The Schwartzwalder uranium deposit. I: Geology and structural controls on mineralization.

A detailed study of the geology of the Schwartzwalder uranium deposit in the eastern foothills of the Front Range of Colorado reveals several geologic features that, in combination, provided a much better setting for uranium deposition than at other sites where much smaller deposits were formed. The Proterozoic host rocks for the Schwartzwalder deposit include garnet-biotite gneiss and quartzite that form a thin transition zone between regionally extensive mica schist and hornblende gneiss units. These foliated rocks were folded into a nearly vertical isoclinal synform that locally doubled the thickness of the transition zone. Subsequent fault movement during Proterozoic, Paleozoic, and Laramide tectonism generated a complex, three-part fault system that obliquely transected the nose of the synform. The steeply dipping fault system cut the rocks of the transition zone at the axial trough of the fold. This generated a very deep but laterally narrow zone of open breccia in the brittle rocks of the transition zone that was bounded by gouge-filled fault zones in the mica schist and hornblende gneiss units. This open conduit provided an excellent avenue for hydrothermal fluids which subsequently formed the uranium deposit.A comparison of the geology of the other uranium deposits along the foothills with that of the Schwartzwalder deposit demonstrates that the smaller sizes of the other deposits may be attributed to the absence of one or more of the lithologic or structural elements that are present at the larger deposit. Although all of the deposits are along concurrently active fault zones, and all are within the same metasedimentary and metavolcanic terrane, geologic variations at each site determined the extent of ground preparation and continuous open channels along which hydrothermal fluids ascended to form the uranium deposits.