The country rock in the area of the Homestake mine is Triassic Texas Creek Granodiorite, which regionally underlies and locally intrudes pelitic metasedimentary and subordinate andesitic metavolcanic strata of the Jurassic or older Mesozoic Hazelton Group; and the Eocene Hyder Quartz Monzonite, which intrudes the Hazelton and Texas Creek rocks (Smith, 1977, Koch, 1996).
The deposit consists of a quartz fissure vein 4-5.5 feet thick in Texas Creek Granodiorite (Buddington, 1925, p. 88-89). The vein was traced in outcrop for about 250 feet. The mineralized parts of the vein contain bands of solid sulfide up to 4 feet thick consisting mainly of argentiferous 'steel' galena, accompanied by pyrite, chalcopyrite, and a little sphalerite. The granodiorite hanging wall of the vein contains stringers of tetrahedrite. So-called 'steel' galena resembles augen gneiss with eyes of granulated pyrite in flow-banded galena; this texture indicates that the mineral deposit was subjected to intense stress since its formation (Buddington and Chapin, 1929, p. 320).
A 9.5-ton test shipment of sorted ore sent to a smelter in 1925 contained 50 percent Pb, 0.7 percent Zn, 22.87 oz Ag per ton, and 0.29 oz Au per ton (Buddington, 1929, p. 95; Cobb, 1978, p. 39).Lead-isotope studies of galena from the Homestake mine (Maas and others, 1929, p. 229-248) indicate that the deposit is Eocene in age, contemporaneous with emplacement of the Hyder Quartz Monzonite.