SULFIDE MINERALOGY OF THE LOKI’S CASTLE HYDROTHERMAL VENT, ARCTIC MID OCEAN RIDGE

Samples from the active Loki’s Castle hydrothermal vent were collected from the seafloor by ROV during the MarMine cruise in 2016. The hydrothermal vent sites consist of black smoker systems, and as such have diverse mineralogy with highly heterogeneous macro and micro textures. The samples currently under investigation represent chimney collapse breccia rather than massive sulphide from the core of the deposit, and include sulfide-poor, white components, and sulfide-rich dark components. Sulfide mineralization in the white material is limited with minor sphalerite, galena and pyrite. The less common black smoker material is dominated by silica and sulfate gangue, with sulfides occurring as lenses and patches of fine-grained material and, locally, larger grains.

In the sulfide-rich areas, the sulfides occur in mineralogically and metallogenically distinct zones on the cm scale. The Fe-rich zones comprises intergrown pyrite and marcasite, the Zn-rich zones comprises sphalerite, and the Cu-rich zones comprises an isotropic Cu-Fe-S phase, likely isocubanite, with fine (< 500 nm) lamellae of chalcopyrite, and larger exsolutions (~1 µm) with an identical composition to the matrix phase, which are possibly cubanite. Minor pyrrhotite and galena was observed. There is significant overlap between the zones, especially the Zn-rich and Cu-rich zones, where the isocubanite replaces sphalerite. The sphalerite is infected with chalcopyrite disease, but under the optical microscope several grains and zones appear to be inclusion-free. However, SEM investigations show lamellae in sphalerite less than 100 nm wide, oriented in the same crystallographic planes as chalcopyrite lamellae in neighboring isocubanite, indicating that not all lamellae in sphalerite may be visible in the optical microscope.