Samples collected during this period contained a suite of minerals that included species not described before from Colorado.  Later work by the Bendix Corp, under contract to the Atomic Energy Commission, expanded the list of species found there and included one new species. The new mineral, theisite, is an orthorhombic Cu-Zn(As,Sb) arsenate described as sectile, bluish-green scales to 2mm. This species has since been recognized from a locality in Utah and from a number of localities in Europe. A partial listing of additional species includes:

adamite, austinite, azurite, chalcocite, chonichalcite, covellite, cuprite, digenite kolwezite, malachite, parnauite, partzite, pyrite, tetrahedrite, theisite, uraninite, zeunerite.

This unusual suite of elements and minerals have similarities to uranium vein deposits with complex paragenesis in the Co-Ni arsenide group. These could include nearby deposits in Precambrian rocks as well as  uranium bearing, silver, base metal vein deposits in Europe. The deposit at Tucker’s Tunnel contains significant amounts of As, Cd, Sb, Mo, Ni, Co and Ag as well as large concentrations of Cu, Pb and Zn. A uranium-copper-cobalt-silver-arsenic vein in Precambrian rocks at Elk Park, south of Silverton and a uranium vein deposit high in the Needle Mountains northwest of the Tucker’s Tunnel occurrence are of similar interest.

Like many of the lessor mineralized areas in the San Juan Mountains, these prospects were first discovered and tested in the latter part of the 1800’s and in the early 1900’s. First recorded production is in 1913 and included some 57 tons of ore with 12 ounces of gold, 237 ounces of silver, 2,900 pounds of copper and 1,700 pounds of lead.

In the mid-1950’s, Mr. Reno Montonati, a local prospector interested in the silver in the area, discovered radioactivity at Tuckers Tunnel, did some development/exploration work and promoted the prospects for uranium. An engineer, he actually built his own helicopter and flew it in the area!

The collecting site, named the Tuckers Tunnel prospect, consists of a series of pits, trenches and short adits on an east-west trending structure cutting Paleozoic sedimentary carbonate-rich rocks. All minerals noted here have been found in the dumps at this prospect. The best mineralization was found in small openings in fractures and veins in blocks of angular carbonate-rich breccia with visible secondary copper mineralization. There are, however, many prospects, (the Mary Murphy Mine, for example) scattered in the area to the north that show copper mineralization and associated radioactivity. These prospects, unlike those at Tuckers Tunnel, do not show the variety of mineral species or the breccia that is the host for the minerals described here.

The Dexter No. 7 Mine at Calf Mesa is a good source for some unusual sulphate species and may remain so after more than 20 years since my last visit. In general, because of the remote locations and sometimes difficult access, the San Rafael Swell in East-Central Utah is one of significant potential, particularly for species related to uranium deposits.

For a fairly complete list of minerals from the “Dexter Group” of claims see Bullock, Kenneth C., “Minerals and Mineral Localities of Utah” Utah Dept. of Natural Resources Bulletin 117, 1981. Of primary interest are sulphates that occur in micro to small thumbnail sizes such as coquimbite, roemerite, voltaite and others in association with alunogen, halotrichite, jarosite and a mix of massive, soluble, crystalline sulphates. Uranium minerals are present but were not seen as ‘collectible’ specimens.

For picture of some of these species see location references in the MINDAT database at www.mindat.org

The Dexter No. 7 mine on Calf Mesa is located about 12 miles north of Interstate (70) on the crest of the San Rafael Swell some 25 miles west of Green River, Emery County,Utah. Access is good in dry weather but it is a remote area so plan accordingly. A 4wd road to the mine was passable in the late 1980’s but there may have been ‘reclamation’ on the property since. With the present interest in uranium there could be exploration activity as well.

The uranium workings at the south end of Calf Mesa in Emery County, Utah (the Dexter Group of claims)  have produced some of the more unusual and attractive sulphates known from the Western U.S.  Workings develop uranium mineralization in sandy units in the Chinle Formation of Triassic Age. While specimen production during actual development of the properties in the 1950’s thought to have been minor, abundant specimens were found as secondary accumulations in waste rock at the mines in the late 1980’s. There has been more recent collecting of specimens there but the extent of that activity is unknown. Because many of the minerals of interest at the Dexter Group are formed in waste rock and ores exposed to weathering there may be a continuing opportunity for collecting good specimens there for some time.

In the early and mid 1970’s I spent several months working on the uranium potential of the Denver Basin and, in particular, the area east and southeast of Colorado Springs and Trinidad. We were evaluating mostly Lower Cretaceous rocks but spent some time checking surface and sub-surface anomalies based on areal and ground water sampling programs. Surface rocks in much of this area consist of Upper Cretaceous shales containing multiple horizons with large concretions.

On a late afternoon in 1972, returning to our base of operations in La Junta from the area near Timpas, we stopped to check a shale unit along the highway and found several promising concretions. We excavated one, about 4′ in diameter, and found what may have been the first reported clear barite in what was later called the “La Junta” barite area. These barites are often water-clear and display a variety habits some of which are remeniscent of those found in concretions near Grand Junction.

The specimen pictured was displayed at the Denver Gem and Mineral show in, I believe, 1976 in a Self-Collected Minerals case. The blocky crystal is about 2.3 cm on an edge. It generated a bit of interest and the location was given to several people, including Don Knowles who reported little success after at least one trip to the area. Some years later I received an inquiry from Dan Kyle about the location and sent maps of the area to him. As most collectors in the Denver area know, Dan has found some very fine specimens there.

While much has been said about the clear barites there are two additional locations that I have always wanted to re-visit. One, not far from the original barite discovery, was near a well where we collected a water sample. Weathered concretions on the surface nearby contained lovely rosettes of white, flattened calcite rhombs up to 3 inches across displayed on a matrix coated with orange-brown lichens. Another, some miles to the east, was a somewhat obscure zone of smaller concretions, up to about 2.5-3 feet in size, with tabular blue to blue-gray and yellow zoned barites up to 2 inches on a calcite-crystal matrix. We spent less than 20 minutes walking contour on this concretion zone and found one weathered open, with the crystals described above and several others that looked promising but we did no digging.

In the mid-1970’s I spent some time working in SW Colorado in the La Plata Mountains near Durango. This area has produced significant gold (the Bessie G mine, for example, was active at that time) and has been investigated for its porphyry copper potential and uranium as well.

The Tomahawk Mine and basin are located above timberline in the western part of the complex. Rocks in the area include a variety of intrusives and metamorphosed sediments that form the high ridges and cliffs around the head of Tomahawk Creek. The Tomahawk mine produced gold in the last century. At the time I worked in the area there was still a very thin, rich vein containing visible gold accessible (if you were a skilled rock climber) on the cliff face at the creek near the mine ruins.

Above: Tomahawk Basin seen from high ridge to SE.

Below: Tomahawk Mine structures in 1975.

What passed for a road ended a few hundred yards west of the mine at a drill site located near a small intrusive complex. Talus from the high cliffs to the south of this intrusive contained scattered vugs that were mineralized with epidote, quartz, K-feldspar and rare fluorite. The fluorite crystals were simple octahedrons of a rich violet color. Their maximum size was about 2 mm.

I have always thought that it would be worthwhile to investigate the area for the source of the fluorite. Perhaps there are larger cavities and better crystals in the larger talus or lower cliff faces a few hundred feet up and to the south of the intrusive. Are there any of you energetic young collectors interested?