HELENA-LAST CHANCE DISTRICT
The Helena-Last Chance district, in the southern part of Lewis and Clark County, in and around Helena, the capital of Montana, includes the famous Last Chance Gulch placer deposits, among the richest and most productive in Montana. Placer gold was discovered in Last Chance Gulch in 1864; other placer deposits in neighboring gulches were discovered soon aftej, and in the fall of the same year lode gold was discovered at the Whitlatch-Union mine, the most productive lode in the district. Gold accounts for more than 99 percent of the total value of the mine production from the district.
Most of the rich placers and lodes were mined out before 1900, and mining operations after that time were intermittent and on a small scale until 1934, when the gold price increase reawakened activity. During 1935-50 a successful dredging operation yielded considerable gold. Lode mines were also reactivated and were productive to 1940 but declined thereafter (Lyden, 1948, p. 56-57). There was no recorded production from lodes or placers during 1954-59.
The estimated value of the early placer gold production, most of which was taken out before 1868, ranges from $10 to $35 million (Knopf, 1913, p. 15, 86). Pardee and Schrader (1933, p. 186) credited the district with a placer production of $17,079,000 (826,275 ounces) and with lode production of $6,304,000 (305,000 ounces) from 1864 through 1928. From 1929 through 1959 the district produced about 40,120 ounces of lode gold and about 110,600 ounces of placer gold. Thus the total production of the district through 1959 was at least 345,000 ounces of lode gold and about 940,000 ounces of placer gold.
The Helena-Last Chance district lies along the north edge of the Boulder batholith, a mass of quartz monzonite and related rocks of Late Cretaceous or early Tertiary age which has intruded a thick sequence of sedimentary rocks of late Precambrian, Paleozoic, and Mesozoic age and volcanic rocks of Late Cretaceous age. Other igneous rocks in the district are porphyry dikes and sheets of pre-Tertiary age and small intrusive masses, lava flows and tuffs of rhyolite, all of Miocene age.
The sedimentary and volcanic rocks were folded into a large dome about 25 miles in diameter which extends beyond the district. The folds were ruptured by the intrusion of the Boulder batholith, and additional faulting occurred as the intrusive forces relaxed.
The ore deposits are mainly near the contact of the batholith with the sedimentary rocks; some are in the granitic rocks, and others are in the adjacent hornfels or tactite. The ore minerals in the contact deposits are pyrite, pyrrhotite, and gold, and local chalcopyrite and galena; they occur in aggregates of lime-silicate minerals, tourmaline, quartz, ankerite, and chlorite.
The Whitlatch-Union lode, the most productive in the district, lies partly in the granite and partly in hornfels. The vein ranges in width from a thin seam to 15 feet and averages about 4 feet. The ore taken out in the early years averaged from $20 to $25 per ton in gold (Knopf, 1913, p. 99).
Page 2 of 6