Abstract

Structurally and petrologically the Black Buttes area is part of a northwest trending zone of Tertiary hypabyssal intrusions in the northern Black Hills. Cambrian to Triassicage sedimentary rocks were domed and broken during Eocene intrusion. The intrusive rock types, each representing a separate intrusive event, include trachyte porphyry, alkali trachyte porphyry, aegerine-augite trachyte porphyry, phonolite, and nordmarkite. Flow structures, defined as those fractures in igneous rock which parallel oriented tabular and platy mineral grains, are usually more closely-spaced than 5 cm. Differential flow of magma caused by rapid chilling along the margin of the intrusive body relative to its center formed planes of shear which are preserved as flow structures parallel to igneous-sedimentary contacts. Because flow layering is oriented parallel to igneous-sedimentary contacts, detailed mapping of these flow structures helps delineate intrusive shape. The largest intrusive body (trachyte porphyry) is located in the northern Black Buttes and has the general character of a laccolith. The principal level of concordant trachyte porphyry emplacement is inferred to be in the thin-bedded carbonates of the upper Deadwood Formation. The domed sedimentary rocks, however, were broken by marginal faulting during continued trachyte porphyry intrusion and by cross intrusion of younger rocks. South of the trachyte porphyry laccolithic body, five smaller uplifts are exposed. Four are domal or anticlinal structures (2.5 to 4 km²) in Paleozoic and Mesozoic sedimentary rock which are inferred to have covered intrusive cores. The fifth is a nearly circular nepheline syenite porphyry body (1.5 km²) in fault contact with the adjacent sedimentary beds. Lead-zinc mineralization is localized along steeply-dipping fractures in carbonates of the Pahasapa Formation immediately above a trachyte porphyry sill.