Uranium minerals occur with ores of copper, tin, and silver at Majuba Hill, Nevada. During World War I, the Majuba Hill mine
produced about 4,000 tons of 12 percent copper ore, and during World
War II about 23,000 tons of ore containing 2 to 4 percent copper and
enough tin ore to furnish from 10 to 15 tons of metallic tin. No
uranium has been produced.
The bedrock geology of Majuba Hill consists of a complex;
volcanic neck or plug composed of rhyolite porphyry, porphyritic
rhyolite, quartz feldspar porphyry, breccia dikes, and irregular breccia
masses. This plug intruded Triassic (?) sedimentary rocks, chiefly shale.
The principal loci of ore deposition were (l) parts of the
breccia bodies, particularly where they were porous and where they were
silicified and tourmalinized, and (2) a few small pre-mineral fractures
adjacent to the breccia masses, Intense hydrothermal alteration resulted
in the formation of sericite, quartz, and tourmaline in the wall rocks.
This was accompanied or followed by deposition of arsenopyrite, pyrite,
cassiterite, fluorite, and several primary sulfide minerals of copper.
All of the ore bodies exposed in the mine are highly oxidized,
and a profusion of secondary minerals of copper and iron have been found.
The only uranium minerals detected are the phosphates torbernite and
matatorbernite and the arsenate zeunerite. These are widely disseminated
throughout the mine in small amounts associated with the secondary
minerals of copper and iron along minor fault surfaces, bedding planes in
the shale, and in porous parts of the breccia. A primary uranium mineral
has not been found.
The secondary uranium minerals appear to be most abundant
where copper ore is highest grade. Although unoxidized uranium ore
has not been found, it is inferred from this relation that the deposition
of the original uranium minerals favored those areas where primary copper
mineralization was most intense. There may have been an intimate association
of uranium with silver in the ore, but this has not been established.
The U3O8 content of samples of copper ore ranges from .0.002 to 0.30 percent.
Because the bottom of the oxidized ore has not been reached in
mining or exploration, the downward extensions of the known ore bodies
of secondary minerals would appear to be worthy of investigation to determine
the grade and extent of the primary ore.