PRELIMINARY GEOPHYSICAL FRAMEWORK INTERPRETATIONS OF THE PALEOPROTEROZOIC GREEN MOUNTAIN ARC IN THE MEDICINE BOW MOUNTAINS, SOUTHERN WYOMING

The Paleoproterozoic Green Mountain arc (GMA) formed and was accreted to the southern margin of the Archean Wyoming Province during the 1780-1740 Ma Medicine Bow orogeny. The Cheyenne Belt forms the boundary between the GMA and Wyoming Province and is expressed as multiple northeast-striking shear zones in the Medicine Bow Mountains. The GMA includes large volumes of both felsic and amphibolite gneisses, felsic to intermediate intrusions, and large ca. 1778-1774 Ma mafic intrusions, the Lake Owen layered mafic complex (LOLMC) and Mullen Creek layered mafic complex (MCLMC). Both mafic intrusions host known vanadium and platinum group element mineralization. New geologic framework interpretations are based on an Earth MRI-sponsored aeromagnetic dataset, new ground gravity data, and old and new petrophysical data. The LOLMC is an intact, dense, and strongly magnetized funnel-shaped intrusion that produces a 10 mGal gravity high and a complex, 9000 nT aeromagnetic anomaly. Curvilinear aeromagnetic highs lie over magnetite-rich zones associated with elevated vanadium and facilitate mapping of the LOLMC’s layering. Skewed and complex aeromagnetic anomaly shapes are consistent with a previous study that showed the LOLMC has very strong remanent magnetization. The MCLMC produces a 10 mGal gravity high and three spatially isolated aeromagnetic highs <1500 nT. The isolation of the aeromagnetic highs may reflect spatial variations of metamorphism and alteration, and/or partial dismemberment of the MCLMC by tectonism along the Cheyenne Belt. 3D modeling indicates that the LOLMC and MCLMC have preserved thicknesses of 3-5 km. The poorly exposed Bear Creek shear zone (BCSZ) trends east-west across the study area, forming the boundary between an area dominated by dense, weakly magnetized amphibolite gneiss on the north from an area dominated by lower density, variably magnetized felsic gneiss on the south. A strong gravity gradient follows the BCSZ trend. The LOLMC and MCLMC intrude the area north of the BCSZ dominated by amphibolite gneiss. Smaller volume mafic bodies intrude both gneiss units and tend to produce aeromagnetic and gravity highs. Felsic to intermediate intrusions are variably magnetic and produce local gravity lows. The ca. 1433 Ma, low density Sherman Granite intrudes rocks of the GMA and produces a 20 mGal gravity low in the northeast part of the study area.