Petrology of the Cenozoic igneous rocks of the Tinton district, Black Hills, South Dakota - Wyoming
Date of Award
Master of Science (MS)
The Tinton district is located on the northwest flank of the Black Hills uplift, 20 km west of Lead, South Dakota, along the Wyoming South Dakota border. The Tinton or Nigger Hill uplift comprises a central area of domed Precambrian Xstrata with marginal exposures of Paleozoic sedimentary rocks which dip radially outward on all sides. The area was intruded by Cenozoic igneous rocks that include a stock of quartz latite/quartz trachyte porphyry, an alkalic ring dike complex, lamprophyre dikes and intrusive breccia.
Precambrian rocks are represented by a sequence of metapelites thst have been intruded by metabasite and granitic pegmatite dikes. The schistosity of the metapelites strikes NNW with a consistent westerly dip of 28° to 90°. The metamorphic conditions were determined to be 3.5-4 kbar at 520-540°C on the basis of textures and mineral para geneses of the metapelites and metabasites.
Quartz latite/quartz trachyte porphyry is exposed as large masses in a steep-walled, columnar jointed marginal zone and as high angle sills with parting screens of schist in the central area. The geometric relations with confining rock units and igneous foliation suggests that a stock is present at depth. Major element chemistry indicates a differentiated series with an alkali basalt affinity though a tholeiitic derivation was not disproved. Feldspar structure and chemistry were utilized to trace the fractionation path and to estimate intrusion temperatures.
The intrusion of alkalic lamprophyre dikes post-dates the solidification of the quartz latite porphyry mass. Model mineralogy, textures,and clinopyroxene chemistry indicates an apparent consanguinity between lamprophyre dikes and pyroxenite of the Mineral Hill alkalic ring dike complex. The lamprophyres may have evolved from a volatile rich fraction of the pyroxenite magma.
Late-stage hydrothermal alteration affected all rocks in the Tinton district. A classic alteration halo was observed with progressive intensity of sericitic, propylitic, and pyritic alteration toward the center of the uplift accompanied by intrusive breccia. The chemistry, fabric, geometry, age, and alteration conforms to the porphyry copper environment.
Ray, John T., "Petrology of the Cenozoic igneous rocks of the Tinton district, Black Hills, South Dakota - Wyoming" (1979). Theses and Dissertations. 239.