Trace element concentrations and chemical zoning of spodumene from magmatic and hydrothermal origins

Abstract

Spodumene is a major lithium ore mineral mined from granitic pegmatites and can crystallize (1) from magma, (2) from hydrothermal fluid in miarolitic cavities or fracture-filling units, or (3) as a product of petalite breakdown. Although spodumene’s purity influences the economic value of lithium ore, its trace-element geochemistry is not entirely understood. This study investigated 24 unaltered spodumene samples from 21 localities formed through each of these three modes of origin. The chemical zoning of each spodumene sample was characterized using cathodoluminescence (CL), micro-X-Ray fluorescence mapping (µXRF), and/or qualitative laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) line scans, whereas the concentrations of trace elements were quantified using LA-ICP-MS point analyses. Intra-crystalline chemical zoning was present in all three spodumene types as either oscillatory growth bands and/or sector zoning. For the four most abundant transition and post-transition metals analyzed (Fe, Mn, Ga, Sn), the concentrations ranged from below detection to 10510 ppm (Fe), 1850 ppm (Mn), 300 ppm (Ga), 320 ppm (Sn), whereas Rb and Cs were below detection. Magmatic spodumene had the highest mean concentrations of Fe (2345 ppm) and Sn (94 ppm), hydrothermal spodumene had the highest mean concentrations of Mn (641 ppm) and Ga (94 ppm), and spodumene formed from the breakdown of petalite consistently had the lowest mean concentrations, though there was significant overlap in the datasets. In addition, texturally similar spodumene-quartz intergrowths of primary and secondary origins were distinguished via trace-element geochemistry. Principle component analysis of Fe, Mn, Ga, Sn, Ti, and Zn values from all 325 LA-ICP-MS measurements showed discrimination by CL color, as well as color in handsample for gem-varieties of hydrothermal spodumene.