Name applied to a granite penetrated in the subsurface by drilling northeast of the Homestake Mine in Lawrence Co, SD in the Midcontinent region. Named for Crook Mountain, a topographic feature 11 km northeast of the Homestake Mine. Crook Mountain is designated the type area. Has a pegmatitic texture. The CIPW norm is corundum normative; plagioclase composition indicated by the norm is An10. A peraluminous nature, low CaO content, and presence of minor coarse-grained tourmaline combined with a K:Rb ratio of 160 indicates an intermediate di Herentiate from a more primitive granite akin to the Harney Peak Granite of the southern Black Hills. Known only from drillholes; therefore extent and form are poorly understood. Emplaced into previously metamorphosed rocks. The contact metamorphic effects overprinted preexisting regional metamorphic fabric. Early Proterozoic--about 1.72 Ga, the age of Harney Peak Granite --age assigned, although an absolute age cannot be assigned. Probably coeval with the Harney Peak.

Source: GNU records (USGS DDS-6; Denver GNULEX).