CHARACTERIZING OXIDE AND SULFIDE MINERALIZATION IN THE NEMO SHEAR ZONE, SD

The Nemo shear zone (NSZ), exposed near Nemo, SD in the NE portion of the Black Hills, is a high strain zone that developed during the final stages of suturing between the Wyoming and Superior provinces during the Proterozoic assembly of North America. The vertical NW-striking shear zone is host to oxide and sulfide mineralization. This research characterizes the mineralization and proposes the shear zone acted as a conduit for hydrothermal fluid flow.

The study area is 14 km² following the shear zone from the Benchmark Iron-formation in the SE, and extending NW for 6 km, cutting across the Boxelder Creek Fm and Blue Draw Metagabbro. The Boxelder Creek Fm is dominantly a nearly pure quartz arenite with less abundant iron-stained quartz and chert metaconglomerate. The metagabbro is an amphibolite-facies gabbroic sill with non-aligned hornblende outside the NSZ. Within the NSZ the Blue Draw Metagabbro is a greenschist facies phyllonite characterized by talc, actinolite, and chlorite, and the quartzite occurs as an ultra-mylonite locally containing abundant fuchsite.

In the NW, within the NSZ, the quartzite locally contains iron oxides/sulfides adjacent to meter-scale vertical NW-striking quartz veins bounded by a cliff-forming vertical NW-striking fuchsite-schist to the west. The oxide/sulfide heavy areas within the quartzite contains 1-5mm euhedral crystals that grew preferentially parallel to shear fabric. The oxides/sulfides are syn-deformational pyrite cores with post-deformational magnetite rims overprinting the shear fabric. Within the NSZ the gabbro exhibits alignment of amphibole minerals parallel to the shear zone, magnetite grains, and post-deformational non-aligned ilmenite lathes cross-cutting the shear fabric.

Constraining the timing of the mineralizations via microstructural relationships between the oxides, sulfides, and shear fabric, in addition to evidence of hydrothermal fluid flow, and absence of similar mineralization outside of the NSZ, supports the structural control of the NSZ on hydrothermal fluid flow as well as prolonged-duration fluid flow with pulses of varying chemistry. The Black Hills contain igneous intrusions of the Harney Peak age with the potential to introduce magmatically expelled hydrothermal fluid into the NSZ zone, driving mineralization.