PRELIMINARY GEOCHEMICAL ANALYSIS OF SOILS FORMED ON LITHIUM-CESIUM-TANTALUM PEGMATITES IN OXFORD COUNTY, MAINE

Lithium (Li) is a critical metal that requires additional granitic pegmatites of the lithium-cesium-tantalum (LCT) family to meet demands. Currently, pegmatite exploration in Southwestern Maine relies on drilling. Potentially, soil geochemistry could be used. This study explores the fate of LCT pegmatite pathfinder elements during soil formation processes. The study area is covered by glacial till, but natural soil has formed along slopes directly on pegmatite bedrock. Additionally, technosols developed on historic and active quarries can be identified to have been derived directly from bedrock.

Soil samples from the Plumbago Mtn. near Newry, Maine, were collected at two LCT pegmatite quarries: 1) Plumbago North (PBN), a large spodumene pegmatite, actively mined during the last decade, and 2) Rose Quartz (RQ), a smaller pegmatite, historically mined for pollucite and spodumene. For comparison, we analyzed Fisher Quarry (FQ), a niobium-yttrium-fluorine (NYF) less fractionated pegmatite near Topsham ME, historically mined for feldspar. Soils developed on pegmatites or surrounding micaschists were collected from the organic-rich A horizon; eluvial, leached, gray E horizon; brown B horizon; and debris directly formed on spodumene-rich zone. To quantify the soluble elements, soils were dried, pulverized, and shaken for 24 hours at a sample to pure H₂O ratio of 4:1. After centrifuging and filtering, the leachate solution was analyzed by ion chromatography to quantify inorganic anions and cations, including Li⁺.

PBN samples formed on nonmineralized pegmatite or host rock have low water-soluble Li levels from below detection to ~30 ppb, with one outlier of ~1740 ppb. In contrast, soils formed on PBN spodumene zone have slightly higher average Li levels of ~75 ppb. Weathered debris samples taken directly above a spodumene crystal yielded ~1700 ppb of Li. Soils from the extremely fractionated RQ pegmatite have, on average, ~4740 ppb of Li. FQ averaged 1225 ppb of Li.

Preliminary technosol-leachate data suggest that soluble Li does not always correlate to the abundance of Li minerals in the subsurface or to the soil horizon. The soils are highly heterogeneous. Factors that may explain this variability include: Li retention in clays; scattered debris blasted during mining; age of the technosols, and the vegetation.