A NEW SPODUMENE-BEARING LCT PEGMATITE OCCURRENCE IN MID-COASTAL MAINE

Lithium-Cesium-Tantalum (LCT) pegmatites are rare, highly differentiated late-stage crystallization products of granitic magmas. These rocks are of scientific interest because they have the potential to provide information on granitic magma evolution. LCT pegmatites are of economic interest as they may contain ore minerals of lithium (spodumene) and tantalum (columbite-tantalite), semi-precious gemstones, and museum quality specimens of rare minerals. Here we report our preliminary findings on recently discovered LCT pegmatite occurrences near the village of Appleton in mid-coastal Maine.

The small pegmatite bodies (< 100 m²) are exposed west of Penobscot Bay, just east of the boundary between the Fredericton and St. Croix terranes. The pegmatites are discordant bodies intruding into steeply dipping, lower amphibolite facies rocks of the Jam Brook Complex. Although the protolith age of these country rocks is uncertain, field relationships indicate the pegmatites post-date regional metamorphism and deformation which is presumably Late Silurian-Early Devonian in age. Small exposures of tourmaline-bearing, two-mica granite, likely related to the Late Devonian Waldoboro Plutonic Complex exposed to the south, are found in the area and offer a possible genetic connection to the LCT pegmatites.

Mineralogically, the pegmatites are composed of quartz, alkali feldspar, euhedral spodumene up to a meter in length, cleavelandite, and white mica. Accessory minerals include columbite (< 2 cm), zircon (< 1 mm), and rare montebrasite nodules up to 6 cm across. One of the well exposed dikes shows evidence of compositional and textural zonation. Along the hanging wall contact is a narrow 1-2 cm wall zone composed of coarse-grained feldspar and quartz. The wall zone grades into a very coarse-grained bimodal core zone (up to 2m) rich in spodumene and quartz. Near the wall zone-core boundary, spodumene pseudomorphs (< 6 cm) oriented perpendicular to the contact have been replaced by white mica. Large spodumene crystals oriented perpendicular to the contact taper into the core zone.

Geochemical studies of both the whole rock composition and individual mineral compositions are underway. Additionally, attempts to determine the crystallization age of the pegmatites using ⁴⁰Ar/³⁹Ar and U-Pb techniques are in progress.