Geology and mineralogy of the Apex germanium-gallium mine, Washington County, Utah

Abstract

Abstract The Apex Mine, in Washington County, southwestern Utah, is the first mine in the world to be operated primarily for germanium and gallium. The ore, consisting of goethite, limonite, hematite, jarosite, azurite, malachite, conichalcite, and several other metal oxides, carbonates, arsenates, and sulfates, is thoroughly oxidized to at least 425 m below the surface. Most of the copper-rich ore was removed during previous mining operations, leaving behind the iron-rich minerals that contain most of the germanium and gallium. Germanium is concentrated chiefly in goethite (as much as 0.5 percent), hematite (as much as 0.7 percent), and limonite (as much as 0.5 percent), while gallium is concentrated mostly in jarosite (as much as 0.7 percent), and in some limonite (as much as 2 percent). Copper minerals are concentrated in and adjacent to carbonate rock, apparently replacing it. The ore at the Apex Mine forms an irregular, branching, chimney-like body (or bodies) in fault breccia, gouge, and fissures associated with steeply dipping subparallel fault zones in the Pennsylvanian Callville Limestone. Within and immediately adjacent to the fault zones, limestone has been dolomitized and locally silicified. Only a few small pieces of unreplaced primary sulfide ore have been found; these contain pyrite, galena, sphalerite, chalcopyrite, quartz, and traces of barite. The abundance of copper, germanium, gallium, and arsenic in the supergene ore implies the former presence of sulfides and sulfosalts containing these metals in the primary ore, which would make it similar to dolomite-hosted ores at Kipushi, Zaire; Tsumeb, Namibia; and Ruby Creek, Alaska. High concentrations of germanium and gallium should be looked for in gossans and other oxidized zones of copper-rich, arsenic-bearing sulfide deposits, particularly those in carbonate host rocks.