Geologic map of the Stibnite mining area, Valley County, Idaho

Abstract

The Stibnite mining area, as used herein, is bounded by the map extent that includes the Yellow Pine, West End, and Hangar Flats ore bodies. Other ore bodies are nearby, but the purpose of this map is to offer a detailed (1:8,000 scale) geologic map with new cross sections in the immediate area of Stibnite, Idaho. This geologic map is very similar to the Stibnite quadrangle map (Stewart and others, 2016) particularly the units and structure descriptions, because of the overlap of map extent. The new work by the author includes: (1) the topographic lines generated from the LiDAR base (courtesy of Midas Gold Corporation); (2) additional structural measurements; (3) revision of geologic unit contact placements particular around West End and Stibnite pits among other locations; and (4) seven new cross sections. New structural measurements from field work account for 20 percent of measurements shown with the remaining from Midas Gold Corp., Smitherman (1985), and the Stibnite quadrangle map (Stewart and others, 2016). Locations of many shallow features in the cross sections are controlled by core logs of 48 drillholes provided by Midas Gold Corp. The logs include dike placement, dike to plutonic bodies relationships, metasedimentary body localities, and dips of stratigraphic units. The law of sines was used to calculate dip of contacts between metasedimentary units for each cross section. Other features at depth in the cross sections are schematic based on nearby surface features and overall geologic interpretation. The map area contains metamorphosed sediments of Neoproterozoic and Paleozoic age within the Stibnite roof pendant. This rock package is open to tightly folded and reached lower amphibolite facies metamorphism during the Cretaceous Period. Most of the metasedimentary rocks are nearly vertical to overturned and young to the southwest, except on the southwestern flank of the Garnet Creek syncline. Pulses of the Idaho batholith granitoids intruded the metasedimentary units found in the Stibnite roof pendant. Faulting with apparent reverse, normal, and/or strike-slip offset are all present within the map area. Mineralization is largely fault controlled with some stratigraphic control. Volumetrically minor dikes, sills, and small intrusions are of Eocene age, and these intrusions are mostly depicted on the cross sections. Quaternary surficial deposits occur in stream beds and glaciated areas. Field work was conducted during the summers of 2013, 2015, and 2016. For consistency with recent research, most of the Stibnite quadrangle geologic map units (Stewart and others, 2016) are used for this geologic map. Intrusive units Kqd and Tba are new. The additional geologic mapping by the authors and compilation of detailed geologic maps from Midas Gold Corp. enhanced resolution. Cross sections incorporated drill core data including rock type, unit thickness, and oriented structural measurements offering detailed subsurface control. Data access was courtesy of Midas Gold Corp. Reed S. Lewis, Russell V. Di Fiori, and Claudio Berti provided constructive reviews that significantly improved this maps and cross sections. Previous studies that focus on mineralization include Schrader and Ross (1925), Currier (1935), White (1940), Cooper (1951), Cookro and others (1988), and more recently Gillerman and others (2019). Digital map files are available online (Wintzer, 2022).