Copper became the focus of mining in the Santa Cruz Valley and elsewhere in Southern Arizona beginning in the late 1880s. By 1907, Southern Arizona led world copper production. Sporadic prospecting reportedly began in the northwestern portion of the Rosemont property, in the Helvetia Mining District, in the mid-1800s. Production from mines on both sides of the northern Santa Rita Mountains brought forth construction and operation of the Columbia Smelter at Helvetia on the west side of the Santa Rita Mountains and the Rosemont Smelter in the Rosemont Mining District on the east side of the Santa Rita Mountains.