Chemistry of potassium feldspars from three zoned pegmatites,Black Hills,South Dakota: Implications concerning pegmatite evolution

An initial phase of an extensive geochemical study of pegmatites from the Black Hills, South Dakota, indicates potassium feldspar composition is useful in interpreting petrogenetic relationships among pegmatites and among pegmatite zones within a single pegmatite. The $\text{K}\text{Rb}$ and $\text{Rb}\text{Sr}$ ratios and Li and Cs contents of the feldspars within each zoned pegmatite, to a first approximation, are consistent with the simple fractional crystallization of the potassium feldspar from a silicate melt from the wall zone to the core of the pegmatites. Some trace element characteristics (i.e. Cs) have been modified by subsolidus reequilibration of the feldspars with late-stage residual fluid.$\text{K}\text{Rb}$ ratios of the potassium feldspar appear to be diagnostic of the pegmatite mineral assemblage. The relationship between $\text{K}\text{Rb}$ and mineralogy is as follows: Harney Peak Granite (barren pegmatites) > 180; Li-Fe-Mn phosphate-bearing pegmatites = 90?50; spodumene-bearing pegmatites = 60?40; pollucitebearing pegmatites < 30. Although the $\text{K}\text{Rb}$ ratios suggest that the pegmatites studied are genetically related by fractional crystallization to each other and the Harney Peak Granite, overlapping $\text{Rb}\text{Sr}$ ratios and the general increase in Sr and Ba with decreasing $\text{K}\text{Rb}$ indicate the genetic relationship is much more complex and may also be dependent upon slight variations in source (chemistry and mineralogy) material composition and degrees of partial melting.