Fluid exsolution during the crystallization of a volatile-rich pegmatitic melt can result in the formation of extensive dispersion halos around the pegmatite through fluid transport of relatively incompatible elements out of the pegmatite system. The consequence of interaction between pegmatite-derived fluid and quartz-mica schist surrounding selected Black Hills pegmatites (Etta, Bob Ingersoll No. 1, and Peerless pegmatites) is alteration of primary metamorphic mineral assemblages to secondary mineral assemblages and modification of the original compositions of primary metamorphic minerals.
The alteration assemblages are in immediate contact with the pegmatite and consist of either B-rich assemblages (quartz + biotite + muscovite + tourmaline and quartz + muscovite + tourmaline) or aluminous, B-poor assemblages (muscovite + plagioclase + quartz). Both assemblages result from the instability of biotite with increasing B2O3 and/or [Al/(Na + K)].
The modification of mineral compositions through mineral-fluid exchange reactions (e.g., biotite Rb/K, F/OH) results in the formation of extensive exomorphic halos around the pegmatites. The compositional characteristics of the dispersion halos are related to pegmatite mineralogy. The dispersion halo associated with the Etta pegmatite (spodumene-bearing) is enriched in alkali elements (Li, Rb, Cs), As, and U, whereas the dispersion halos associated with the Bob Ingersoll No. 1 and Peerless pegmatites (lepidolite- or lithia mica-bearing, spodumene-absent) are enriched in alkali elements (Li, Rb, Cs), As, U, B, and F. The dispersion halos extend 21 to 90 m from the pegmatite contact along sampling traverses. The relative mobilities suggested from the enrichment of the quartz-mica schist and its mineral components are As = U < B = F < Rb ≤ Cs < Li.