Sphalerite from the sediment-hosted Dolostone Ore Formation (DOF) Cu-Co-Zn
deposit, in northwestern Namibia, has previously been shown to contain
extremely high concentrations of the critical metal Co (up to 1 wt%). These
concentrations are the highest reported in sphalerite to date, and the how and
why of sphalerite being able to incorporate such high concentrations of Co are
poorly understood. We use correlative electron probe microanalysis, electron
backscattered diffraction, and atom probe to reconstruct the likely incorporation
mechanisms and modes of occurrence of such high Co concentrations in natural
sphalerite. While over twenty samples were studied, the comprehensive analytical
workflow was executed on one representative sample to gain a detailed
understanding of Co enrichment. The sulfides of the studied sample are Corich
pyrite, chalcopyrite, Co-rich sphalerite, linnaeite, cobaltpentlandite, and
cobaltite, mentioned in order of abundance. Detailed petrography of these
sulfides indicates that they formed through three stages during the main Cu-
Co-Zn ore stage of the DOF. Cobalt was initially contained in pyrite that grew
during Ore Stage 1 and was later affected by oxidizing fluids (Ore Stage two). This
led to remobilization and growth of linnaeite (Co2+Co3+
2S4). A later change in fO2
(Ore Stage three) led to the breakdown of linnaeite and the further growth of
accessory cobaltite along with the Co-rich sphalerite and chalcopyrite. The hyperenriched
Co-sphalerite then is the last major sink for Co in the DOF deposit. A low
Fe and Co and high Zn sub-grain boundary network within the Co-rich sphalerite
was identified by EPMA and EBSD. This sub-grain network is believed to have
formed during a later, secondary metamorphic stage (Cu-Zn (-Pb) Ore Stage 4),
which developed during ductile deformational mineralization styles such as
pressure shadows and veins. Our APT data reconstructions show no evidence
for Co-inclusions within the Co-sphalerite, and spatial ion correlation analyses of
the data suggest that Co occurs in the sphalerite through simple substitution of Zn. This study demonstrates that sphalerite may contain significant concentrations of the Co through simple substitution, potentially representing an important nontraditional Co source in future critical metal exploration.