Sumdum is a historical mine and community located at the head of Sanford Cove, an inlet on the southern shore of Endicott Arm, approximately 64 miles (103 km) north-northwest of Petersburg and 55 miles (89 km) southeast of Juneau, Alaska. The mining camp hosted a post office from 1897 to 1942, named after the Sumdum Glacier, which is situated 7 miles (11 km) north across Endicott Arm. The name “Sumdum” derives from a Tlingit word, reportedly referring to the booming sound of ice breaking off the glacier. The U.S. Coast and Geodetic Survey first published the name as “Soundon” in 1892. Endicott Arm is a deep and narrow fjord located within the Tracy Arm-Fords Terror Wilderness, which was designated by the U.S. Congress in 1990. The area encompasses 653,179 acres (264,332 ha). During the last glacial period, the fjord was filled by the Brown, North Dawes, and Dawes glaciers. Today, only the Dawes Glacier remains tidal, contributing a substantial amount of floating ice to the fjord from its calving ice face.
The Sumdum mines are situated in the Sumdum Glacier mineral belt, which extends approximately 32 miles (52 km) along the southwest side of the Coast plutonic Complex. This complex is a significant geological feature of the Earth’s crust, stretching nearly 1,100 miles (1,770 km) uninterrupted from the British Columbia-Washington border in the south to the Yukon Territory in the north. It varies in width from about 60 to 120 miles (100 to 200 km). The mineral belt is part of the larger Juneau Gold Belt, believed to have formed during the Cretaceous period. This formation resulted from fluids generated by metamorphism associated with the emplacement of the Coast Range Batholith. The Coast Plutonic Complex is divided by a tonalite sill of Cretaceous or Paleogene age, trending north-northwest. The main part of the complex, lying northeast of the sill, consists of highly deformed gneiss, marble, and some schist. Near the international boundary with Canada, Paleogene-age granodiorite plutons have intruded these rocks. Southwest of the sill, metamorphic rocks are locally intruded by granitic and other igneous rocks. Adjacent to the sill are highly deformed gneiss, schist, quartzite, and calc-silicate rocks. The Sumdum Glacier mineral belt is largely within these formations.
The Sumdum Glacier mineral belt once featured a significant deposit: the gold-bearing quartz veins of the Sumdum Chief gold mine. Before 1905, the mine produced approximately 24,000 ounces (680 kg) of gold from ore containing about 0.4 ounces (13.6 g) of gold per ton, along with a similar amount of silver. The mine’s portal is located roughly 1.8 miles (3 km) south-southeast of the abandoned town of Sumdum on Sanford Cove. Discovered in 1889, the Sumdum Mine comprised two veins: the Bald Eagle and the Sumdum Chief. Initially, these were separate properties but were consolidated into a single mine in 1899. Most mining operations took place from 1895 to 1903, when the ore bodies were exhausted and diamond drilling failed to locate additional ore. The veins were mined from a tunnel 3,500 feet (1,067 m) long. During this period, the mill housed 10 stamps, and at full capacity, it milled 40 tons of rock per day at a cost of 30 cents per ton. A pipe 12,000 feet (3,658 m) in length supplied water to two Pelton wheels, generating 170 horsepower. A wagon road from the mill, along with an aerial tramway to the wharf, was used to transport the concentrates. Read more here and here. Explore more of Sumdum Mine and Sanford Cove here: