Geology and mineral deposits of the Minnie Moore and Bullion mineralized areas, Blaine County, Idaho

In the early 1880?s the discovery of rich ores in the Minnie
Moore and Bullion mineralized areas sparked a rush to settle and
develop the Wood River valley. Silver and lead discoveries in
these areas spurred the boom in mining after completion of the
Oregon Short Line Railroad to Hailey in 1883.
In both areas the ore comprises galena, sphalerite, and tetrahedrite
in a gangue of siderite, calcite, or quartz. Minor goldbearing
quartz veins are also present. The ore is in fissure and
replacement veins along fracture systems that formed in Late
Cretaceous time, after intrusion of nearby granodiorite or quartz
diorite stocks. The ore formed under mesothermal conditions
and heat was supplied by the nearby plutons. In the Minnie
Moore area, the mineralized veins are cut by low-angle normal
faults that are of probable Eocene age.
In the Minnie Moore mineralized area, the host rock is the
middle part of the Devonian Milligen Formation, (the informal
Lucky Coin limestone and Triumph argillite), which is the same
stratigraphic level as the host ore in the rich Triumph mine
northeast of Hailey.
In the Bullion mineralized area, the ore is hosted by the
lower member of the Middle Pennsylvanian to Lower Permian
Dollarhide Formation. Rich ore was mined in several tunnels
that reached the Mayflower vein, a northwest-striking mineralized
shear zone.
The deposits are thought to be mainly mesothermal veins
that formed in association with Cretaceous magmatism. The
syngenetic stratiform model of ore formation has often been
applied to these deposits, however, no evidence of syngenetic
mineralization was found in this study. Faulting has displaced
most of the major orebodies and thus has made mining these
deposits a challenge.