GEOCHEMICAL SURVEY OF PEGMATITES FROM SOUTHERN MAINE

Pegmatites are extremely coarse-grained, igneous rocks characterized by textures in which the crystals are predominantly three centimeters or more in length. They possess a unique petrogenesis, petrology and mineralogy making these rock bodies fascinating to study and economically important to our society. They are the primary source of some rare-elements and provide the world’s largest, most valuable gems. Single pockets within a pegmatite have yielded several million dollars’ worth of gem rough. The factors that allow these rock bodies to grow gigantic crystals and comprise exotic-elements are a combination of a low nucleation rate, high water pressure, high diffusivity, high concentration of volatiles, and a low geothermal gradient.

This study explores how pegmatite minor and trace element geochemistry correlates with the degree of pegmatite evolution. Whole rock, wall zone samples of pegmatites from 17 different locations in southern Maine were analyzed using an ARL-SEMQ Electron Microprobe and a Direct-Coupled Plasma Spectrophotometer.

Results show that rubidium, manganese and cesium correlate positively with pegmatites that contain highly evolved mineral assemblages such as spodumene, lepidolite, spessartite, manganese and tantalum enriched columbite group minerals, pollucite and elbaite tourmaline. Magnesium, iron and potassium negatively correlate with highly evolved pegmatites. In conclusion the more evolved pegmatites in the study area with significant concentrations of rubidium, manganese and cesium are Mount Mica, Mount Marie, Black Mountain Quarry, Bennett Quarry, and the Orchard Quarry. The more primitive pegmatites with high concentrations of magnesium, iron and potassium are the Granitic Roadcut, Songo Pond, Mount Marie, the La Flame Outcrop, and Consolidated Quarry.