GEOLOGIC MAP OF THE BALD MOUNTAIN QUADRANGLE, NORTHERN BIGHORN MOUNTAINS, WYOMING

The Bald Mountain 7.5-Minute Quadrangle includes an outlier of Archean crystalline rocks about 120 km northwest of the greater Bighorn Archean massif. Bald Mountain Quadrangle is located on the northern border between Sheridan and Big Horn counties, Wyoming. Here we present the results of detailed (1:24,000 scale) mapping of the Bald Mountain 7.5 Minute Quadrangle Wyoming, which mainly consists of Archean basement rocks of the Laramide Bighorn uplift. These basement rocks are mantled by Paleozoic cratonic strata of the Flathead, Gros Ventre, Gallatin, Bighorn, Madison and Amsden formations. This region was explored for gold associated with the basement rocks, and Th in the basal Flathead sandstone. The Flathead is currently being prospected as a paleo-placer REE deposit. Our mapping revealed three different lithologic phases of the Archean basement quartzofeldspathic rocks, amphibolite, and diabase dikes. U-Pb isotope analysis of basement rocks shows LA-ICPMS ages ranging from 2879.7±6.7 Ga to 2894.7±7.3 Ga. Coarse alkali feldspar granite is the most prominent phase of the Archean basement, and displays a strong and consistent ~E-W fabric. This alkali feldspar granite is locally pegmatitic, and is rich in accessory apatite, monazite, and zircon, and likely sourced the paleo-placer REE deposit in the overlying Flathead Formation. Smaller bodies of granite and adamellite also are present. The basement fabric aligns closely with a south-verging Laramide range bounding reverse fault/hanging wall anticline along the southern margin of the Quadrangle. Here Paleozoic strata dip moderately to steeply to the southwest. In the central and northern areas of the Quadrangle, Paleozoic strata dip very gently to the northeast.