Biotite-Rich Rock in the Gunsight Formation
A thick lens of biotite-rich rock, interlayered with thick beds of cross-bedded meta-sandstone + scapolite of the Gunsight Formation of the Lemhi Group in the Lemhi sub-basin of the Mesoproterozoic Belt Basin. The biotite-rich rock is exposed in a prospect pit above the Sweet Repose mine adit. A claim post in the foreground marks the discovery location for a quartz vein that cuts the biotite-rich rock and contains copper- and cobalt-bearing minerals. USGS intern, Shane Kostka, prepares to collect representative samples of rocks from this site.