ND-ENRICHMENT IN THE KINGMAN FELDSPAR PEGMATITE, MOJAVE PEGMATITE DISTRICT, NORTHWESTERN AZ

The Kingman Feldspar pegmatite, located in the Mojave Pegmatite district in northwestern AZ, intrudes syn- to post-collisional Paleoproterozoic metamorphic and granitic rocks. This sill-like body trends N50-65°E, dips 60-75°NW, is ~460 m in length, and ranges in thickness from 20 to 60 m. This post-orogenic pegmatite is strongly enriched in LREE and depleted in F, Nb and HREE. This is atypical of classic NYF pegmatites and has led to an unusual REE mineral assemblage. Allanite, the only REE mineral present, occurs as large, extensively fractured, pods or crude crystals up to a half meter in size, some with thorogummite inclusions. The individual crystals are dark brown to black, euhedral to subhedral, and reach 25 cm in length. All of the exposed surfaces of allanite are coated with a reddish iron oxide crust and Nd-enriched bastnäsite-(Ce).

EPMA were initially recalculated on the basis of 6 M + T sites and water calculated to yield a total by stoichiometry (OH + F = 1). Subsequently, Fe²⁺/Fe³⁺ was partitioned to yield 25 positive charges. Based on REE distribution, there are currently three allanite species recognized; allanite-(Ce), allanite-(La) and allanite-(Y). Allanite from the Kingman pegmatite is predominantly Nd-rich allanite-(Ce). However, some of the samples are Nd dominant, thus indicating a new species in the allanite group; “allanite-(Nd)”. An average of 5 analysis of a Nd-dominant sample yields the following formula:

(Ca_0.90Mn_0.05□_0.05)_Σ1.00(Nd_0.30Ce_0.24Ca_0.16La_0.15Pr_0.07Sm_0.04Th_0.02Yb_0.01Y_0.01)_Σ1.00 (Fe²⁺_0.99Mn_0.05Mg_0.04)_Σ1.08(Al_1.53Fe³⁺_0.44Ti_0.04)_Σ2.01(Si_2.99Al_0.01)_Σ3.00O₁₂(OH_0.99F_0.01)_Σ1.00

The unusual extreme LREE enrichment and HREE depletion of the Kingman pegmatite may be the result of either (a) having been formed by partial melting of within plate granites where the HREE were sequestered into residual garnet, thus were never present or (b) HREE were partitioned into a late-stage fluid that was subsequently lost to the system. The former process is favored because, while the latter is common in pegmatites, the absence of fluorine to act as a HREE complexing agent, as well as the absence of HREE-enriched minerals in either the quarry or the dump material, suggests that they were never present. The Nd enrichment may reflect the composition of the protolith but is poorly understood.