Granite Mountain is composed of Paleoproterozoic biotite granodiorite to granite that is known by geologist as the Mint Wash Granodiorite. The Mint Wash Granodiorite consists of medium to coarse-grained, weakly to strongly porphyritic granite with phenocrysts of gray to pink potassium feldspar up to in length and as much as eight percent biotite that is less than in size. This granite grades laterally into a fine grained, equigranular leucogranite with less than two percent biotite that is less than in size. The Mint Wash Granodiorite is almost everywhere undeformed. The porphyritic granite was named the Mint Wash Granodiorite by DeWitt and others for exposures along Mint Wash east of Granite Mountain. In composition, it is an alkali-calcic granodiorite to granite. Zircons from this granite yielded a U-Pb date of 1680 +/- 16 Ma.