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.
A careful newspaper wrap is usually fine for transporting specimens in flats or boxes. Cleaning using air tools is recommended but a quick rinse in alcohol will help in some cases. Most species are highly water soluble and may need a preservative spray in humid climates. Matrix rock is very reactive and can create an acid environment in your display case or in boxes. A limited soak in dilute Elmer’s Glue or a sealing spray may help to avoid these problems.
Minerals of interest were found mostly in piles of mineralized sandstone blocks near the portals to the mine. This mineralized material has been exposed for years and soluble species have been dissolved and re-formed in cracks and on surface areas within the pile. You will need a typical assortment of tools that includes heavy sledges and bars to break up large blocks of sulphate-cemented sandstone. I recommend face masks and eye protection as dust from the pile is highly irritating and may contain radioactive species.
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
Workings at the Dexter No. 7 mine include short drifts and slopes largely open to the surface of the cliff face. Other than the access road the only other evidence of the operation remaining in the late 80’s was a cable, part of a tram system from the portal area to the base of the mesa, and remnants of shacks and trash dumps. While the literature describes coquimbite specimens from parts of the underground workings, there was nothing significant found underground.
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.
