EVALUATION OF FORMATIONAL CONDITIONS FOR A LI-RICH PEGMATITIC MICA

A pink-to-red lithium mica-rich pegmatite from the Keystone District, Black Hills, South Dakota, was analyzed via SEM and microprobe to obtain precise chemical compositions with the goal of estimating conditions of formation. The sample is a coarse-grained pegmatite, comprising approximately 70% mica, 25% quartz, and 5% accessory minerals, including zircon, orthoclase, rutile, magnetite, and monazite. The micas range in color from light pink in the main body to deep cranberry red with black magnetite flakes edging the white quartz veins. Further analysis through XRD shows the mica contains high amounts of lithium with trace amounts of manganese, resulting in the red color of the lepidolite.

Calculated mineral formulae for the accessory minerals includes: zircon (Zr_.83Ti_.10K_.06) (Si_.83Al_.14)O ₄, monazite (Ce_.51La_.37Nd_.12) (P,Si,Al), rutile (Ti_.92Fe_.04K_.04) O₂, and magnetite (Fe_.95Mn_.03Ti_.02)₃ O₄.

Data suggests a late-stage, vapor-rich magma, high in incompatibles Li and Zr, with smaller amounts of K, Na, Ce, and La. Magnetite and rutile suggest an excess of Fe and Ti in the magma.

Pressure-Temperature conditions can be calculated from the assemblages in the trace minerals and extrapolated to suggest possible other mineral assemblages existing in the same pegmatite.