The geochemistry, origins and metallurgical implications of different textural types of spodumene-quartz intergrowths (SQUI) from the Tanco pegmatite, Manitoba, Canada

Abstract

The Archean Tanco deposit in southeastern Manitoba is a complexly zoned petalite-subtype pegmatite. Lithium mineralization primarily occurs as spodumene-quartz intergrowths (SQUI), which are historically interpreted as pseudomorphs of petalite breakdown. This breakdown would produce a spodumene-to-quartz ratio of 56.3:43.7 vol%, volume reduction textures, and low trace element contents. However, SQUI at Tanco shows variable spodumene-to-quartz ratios, remarkably little petalite preservation or volume reduction textures and green spodumene with up to 8330 ppm Fe.

Three textural varieties of spodumene-quartz associations were identified and described in zones 45 and 50 of the pegmatite: (1) Classic SQUI with c-axis-oriented, oscillatory-zoned spodumene and quartz, which locally occur within relict crystal outlines; (2) spodumene-quartz symplectites (SQS) characterized by pervasive symplectic intergrowths with elevated Cs and depleted Mn and Sn; and (3) macro spodumene-quartz intergrowths with randomly oriented, stubby crystals and chaotic zonation.

Metastable virgilite exists as a solid solution with quartz and is known to crystallize from experimental pegmatite crystallization studies. The breakdown of virgilite into classic SQUI could explain the lack of petalite and volume reduction textures, variable mineralogical ratios, and iron-bearing spodumene in SQUI. SQS likely resulted from coupled dissolution-reprecipitation in the presence of a fluid, while macro intergrowths represent primary magmatic textures. The occurrence of SQS at Tanco poses a metallurgical issue due to the fine sizes and hardness of the intergrowths leading to Li recovery issues. Understanding the origins of lithium mineralization is important as the quartz-spodumene textural associations have metallurgical recovery implications, affecting the overall economic viability of deposits.