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Alan R. Wallace

Publications and source records attributed to Alan R. Wallace.

24 records · Page 2Linked to original sources
USGS1983

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

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 carbonate-sericite 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 CO2 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 CO2 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.

Open-File Report↗
USGS1979

Alteration and vein mineralization, Ladwig uranium mine, Jefferson County, Colorado

Uranium ore at the Ladwig mine, Jefferson County, Colo., occurs in steeply dipping, northwest-striking faults and related fractures with a carbonate-adularia assemblage that forms in altered wallrocks and fills veins. The faults occur between large intrusive pegmatites and garnetiferous gneisses of Precambrian age, and were reactivated as the result of the early Paleocene uplift of the Front Range foothills. Mineralization in the deposit includes both wallrock alteration and vein filling. Alteration was intense but local, and chiefly involved the carbonatization of mafic minerals in the wallrocks. Felsic minerals in the wallrocks are relatively unaltered. The veins are filled with an adularia-pitchblende-carbonate assemblage with minor related sulfides and coffinite. Many of the iron-bearing carbonates in both the alteration and vein assemblages have been altered to hematite. The mineralization and alteration are believed to have formed in response to initially high amounts of CO2 and the subsequent release of dissolved CO2 by boiling or effervescence. Uranium, carried in a dicarbonate complex, was precipitated directly as pitchblende when the CO2 was released. The expulsion of H+ during boiling created a net oxidizing environment which oxidized the iron-bearing carbonates. Late stage calcite and sulfides were deposited in existing voids in the veins.

Open-File Report↗
USGS1979

Occurrence of uranium in rocks of the intrusive complex at Ekiek Creek, western Alaska

Uranium in the Ekiek Creek Complex of western Alaska is related to a niobium-rich pyrochlore in the nepheline syenite of the complex. The complex consists of an aegirine-phlogopite pyroxenite that has been intruded and partly replaced by nepheline syenite. The contact zone between the two igneous units varies from a sharp contact to a diffuse zone where the pyroxenite has been metasomatically replaced by the syenite. The entire complex was intruded into an older Cretaceous monzonite. The pyrochlore occurs as an accessory mineral in the syenite, and is visible in rocks containing over 50 ppm uranium. Chemical analyses indicate that, in all samples of syenite, there is a positive correlation between uranium and niobium; this suggests that the uranium-pyrochlore association persists even when pyrochlore is not readily visible in thin section. The small amount of pyrochlore, and its refractory nature, make the complex an unfavorable source for secondary uranium leaching or heavy-mineral concentration.

Open-File Report↗