New Mexico Mineral Symposium — Abstracts

Edward S. Dana (1892) first described onegite thus: "onegite is acicular goethite penetrating quartz, like rutile, from an island in Lake Onega, Russia." The term "onegite" has fallen into disuse because it is not a mineral name but is descriptive of an inclusion of one mineral within another.

The term "onegite" is still used by many collectors in Colorado, probably because it is easier to say "onegite" than "quartz with goethite inclusions." This combination of minerals has been found in the Pikes Peak Granite pegmatites since the late 1800s, but little has been written about it, and few specimens are found in museums or other major collections. Over the past 30 yrs, I have found many pockets containing onegite, including particularly fine material on the Second Mesabi Claim, located in the southern edge of the Lake George intrusive center near Lake George, Colorado.

Peter Modreski (1996) of the U.S. Geological Survey in Denver gave a presentation at the Seventeenth New Mexico Mineral Symposium, titled Onegite (amethyst with goethite inclusions) from the Pikes Peak batholith, Colorado. His presentation covered what was then known about the material, including his own estimation of the paragenetic sequence and temperatures of crystallization. My presentation will review Modreski's description and photos of onegite, and then, present new find¬ings from fluid-inclusion testing on the minerals.

In conjunction with the recent publication of a description of the occurrence of onegite (Berry, 2001), I had fluid-inclusion testing done by Virgil Lueth at the New Mexico Bureau of Geology and Mineral Resources in Socorro.

The fluid inclusions showed that the primary quartz formed at a temperature of 360° C, indicat¬ing that this pocket, at least, formed after the granite magma had cooled and crystallized—not as part of the magma, but as truly hydrothermal in nature (crystallizing from water carrying elements through cracks and crevices in the now cooled granite.)

The fluid inclusions in the quartz with goethite inclusions had temperatures much lower than expected. They were found to be a single-phase growth of 40° C or less. Using the known tempera¬ture gradient (the natural increase in temperature with depth) for the Pikes Peak Granite, which is approximately 25° C/km, and by subtracting the average ambient temperature of 3.4° C at the Lake George U.S. Weather Station (Normals and extremes, 2001), one finds that a temperature of 36.6°C divided by the 25° C/km temperature gradient yields a crystallization depth minimum of 1.4 km.

The Pikes Peak pluton was emplaced at a depth of approximately 4-5 km, 1.06-1.09 b.y. before present (Unruh et al., 1995). It is also generally accepted that the Laramide orogeny (present Rocky Mountain uplift) began about 60-70 m.y. ago and continued rather slowly until about 10 m.y. ago, when rapid uplift began. I reached the conclusion that the pegmatites yielding the onegite could not have reached a depth of 1.4 km until sometime during the last 10 m.y.

Knowing that there were two other ancestral Rocky Mountains, one may rightly question why formation of the onegite could not have occurred earlier. I further postulate that during those peri¬ods, which were at times of a more tropical climate (dinosaurs, etc.), ambient temperatures were probably higher than 40° C.

Photos from this presentation show that there were several variations in deposition, particularly of goethite growth under and on top of the onegite, proving that goethite can also crystallize at very low temperatures and pressures. Also, after the publication of the Rocks & Minerals article (Berry, 2001), I corresponded with a collector in Montana who has found quartz with goethite inclusions (some amethyst) in the Montana batholith in at least two different locations (Van Laer, pers. comm. 2001). Photos of the material show that it is similar to the Pikes Peak material, with the possible exception of euhedral growths of goethite. The onegite in Montana appears to have been deposited directly on the quartz. I have found the underlying goethite depositions are often missing when euhedral amethyst crystals with goethite inclusions are found, and when goethite sub-strata is present, the onegite is very heavily included making it opaque.

Little other modem scientific investigation of the paragenesis of the miarolitic cavities in the Pikes Peak Granite has been done. It would be of great interest to expand this investigation to other pockets within the batholith, extending the testing to other species. I extend an offer to take a professional mineralogist to a variety of pockets I have collected, securing from the dumps: quartz and microcline; from the walls: fluorite, amazonite, and smoky quartz; and other minerals that might be present. This investigation should be able to show if the occurrences noted in this paper are different from those in other areas or those lacking onegite. It would even be valuable to know if there is a difference in crystallization temperature from the base of a quartz crystal to the termination

References:

1. "Normals & extremes, Sept. 1960???Dec. 1997, for Lake George," in Climatology of Colorado [data??base online] (National Weather Service, Pueblo, Colorado) [cited September 19, 2001] available from www.crh.noaa.gov/pub/.
2. Berry, R. R., 2001, Goethite inclusions in quartz from the Pikes Peak Granite: Rocks & Minerals, v. 76, no. 4, pp. 228-232.
3. Dana, E. S., 1892, The system of mineralogy of James Dwight Dana, 1837-1868: John Wiley & Sons, New York, 6th ed.
4. Modreski, P. J., 1996, Onegite (amethyst with goethite inclusions) from the Pikes Peak batholith, Colorado; in Seventeenth annual New Mexico Mineral Symposium abstracts, November 9-10, 1996, Socorro, New Mexico: New Mexico Bureau of Mines and Mineral Resources.
5. Unruh, D. W., Snee, L. W., Foord, E. E., and Simmons, W. B., 1995, Age and cooling history of the Pikes Peak batholith and associated pegmatites (abs.): Geological Society of America, Abstracts with Programs, v. 27, no. 6, p. A468.