Diamond Exploration in Wyoming

The Wyoming State Geological Survey (WSGS) is active in identifying, categorizing, and finding new mineral deposits within Wyoming (http://www.nrcan.gc.ca/mms/diam/Kimberlite-EN/Kimberlite.swf). During the past two decades, the WSGS has discovered several gold, ferrous, base metal and gemstone deposits, and some platinum, palladium, silver, nickel, and cobalt. In addition, the Metals and Precious Stones section has mapped more than 500 mi² of greenstone terrain, and associated mining districts.

Since 1977, the WSGS has also discovered dozens of kimberlites and identified more than 300 kimberlitic indicator mineral anomalies within the Wyoming craton (see Hausel and others, 1988, 1997). The Wyoming craton is host to the two largest kimberlite fields (State Line and Iron Mountain), and the largest lamproite field (Leucite Hills) in the United States (Hausel, 1995). This is significant in that kimberlite and lamproite are the only rock types that have been successfully mined for diamonds.

Following a mapping project in the State Line district south of Laramie in 1977 and 1978 by the WSGS (Hausel and others, 1981), several kimberlites were tested for diamonds by various mining companies. More than 130,000 diamonds, including gemstones weighing more than 28 carats, were recovered from kimberlite and associated placers in the Colorado-Wyoming State Line district (see Hausel, 1998). Currently, the WSGS is working in the Iron Mountain district in the Laramie Mountains 40 miles north of the State Line district.

STATE LINE DISTRICT

Geological Mapping

The Wyoming portion of the Colorado-Wyoming State Line district includes 20 known kimberlites. All that have been tested to date, have yielded diamonds. Diamonds recovered from the Wyoming Portion of the district have included nearly 50% high-quality gemstones.

The ‘known’ kimberlites in the district are located to the west of highway 287 south of Laramie and are deeply eroded pipes of diatreme facies kimberlite (Early Devonian and Early Cambrian) emplaced in granitic facies of the Sherman Granite (1.4 Ga) ( see State Line geological map)(Need MrSid Browser Plugin). The Sherman Granite intrudes Proterozoic (1.7-1.9 Ga) basement rocks of the Colorado Province.

The kimberlites are not easy to find. Being deeply eroded and weathered, the kimberlites typically consist of montmorillonite clays with considerable carbonate, and tend to blend into the local topography. However, the clays do not support growth of trees, and in woody areas, some kimberlites form open parks with grass growing in the kimberlite.

IRON MOUNTAIN DISTRICT

Geological Mapping

Mapping by the WSGS began in the Iron Mountain district in 1998, as a follow-up to earlier work by Smith (1977) and Hausel and others (1988). Smith had identified a small group of tiny kimberlite dikes and blows (Devonian) that were emplaced in Sherman Granite (1.4 Ga).

On the extreme western edge of the district ( see geological map ), sample concentrates with kimberlitic indicator minerals (pyrope, picroilmenite, and chromian diopside) were recovered from stream sediments collected by the WSGS in the 1980s (Hausel and others, 1988). The indicator minerals from five samples along the southwestern margin of the district most likely originated from the dike-blow complex immediately to the east. However, the source of the indicator minerals from two samples taken in an intermittent stream along the northwestern margin of the district, was unknown until recently. These samples were separated by a divide from the main body of kimberlites mapped by Smith, and the source was implied to be undiscovered kimberlites somewhere in that drainage basin. Based on these samples, the WSGS initiated the current project in the Iron Mountain district.

By the end of the 1999 field season, the WSGS discovered some kimberlites within the drainage basin along the northwestern edge of the Iron Mountain district (note the kimberlite dikes adjacent to the anorthosite xenolith). These are part of a group we refer to as the Indian Guide kimberlites. It is anticipated that other kimberlites associated with this group will be found in the future.

During mapping in 1998 and 1999 of the main body of the Iron Mountain kimberlites, the WSGS discovered considerably more kimberlite than had previously been mapped. We have now identified a major kimberlite dike-blow-pipe complex within a 12 mi² area that has more than 4 miles strike length. We have also found evidence of several hidden kimberlites , and hidden fractures and shears that could potentially host other undiscovered kimberlites. Several of the Iron Mountain Kimberlites are visible on aerial photos ( 1,2,3 ).

Geophysical Surveys.

Several targets were selected for magnetic and electromagnetic surveys. The targets were selected because kimberlites were projected into boggy depressions with distinct vegetation anomalies. The magnetic data was inconclusive, typical of many kimberlites examined in the past (see Hausel and others, 1979), but the EM data proved to be valuable.

Geophysical line 16 ( see map ) was run along a north-south trend in a depression in section 16. A 450-ft long conductor was detected in the depression. The conductor is probably a reflection of a hidden kimberlite.

To the northwest in section 8 (Images 1,2 ), a group of grassy springs with some 'blue ground' were found. A north-south line across the southwestern-most anomaly ( geophysical line 8d) yielded an excellent 600-ft long conductor. A short distance to the northeast, ( geophysical line 8f) produced a distinct 400-ft conductor. These are interpreted to reflect hidden pipes lying under the vegetation anomaly. Geophysical surveys are scheduled for the line of vegetation anomalies continuing to the northeast from these two pipes. We suspect that we have as many as 4 to 5 other pipes in this group.

In section 2, along the eastern flank of the district, a depression occurs in a boggy area. This bog partially obscures a kimberlite dike. Geophysical line 2d was run along a north-south trend east of the projected northeasterly-trending kimberlite dike. A 60-ft conductor was identified, and suggests the presence of a dike, or small blow, east of the kimberlite outcrop. A parallel profile, ( Geophysical line 2c )was run across the projected trend of the dike. This produced a 90-ft long conductor over the projected trend of the dike, and is interpreted to be a reflection of the kimberlite beneath the heavily vegetated depression. Another line ( Geophysical line 2b )was run in a northwesterly direction across the projected trend of the kimberlite. A 45-ft long conductor was detected over the kimberlite.

As you gaze at the map, several other targets become obvious. Look in the S/2 section 8 and N/2 section 17, where a large Tertiary paleoplacer bisects the kimberlite dike complex. Since kimberlites have been traced into the paleoplacer from both sides, in all probability, some kimberlites underlie this boulder conglomerate.

Exploration targets have also been identified north and west of the Iron Mountain district. A strong kimberlitic indicator mineral anomaly that yielded a few hundred pyrope garnets was detected by the WSGS in the Grant Creek area a few miles north of the district (see Hausel and others, 1988, Hausel, 1998). Another area of interest lies west of the district where a group of structurally-controlled, water-filled, depressions are found in anorthosite (1.5 Ga) on aerial photography.

Geochemistry

The chemistry of kimberlitic indicator minerals is used to identify potential diamond targets. High-chrome, sub-calcic, magnesian peridotitic pyrope garnets are often associated with diamond. These garnets are termed 'G10', as opposed to 'G9' pyropes which are more calcic. The G10 pyropes are derived from diamondiferous harzburgitic upper mantle, and G9 pyropes are derived from lherzolitic upper mantle, where diamond is most often unstable. Thus the presence of G10 pyropes indicate that the kimberlite magma began its journey within the diamond-stability field of the upper mantle.

Electron microprobe analyses of peridotitic pyropes, and eclogitic pyropes, chromite, and chrome-potassic pyroxenes is still in progress. After all microprobe analyses are completed, the WSGS will select the best targets, and collect samples (a few hundreds pounds each) to be processed on the WSGS's grease table for diamonds. Note that several sites are labeled with G10 on the geological map. These extend from the western end of the district, all the way to the eastern edge, with several anomalies in-between.

One consideration about the diamond potential of the Iron Mountain kimberlites is that diamonds will burn. It is suggested that picroilmenite compositions reflect the oxidation state of the kimberlite magma during emplacement, and may provide an indication of diamond preservation. Iron Mountain ilmenite compositions were reported by McCallum and Waldman (1991) to be depleted in magnesium with slight enrichment in chromium, which has been interpreted to correlate with increased oxidation state of the magma, implying diamond preservation to be unlikely. However, the ilmenite compositions from Iron Mountain are similar to ilmenites from kimberlite at Kelsey Lake (the first commercial diamond mine in North America!). Diamonds from Kelsey Lake include well-preserved, gem-quality, octahedrons with no evidence of resorption (Coopersmith and Schultze, 1996). Thus Mg-depletion and Cr-enrichment in ilmenite does not reflect oxidation of the magma as previously thought.

IRON MOUNTAIN DISTRICT

Geological Mapping

The Leucite Hills located north of Rock Springs, is the largest field of lamproites in North America. These rocks are similar to lamproites found in the Ellendale field in Western Australia. The lamproites consist of Wyomingites, madupites, and orendites but olivine lamproites are in general lacking. The possibility of hidden olivine lamproites is considered high, similar to that seen in the Ellendale field where the diamondiferous olivine lamproites are hidden under a few inches of soil. Even so, olivine is found in some of the lamproites in the northeastern portion of the field. A few years ago, the WSGS found considerable olivine in anthills adjacent to Black Rock. In two anthills, the WSGS recovered more than 13,000 carats of olivine. Much of the olivine is gem-quality and produces a very attractive peridot gemstone.

The Leucite Hills lamproites were emplaced along east-west trending fractures along the flanks of the Rock Springs uplift. The lamproites erupted through Cretaceous sediments that lie on top of an Archean basement (Archon) at depth. The lamproites are reported to have erupted only 0.9 to 3.1 million years ago. Some spectacular volcanic features can be seen associated with this volcanoes and flows. Mapping in the district was completed and submitted to the WSGS publications office in 2001. The preliminary map (see geological map of Leucite Hills) (Need MrSid Browser Plugin) shows a group of scattered lamproites on the flank of the Rock Springs anticline.

References Cited

Coopersmith, H.G., and Schultze, D.J., 1996, Development and geology of the Kelsey Lake diamond mine, Colorado in T.B. Thompson, ed., Diamonds to gold - I. State Line Kimberlite district, Colorado, II. Cresson mine, Cripple Creek district, Colorado: Society of Economic Geologists guidebook series, p. 5-19.

Hausel, W.D., McCallum, M.E., and Woodzick, T.L., 1979, Exploration for diamond-bearing kimberlite in Colorado and Wyoming: an evaluation of exploration techniques: Geological Survey of Wyoming Report of Investigations 19, 29 p.

Hausel, W.D., Glahn, P.R., and Woodzick, T.L., 1981, Geological and geophysical investigations of kimberlites in the Laramie Range of southeastern Wyoming: Geological Survey of Wyoming Preliminary Report 18, 13 p., 2 plates (scale 1:24,000).

Hausel, W.D., Sutherland, W.M., and Gregory, E.B., 1988, Stream-sediment sample results in search of kimberlite intrusives in southeastern Wyoming: Geological Survey of Wyoming Open-File Report 88-11, 11 p. (5 plates) (revised 1993).

Hausel, W.D., 1995, Diamonds and their host rocks in the United States: Mining Engineering, v. 47, no. 8, p. 723-732.

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., 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.

McCallum, M.E., and Waldman, M.A., 1991, The diamond resources of the Colorado-Wyoming State Line district: kimberlite indicator mineral chemistry as a guide to economic potential: in Wyoming Geological Association 42nd Field Conference Guidebook, p. 77-90.

Smith, C.B., 1977, Kimberlite and mantle derived xenoliths at Iron Mountain, Wyoming: M.S. thesis, Colorado State University, 218 p.

Geophysical Profiles

FIGURES

Figure 1: Geological Map Of The Wyoming portion of the Colorado-Wyoming State Line district, by W. Dan Hausel.
Figure 2: Preliminary Geological Map Of The Iron Mountain Kimberlite District, Southeastern Wyoming, by W. Dan Hausel
Figure 3: Photo of depression in Section 2.
Figure 4: Photo of vegetation anomaly over spring in section 8.
Figure 5: Another photo of vegetation anomaly over spring in section 8.
Figure 6: Strong grassy vegetation anomaly growing over kimberlite in the Iron Mountain district.
Figure 7: Wyoming Diamonds
Figure 8: Robert Gregory standing on an outcrop of Iron Mountain Kimberlite.
Figure 9: False color infrared air photo over circular depressions west of the Iron Mountain district.
Figure 10: Preliminary geological map of the Leucite Hills lamproite field, by W. Dan Hausel.

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