New Mexico Mineral Symposium — Abstracts

In 2011, we reported in this conference the initial results of a project to identify some of the complex rare earth element (REE) minerals from the Petaca District pegmatites (Spilde et al., 2011). Several of the important historical publications on this district, e.g. Jahns 1946, simply referred to the dark glassy minerals collected from these mines as samarskite. The rationale for our project was to use modern instrumentation to analyze and finally identify what we believed were misidentified specimens. At the time of our 2011 paper, we had sampled only a few of the pegmatites and had already identified several minerals new to the district or even to New Mexico. There are 88 mines and prospects at Petaca, and Jahns (1946) and his co-workers examined 76 during their fieldwork. Nearly all of these sites have now been visited by the authors, in addition to some not examined by Jahns. Most, but not all, locations yielded samples of complex REE minerals and/or monazite-Ce. Thus, with this paper, we present an update on this continued work.

The Petaca pegmatite district is located in north-central New Mexico in Rio Arriba County and lies within the Carson National Forest. The district forms a north trending belt about 7.2 kilometer wide by 24.1 kilometer long. Many of the individual pegmatites were mined for sheet and scrap mica from about 1870 through the end of World War II (1945), with perhaps one of the deposits mined as late as 1965. They are considered rare-element pegmatites, and with their Y- and Nb-bearing minerals, amazonitic potassium-feldspar, and common occurrence of fluorite as an accessory mineral, fall within the NYF (Niobium-Yttrium-Fluorine) category of pegmatites (London 2008).

In general, a border zone, wall zone, usually one or more intermediate zones, and a core zone usually comprise the pegmatites. The mineral composition is mainly microcline (perthite), quartz, albite, and muscovite, which occur as medium- to coarse-grained aggregates with granitoid textures in wall zones, and as large or giant anhedral to euhedral crystals in intermediate zones. Replacement bodies and fracture fillings of quartz, albite and muscovite are common. Accessory minerals include spessartine, fluorite, columbite-tantalite, monazite-(Ce), ilmenite, and less commonly, beryl, bismutite and various oxides that contain rare earth elements (REE), Y, Nb, Ta, and/or Ti. Some fluorite specimens from the Petaca district, particularly those from the Globe mine, exhibit extraordinary phosphorescence, glowing for 8 minutes or longer after short-wave ultraviolet light is removed. Several of the accessory minerals are radioactive due to essential uranium or thorium, or because they contain inclusions or fracture fillings of radioactive species. During our fieldwork in the district, radioactive minerals were collected using radiometric (scintillation) detectors.
To date, Y-REE-Nb-Ta-Ti oxides and other minerals have been collected from 32 individual pegmatites and analyzed by electron microprobe. Several new minerals were identified or confirmed for the district, including euxenite-(Y), samarskite-(Y), polycrase-(Y), xenotime-(Y), fergusonite-(Y), betafite, microlite (and variety uranmicrolite), tantalum-bearing rutile (variety strüverite), and pyrochlore. See Table 1 for formulas.

Monazite-(Ce) occurs as blocky masses and crystals weighing up to a half kilogram, and is the most abundant of the REE-bearing minerals in the Petaca district. Xenotime-(Y) was observed as vein-like alterations of and as inclusions in monazite-(Ce). Thorite, containing significant phosphate, was found at the Coats mine, and a sample likely to be thorite based on gamma spectroscopy was also found at the Apache mine. Thorite also occurs as minute inclusions in monazite-Ce at the Coats, Fridlund, La Paloma, and North Star deposits. Zircon was found at the La Paloma pegmatite as millimeter-sized crystals in quartz, associated with samarskite and bismutite. Uranmicrolite, associated with strüverite, occurs at the La Jarita pegmatite in cm-sized brownish-amber masses, and visually similar material of the same size was collected at the La Paloma. All specimens of columbite-tantalite analyzed in our study were found to be columbite-Mn.

The Y-REE-Ta-Nb-Ti oxide minerals examined in this study are highly complex. Differentiating between Y-REE-Ta-Nb-Ti oxide minerals is hampered not only by the complex chemistry of the minerals, but also because they are often metamict and display weak or no X-ray diffraction patterns. Furthermore, gross similarity between mineral formulae (e.g. samarskite vs. euxenite vs. fergusonite) adds to the confusion. The mineral composition reflects an evolving melt chemistry during crystallization, which was overprinted by late-stage igneous processes, and finally by hydrothermal alteration. Chemical analysis is required to classify many of these minerals, and even that requires statistical analysis methods to properly identify them. Nevertheless, the Petaca District is yielding a diverse array of rare and unusual minerals. Further exploration will undoubtedly uncover additional rare-element pegmatites and add new species to the growing list of minerals from this district.

References:

1. Jahns, R.H., 1946, Mica deposits of the Petaca district, Rio Arriba County, New Mexico: NM Bureau Mines and
   Mineral Resources, Bulletin 25, 294 p.
2. London, D., 2008, Pegmatites, Mineralogical Association of Canada, Special Publication 10, 347 p.
3. Spilde, M.N., Dubyk, W.S., Salem, B., and Moats, W.P., 2011, Rare and unusual minerals from the Petaca Pegmatite District of New Mexico, Abstracts volume of 32nd Annual New Mexico Mineral Symposium, 5–6.