Pegmatite/wallrock interactions, Black Hills, South Dakota: Progressive boron metasomatism adjacent to the Tip Top pegmatite

Interaction between country rock and fluids derived from the Tip Top pegmatite has resulted in a series of boron enriched assemblages. Between unaltered quartz-mica schist to the pegmatite contact is a succession of four mineral assemblages:

• 1.

  (1) Quartz-Biotite-Potassium Feldspar assemblage (Q-B-K), which consists essentially of the original metamorphic silicate assemblage plus anomalously high amounts of modal tourmaline

• 2.

  (2) Quartz-Biotite-Tourmaline assemblage (Q-B-T)

• 3.

  (3) Tourmaline-Quartz-Muscovite assemblage (T-Q-M)

• 4.

  (4) Tourmaline-Quartz assemblage (T-Q). Alkali elements (Cs, Rb, K, Li), SiO₂, and Ba show a decrease from the Q-B-K assemblage to the T-Q assemblage. A1₂O₃, Ga, B, total Fe and Zn increase moderately from the Q-B-K assemblage to the T-Q assemblage.

The mineral chemistries also change considerably. The Mg/(Mg + Fe²⁺) ratios in biotites range from 0.54 to 0.50 in samples from the Q-B-K assemblage to 0.39 in the (Q-B-T) assemblage. The range in tourmaline end-member components from the Q-B-K assemblage to the T-Q assemblage is as follows: Q-B-K: Dravite_.63 Schorl_.23 Elbaite_.05 Buergerite_.09 T-Q: Dravite_.23 Schorl_.37 Elbaite_.17 Buergerite_.23.

Observed variations in mineral assemblage and whole rock chemistry within the alteration zone appear to a first approximation to be a function of μB₂O₃ (boron metasomatism) and μK₂O (alkali leaching). The breakdown of feldspar and biotite may be approximated by reactions: 2HCl + 2(K, Na)AlSi₃O₈ /ai 2(K, Na)Cl + Al₂SiO₅ + 5SiO₂ + H₂O and 2 Annite + SiO₂ + 5Al₂SiO₅ + 2NaCl + 6H₃BO₃ /ai 2 Tourmaline + 2KCl + 7H₂O.

The alteration zone may represent either a single episode (B-, Cs-, Li-, Rb-enriched fluid) or multiple episodes (B, Zn, Mn fluid and Cs, Li, Rb fluid) of pegmatite fluid-schist interactions. In both situations, B in the aqueous fluid from the pegmatite reacts with the schist breaking down sheet silicate “traps” for Cs, Rb, Li, and K and forming tourmaline-rich assemblages.