New Mexico Mineral Symposium — Abstracts

The Zuni mine is about three mi northwest of Silverton in the San Juan Mountains of southwestern Colorado and is the type locality for zunyite and guitermanite(?). Both of these minerals were described in 1884 by Hillebrand although the validity of guitermanite(?) as a mineral species is presently uncertain. The mine is near timberline on the south face of Anvil Mountain at elevations ranging from about 11,800 to 12,000 ft. Collecting is limited mainly to the summer months, from early July to late September, because of winter snow accumulations.

The Zuni mine lies in a geologically complex region referred to as the western San Juan caldera complex that locally includes the Silverton, San Juan, Uncompagahgre, and Lake City calderas. The entire caldera complex developed in the Oligocene-Miocene in a large cluster of earlier intermediate-composition stratovolcanoes that, at minimum, include the Cow Creek, Larsen, Carson, and Cimarron centers (Sanford, 1992). The mine is near the 26-27 Ma Sultan Mountain stock that intruded along the ring-faulted margin forming the southern and southwestern edges of both the San Juan and Silverton calderas (Hon and Lipman, 1989). Hydrothermal solutions rising along the faulted rim belt have extensively altered the volcanic rocks forming the southern slopes of Anvil Mountain (Molenaar et al., 1968).

The Zuni was originally located in 1881 as a silver mine. The property consists of five patented claims covering approximately 50 acres (Groben, 1976). Workings consist of an upper and a lower adit, a shaft, and a small open cut near the portal of the upper tunnel. Upper and lower workings are separated by about 200 vertical ft with no connection between them (Ransome, 1901). None of the underground workings are presently accessible. This mine exploited a small Ag-Pb-Cu orebody trending N15°W and dipping 75-80° westward (Ransome, 1901). The deposit, possibly a small breccia pipe (Hillebrand, 1884), is hosted by lathes and andesites of the Silverton Volcanic Series (Ransome, 1901). Country rock adjacent to the deposit has been subjected to advanced argillic alteration and contains ubiquitous disseminated pyrite.

Zunyite, quartz, pyrite and possibly enargite occur in attractive crystals of interest to micromount collectors. At the end of this abstract, we have included a page of drawings illustrating the wide variety of morphologies exhibited by the zunyite and pyrite crystals. Other minerals previously reported from the Zuni mine include anglesite, barite, bournonite, cosalite, dickite, galena, guitermanite(?) (presently under re-investigation), jordanite, kaolinite, pearceite, and native sulfur. To this list we can add acanthite, alunite, anatase(?), apatite, bornite, kuramite (first recorded occurrence in Colorado), sphalerite, stibnite, and an unknown Pb-As sulfide, which were all observed during micromount or microprobe studies.

Acknowledgments
The authors thank Patrick E. Haynes of Cortez, Colorado for helpful discussions of the mineralogy of the Zuni mine and for the donation of several specimens examined in this study. We also wish to thank Jack Murphy of the Denver Museum of Natural History for the loan of a piece of sample DMNH#547 for analysis and for discussions of the mineralogy of this mine and the area around it. The crystal drawings were produced using the SHAPE° crystal drawing program (copyright by Eric Dowty and R. Peter Richards, 1987 & 1988).

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References:

1. Groben, M. M., 1976, Zuni mine: The Mineralogical Record. v. 7, p. 270.
2. Hillebrand, W. F., 1884, On zunyite and guitermanite, two new minerals from Colorado: Colorado Scientific Society Proceedings, v. 1, pp. 124-131.
3. Hon, K., and Lipman, P. W., 1989, Western San Juan caldera complex; in Chapin, C. E., and Zidek, J. (eds.), Field excursions to volcanic terranes in the western United States, Vol. I: Southern Rocky Mountain region: New Mexico Bureau of Mines and Mineral Resources, Memoir 46, pp. 350-380.
4. Molenaar, C. M., Baars, D. L., Mayor, J., and Kelly, V. C., 1968, Road log from Ouray, Colorado to Farmington, New Mexico via Silverton, Eureka, Durango, and Aztec: San Juan-San Miguel-La Plata region: New Mexico Geological Society, Guidebook to 19th Field Conference, pp. 104-117.
5. Ransome, F. L., 1901, A report on the economic geology of the Silverton quadrangle, Colorado: U.S. Geological Survey, Bulletin 182, 265 pp.
6. Sanford, R. F., 1992, Lead isotopic compositions and paleohydrology of caldera-related epithermal veins, Lake City, Colorado: Geological Society of America, Bulletin, v. 204, pp. 1236-1245.