MINERALOGY OF PRIMARY PHOSPHATE MINERALS IN LI PEGMATITES OF THE KINGS MOUNTAIN DISTRICT, NORTH CAROLINA

Earlier studies (Stander et al, 2011; Cameron et al., 2011) of Li pegmatites in the Kings Mountain Belt of North Carolina revealed several primary phosphate minerals included in the feldspar-quartz-muscovite-spodumene-beryl matrix of the ore-bearing pegmatites. Secondary phosphate phases occur as vein-fillings and as coronas on the primary phosphates. The purpose of this study is to determine the mineralogy of the primary phosphate phases in samples from the Foote Mineral Mine [FMM] and Hailman-Beam Mine [HBM]. Phosphate phases were judged to be primary based on their grain size and inclusion within the silicate fabric.

Compositions of phosphate phases were determined by electron microprobe at UGA. Phosphate phases stand out against the background of silicate minerals on back scattered electron images. Some of the phosphate phases were sensitive under the electron beam and analytical conditions (15 Kv, 4nA, 5 micron spot size) were adjusted to minimize sample damage.

Four different phosphate minerals were identified as primary phases. Fluroapatite is the most common phosphate and occurs in the amphibolite country rock, barren and ore (spodumene-bearing) pegmatites. Manganese-bearing phopsphates are restricted to the ore pegmatites. Mn-fluroapatite (1.0 to 3.9 wt % MnO) occurs in all of the ore pegmatities. Fairfieldite (no F, Mn/Mn+Fe = 0.5 - 0.9) occurs with triphylite (no Ca or F, Mn/Mn+Fe = 0.2 - 0.5) in the HBM spodumene-bearing pegmatites, but only one sample from the KMM contains triphylite (Mn/Mn+Fe = 0.3). Textural relations show Mn-fluroapatite overgrowths on triphylite and veins of fairfieldite cutting triphylite.

The occurrence of triphylite is an excellent guide to spodumene-bearing pegmatites in the HBM. The scarcity of triphylite in the KMM samples may be related to alteration associated with the more fractured character of the KMM pegmatites (see abstract by Hogan et al, this meeting) and the abundance of secondary phosphates identified in the FMM.