PETROLOGY OF SEVERAL PEGMATITES IN THE KEYSTONE DISTRICT, BLACK HILLS, SOUTH DAKOTA

Three pegmatites in the vicinity of Keystone, South Dakota, of simple mineralogy and without concentric zoning have been studied in detail. They occur within Precambrian quartz-mica schist and are peripheral to the Precambrian Harney Peak granite. Two are concordant to schistosity and bedding of the country rock, and the third is sharply discordant. These pegmatites are of very similar average modal and chemical compositions. Samples of the Harney Peak granite from two localities have compositions close to those of the pegmatites. All the compositions fall near the quartz-feldspar field boundary in the “synthetic-granite” system. The average composition of pegmatites and granite is approximately 30 per cent quartz, 44 per cent plagioclase, 17per cent microcline, 8 per cent muscovite, and less than 1 per cent tourmaline, garnet, and apatite.

Large perthite crystals appear to have grown, at least in part, by pushing aside previously crystallized fine-grained quartz and plagioclase. This relationship is compatible with experimental findings which indicate that in a melt of this composition, crystallization, whether of the one- or two-feldspar types, should begin with quartz and a Na-rich feldspar.

At one locality thin, steeply dipping tabular pegmatites are sharply separated into an upper porphyritic perthite-bearing unit and a lower fine-grained albite-quartz unit. The upper, perthite-bearing unit of the layered pegmatites has a composition near that of the “granite minimum” in the “synthetic-granite” system and is thought to represent a rest liquid which has been separated from the phases which crystallized earlier.

The pegmatites and granite are believed to have been intruded as magma derived from a common source.