PETROLOGY OF A TERTIARY DIKE-SILL INTRUSION INTO THE DEADWOOD FORMATION OF THE NORTHERN BLACK HILLS, SOUTH DAKOTA

Tertiary age phonolite dikes occur in the Precambrian and overlying Cambrian Deadwood Formation of the northern Black Hills of South Dakota. The study area is located 4 kilometers southeast of Lead South Dakota on US85. The outcrops at this location contain a steeply dipping porphyritic phonolite dike (>100 meters thick) crosscutting horizontal Deadwood Formation sandstone, siltstone and mudrock. Two sills (0.5 m and 1.4m thick) occur at the margin of the dike and extend out along bedding planes between the sandstones and mudstone layers of the Deadwood Formation. Ten samples were collected at the study area (3 metasedimentary and 7 igneous) for petrographic analysis. Chemical characterization of the minerals using EDAX line scans across the grains to gain a better understanding of the petrogenesis is in progress.

Petrographic examination of the Deadwood sandstone/mudstone, phonolite dike and the sill samples was done to characterize the petrology of the units. The metasedimentary rocks are fine- to medium-grained sandstones and siltstones containing quartz, calcite, plagioclase and muscovite. Texturally the samples show evidence of recrystalization along with metamorphic mineral growth of muscovite and plagioclase. The five dike samples are medium- to coarse-grained porphyritic phonolite containing the minerals; plagioclase, nepheline, aegirine-augite, titanite, apatite and sodalite. The coarse grained phenocrysts are typically plagioclase and nepheline. The two sill samples are highly altered and contain sericite and carbonate and are interpreted to be pseudomorphs of plagioclase and nepheline phenocrysts of similar size and shape to those contained in the porpyritic phonolite samples.