RARE-EARTH ELEMENT DISTRIBUTION IN THE MOHAVE COUNTY PEGMATITES, NORTHWESTERN ARIZONA

Mohave County pegmatites, located in northwestern AZ, can be divided into two geographical groups. The first lies just northwest of Kingman in the Cerbat Range and the second approximately 65 km southeast in the Aquarius Range. Pegmatites from both groups are zoned with a composite quartz-feldspar core. REE-bearing mineral assemblages show that pegmatites in the Cerbat Range are extremely light-rare-earth element (LREE) enriched whereas those to the southeast are less REE enriched with heavy-rare-earth-elements (HREE) slightly dominant.

In the Cerbat Range, HREE bearing minerals are conspicuously rare to absent. The Kingman Feldspar Mine exposes a portion of a large, NE trending, ~400 m long, sill-like pegmatite. Allanite-(Ce), the only REE mineral present, occurs as large, anhedral to subhedral crystals up to 50 cm in length that contain micron scale thorogummite inclusions. These crystals are present in the quarry walls as well as the dumps. Just to the west, a prospect pit exposes a small portion of another pegmatite. Two large (~30 cm) monazite-(Ce) pods are exposed in the western wall of the cut. A small (1 cm) aeschynite-(Y) mass from this quarry was the only HREE mineral recovered from the Cerbat pegmatites.

REE-bearing minerals are less abundant in the Aquarius Range pegmatites which occur as east trending dikes ranging in thickness from < 1 meter to ~15 meters. The Rare Metals mine is host to several centimeter masses of euxenite-(Y), polycrase-(Y) and less abundant monazite-(Ce). Several smaller nearby pegmatites contain rare, several cm crystals of xenotime-(Y), ilmenorutile, and monazite-(Ce). Inclusions of xenotime-(Y) (< 1 mm) are present in the monazite-(Ce).

These pegmatites are hosted by early Proterozoic granitic rocks that are exposed on mountain ranges formed during Basin and Range extension. Pegmatites exhibit distinctly different REE chemistries with the western group exhibiting extreme LREE enrichment and the southeastern group showing a more typical, slightly HREE enriched distribution. Additionally, the eastern pegmatites display late stage Ti enrichment as indicated by the presence of Ti-rich REE oxides. These pegmatites, similar in age and in close geographic proximity (prior to extension), have diverse REE distributions suggesting heterogeneity of the Precambrian host granite.