Abstract

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.

The extent of fluid infiltration into the country rock surrounding the pegmatites and fluid composition can be approximated through trace-element modeling. At the pegmatite contact, less than one equivalent mass of fluid equilibrated with the schist. The pegmatite-derived fluids have relatively high solute concentrations (> 1000 ppm) and exhibit compositional differences between pegmatites. The most noticeable differences in fluid composition are the high B content and low ratio
fH2O/fHF
ratio of fluids derived from the Bob Ingersoll No. 1 and Peerless pegmatites compared to the Etta pegmatite. The low
fH2O/fHF
ratio of the fluid phase may be responsible for the stability of lepidolite and lithia mica relative to spodumene in the Bob Ingersoll No. 1 and Peerless pegmatites.
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