Geology of the Lincoln Area, Lewis and Clark County, Montana

Abstract

The Lincoln area (townships 13 and 14 N, ranges 7 and 8 W, about thirty miles northwest of Helena, Montana) is underlain by Pre-Cambrian Belt sedimentary rocks intruded by late Cretaceous (?) granitic stocks with concomitant widespread contact metamorphism and mineralization. The granitic stocks are probably related to the Boulder batholith. The pre-intrusion structure is characterized by high angle faults and broad open folds of Cretaceous age (Laramide). Oligocene (?) volcanic rocks were extruded on an eroded surface of the Belt rocks and granitic stocks. A second period of mineralization followed extrusion of the volcanic rocks. Fracture cleavage which dips consistently to the southwest as well as the overall structure show that a southeast plunging syncline which marks the north end of the Boulder batholith continues into the Lincoln area. The syncline extends at least twenty miles north of the batholith and dominates the structure over an area of about 350 square miles. About forty square miles of middle Tertiary volcanic rocks are composed of a lower series of andesitic to latitic flows and an upper series of rhyolitic welded ash flows. The features of the welded ash flows suggest that they were deposited in part by a vesiculating mass of rhyolitic magma (pumice froth flows). The volcanic rocks are presumably about the same age as the Lowland Creek volcanics of the Butte area. The area and the region several miles to the north are about the northern limit of Boulder batholith activity, Tertiary volcanism, and associated mineral deposits. The superposition of these two periods of igneous activity and their gross similarities imply that they are genetically related. Gold and silver have been produced from epithermal fissure veins. The scant available data suggests that the veins are vertically zoned. There were probably at least two periods of epithermal mineralization: one during the late stage cooling of the stocks, and a second after extrusion of the lower volcanic series. Remnants of Tertiary surfaces preserved under the volcanic rocks imply that there have been topographic inversions since the middle Tertiary. Glacial deposits suggest at least one early period of valley glaciation and later, perhaps recent, periods of restricted mountain glaciation. Rich gold placer deposits, such as in McClellan Gulch, accumulated after the earliest period of valley glaciation.