Tertiary rhyolite dikes cutting Precambrian rocks at the Homestake “open cut” in Lead. Notice that the dikes flatten out near the surface where they met the Cambrian Deadwood Formation.
For rockhounds exploring the Hills, the Harney Peak Granite is a paradise since in excess of 20,000 pegmatites have been identified in, and surrounding, the intrusion and these pegmatitic units contain at least 175 mineral species (Gries, 1996)! I have collected in the Hills, off and on, since my graduate school days in the mid 1960’s. My collection is not large, but I find the specimens fascinating (I am easy to please) and will describe a few.
Giant spodumene crystals in the wall of the Etta Mine near Keystone, SD. Note miner for scale. Photo taken in 1904 and courtesy of W.T Schaller and the U. S. Geological Survey archives.
Fragment of spodumene showing greenish sheen. Fragment of pink kunzite, ~3.4 cm, placed on specimen for scale.
Roadcut pegmatite west of Custer displaying large specimens of schorl tourmaline, the largest is ~ 30 cm.
Between miles six and seven west of Custer on US 16 are fantastic exposures of a wavy, shimmering micaceous schist along the west side of the road. At the seven mile mark and west along FDR 287, the schist produces “gemmy, transparent, ruby-red modified dodecahedral crystals of almandite [almandine garnets]” (Roberts and Rapp, 1965). These garnets are the iron-rich end member of a solid-solution series with pyrope garnets having magnesium substituting for the iron. They may be collected in the schist host rock or loose in the associated wreathing product (sediment).
Micaceous schist, nickel for scale. Collected near the garnet locality where the rock unit contains numerous small garnets.
Greenish columns of actinolite with bronze-colored flakes of phlogophite in a white dolomitic marble. Width of specimen ~5.5 cm.
I have additional specimens from the Hills and other postings will be forthcoming.
Arrojadite; A Rare Phosphate---April 2, 2013