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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 4. (April)

First Page: 757

Last Page: 758

Title: Uranium in Volcanic and Volcaniclastic Rocks: Discussion of Examples from Canada, Australia, and Italy: ABSTRACT

Author(s): Laurence Curtis

Article Type: Meeting abstract


Uranium deposits and prospects in volcanic and volcaniclastic rocks encompass a broad spectrum of genetic types, as illustrated by examples from Canada, Australia, and Italy. The hosts are commonly enriched in Zr, Ba, Sr, Rb, REE ± Th, and anomalous amounts of F are usually present. Additional metals such as Mo, Zn, and Cu may be present in economic concentrations in some of the occurrences, but there is no consistent U-base metal association. Associated comagmatic intrusives are locally enriched in Sn, W, Mo, U, and F ± Th. Mineralization is associated with both volcanic and volcaniclastic rocks surrounding subaerial volcanic complexes, and in some places with intercalcated epiclastics. Uranium

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mineralization is also hosted by euxinic sedimentary-tuffaceous facies adjacent to the volcanic complexes. The volcanics are generally, though not exclusively, acid in composition, have alkaline affinities, and are typical of the variety developed in continental rift systems. The mineralization within the volcanic and volcaniclastic rocks is generally conformable showing a preference for more clastic permeable units. Structurally controlled mineralization may occur and is sometimes of economic importance. In both places the controlling features have acted as channelways for migrating hydrothermal and ground-water solutions. Alkali, CO2, and H metasomatism commonly accompany the ore-forming process.

The uranium is considered to be of magmatic origin, transported by F- and CO2-rich hydrothermal fluids which have percolated through the volcanic pile. Under favorable conditions additional uranium may have been scavenged during transport of the ore fluids, during metasomatism or by ground waters circulating on the flanks of the caldera(s). Subaqueous venting of hydrothermal fluids distal to the volcanic centers may give rise to uranium concentrations within reducing (sulfide-rich) sedimentary facies. Precipitation of uranium in the subaerial or subaqueous environ may have been influenced by H2S exhalation in the vicinity of fumaroles or by dissolved H2S provided by a plumbing system. For these deposits in which F is a significant component it is proba le that U-F complexes transported by acid solutions have been destabilized by changes in pH (and Eh) due to mixing with mildly acid to alkaline ground waters or due to precipitation of the fluoride ion. Precipitation of any free or clay-absorbed uranyl ion would also be promoted by the presence of H2S.

Although some later supergene processes may have upgraded the primary uranium concentrations, I interpret the mineralizing episode(s) to be synvolcanic. This does not deny the importance of the intermixing of ground waters and the uranium-rich hydrothermal fluids as a means of inducing uranium precipitation, or the scavenging of uranium by these fluids as they percolate through the volcanic pile.

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