REE-MINERALS AT THE PETACA PEGMATITE DISTRICT, NEW MEXICO

The Petaca Pegmatite District, in north-central New Mexico, contains at least 88 individual pegmatite bodies in an area about 7 km wide by 24 km long. The bodies are zoned granitic pegmatite sills, dikes, and other cross-cutting forms, ranging from a few meters to large deposits such as the Kiawa Mine, with an outcrop of over 450 x 80 m. Many of the pegmatites were mined for sheet and scrap mica from about 1870 through the end of World War II, with some mining continuing at the Globe mine into the late 1950’s and perhaps as late as 1965. The Petaca deposits are classified as rare-element pegmatites, and with their high concentration of Y- and Nb-bearing minerals, amazonitic potassium-feldspar, and common occurrence of fluorite as an accessory mineral, are considered to be NYF-type.

REE-bearing Nb-Ta oxides are among the more interesting minerals that occur in the Petaca district; such minerals previously reported include those from the euxenite, gadolinite, aeschynite, fergusonite, and samarskite groups. However, the identities of many of these minerals or mineral groups are sometimes tentative, and for many years, published information simply classified all black, metamict minerals as “samarskite.” Due to the uncertainty of which REE-bearing Nb-Ta oxides actually occur in the Petaca District, we are in the process of conducting a district-wide survey of the Petaca REE minerals. In addition to Nb-Ta oxides, samples of other minerals, including monazite, columbite-tantalite, and xenotime have been collected and analyzed by electron microprobe. 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), britholite-(Ce), microlite, pyrochlore, and uranmicrolite.

Accessory minerals from many of the pegmatites contain considerable Th, U and REE. For example, monazite exhibits as much as 17 wt% ThO₂ and samarskite up to 19 % UO₂. Monazite is enriched in most LREE (La, Ce, Nd, Sm and Gd). Conversely, polycrase/euxenite is enriched in HREE, especially Dy and Yb. The district displays differentiation of Ta/Nb and Mn/Fe in columbite between individual pegmatite bodies. In general, deposits in the southern portion of the district display higher Ta/Nb and Mn/Fe ratios than those in the north.