INVESTIGATION OF COMPLEX PEGMATITES, BLACK HILLS, SOUTH DAKOTA

Within the Black Hills of South Dakota there are hundreds of pegmatites of complex mineralogy. The Cerny-Ercit classification (2005) separates pegmatites into 5 classes, 3 of which are interpreted as mid-crustal: muscovite, muscovite rare-element, and rare-element. Muscovite-class pegmatites are interpreted to be anatectic melts, whereas those in the rare-element class are interpreted as associated with large granitic intrusions. Muscovite rare-element pegmatites are ill-defined and possess characteristics of both the rare-element and muscovite classes. A weakness with the Cerny-Ercit classification is that it lacks explanation for the petrogenesis of muscovite-rare-element pegmatites. This project investigates field relationships for muscovite rare-element and rare-element pegmatites both present in the Black Hills, in order to understand the processes responsible for their petrogenesis.

This project investigates the orientation of the pegmatite with respect to the structure in the country rock, the contact relationships between pegmatite bodies and country rock, and the mineralogy and textures of the pegmatite bodies. In all cases rare-element pegmatites were concordant to the structure in the host rock and contained only minor migmatization of the country rock if at all. Muscovite rare-element pegmatites, however, were both concordant and cross-cutting, and they are associated with significantly more migmatization and late porphyroblast growth in the country rock. The mineralogy in pegmatites from these two classes are similar in the presence of rare-element minerals (e.g.: cassiterite, lepidolite, spodumene, beryl and tourmaline) , yet increased grain size and concentration of muscovite and schorl near the contacts with migmatized country rock, and schorl grains oriented perpendicular to the contact, are common in muscovite rare-element pegmatites.

Similar rare-element compositions in both pegmatite types suggest both are related to a large granite intrusion. However, migmatized country rock and textures near margins in muscovite rare-element pegmatites may be a result of mobilization of elements from the partially melted country rock and contamination of the pegmatite margins. If so, the composition of country rock intruded may control which type of pegmatite forms.