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 approximately 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. 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 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, palladium, gold, silver, copper, titanium, chromium, and vanadium anomalies occur within the Proterozoic terrain – most notable are the Mullen Creek, Lake Owen, and Puzzler Hill complexes. The New Rambler mine is located along the northeastern edge of the Mullen Creek mafic-ultramafic complex in the Medicine Bow Mountains and is Wyoming’s only mine known to have historically produced platinum and palladium. The mineralization, occurring in hydrothermally altered mafic shear-zone cataclastics, may have been remobilized from the layered complex. 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 platinum 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, thorium, and rare earth elements. Similarities between these rocks and the gold-rich quartz-pebble metaconglomerates of the Witwatersrand, South Africa, suggest that they also have potential to host significant copper, gold, and silver mineralization.
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 usually ceased due to factors such as declining metal prices, lack of technological developments, 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 offset by faulting. The extension of the ore deposit was never found.
Centennial Ridge District
Placer gold, discovered in gravels along the Middle Fork of the Little Laramie River, led to the organization of the Centennial Ridge mining district in the east-central Medicine Bow Mountains in 1876. Placer activity was followed by several lode discoveries including the Centennial mine. A new wave of prospecting and development followed the 1901 discovery of platinum associated with copper ores at the New Rambler mine 5 miles to the southwest. Structural fabric within the district is generally northeast-trending and parallel to the Cheyenne belt. Lode mineralization includes foliation/schistosity parallel gold-bearing quartz veins in biotite and hornblende gneisses and schists, and gold-platinum fracture-filling and replacement veins in shear zones and faults cutting the gneisses and schists. Sulfides and arsenides accompany gold-platinum in the fracture fillings. Sulfide-rich zones, dominated by pyrite and occurring in mafic host rocks, usually accompany the richest ores in the district. Actual production from the Centennial Ridge district is unknown. However, the Centennial mine produced an estimated 4,500 ounces of gold.
Douglas Creek District
The Douglas Creek district in the central Medicine Bow Mountains includes all placer deposits along Douglas Creek and its tributaries, from Rob Roy Reservoir southward for 6 miles to below Lake Creek. Gold was discovered in Moore’s Gulch, a tributary of Douglas Creek, by Iram Moore in 1868. Lode gold discoveries in both the New Rambler and Keystone districts resulted from placer gold being traced upstream to its primary sources. Heavy placer activity along the creek included elaborate hydraulic ditches in use by 1876. Resurgent placer activities during the 1930s, 1940s, and 1950s used gasoline-powered draglines and floating washing plants. Gold recovered from gravels up to 20 feet thick varied from 890 to 960 fine, with some silver and traces of platinum. Currey (1965) estimated total gold production from the Douglas Creek placers at about 4,000 ounces. The Douglas Creek district remains a popular area for amateur placer mining.
New Rambler District
The New Rambler district, just west of Douglas Creek, is near the south edge of the Cheyenne belt along the Rambler shear zone, an east-trending branch of the Mullen Creek-Nash Fork shear zone. The Rambler shear zone, numerous local northeast-trending shears, and a few northwest-trending faults cut foliated granodiorite, the younger Rambler Granite, and the distorted northeast extremities of the Mullen Creek mafic-ultramafic complex.
Primary copper sulfides and gold occur in quartz veins, as fracture fillings, and in zones of brecciation. Significant secondary mineralization, found only in the New Rambler mine, often assayed more than 35 percent copper. The New Rambler mine first opened as a gold mine in 1870. Copper was discovered in 1900 at a depth of 65 feet, and platinum within the covellite ore was discovered in 1901. Estimated production from the New Rambler mine totaled 171.3 ounces of gold, 7,346 ounces of silver, 1,753,924 pounds of copper, 910 ounces of platinum, and 16,870 ounces of palladium. The New Rambler area is considered an attractive target for platinum group metals exploration.
Keystone District
The Keystone district, about 3 miles southeast of the New Rambler district, hosts lode gold mineralization concentrated along northwest-trending shears that cut quartz diorite, quartz-biotite schist, and foliated granodiorite. These shears, subsidiary to the Mullen Creek-Nash Fork shear zone, provide loci for quartz veins and veinlets and copper-gold mineralization with associated epidotization of wallrock. Currey (1965) estimated the total lode gold production from the mines in the Keystone district at about 8,500 ounces.
Sierra Madre / Encampment District
The Encampment mining district in the Sierra Madre mainly produced copper after its discovery in 1874. However, gold and silver were significant byproducts of copper mining. The Encampment (also known as the Grand Encampment) district includes the entire Sierra Madre in south-central Wyoming and extends into Colorado. The district is bisected by the generally east-trending Mullen Creek-Nash Fork shear zone, which is more than one-half mile wide in places. This shear zone forms part of the Cheyenne belt suture (see Principal Metal Districts Map above) that separates the Archean Wyoming Province to the north from cratonized Proterozoic basement of the Colorado Province to the south.
Thick successions of Late Proterozoic metasediments that overlie the Archean basement characterize the northern part of the district, where mineralization is typified by copper-bearing quartzites, pegmatites, quartz veins, and unaniferous metaconglomerate. Middle Proterozoic calc-alkaline metavolcanics intruded by granitic plutons characterize the southern part of the district, where rocks host stratiform volcanogenic sulfides and related mineralization. Fracture-controlled, copper-dominated base metal deposits typify mineralization within the shear zone.
The Ferris-Haggarty mine in the central Sierra Madre was one of the world’s more important copper deposits during the early 1900s. This significant deposit with accessory gold and some silver was developed in a sheared metaconglomerate. The mine produced an estimated 21 million pounds of copper with some byproduct gold and silver. A 1988 estimate of unmined ore included 928,500 tons of 6.5 percent copper containing 116,800 ounces of gold. Current high metals prices have sparked re-evaluation and exploration of the Ferris-Haggarty and adjacent areas.
Amateur prospectors still search the Sierra Madre for gold although it was always secondary to copper in the Encampment district.
Contact:
Patty Webber, patty.webber@wyo.gov