Alteration and vein mineralization, Schwartzwalder uranium deposit, Front Range, Colorado

Abstract

The Schwartzwalder uranium deposit, in the Front Range west of Denver, Colorado, is the largest vein-type uranium deposit in the United States. The deposit is situated in a steeply dipping fault system that cuts Proterozoic metamorphic rocks. The host rocks represent a submarine volcanic system with associated chert and iron-and sulfide-rich pelitic rocks. Where faulted, the more competent garnetiferous and quartzitic units behaved brittlely and created a deep, narrow conduit.The ores formed 70-72 m.y. ago beneath 3 km of Phanerozoic sedimentary rocks. Mineralization included two episodes of alteration and three stages of vein mineralization. Early carbonatesericite alteration pseudomorphically replaced mafic minerals, whereas the ensuing hematite-adularia episode replaced only the earlier alteration assemblage. Early vein mineralization produced a minor sulfide-adularia-carbonate assemblage. Later vein mineralization generated the uranium ores in two successive stages. Carbonates, sulfides, and adularia filled the remaining voids. Clastic dikes composed of fault gouge and, locally, ore were injected into new and existing fractures.Geologic and chemical evidence suggest that virtually all components of the deposit were derived from major hornblende gneiss units and related rocks. The initial fluids were evolved connate/metamorphic water that infiltrated and resided along the extensive fault zones. Complex fault movements in the frontal zone of the eastern Front Range caused the fluids to migrate to the most permeable segments of the fault zones. Heat was supplied by increased crustal heat flow related to igneous activity in the nearby Colorado mineral belt. Temperatures decreased from 225°C to 125°C during later mineralization, and the pressure episodically dropped from 1000 bars. The CO^ fugacity was initially near 100 bars, and uranium was carried as a dicarbonate complex. Sudden decreases in confining pressure during fault movement caused evolution of 009 and a consequent increase in pH. Uranium was released with destruction of the uranyl complexes; it was subsequently reduced by aqueous sulfur species, thereby leading to the precipitation of pitchblende. ACKNOWLEDGMENTS

Analytical data from host rocks and ores, Schwartzwalder uranium deposit, Front Range, Colorado

“…2). The protoliths for the units were probably mafic volcanic rocks with associated carbonate muds (hornblende gneiss), chert (quartzite), silicate iron formation (garnet-biotite gneiss), and iron-rich shale (mica schist) (Wallace, 1983). All rocks were metamorphosed to amphibolite grade during Proterozoic time.The major controlling fault system for fractures at the deposit is the Rogers fault, a northwest-trending system that, at the deposit, has two parallel segments, about 1,000 m apart, that dip steeply to the east.…”

“…2). The protoliths for the units were probably mafic volcanic rocks with associated carbonate muds (hornblende gneiss), chert (quartzite), silicate iron formation (garnet-biotite gneiss), and iron-rich shale (mica schist) (Wallace, 1983). All rocks were metamorphosed to amphibolite grade during Proterozoic time.…”

“…It is currently the largest known vein-type uranium deposit in the United States. This report provides the analytical data from host rocks and ores that have been (Wallace, 1983) and will be used in reports discussing the genesis of the deposit. Previous analytical data were presented by Young (1979).The deposit is in a complex fracture system that cuts a mixed raetasedimentary and metavolcanic terrane of Proterozoic age (Sheridan and others, 1967).…”

“…It is currently the largest known vein-type uranium deposit in the United States. This report provides the analytical data from host rocks and ores that have been (Wallace, 1983) and will be used in reports discussing the genesis of the deposit. Previous analytical data were presented by Young (1979).…”