Bedrock along Mammoth Creek is mainly quartzite schist and mica schist intruded by granitic bodies and cut by numerous quartz veins (Prindle, 1905; Mertie, 1938). About 12 feet of locally-derived gold-bearing gravel is overlain by 3 feet of overburden. Gold in upper valley is fairly coarse and light colored. Fineness is about 840, increasing downstream. The source of gold is believed to be quartz veins and mineralized zones in the bedrock (Mertie, 1938). Samples of granite talus contained allanite, galena, molybdenite, scheelite, iron sulfide minerals, garnet, topaz and hematitic copper carbonate minerals (Nelson and others, 1954).

In 1906, a small steam shovel capable of handling 50 cubic yards of gravel per hour was installed on Mammoth Creek, and a 9.5 km long ditch was built in 1908 to bring water from Bonanza Creek for hydraulic mining. The largest hydraulic plant in the Circle district was used in mining the entire length of Mammoth Creek from Mastodon to Porcupine Creeks (Ellsworth, 1910; Ellsworth and Parker, 1911). In 1915 the Berry Dredging Company installed a dredge on the creek. It worked upstream along several parallel paths, was dismantled, carried downvalley, and reassembled several times during the following 35 years (Yeend, 1991). The main channel of the creek is completely mined (Menzie and others, 1983). In the 1980s, as many as four separate mining plants were operating along the creek in an attempt to clean the bedrock better than it had been by dredging, as well as to wash pockets of gravel left unmined along the channel margin. In addition, the thin mantle of colluvium on the channel sides yielded some gold following the stripping of the barren overlying ice-rich muck (Yeend, 1991).