THEISITE: Just because you don't understand it doesn't mean it isn't so.

I am always on the lookout for funky and sort of quirky uncommon to rare minerals. My small mineral collection has numerous specimens that I purchased due to the facts that I did not recognize the name, and it was cheap. Anything coming off of a dusty shelf with an older label was a bonus. An extra, extra bonus was created if the new mineral was collected in Colorado. And a three-level bonus appeared if the mineral came from near Tuckerville, Colorado, a town, well really a former town now inhabited by ghosts. After purchasing a three-level bonus mineral I didn’t have the slightest idea about the location of Tuckerville until I noticed it was close to Vallecito Reservoir. That particular body of water, on the Pine River, is located about 18 miles northeast of Durango. I have camped there twice in absolutely beautiful USFS campgrounds and spent the time fishing rather than exploring back roads. Tuckerville is then located about 12 miles northeast of the Reservoir on FS 2274 or Middle Mountain Road, a winding road that feels better in a 4-wheel drive pickup than a low-slung passenger car. The elevation is ~ 10,600 feet. After speaking to the ghosts of “almost nothing left” Tuckerville, rockhounds must take a hike (400 feet elevation gain) on the established “jeep trail” to reach the old mines of Tuckers Tunnel or Tuckerville Prospects. Evidently the tunnel has collapsed as have other adits. Mineral collectors who made it this far have zeroed in on what remains of the Tunnel dump. MinDat lists 31 valid mineral species collected in the dump and noted the Tunnel dump as the Type Locality of theisite [Cu₅Zn₅(AsO₄,SbO₄)₂(OH)₁₄].

WORDS OF THE DAY

Funky Mineral: Theisite

Ghost Town:  Tuckerville

New Geological Term to Learn:  Fahlore Deposits

There is not much to say about the Tuckerville Prospects except I regret not being able to visit the locality (at the time of camping at Vallecito I was chasing fossils). Very little information, at least that I could locate, is published on the mining area. Certainly, the best publication is a Rocks and Minerals article by Haynes and Paul Hlava (1998). In addition, Williams (1982) described theisite as a new mineral in Mineralogical Record; however, I could not locate a copy of that article but did find the abstract.

Google Earth© image of the location of Tuckerville (nothing to see on image) and the Tuckerville Prospects.  The winding road comes north from Vallecito Reservoir.

According to Haynes and Hlava (1998) the Tuckerville Prospects are part of the Cave Basin Mining District where in 1913-1914 speculators and investors were overly optimistic about future production of copper, silver, and gold—many claims were filed but production was minimal. As best I can determine in adding figures together, the Cave Basin mines, mostly the Mary Murphy, Holbrook, and Silver Reef, produced a tad less than 100 ounces of gold, ~270 ounces of silver, ~2900 pounds of copper, and ~1700 pounds of lead---all from ~120 tons of ore shipped (via animal drawn wagons or mule trains????) down the mountain to an unknown processing plant. Mining was sporadic from 1913 to 1936. The above production figures are from Schmitt and Raymond (1977) and Steven and others (1969).

I may be missing something but have been unable to locate base metal production figures for the Tuckerville Prospects (the above production figures are from the entire District).  However, it must have been minuscule. The minerals listed by MinDat do not include silver or gold; however, some minerals do include copper, mercury, zinc and maybe a grain or two of galena (lead). Theisite was discovered in 1980 at Tucker Tunnel by two geologists, N.J. Theis and Michael Madsen (Theis and others, 1981), as they tromped through the area looking for uranium—evidently there are some rocks that excite a Geiger Counter, perhaps uraninite and/or zeunerite, as Haynes and Hlava stated, “the prospect [Tuckerville] is radioactively anomalous (up to 700 gamma counts per second).” Steven and others (1969) noted that “base and precious metals in the Cave Basin District were replacement deposits in lower Paleozoic sedimentary rocks”—perhaps the Ouray of Leadville formations.

Now down to the new mineral from Tucker Tunnel—theisite, a copper zinc arsenate antimonate. We know the chemical makeup of the mineral due to analyses by X-ray Powder Diffraction and Electron Microprobe studies. Unfortunately, I don’t have either gizmo in my office.

Visually (with a microscope) theisite is very difficult to identify, especially for an ole plugger like me. First of all, specimens are usually quite small (mine are really, really tiny) and rarely occur as crystals but as crusts, spherical aggregates or simply individual spheres, cleavage plates, micaceous plates, or just plain globs. The color is some sort of blue + green: greenish blue, turquoise-green, turquoise-blue, pale green, or pale blue. Specimens have a pearly luster and are very soft (MinDat states 1.5 Mohs).  I found it very difficult to identify my small specimens and if not for the collection and identification by David Shannon I could have guessed any number of copper zinc minerals.

Two dark green spheres of theisite and two lighter green spheres of "your guess." The dark mineral is a manganese oxide. Width FOV ~4.0 mm. 

Green to blue green theisite spheres or cluster of spheres in and around a vug. Width FOV ~4.0 mm.

Scattered green to blue green theisite spheres or cluster of spheres. Width FOV ~3.0 mm. 

Clear gemmy crystals of platy hemimorphite with black manganese oxide. Width FOV ~2.0 mm. 

MinDat pointed out that theisite was a rare secondary mineral in fahlore deposits. Well, that piece of info rattled my brain and sent me scrambling. The best I could do with an understanding definition of fahlore was from Wikipedia: Fahlore refers to an ore consisting of complex sulfosalt minerals (a metal + semi-metal + sulfur) and in the case of theisite the mineral is formed due to oxidation of a mineral(s) in the tennantite--tetrahedrite solid solution series.

Just because you don't understand it doesn't mean it isn't so.

            Lemony Snicket

REFERENCES CITED

Haynes, P.E. and P.F. Hlava, 1998, Mineralogy of Tuckers Tunnel: Tuckerville, Hinsdale County, Colorado: Rocks & Minerals, vol. 73, no. 5.   

Schmitt, L. J. and W. H. Raymond, 1977, Geology and mineral deposits of the Needle Mountains district, southwestern Colorado: U.S. Geological Survey Bulletin 1434.

Steven, T. A., L. J. Schmitt Jr., M. J. Sheridan, and F. E. Williams, 1969, Mineral resources of the Sun Juan primitive area, Colorado: U.S. Geological Survey Bulletin 1261-F.

Theis, N. J., M. E. Madsen, G. C. Rosenlund, W. R. Reinhart, and H. A. Gardner, 1981, National uranium evaluation, Durango quadrangle, Colorado, Grand Junction, Colorado: Bendix Field Engineering Corp.

Williams, S. A. 1982, Theisite, a new mineral from Colorado: Mineralogical Magazine, vol. 46.