New Mexico Mineral Symposium — Abstracts

he Trout Creek Pass Pegmatite District consists of numerous, zoned granitic pegmatites that intrude the 1.7 Ga Denny Creek Granodiorite and which are located east and southeast of the town of Buena Vista, Chaffee County, Colorado. Although mined chiefly for feldspar, the pegmatites are enriched in niobium (Nb), tantalum (Ta), titanium (Ti), and rare earth elements (REE). This presentation will focus on four well-known deposits in the district that have been popular collecting sites: the Clora May, Yard, Crystal No. 8 (also known as the Luella), and the Tie Gulch pegmatites. The Clora May, Yard, and Crystal No. 8 are located on public land within the San Isabel National Forest; the Tie Gulch pegmatite appears to lie on private land north of, but not within, the Tie Gulch drainage. A high clearance vehicle is needed to reach the Yard. The other deposits are accessed via good, but unpaved roads.

Each of the four pegmatites exhibits a wall zone dominated by quartz and microcline, often in graphic intergrowth. Magnetite is also present in small amounts. Quartz and microcline comprise the core of each pegmatite as individual masses of giant crystal size, commonly 1-5 meters across. "Books" of biotite, 0.3 meters or more, occur along the core margins. Intermediate zones, or replacement zones, occur locally as small pods adjacent to the core and contain albite, ± muscovite, and accessory minerals such as fluorite, allanite-Ce, bismutite, gadolinite-Y (very rare), xenotime-Y (very rare), monazite-Ce, polycrase-Y, and aeschynite-Y. Rose quartz of light pink color occurs at the Clora May and reportedly at the Crystal No. 8, and is present in trace amounts at the Tie Gulch pegmatite.

REE-Nb-Ta oxides are the main attraction for mineral collectors, however, chemical analysis is needed to identify specific mineral species, and even with modern methods, identification is difficult. In the past, most specimens from the Trout Creek Pegmatite District were labeled as "euxenite". Hanson and others (1992) report from microprobe analyses and recrystallization studies that specimens of black, metamict samples from the Yard and Crystal No. 8 pegmatites are most likely polycrase-Y, while visibly similar samples from the Clora May appear to be aeschynite-Y (they did not suggest an identity for REE-Nb-Ta oxides that occur at Tie Gulch). Most collectors label their specimens of REE-Nb-Ta oxides from these localities accordingly.

Although all four pegmatites have been popular collecting sites for decades, with the exception of the Crystal No. 8, recent visits by the authors have been productive. For example, a 10 cm crystal aggregate of polycrase-Y was recovered from the Yard, as well as smaller clusters and individual crystals and many broken pieces. Several years ago, a Colorado collector removed literally a wheelbarrow load of aeschynite-Y from the Clora May, which included many crystals and crystal groups, and perhaps the largest single, well formed aeschynite crystal ever recovered from a Colorado pegmatite. Additionally, dozens of 1-2 cm crystals of polycrase-Y(?) were screened from the dump at Tie Gulch by another Colorado collector. Even the Crystal No. 8 likely has something left if one is willing to dig into hard rock, as radioactive targets were noted this past summer, but not investigated.

References:

1. Hanson, S. L., Simmons, W. B., Webber, K.L., and Falster, A. U., 1992, Rare-earth element mineralogy of granitic pegmatites in the Trout Creek Pass District, Chaffee County, Colorado: Canadian Mineralogist, V. 30, pp 673-686.