File:Chlorite schist (Neoarchean; Soudan Mine, Soudan, Minnesota, USA) 1 (22286542128).jpg

Chlorite schist from the Precambrian of Minnesota, USA. (public display, Soudan Underground Mine State Park visitor center, Soudan, Minnesota, USA)

This is a chlorite schist from a shear zone in the deep subsurface of Minnesota's Soudan Mine. The mine targeted high-grade iron ores (specularites) from the Neoarchean-aged Soudan Iron-Formation (see: <a href="https://www.flickr.com/photos/jsjgeology/albums/72157652553006284">www.flickr.com/photos/jsjgeology/albums/72157652553006284</a>). The site is now a state park and underground tours are offered to the mine's 27th Level. Mapping of the Soudan Mine's 27th Level (see Vallowe et al., 2010) shows that iron-formation is "interbedded" with various schists that occur along old shear zones (fault zones). These shear zone schists are Neoarchean in age and may be related to (just predating or coeval with) the development of the ~2.68 billion year old Murray Shear Zone.

Vallowe et al. (2010) provides this description of the sheared zone rocks and chlorite schist in particular: ". . . curvilinear bands of schistose rocks found along discrete ductile to brittle-ductile structures. Sheared rock units typically have anastomosing fabric composed of discrete foliation and shear banding (C-S structures)." and "Chlorite Schist - Green to dark-green, very fine- to fine-grained chloritic schist. Generally composed of fine-grained chlorite folia anastomosing around small (1 mm x 6 mm) domains of more massive grains, but locally can consist entirely of chlorite phyllite with 2-4 mm sugary textured quartz eyes. Weathered surfaces commonly pock-marked with elongate pits aligned along the dominant foliation trend. Near ore body boundaries, often shows hematite inclusions."

Locality: 27th Level of the Soudan Mine, Soudan Underground Mine State Park, Soudan, northeastern Minnesota, USA

Site-specific geologic info. synthesized from: