The Bureau of Mines, as part of a program to assess Alaskan reserves of certain critical and strategic minerals, has intermittently investigated niobium (Nb) associated with placer gold-tin deposits near Tofty, Hot Springs district, Alaska. These investigations have identified placer and

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lode sources that together contain inferred reserves of approximately 300,000 lb of Nb.

Splits of samples collected by the Bureau of Mines in the early 1950s from tailings piles of now-inactive drift mines were reanalyzed. Most of these samples contain between 0.2 and 4.5% Nb. The presence of relatively higher concentrations of Nb in the Deep Creek-Miller Gulch-Idaho Gulch area suggests proximity to an Nb lode source in that area.

In an effort to locate the lode source of the placer Nb, trenches excavated by the Bureau of Mines in 1956 on upper Idaho Gulch were reexamined. Between 0.02 and 0.10% Nb is present in two lenses of radioactive ferruginous regolith exposed in these trenches. In the regolith, the Nb mineral, aeschynite [(Ce, Ca, Fe, Th)(Ti, Nb)₂(O, OH)₆], occurs with up to several percent apatite and zircon and trace amounts of monazite. The lenses persist 1,200 ft along strike, and drilling shows them to continue 150 ft downdip. Apparently, they are derived from the selective weathering of dolomite marble containing magnetite, pyrite, and pyrrhotite and traces of zircon and apatite. The inferred reserve of the regolith lenses is approximately 200,000 lb of Nb, which is twice that of the inferred placer reserves.

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