HETEROGENEOUS TRACE-ELEMENT DISTRIBUTION IN SPODUMENE FROM THE BLACK HILLS, SOUTH DAKOTA AND OXFORD COUNTY, MAINE PEGMATITES

Spodumene (LiAlSi₂O₆), a pyroxene-group mineral and the principal magmatic Li resource, can form meter-long crystals during crystallization of Li-rich pegmatites. Although spodumene purity is important for the extractive industry, its compositional heterogeneities are poorly understood. Trace-element distribution in mega crystals of spodumene can be controlled by crystallographic factors, primary compositional variations and inclusions trapped during magmatic growth, and secondary alteration.

Here we report portable X-ray Fluorescence (p-XRF) quantitative analysis of Fe, Mn, Ga, and Sn in spodumene, calibrated by Laser Ablation ICP-MS on matrix-matched standards. We aim to 1) compositionally map subhedral crystals and 2) discriminate between primary and secondary chemical signatures. Additionally, Scanning Electron Microscopy was used to identify solid inclusions as a potential source of p-XRF chemical anomalies. For comparison, semi-gemmy to highly altered crystals were analyzed from two classic localities in the Black Hills, South Dakota: Tin Mountain (TM) and Etta (ET); and two relatively new lithium prospects from Oxford Co., Maine: Plumbago North (PBN) and Rose Quartz (RQ).

3D primary zoning and alteration effects were studied on 28 x 11.5 cm subhedral PBN crystal, sliced parallel and perpendicular to c-axis along (010) and (001) planes, respectively. P-XRF heatmaps were acquired based on 895 spectra collected along 5x5 mm grids of points. Growth zoning is suggested by Fe-Ga rich, Mn-Sn poor core relative to (110) rims.

Comparative p-XRF results for unaltered spodumene:

Pegmatite field	Spod. crystals	Fe (ppm)	Mn (ppm)	Ga (ppm)	Sn (ppm)
Oxford Co., ME	7	300-3000	300-700	50-80	60-350
Black Hills, SD	6	50-2100	200-500	40-100	140-350

Dull, discolored spodumene has much higher ppm levels of Fe (8000), Mn (2000), Ga (130), and Sn (600). Some anomalous Sn values are caused by cassiterite inclusions seen in SEM. This study shows that p-XRF can be used to map trace elements in spodumene mega crystals and to screen areas of interest for further micro-analytical techniques.