Fluvial origin of the lower Proterozoic Sioux Quartzite, southwestern Minnesota
The Sioux (Quartzite of inferred Early Proterozoic age (1,760−1,630 m.y.) occurs in southwestern Minnesota, eastern South Dakota, and adjoining parts of Iowa and Nebraska where it overlies a regolith developed on an older Precambrian crystalline basement. The rocks constitute a red-bed sequence that was deposited by braided streams flowing over a deeply weathered land surface of moderate relief. Deposition was confined largely to fault-bounded basins in a cratonic setting; the basins subsided slowly and were rarely, if ever, areas of steep relief. The Sioux Quartzite is a texturally and mineralogically mature quartz arenite. Sand grains are mainly monocrystalline quartz with rare grains of chert, granular iron-formation, and quartzite. Scattered conglomeratic layers contain lithic clasts that include red quartzite, chert, iron-formation, vein quartz, and rhyolite, together with rare welded rhyolite tuff and granitoid gneiss. Stratigraphic intervals of conglomeratic orthoquartzite are interspersed throughout the basal two-thirds of the formation, whereas thin units of sericitic mudstone are most abundant in the upper third. Overall, the upward fining of size grades in the Sioux may indicate diminishing stream gradients and reduction of relief in source areas during deposition. The diagenetic minerals diaspore, kaolinite, and quartz cement indicate an environment of intense leaching, probably under warm, humid climatic conditions, during or closely following deposition. Detrital feldspar, present in trace amounts in deeper parts of the stratigraphic section, may have been more abundant originally but was destroyed by intrastratal reactions. The sub-Sioux regolith is characterized by (1) fine-grained kaolinite and sericite formed from intensively altered coarse-grained metamorphic feldspar and (2) the presence of abundant secondary hematite and silica. By virtue of its Early Proterozoic age, its alluvial origin, and its unconformable position above older Proterozoic and Archean rocks, the Sioux is inferred to have some potential for unconformity-related, Athabasca-type uranium deposits. Its provenance, which includes older Proterozoic iron-formation and volcanic rocks (greenstone), enhances its potential for paleoplacer gold deposits.