TEXTURAL AND MINERALOGICAL CHANGES DURING CRYSTALLIZATION OF A LITHIUM PEGMATITE, FLORENCE COUNTY, WISCONSIN, USA

The spodumene-amblygonite bearing King’s-X pegmatite (KX; Falster, 2005) is a 1-3 meter thick dike that intruded Penokean-age amphibolites. The mechanisms of lithium diffusion across more than 50 m in the KX contact aureole have been studied (Liu et al., G³, 2010), but little is known about the internal distribution of lithium. This preliminary petrographic study focuses on a KX pegmatite exposure situated ~400 m south from the outcrop investigated previously. Textural and mineralogical changes during inward crystallization of the KX magma sheet were investigated with emphasis on distribution and post-crystallization changes of lithium minerals.

Internal mineral zoning is well defined but complicated by large screens of country rock included within the dike. The outer zones are symmetrical and include a fine-grained tourmaline-apatite-feldspar border (<1 cm thick) and a coarser comb-textured albite-tourmaline-quartz wall zone (<10 cm thick). The asymmetrical inner zones are more variable along strike. They include a <70 cm thick, K-feldspar-quartz intermediate zone with coarse crystals of spodumene, beryl, and amblygonite and a ~80 cm thick K-feldspar-spodumene-quartz core zone. A late unit of spherulitic tourmaline in a fine albite-lepidolite matrix crosscuts the preexisting intermediate zones.

Outer-zone textures such as comb and radiating tourmaline and albite, skeletal myrmekite-like lepidolite-albite, and quartz-albite intergrowths suggest rapid cooling against country rock and screens. Additional comb and spherulitic tourmaline and albite were noted in the inner zones, indicating that nucleation and growth kinetics varied during sequential crystallization. Spodumene, the main lithium mineral at KX occurs as fresh, < 10 cm long laths rimmed by graphic lepidolite-albite intergrowth. Spodumene can be partially replaced along thin fractures by a fine mixture of phyllosilicates. Amblygonite (Li-Al phosphate) crystals are <10 cm long, relatively fresh, and rimmed by Li±Fe±Mn±Ca phosphates. The abundance of lithium minerals throughout the KX mineral zones, lack of deformation, and good preservation of magmatic textures warrant a further geochemical study to elucidate the partitioning of Li and Li isotopes between magma, fluids, and minerals in a rapidly cooled system.