New Mexico Mineral Symposium — Abstracts

The Petaca district, located in north-central New Mexico in Rio Arriba County, lies within rugged, forested country that is a part of the Carson National Forest. The district is accessed by unpaved roads extending from New Mexico state highways 111 and 519. Granitic pegmatites in the Petaca district were mined for sheet and scrap mica from about 1870 through the end of World War II (1945), and perhaps as late as 1965. Mica may have been recovered from mines at the Cribbenville town site (abandoned) as early as the 17th century, which may represent the earliest systematic effort to recover sheet mica in the United States (Jahns 1946). Although mica (muscovite) was the chief commodity produced, small amounts of beryl, monazite, columbite-tantalite, and rare-earth-element (REE) bearing niobium (Nb)- tantalum (Ta) minerals were also recovered. The Petaca district offers the collector the opportunity to find minerals in which REE, as well as uranium and thorium, are essential constituents. The acquisition of such rare and unusual minerals is becoming increasingly popular among mineral enthusiasts.

The Petaca pegmatites are classified as rare element pegmatites and are found in an area about 1 to 4.5 mi wide by 15 mi long. The pegmatites form dikes, sills, pods, and irregular bodies that intrude Paleoproterozoic metasedimentary and metavolcanic rocks of the Vadito Group and were emplaced approximately 1400 Ma. The Precambrian terrane hosting the pegmatites is surrounded and locally covered by volcanic and volcaniclastic rocks of Cenozoic age. The Petaca pegmatites, with their high Y- and Nb-bearing minerals, amazonitic potassium-feldspar, and common occurrence of fluorite as an accessory mineral, fall within the NYF type.

In general, a border zone, wall zone, usually one or more intermediate zones, and a core zone generally comprise the pegmatites. Replacement bodies of albite ± muscovite and fracture fillings of quartz ± albite are common. The pegmatites are composed chiefly of 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. The intermediate zones are usually dominated by only one or two minerals, whereas the pegmatite cores are usually composed of quartz, or less commonly quartz + microcline. The common accessory minerals are spessartine, fluorite, columbite-tantalite, monazite-(Ce), beryl, ilmenite, and bismutite.

REE-bearing Nb-Ta oxides are among the more interesting minerals that occur in the Petaca pegmatites; such minerals previously reported from the district include those from the euxenite, gadolinite, aeschynite, fergusonite, and samarskite groups. However, the identities of many of these minerals or mineral groups are tentative, and most are unsupported by published data. For convenience and for practical reasons, Jahns (1946) simply classified all black, metamict minerals he observed as "samarskite" in his classic study of the geology and the economic potential of the Petaca pegmatites.

Due to the uncertainty of which REE-bearing Nb-Ta oxides actually occur in the Petaca district, a number of pegmatites were examined and suspect samples of REE-bearing Nb-Ta minerals collected. Such samples were rare and were found at only a few of the deposits. The suspect samples were analyzed by electron microprobe to identify composition and mineral type and to determine the relative abundances of REE. Of the pegmatites sampled, REE-Ta-Nb minerals were found in six of them, but only the Bluebird, Fridlund, and La Paloma pegmatites provided coarse specimens, where they occurred as masses of dark-brown to black, glassy minerals up to several centimeters in diameter. Minute inclusions of REE-Ta-Nb minerals were found in columbite and other minerals at the Alma, Nambe, and North Star pegmatites.

Based on the results of our microprobe study, several new minerals were identified or confirmed for the district, including euxenite-(Y), samarskite-(Y), polycrase-(Y), xenotime-(Y), and microlite. However, yttrotantalite, fergusonite, gadolinite, and aeschynite were not found, and thus, could not be confirmed. Polycrase-Y is a new mineral for the state, and was collected in situ at the Bluebird mine in the central part of the district.

Samples of other minerals, including monazite, columbite-tantalite, and fluorite, were also subjected to electron microprobe analysis. Monazite-(Ce) was found as reddish-brown or salmon-colored, blocky masses and crystals weighing up to a half kilogram, and is probably the most abundant of the REE-bearing minerals in the Petaca district. Xenotime-(Y) was observed as vein-like alterations of monazite and as sub-millimeter inclusions in monazite and Y-rich fluorite. Inclusions of pyrochlore were found at the Fridlund pegmatite and microlite at Nambe and La Paloma pegmatites.

A kilogram mass of yellowish-brown, pulverulent (i.e., powdery) thorite, containing significant phosphate, was found at the Coats mine. The PO4-rich material occurs in veinlets scattered throughout the larger thorite sample. Thorite also occurs as inclusions in monazite at the Coats, Fridlund, La Paloma, and North Star deposits, where the inclusions range from individual millimeter size to swarms of micrometer-sized masses. Zircon was found at La Paloma as millimeter-sized black crystals in quartz, associated with samarskite and bismutite. All specimens of columbite-tantalite analyzed in our study were found to be columbite-Mn.

Specimens of columbite-tantalite and bismutite continue to be found by collectors at many of the pegmatites, including a recently collected partial crystal of columbite-tantalite weighing in excess of 3.6 kilograms. Microcline, variety amazonite, is locally common at several pegmatites near and at the Mica Lode and Vestegard mines. Galena, associated with fluorite, was found loose in soil at the North Star mine.

Additional investigation of the mineralogy of the Petaca pegmatites is ongoing, with emphasis on the REE-bearing mineral species.

References:

1. Jahns, R. H., 1946, Mica deposits of the Petaca district, Rio Arriba County, New Mexico: New Mexico Bureau Mines and Mineral Resources, Bulletin 25, 294 pp.