Slocum Mountain and the majority of the surrounding hills are mostly composed of highly weathered Mesozoic-age granitic rock. A few small exposures of Tertiary volcanic rocks occur on Slocum Mountain; much larger exposures of similar rock crop out 10 miles northwest of Slocum Mountain at Eagle Crags. To the south of Slocum Mountain, in the Opal Mountain/Black Mountain area, Miocene volcanic and pyroclastic rocks and younger Pleistocene basalts are abundant.
Turquoise is found only in the oxidized zone of primary copper deposits, usually within about 150 feet of the surface. It almost invariably occurs in arid regions and is generally associated with silicic igneous rocks like granite, rhyolite, and trachyte. Extreme alteration of the host rock combined with copper and phosphorus-rich fluids produces an environment conducive to the formation of turquoise deposits. Slocum Mountain and most of the surrounding hills are predominantly composed of highly weathered granitic rock.