Abstract
Tourmaline occurs as an accessory mineral in five of six zones in the Li-, B-, Be-, Nb-, Ta-, and Sn-enriched, internally zoned Bob Ingersoll No. 1 pegmatite located in the Precambrian core of the southern Black Hills near Keystone, South Dakota. The purpose of this investigation is (1) to examine the usefulness of tourmaline as a petrogenetic indicator in this setting and (2) to apply this indicator to interpret the internal evolution of the pegmatite. Tourmaline occurs in ten distinct types based on texture or habit and coexisting mineral assemblage. Chemical analyses of the tourmaline show compositional differences between types that may be useful in determining the crystallization sequence and differentiation mechanisms. Trends of major-element variations in tourmaline from the country rock to the core include the following: (1) Mg and Ti decrease abruptly from the country rock through the border zone to the wall zone; (2) Fe decreases and (Li + Al) increase from the wall zone to the core; and the minor elements Mn, Zn, and Ca generally increase toward the core. Tourmaline compositional trends in this pegmatite, coupled with textural features and mineral associations, provide evidence of an inward, generally sequential crystallization of the border zone, wall zone, intermediate zones, and finally the core, but with overlap between the inner wall zone and intermediate zones. The pegmatite melt was probably saturated at the onset of crystallization; however, fluid exsolution may have temporarily stopped during the initial crystallization of the third intermediate zone and a period of tourmaline instability, but then resumed, along with tourmaline crystallization, during the formation of the pegmatite core.
