New Mexico Mineral Symposium — Abstracts

Tellurium (abbreviated Te) is one of the rarest elements found in the earth's crust, with an average abundance far lower than platinum's. Though rare, telluride minerals (those bearing tellurium) are noted in all 13 mining states in the western U.S., including the four-corner states of Arizona, Colorado, New Mexico, and Utah. For the remainder of this discussion, the term telluride minerals will be restricted to unoxidized species, excluding tellurites and tellurates.

Curiously, certain areas are broadly enriched in telluride minerals such as Colorado, whereas adjacent areas like Wyoming contain few Te-bearing deposits. In fact, Colorado has roughly 38 times as many telluride occurrences per square kilometer as Wyoming, and five times as many per square kilometer as an average western mining state. New Mexico and Utah are examples of typical western mining states in terms of Te occurrences per square kilometer, whereas Arizona contains roughly one third the number of Te occurrences of average western mining states.

In addition, Colorado contains the broadest telluride mineral diversity of the four-corner states. One area, the Boulder County telluride belt, contains at least 16 unoxidized telluride minerals, which represents the largest telluride diversity of any deposit that the author is aware of in the world. In fact, 65% of Colorado's telluride mineral varieties are found in the Boulder Telluride belt alone. In Arizona, the Bisbee district contains over half of that state's telluride minerals. Surprisingly, Utah has the least diverse telluride minerals of the four corner states (perhaps because of inadequate identifica¬tion techniques), even though it has the typical number of occurrences per square kilometer. Another unexpected result of this investigation is that certain statistical and geologic evidence suggests that Arizona should contain more Te-bearing mining districts than are presently known.

The mineralogy of the telluride minerals in the four-corner states is fairly typical of Te¬bearing districts worldwide. The most reported telluride minerals in the region are (in decreasing order of reported district-wide occurrences, with the number of reports in parentheses): petzite (24), hessite (21), altaite (16), sylvanite (16), calaverite (13), native tellurium (13), and tetradymite (12). Detailed literature examination indicates that altaite, native tellurium, hessite, and tetradymite are perhaps the most commonly reported telluride minerals worldwide, occurring in a broad range of geologic environments. The presence of petzite, sylvanite, and calaverite in the above list suggests the relative prevalence of Au-tellurides in the four-corner states.

One might infer from above that the general trend is for areas with the most reported Te¬bearing minerals to contain the widest diversity in their telluride mineral chemistry. This is basically true. For example, Colorado has 40 Te-bearing districts, 23 reported telluride minerals, and tellurides with more varied chemical elements than other areas in this study. On the other hand, Utah has the least number of verified telluride minerals, hence the least diversity in telluride mineral chemistry.

The tellurides present in the four-corner states are representative of many worldwide hydrothermal systems with low to moderate temperatures of formation. Tellurides were deposited late in the paragenetic sequence of mineralization and are associated with certain Mesozoic/Cenozoic metal-bearing intrusions, in many cases Cu and or Mo alkalic porphyry systems. Tellurides are found either in the porphyries, limestone replacements, skarns, or most commonly in epithermal veins. An entirely different origin is illustrated by the Vulcan deposit near Gunnison, Colorado, which is a submarine exhalative massive-sulfide deposit of Proterozoic age.

Most of the telluride occurrences in the western U.S. do not occur in distinct belts. Telluride occurrences in Arizona are confined to the far southeast corner of the state except for the Tiger district near the state's center. Colorado's telluride occurrences generally fall within a triangle, with one leg the northeast—southwest-trending Colorado Mineral Belt, another leg trending east from La Plata to an area southeast of Westcliffe (west of Pueblo), and a third leg trending roughly north from Westcliffe to Jamestown, northwest of Boulder. All of New Mexico's telluride occurrences lie in the southwest part of the state except for two districts northeast of Taos. Utah's telluride occurrences are confined to the western half of that state.

Four hypotheses were tested to explain these large-scale distribution patterns. The first concerns the distribution of alkalic rock complexes, with which telluride deposits are known to commonly affiliate (Mutschler et al., 1985). The four-corner states have anomalously high proportions of alkalic intrusions per square kilometer of ground surface, yet they do not uniformly contain above-average numbers of telluride occurrences within their boundaries. The second concerns the timing of peak alkalic igneous activity. In Colorado this occurred 26-76 Ma ago, which is identical to peak alkalic activity in New Mexico and three other western states (Mutschler et al., 1992), yet Colorado's tellurides/km2 far surpasses the other four states'. The third hypothesis concerns plate tectonic relations, which do not provide a clear answer. Finally, metal inheritance tied to underlying basement lithology/age was examined and was the most successful of the four hypotheses. Although it cannot yet be proved, the composition of Colorado's basement appears to be the most reasonable explanation for its anomalously high density of telluride occurrences per square kilometer.
 

References:

1. Mutschler, F. E., et al., 1985, Precious metal deposits related to alkaline rocks in the North American Cordillera???an interpretive review: Trans. Geol. Soc. S. Afr. 88:355-77.
2. Mutschler, F. E., et al., 1992, Alkaline and related igneous rocks and associated mineral deposits???an album of space-time snapshots of the Western U.S. and Canada: U.S.G.S. MF Map Series, 174 p.