North-Central Section - 54th Annual Meeting - 2020

Paper No. 16-20
Presentation Time: 8:30 AM-5:30 PM


GAMET, Nolan George1, DORAN, Krystyna E.1, STUDENT, James J.1 and SIRBESCU, Mona-Liza2, (1)Earth and Atmospheric Sciences, Central Michigan University, 314 Brooks Hall, Mount Pleasant, MI 48859, (2)Earth and Atmospheric Science, Central Michigan University, 314 Brooks Hall, EAS, Mount Pleasant, MI 48859-0000

Lithium (Li) is an economically important element that is on high demand for rechargeable batteries and grid-scale energy storage. LCT-type pegmatites are important resources of Li and other rare metals such as niobium (Nb), tantalum (Ta), and cesium (Cs). LCT pegmatites are typically internally zoned, starting with simple composition in their borders and ending with highly fractionated, rare-metal rich cores. The existence of compositionally-modified boundary layers (BL’s), rich in incompatible elements and depleted in compatible elements that surround rapidly grown crystals, has been postulated for internally-zoned pegmatites. However, the role of BL’s in rare metal ore-forming processes is poorly understood. Here we present preliminary trace element concentrations and distribution in large, magmatic spodumene (LiAlSi2O6) crystals from Tin Mountain (TM), South Dakota and Plumbago North (PBN), Maine to investigate whether crystallization of spodumene generated BL’s in the surrounding melt.

The TM pegmatite is an inclined, pipe-shaped body, 200 x 20 x 30 m in size that has been historically mined for spodumene and Cs in pollucite. The recently discovered PBN pegmatite is an irregularly-shaped, internally-zoned body, 300 x 600 m in surface exposure, that is being evaluated for Li ore potential (Simmons et al., 2019). Both pegmatites contain giant crystals of spodumene in their inner zones, ranging from dm’s to m’s in size, as well as pollucite, beryl, amblygonite, cassiterite, and columbite-group minerals.

We investigated the trace-element distribution along slabbed, crystallographically-oriented, subhedral spodumene collected from TM and PBN pegmatites. We used a Tracer 5i portable X-Ray Fluorescence unit to produce 2D compositional maps for trace elements including Mn, Fe, and Sn. Average Fe2O3 contents (wt%) are lower for PBN (0.22 ± 0.01 N=143) than for TM (0.59 ± 0.20 N=596). Through transmitted light and cathodoluminescence (CL) observations, we documented the effects of microscopic inclusions and fractures, which yielded chemical anomalies of apparent secondary nature. Using these techniques, we were also able to reveal the presence of chemical heterogeneities resembling growth zones, suggesting the existence of BL’s in large to giant spodumene crystals.