REE-MINERALS AT THE PETACA PEGMATITE DISTRICT, NEW MEXICO
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.