| Metal deposits spatially associated with the Cretaceous Boulder and Idaho
batholiths of southwestern Montana and southern and central Idaho have
been exploited since the early 1860’s. Au was first discovered in
placer deposits; exploitation of vein deposits in bedrock soon followed.
In 1865, high-grade Ag vein deposits were discovered and remained economically
important until the 1890’s. Early high-grade deposits of Au, Ag and
Pb were found in the weathered portions of the veins systems. As mining
progressed to deeper levels, Ag and Pb grades diminished. Exploration for
and development of these vein deposits in this area have continued until
the present. A majority of these base- and precious-metal vein deposits
are classified as polymetallic veins (PMV) and polymetallic carbonate-replacement
(PMR) deposits in this compilation. Porphyry Cu and Mo, epithermal (Au,
Ag, Hg and Sb), base- and precious-metal and W skarn, W vein, and U and
Th vein deposits are also common in this area. The world-class Butte Cu
porphyry and the Butte high-sulfidation Cu vein deposits are in this study
area. PMV and PMR deposits are the most numerous in the region and constitute
about 85% of the deposit records compiled. Several types of syngenetic/diagenetic
sulfide mineral deposits in rocks of the Belt Supergroup or their equivalents
are common in the region and they have been the source of a substantial
metal production over the last century. These syngenetic deposits and their
metamorphosed/structurally remobilized equivalents were not included in
this database; therefore, deposits in the Idaho portion of the Coeur d’Alene
district and the Idaho Cobalt belt, for example, have not been included
because many of them are believed to be of this type. |
