First report of linnaeite (Co3S4) and millerite (NiS) from active submarine hydrothermal deposits: Rainbow hydrothermal field, Mid-Atlantic Ridge at 3614'N

Abstract

Linnaeite, millerite and cobalt pentlandite have been discovered in an active seafloor hydrothermal deposit at the Rainbow hydrothermal field at 36° 14'N on the Mid-Atlantic Ridge. All three minerals are exclusively associated with born-ite and chalcopyrite. Textural relationships indicate that chalcopyrite and cobalt pentlandite were the earliest-formed minerals, followed by bornite, and then by linnaeite and millerite. These minerals exhibit a very narrow range of sulfur contents, whereas their contents of Co, Ni, Fe and Cu show a wide variation. The close paragenetic association of linnaeite and millerite with bornite suggests their late precipitation from vent fluids enriched in Co and Ni leached during intense high temperature hydrothermal circulation through ultramafic rocks. The hydrothermal setting of the Rainbow hydrothermal field can be compared with that of the Logatchev hydrothermal field at 14° 45'N on the Mid-Atlantic Ridge, where cobalt pentlandite was first discovered, and with other hydrothermal fields at spreading centers. This comparison indicates that the presence of the Co-Ni-S minerals in present-day submarine massive sulfide deposits is closely linked to ultramafic rocks hosting the hydrothermal field.