1999 Mineral Resource Survey of the Medicine Bow National Forest

Fifty-six 1:24,000 scale topographic quadrangles for the Medicine Bow National Forest (MBNF) were prepared and annotated with mineral occurrence, prospect, and historic mine data. An accompanying extensive spreadsheet was also completed with assay, production, rock-type, and other data. These maps and the spreadsheet information provide a general overview of our current knowledge of the known occurrences and mineral potential of the MBNF. The data supports that much the MBNF is highly mineralized and may be one of the most highly mineralized Proterozoic-Archean mobile belts in North America. All future planning policies of the US Forest Service should consider the mineral potential of the lands contained within the Forest. In particular, the data supports that the MBNF has high potential for the discovery of significant diamond, base, and precious metal resources. The presence of several 'blind', or hidden ore deposits, is likely.

Many more mines and prospects appear on the 7.5 minute topographic maps than are recorded on the spread sheet. The location of these were derived from various sources that contained no information on the type of mineral commodity found at the site. Where mineral and/or metal notations accompany mine and prospect locations, the information is noted on both the spreadsheet and the respective map. If assay data is available, the metal occurrences are noted on the maps when assay values meet or exceed the following values:

Au, Ag, Pt, Pd, REE, Th, U = 500 ppb

Cu, Co, Cr, Ni, Pb, Zn = 500 ppm

Mn, Mo = 1000 ppm.

Placer metals, gemstones, and kimberlitic indicator minerals (i.e., pyrope garnet, chromian diopside, picroilmenite, chromian spinel, diamond) are noted where they have been recovered.

As an example of this data, the Encampment Quadrangle shows many occurrences, mines, and prospects. The other 55 quadrangles are available for examination at the Wyoming State Geological Survey Building on the University of Wyoming Campus, or at the US Forest Service office in Laramie. An extensive list of references were used to compile this data.

The entire MBNF is considered to have high to moderate potential for the discovery of diamondiferous kimberlite. This is based on the favorable geology (i.e. cratonic terrain with a major suture zone), the discovery of kimberlitic indicator minerals, and the presence of the two largest kimberlite districts in the US within 10 to 20 miles of the MBNF boundary. To date, minimal sampling has resulted in the identification of several kimberlitic indicator mineral anomalies within the MBNF, resulted in the discovery of two placer diamonds on Cortez Creek, and the reported discovery of a diamond from a Proterozoic metaconglomerate within the Medicine Bow Mountains, and another diamond immediately north of the Sierra Madre Mountains. The large concentration and widespread occurrence of kimberlitic indicator minerals in the Eagle Rock-Happy Jack area east of Laramie, and the Elmers Rock greenstone belt north of Sybille Canyon in the Laramie Range, supports the potential for the discovery of several kimberlitic intrusions (and possibly diamonds) in those regions.

The Medicine Bow and Sierra Madre Mountains are bisected by a major Precambrian suture locally known as the Mullen Creek-Nash Fork Shear Zone which is part of the Cheyenne Belt. This suture separates the Medicine Bow, Sierra Madre, and Laramie Ranges into a Proterozoic volcanic island arc to the south, and a Archean cratonic margin to the north. Although mineralization has been found throughout the region, the broad region underlain by sheared rocks of the Cheyenne Belt is especially abundant with prospects and mines. The presence of common mineralization in this area is thought to be primarily due to the increased permeability of the rocks due to shearing. This is especially prevalent on the Encampment Quadrangle.

To the south of the Cheyenne Belt, volcanogenic island arc volcaniclastic rocks provide excellent hosts for magmatic massive sulfide mineralization (copper, zinc, lead, silver, gold), and some shear zone copper, gold, and associated gold placers. Several massive sulfide deposits were recognized in the Huston-Fletcher Park region of the Sierra Madre Mountains by Conoco Minerals in the early 1980s, and by later thesis studies at the University of Wyoming. This area in all probability hosts some economic mineral deposits which apparently were not considered significant enough during past planning processes by the USFS to outweigh the wilderness potential of this region. As commercial ore deposits are an extreme rarity, such significant mineralization should be given higher priority in future planning processes.

Layered mafic-ultramafic intrusives, ultramafic massifs and fragments with anomalous platinum (Pt), palladium (Pd), gold (Au), silver (Ag), copper (Cu), titanium (Ti), chromium (Cr), and vanadium (V) anomalies occur within this Proterozoic terrain – most notable are the Mullen Creek, Lake Owen, and Puzzler Hill complexes. The Mullen Creek mafic-ultramafic complex in the Medicine Bow Mountains hosts one of the only known historic Pt-Pd mines in North America, known as the New Rambler mine. This property lies adjacent to the Savage Run Wilderness (see Keystone Quadrangle), and the host ultramafic-mafic complex extends into the nearby Savage Run wilderness. The complex is considered to have high potential for the discovery of platinum-palladium mineralization.

South of the Mullen Creek complex, the Lake Owen layered mafic complex is a Proterozoic gabbroic intrusion which hosts significant mineralization that only recently come to focus during modern exploration efforts. Lake Owen is one an example of a hidden ore deposit, which may be the rule rather than an exception within this geologically complex terrain of the MBNF. Loucks and Glasscock (1989) noted 18 cyclic units within the Lake Owen complex that were defined by large scale repetitions of two or more lithologic units, and by compositional variations in the rock mineralogy. At least 12 stratigraphic horizons in the complex exhibit cumulus sulfide mineralization: four of which are known to contain PGE + Au at grades > 1 ppm (part per million)! One bornite-rich stratiform unit with anomalous Au + Pt +Pd is continuous for at least 2 km, while similar mineralization in another unit extends for 10 km!

Loucks and Glascock (1989) examined the vanadiferous magnetite horizons within the complex ignoring the value of other metals. An estimate of surface mineable oxide cumulates of 1.4 billion tons valued at $33 billion in 1988, was made! This does not include the platinum group metals or gold values within the deposit.

Much of the surface within the MBNF was prospected during the late 1800’s and early 1900’s. Many areas show an almost continuous coverage by historic prospects and mines. Even though most mines and prospects were not developed to any great extent, a few yielded attractive base and/or precious metal assays and were developed into commercial ventures. These included the Ferris-Haggarty , Doane-Rambler, Keystone , New Rambler, Centennial and Douglas Creek mines. The Ferris-Haggarty mine, in particular, was considered a major mine during its lifetime in the first decade of the 20^th century. Several other mines were also developed within the MBNF on a smaller scale than these; and of course, there were many failures. It should be noted that there is no evidence that any of the significant commercial mines (with the exception of the Centennial mine), were ever mined out. Mine operations ceased at many of these mines due to a variety of circumstances 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.

It is the conclusion of this study, that the MBNF may be one of the more highly mineralized forests in the United States. As such, the US Forest Service needs to seriously consider and weigh the potential of the mineral resources in any and all future planning processes. The attached maps and table should provide the USFS planners with the basic surface data on known mineral resources and occurrences, but the USFS also needs to go one step further and consider the potential for the discovery of hidden mineral deposits and evaluate this potential based on the geology of the area. In short, the USFS planners need to consult their geologists as well as 'economic geologists' in the US Geological Survey and the Wyoming State Geological Survey. This is especially important in a resource-minded State such as Wyoming.

*PDF note