the central Pacific region. Elsewhere, precious metals show significant enrichment, possibly to economic grades, in massive sulphide deposits associated with active spreading centres such as the East Pacific Rise and the Red Sea6–8. We now report an unusual occurrence of silver and gold in ferromanganese crusts from a back-arc setting in the south-west Pacific; to our knowledge, the first report of material of this kind.