Medicine Bow Mountains and Sierra Madre

The Medicine Bow Mountains and Sierra Madre are Precambrian-cored Laramide uplifts that straddle the margin of the Wyoming Craton. The Wyoming craton was established more than ~2.7 billion years ago (2.7 Ga, or Giga-annum), but was later affected by a regional metamorphic event 1.9 – 1.7 Ga (Eggler and others, 1988). The south boundary of the Wyoming Craton in the Medicine Bow Mountains and Sierra Madre terminates against the Mullen Creek-Nash Fork shear zone. This shear zone, which forms part of the Cheyenne Belt suture, represents a continental-arc collision zone (Graff, 1978; Hills and Houston, 1979; Karlstrom and Houston, 1984) separating the Wyoming Province to the north from cratonized (1.7 Ga) Proterozoic basement of the Colorado Province to the south.

Within the Colorado Province south of the Cheyenne Belt, metavolcanic and metasedimentary rocks provide excellent hosts for magmatic massive sulfide mineralization (copper, zinc, lead, silver, gold), and some shear zone copper, gold, and associated gold placers. Layered mafic-ultramafic intrusives, ultramafic massifs, and fragments with platinum (Pt), palladium (Pd), gold (Au), silver (Ag), copper (Cu), titanium (Ti), chromium (Cr), and vanadium (V) anomalies occur within the Proterozoic terrain – most notable are the Mullen Creek, Lake Owen, and Puzzler Hill complexes (Sutherland and Hausel, 2004). The New Rambler mine is located along the northern edge of the Mullen Creek mafic-ultramafic complex in the Medicine Bow Mountains and is Wyoming’s only known historic Pt-Pd mine. The mineralization, occurring in hydrothermally altered mafic shear-zone cataclastics, may have been remobilized from the layered complex (McCallum and Orback, 1968). Because of their high potential for platinum-palladium mineralization, Mullen Creek, Lake Owen to the east, and Puzzler Hill in the Sierra Madre continue to be exploration targets.

North of the Cheyenne Belt in the Medicine Bows and Sierra Madre, the Wyoming Province includes amphibolite-grade schists and gneisses overlain by younger Archean and Proterozoic metasedimentary and metaigneous rocks. Metaconglomerates found in several of the Precambrian units are considered potential sources for uranium and thorium. Similarities of these rocks to the gold-rich quartz-pebble metaconglomerates of the Witwatersrand, South Africa, suggest that they also have potential to host significant copper-gold-silver mineralization (Hausel, 1986).

During the late 1800s and early 1900s, intense prospecting left numerous remnants of mines and prospects concentrated in the broad region underlain by sheared rocks of the Cheyenne Belt, although scattered mineralization occurs throughout both mountain ranges. There is no evidence that any of the significant historical mines (with the exception of the Centennial mine), were ever mined out. Mine operations ceased due to factors including declining metal prices and values, ore complexity below the zone of oxidation, outbreak of war, and other political or human-related factors. The Centennial mine ceased operations because the mineralized lode was off-set by faulting – the extension of the ore deposit was never found (Hausel, 1989).

Additional recommended WSGS publications and maps related to gold and geology in the Medicine Bow Mountains and Sierra Madre include:

WSGS Public Information Circular 32, Guide to the geology, mining districts, and ghost towns of the Medicine Bow Mountains and Snowy Range Scenic Byway by W. Dan Hausel (1993), 53 p.

WSGS Open File Report 04-10, Preliminary geologic map of the Saratoga 30′ x 60′ Quadrangle (Link to Geologic Mapping page), by Wayne M. Sutherland, and W. Dan Hausel, (2004), scale 1:100,000, 35 p.

WSGS Memoir 1, A regional study of rocks of Precambrian Age in that part of the Medicine Bow Mountains lying in southeastern Wyoming – with a chapter on the relationship between Precambrian and Laramide structure, by Robert S. Houston and others (1968, reprinted 1978), 5 plates, map scale 1:63,360, 167p.