Abstract As 1–70 climbs westward from Denver to the Continental Divide, it traverses the Idaho Springs, Central City, Empire, Georgetown, and Silver Plume mining districts, which lie near the northeast end of the Colorado Mineral Belt. Together these districts comprise a zone of nearly continuous mineralization extending about 16 mi (27 km) east-northeast from Silver Plume to Central City. Originally gold, then later Ag, Cu, Pb, Zn, and U were produced from the area, beginning in 1859. Most mines at present are inactive. Primary precious-metal production was dominated by gold, with some silver, from the Idaho Springs and Central City districts to the northeast and mainly by silver from the Silver Plume and Georgetown Districts to the southwest.Mineralization was associated with shallow Tertiary igneous activity in country rockcomposed of Precambrian (middle Proterozoic) granite and gneiss. The Idaho Springs site is one of the few easily accessible locations where the Tertiary intrusive suite and mineralization are well exposed on the surface. The site is a road cut along the north side of 1–70, just west of Idaho Springs (Fig. 1). A sketch, drawn to scale, is provided to show the features that may be observed (Fig. 2) 2). Access to the outcrop is along a 20-ft (6-m) wide service road protected from 1–70 by a guardrail.