Diamond Exploration Targets in the Wyoming Craton

The craton is separated into two provinces: the Wyoming Province, and the Colorado Province. The Wyoming Province consists of a stable Archean basement underlying much of Montana and Wyoming. The Colorado Province is a cratonized Proterozoic basement that underlies portions of Colorado and southeastern Wyoming. Within these provinces, numerous kimberlitic indicator mineral anomalies have been identified in stream sediments, anthills, and paleoplacers. The presence of some geophysical and remote sensing anomalies also supports the possibility of many undiscovered, mantle-derived, igneous rocks of kimberlitic affinity within the craton.

Cratons

Although diamonds have been found in a variety of host rocks, current exploration models focus on kimberlites and lamproites – both of which have been found in abundance in the Wyoming Craton.

Cratonic terrains can be separated into regions of favorability to facilitate preliminary evaluation for diamonds (Janse, 1994). Regions of high potential are known as Archons. These consist of cratonic cores of Archean basement last affected by a thermal event =2.5 Ga (billion years old). Areas of lower potential are known as Protons, or terrains of cratonized Early- to Mid-Proterozoic belts last affected by a thermal event with a minimum age of 1.6 Ga. Regions of lower potential are called Tectons. These are formed of Late Proterozoic basement that was last effected by a thermal event with a minimum age of 800 Ma (million years old) that stabilized between 1.6 Ga and 800 Ma.

To date, the Archean provinces (Archons) have yielded the richest diamondiferous kimberlites. For example, kimberlites in the South African and Yukutian cratons have yielded some of the highest value macrodiamonds in the world.

The Early to Mid-Proterozoic cratonized mobile belts (Protons), also represent favorable terrains for diamondiferous kimberlite and lamproite. Within the past 20 to 30 years, the two largest kimberlite fields in the U.S., the Colorado-Wyoming State Line district and the Wyoming Iron Mountain district were discovered in Proterozoic basement rocks of the Colorado Province. Essentially, every kimberlite in the State Line district is diamondiferous.

The cratonized Proterozoic belts also provide favorable terrains for diamondiferous lamproite. This was emphasized by the discovery of olivine lamproite at Argyle, Western Australia, which has yielded incredibly high ore grades. Some tuffaceous phases are reported to average 6.8 carats/tonne.

Diamondiferous olivine lamproites are also found in Proterozoic basement near Murfreesburo, Arkansas, far from any known Archon. The reported grade for the Prairie Creek intrusive is only 0.005 carat/tonne (Northern Miner, 2/17/97).

Wyoming Province

The Wyoming Province consists of Archean granite, gneiss, and supracrustal metamorphic rocks with interspersed greenstone belts exposed in the core of several Laramide uplifts. The province continues north into Canada. The Province is bounded along the south by cratonized Proterozoic volcanogenic gneisses and schists of the Colorado Province (Goldich and others, 1966; Hausel and others, 1991).

Colorado Province

The Colorado Province forms a cratonized Proterozoic mobile belt of island arc metamorphic rocks lying along the southern boundary of the Wyoming Province. The Colorado Province contains the largest known field of kimberlites in the United States as well as dozens of unexplored geophysical, remote sensing, and kimberlitic heavy mineral anomalies. The Colorado Province consists of a 1.8-1.7 Ga metamorphic terrain disconcordantly intruded by 1.5 to 1.4 Ga granitic rocks (Peterman and others, 1968).

The island arc succession collided with the Wyoming Province 1.7 Ga ago. The boundary is marked by a major suture zone, known as the Cheyenne Belt. This belt represents one of the more highly mineralized Proterozoic-Archean boundary zones in North American. The Cheyenne Belt contains many base and precious metal anomalies including a relatively unexplored platinum-palladium-nickel province, which extends south into the Colorado Province.

Kimberlites in the Colorado Province are found as far south as Boulder, Colorado, and as far north as Middle Sybille Creek, Wyoming, but only the Colorado-Wyoming State Line district kimberlites in the central portion of this region have yielded diamonds. Sub-calcic G10 harzburgitic pyropes in several of the kimberlites in the Iron Mountain district, suggests those intrusives also tapped the diamond stability field.

The Colorado-Wyoming Kimberlites

The majority of the known kimberlites occur within the State Line and Iron Mountain districts. However, kimberlitic indicator minerals have been found in large regions of the State. Indicator mineral anomalies have been found along the flanks of the Sierra Madre, Bighorn, Owl Creek, and Seminoe Mountains, in the Laramie, Medicine Bow, and Uinta Mountains, and in the Greater Green River Basin. Diamonds have been reported in kimberlites in the Colorado-Wyoming State Line district and in breccia pipes in the Greater Green River Basin. Diamonds have also been reported at several other localities in Wyoming (Hausel, 1997, 1998).

The Wyoming Craton also encloses the largest lamproite field in the US - the Leucite Hills. The Leucite Hills lie northeast of a highly anomalous 500 mi² region containing widespread kimberlitic indicator minerals found in ant hills, soils, stream sediments, and in the Bishop Conglomerate (Oligocene). This anomaly extends into northwestern Colorado and northern Utah (McCandless and others, 1995).

State Line District

The State Line district encloses about 40 kimberlites, most (if not all) are diamondiferous. Both hypabyssal and diatreme facies kimberlite occur in the district, with minor fragments of crater facies kimberlite reported in some of the Schaffer complex kimberlites at the Kelsey Lake diamond mine.

Kimberlites in the State Line district are generally poorly exposed, have negligible relief, and are deeply weathered. Erosional models suggest that many of the kimberlites may have been deeply eroded with as much as 50% of the original pipes having been removed (McCallum and Mabarak, 1976). Thus a potential for placer diamonds may occur downstream.

More than 130,000 gem and industrial stones ranging from microdiamonds to a 28.3 carat octahedron have been recovered. Several other large diamonds have also been found including 28.18, 16.29, 14.2, 11.85, 10.48, 9.4, 6.2, and 5.51 carat gemstones, as well as many in the range of 1 to 3 carats. The gem to industrial quality ratios are favorable, with some deposits reporting as much as 65% gemstones.

Even though the State Line district has periodically been explored for diamonds for more than two decades, evidence supports that undiscovered kimberlites occur. Royal Gold in a Sept. 19, 1996 press release reported finding indicator minerals in the district "... in drainages where kimberlites were not previously known to exist". Of 73 samples collected, indicator minerals were found in 17.

Geophysical studies have identified possible blind and buried diatremes. Some INPUT responses corresponded with known kimberlite intrusives, as well as potential blind diatremes. A group of 10 to 12 prominent magnetic anomalies were detected during the INPUT survey. These were interpreted as the manifestation of buried or 'blind' diatremes (Paterson and MacFadyen, 1984). Another airborne survey by Dighem for Fleck Resources Ltd., identified several EM and magnetic anomalies in a 4 mi² area. The most promising was a 1,600 x 1,000 ft ovoid anomaly two miles north of the Kelsey Lake mine in Wyoming (Fleck Resource Ltd., Press Release, April, 15, 1997). Remote sensing studies also identified color anomalies associated with several known kimberlites, and possibly associated with undiscovered kimberlite (Marrs and others, 1984).

All kimberlites that have been tested in the State Line district are diamondiferous. Ore grades range from <0.005 to 0.461 carats/tonne, with some bulk samples yielding grades as high as 1.35 carats/tonne. The Kelsey Lake diamond mine has a resource of 16.9 million tonnes to a depth of 320 feet, which has been outlined in two pits (H.G. Coopersmith, pers. comm., 1997).

ANOMALIES

Gros Ventre Range. A 7 to 9 carat blue-white gem quality diamond was reported from a prospect pit in the Gros Ventre Range and verified by a gemologist from Jackson (J.D. Love, pers. comm., 1981).

Laramie Range. Nearly 300 kimberlitic indicator mineral anomalies were identified over a 1,200 mi² area. Some highly anomalous areas include the Eagle Rock, Middle Sybille Creek, Grant Creek and the Elmers Rock greenstone belt (Hausel and others, 1988). There are unverified reports of diamonds from the Blue Grass Creek area in the Elmers Rock greenstone belt (R.W. Marrs, pers. comm., 1981). Reports of diamonds from the Sybille Canyon region, are also unconfirmed.

Eagle Rock area. Thirty-five stream sediment samples yielded indicator minerals. During air photo reconnaissance, a prominent circular feature (the Eagle Rock anomaly) was identified lying along a NW-trending lineament. The anomaly forms a small, topographic depression. This cryptovolcanic depression is soil covered with no exposed rock on the surface. A map showing the location of the kimberlitic indicator mineral anomalies is found in our site at http://wsgsweb.uwyo.edu/metals/medbow_survey.asp.

Middle Sybille Creek . This area is lies 6 miles northwest of the Iron Mountain kimberlite district. A single, 40-ft diameter kimberlite (Radical 'blow') was found. A 200 pound sample yielded no diamonds (Hausel and others, 1981).

Thirty stream sediment samples collected near the Radichal kimberlite contained various amounts of indicator minerals (Hausel and others, 1988; Woodzick, 1987). The indicator minerals from this area include sub-calcic, high-chrome, G10 harzburgitic pyropes. G10 pyropes are derived from the diamond-stability field in the upper mantle of the earth, and are indicators for diamonds.

Indicator minerals were also recovered from samples collected along Bear Creek, which drains north into North Sybille Creek, and from a tributary of Strong Creek to the south. Unfortunately, we don’t have any geochemistry for these garnets.

Grant Creek . A group of samples collected in Grant Creek yielded numerous indicator minerals (Hausel and others, 1988). One company, which re-sampled this area as a follow-up to our results, reported finding microdiamonds. We have no verification on this reported discovery.

Other heavy minerals of interest included native gold, fluorite, and sapphire. Pyrope garnets were also recovered in samples from the Deadhead gravel pit in the immediate area, as well as in some nearby drainages.

Elmers Rock greenstone belt One of the more impressive regions for abundant kimberlitic indicator mineral anomalies lies within the Elmers Rock greenstone belt, north of Sybille Canyon. Some of the better samples were collected north of the Wheatland reservoir on the Laramie River and Dodge Creek. Three of the more highly anomalous samples yielded 30, 85, and 129 indicator minerals as well as some gold, fluorite, and sapphire. Some of the indicator minerals collected in this region include sub-calcic, high-chrome, G10 harzburgitic pyropes. Diamond-stability pyropes were also recovered from samples in the Cooney Hills area along the eastern flank of the greenstone belt.

Another distinct anomaly was identified in the Laramie River drainage, east and north of Elmers Rock near the Wheatland tunnel in section 36, T 23N, R72W. Samples collected from this area contained more than 10 indicator minerals, including one sample with more than 70. A few samples also yielded some microscopic gold and sapphire. The anomalies in the greenstone belt continue north of the map area, where 6 of 7 samples collected in Duck Creek, also yielded indicator minerals.

Powder River Basin. Microdiamonds were recovered from a coal seam near Gillette in the Powder River Basin of northeastern Wyoming (Finkelman and Brown, 1989).

Medicine Bow Mountains. Kimberlitic indicator mineral anomalies and some detrital diamonds were recovered from the Medicine Bow Mountains. In 1977, two near-gem octahedrons were discovered in a gold placer on Cortez Creek in the northern Medicine Bow Mountains. No indicator minerals were found on Cortez Creek, however, indicator minerals were reported in a tributary of South French Creek known as Iron Creek to the south, by Superior Minerals Company. The samples contained rounded to subangular eclogitic and G9 peridotitic pyrope garnets.

A map showing the location of kimberlitic indicator mineral anomalies identified in the Centennial Ridge area of the Medicine Bow Mountains is found on our site at http://wsgsweb.uwyo.edu/metals/medbow_survey.asp.

Seminoe Mountains. Pyrope garnets and chromian diopsides with some gold were recovered from a Tertiary(?) paleoplacer along the east and west banks of the North Platte River along the northern flank of the Seminoe Mountains. The source of the indicator minerals is under investigation. Based on geology, we interpret these garnets to have originated from the nearby greenstone terrain in the Seminoe Mountains. Some sub-calcic, high-chrome, G10 harzburgitic pyropes were recovered from the Tertiary paleoplacer.

Wind River Mountains. A diamond was reported from the Beaver gold placer in the South Pass greenstone belt in the southern Wind River Mountains in the late 1800s (Hausel, 1991). A second diamond (~2 cm across) was reported from a flat between Tourist and Well Creeks one mile west of Mount Solitude in the central Wind River Mountains (J.D. Love, pers. comm., 1981).

Greater Green River Basin . One of the largest lamproite fields in the world, lies along the northern flank of the Rock Springs uplift in southwestern Wyoming. This field is located northeast of an extensive kimberlitic indicator mineral anomaly which covers 500 to 1,000 mi² of the Greater Green River Basin of Wyoming extending south into northeastern Colorado and the northern flank of the Uinta Mountains of Utah.

The Leucite Hills lamproites include several 3.1 to 1.4 Ma volcanoes and plugs dominated by flows with lesser volcaniclastics. No diamonds have been found in this region, although the district is unexplored for hidden olivine lamproites. Some gem-quality olivine (peridot) was recently recovered from the field.

The source of some Green River Basin indicator minerals includes a group of 10 cryptovolcanic breccia pipes along the southwestern flank of Cedar Mountain. The pipes lie along a 5 to 10 mile-long, north-south-trending lineament in the Bridger Formation (Eocene).

A 200 pound core sample processed by the Saskatchewan Research Council, was reported to have yielded two diamonds. A third diamond was recovered from a concentrate collected by Guardian Resources north of the DK pipe.

Bighorn and Owl Creek Mountains. Several kimberlitic indicator mineral anomalies including pyropes with G10 compositions, were reportedly identified in conglomerates along the flanks of the Bighorn and Owl Creek Mountains of northern Wyoming in the 1980s (Chuck Mabarak, pers. comm., 1996).

REFERENCES

Finkelman, R.B., and Brown, R.D., 1989, Mineral resource and geochemical exploration potential of coal that has anomalous metal concentrations: U.S. Geological Survey Circular 1035, p. 18-19.

Goldich, S.S., Lidiak, E.G., Hedge, C.E., and Walthall, F.G., 1966, Geochronology of the midcontinent region - Part 2, northern area: Journal of Geophysical Research, v.71, p. 5389-5408.

Hausel, W.D., 1998, Diamonds and mantle source rocks in the Wyoming Craton, with a discussion of other US occurrences: Wyoming State Geological Survey Report of Investigations 53, 93 p.

Hausel, W.D., Edwards, B.E., and Graff, P.J., 1991, Geology and mineralization of the Wyoming Province: Society for Mining, Metallurgy, and Exploration of AIME, Littleton, CO., Preprint 91-72, 12 p.

Hausel, W.D., Glahn, P.R., and Woodzick, T.L., 1981, Geological and geophysical investigations of kimberlite in the Laramie Range of southeastern Wyoming: Geological Survey of Wyoming, Preliminary Report 18, 13 p.

Hausel, W.D., Kucera, R.E., McCandless, T.E., and Gregory, R.W., 1997, Diamond exploration potential of the Wyoming Craton, western United States, USA: Prospect to Pipeline, Wyoming Geological Association Field Conference Guidebook, p. 139-176.

Hausel, W.D., Sutherland, W.M., and Gregory, E.B., 1988, Stream sediment sample results in search of kimberlite in southeastern Wyoming: Geological Survey of Wyoming, Open File Report 88-11, 11 p., (5 plates).

Janse, A.J.A., 1994, Is Clifford's rule still valid? Affirmative examples from around the world, in Meyer, H.O.A., and Leonardos, O.H., eds., Diamonds-Characterization, Genesis, and Exploration, Campanhia de Pesquisa de Recursos Minerais Special Publication, Brazil, p. 215-235.

Marrs, R.W., Marks, J.E., Hausel, W. D., and Albert, K.E., 1984, Detection of diamond-bearing kimberlites in the Colorado/Wyoming Province: University of Wyoming, Remote Sensing Laboratory Open File Report, 70 p.

McCallum, M.E., and Mabarak, C.B., 1976, Diamond in State-Line kimberlite diatremes, Albany County, Wyoming, Larimer County, Colorado: Geological Survey of Wyoming, Report of Investigations 12, 36 p.

McCandless, T.E., 1982, The mineralogy, morphology, and chemistry of detrital minerals of a kimberlitic and eclogitic nature, Green River Basin, Wyoming: M.S. thesis, University of Utah, S.L.C., Utah, 107 p.

McCandless, T.E., 1984, Detrital minerals of mantle origin in the Green River Basin, Wyoming, Society of Mining Engineers of AIME Reprint 84-395, 6 p.

McCandless, T.E., Nash, W.P., and Hausel, W.D., 1995, Mantle indicator minerals in ant mounds and conglomerates of the southern Green River Basin, Wyoming in Jones, R.W., ed., Resources of southwestern Wyoming, Wyoming Geological Association 1995 Field Conference Guidebook, p. 153-163.

Mitchell, R.H., and Bergman, S.C., 1991, Petrology of Lamproites: Plenum Press, New York, 447 p.

Paterson, N.R., and MacFadyen, D.A., 1984, An airborne EM (INPUT)/magnetometer survey, State Line district, Colorado-Wyoming: Society of Mining Engineers of AIME Preprint 84-310, 11 p.

Peterman, Z.E., Hedge, C.E., and Braddock, W.A., 1968, Age of Precambrian events in the northeast Front Range, Colorado: Journal of Geophysical Research 73, p. 2277-2296.

Woodzick, T.L., 1987, Geophysical and remote sensing characteristics of the Colorado-Wyoming kimberlite occurrences: PhD dissertation, Colorado State University, 300 p.

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