DISTINGUISHING FINE INTERGROWTHS IN CU-FE-ZN-S MINERALS FROM THE ARCTIC MID-OCEAN RIDGE USING SEM, EBSD AND OPTICAL MICROSCOPY

Ore minerals in samples collected from black smokers at Loki’s Castle at the Arctic Mid-Ocean ridge show complex intergrowth and possible exsolution textures. Cu-Fe-S mineral(s) are observed in sphalerite as several µm size rounded inclusions, or as fine lamellae parallel to {001}. Sphalerite (Zn,Fe)S is replaced by a Cu-Fe-S phase, which itself has lamellae of mostly submicron lamellae. The matrix phase is isotropic (cubic?) and appears pinkish yellow, and has composition close to cubanite (CuFe₂S₃). The lamellae are less pinkish, and have compositions close to chalcopyrite (CuFeS₂, tetragonal). In addition, a second set of exsolutions with identical backscatter contrast as the matrix phases, but less pinkish in the microscope, appears as wavy laths. The matrix phase is assumed to be isocubanite (CuFe₂S₃, cubic), and the lamellae are likely chalcopyrite and cubanite (orthorhombic). The fine, crystallographically ordered lamellae in sphalerite and isocubanite are similarly aligned, and sphalerite and isocubanite have identical orientations. Thus, the crystal lattice has been retained during replacement of sphalerite by isocubanite whereas the atomic arrangement has changed. This indicates two stages of Cu-sulfide crystallization: Replacement or co-precipitation during sphalerite growth causing the chalcopyrite disease, and replacement along the rim by isocubanite.

Further work will reveal more detail on the relations between the phases. During the first EBSD data collections only sphalerite gave interpretable patterns. After polishing with Ar-ions the pattern of isocubanite improved such that they were indexable. However because sphalerite and isocubanite have very similar structures they could only be differentiated using combined optical, backscatter, eds and EBSD. Different methods of indexing the patterns and improved collection may enable to distinguish also the chalcopyrite lamellae from the host isocubanite and sphalerite though the chalcopyrite structure is very similar to the host phases.