Late Cretaceous caldera volcanism and porphyry copper mineralization at Silver Bell, Pima County, Arizona: geology, petrology, and geochemistry
Late Cretaceous igneous activity associated with the porphyry copper deposit at Silver Bell was related to caldera volcanism. Caldera volcanism is documented by several lines of evidence: (1) the Confidence Peak Tuff, a phenocryst-rich low-silica rhyolite, ponded to a thickness of greater than 1.5 km.; (2) a belt of Paleozoic sedimentary rocks are enclosed by the tuff as caldera collapse megabreccia; and (3) caldera structure expressed by a 150/sup 0/ arcuate fault, later intruded by plutons and dikes of the QMP (quartz monzodiorite porphyry) suite host to porphyry copper mineralization. Geochemical data show that these rocks are calcalkaline and have close petrotectonic affinities with subduction-related continental arc volcanic provinces. Major element compositions range from high-K andesites to high-K dacites and low-silica rhyolites with the dacites and rhyolites being volumetrically most abundant. Caldera-related rocks have strong LREE/HREE fractionation, except that QMP suite plutons have lower HREE resulting from hornblende fractionation. The chemistry of the common dacites and rhyolites can be derived from andesite by relatively small amounts of plagioclase + pyroxene (or hornblende) + Fe-Ti oxide + apatite fractionation. The composition of the precaldera syenogranite intrusion is anomalous with its high Th, Rb, Ta and flat HREE. Lead isotope ratios for rocks and ores yield a secondary isochron age of 1700 +/- 365 Ma, similar to the age of the oldest Proterozoic crust in southern Arizona. Quartz monzodiorite porphyry plutons have similar Pb isotopic compositions as the porphyry copper ores. Coupled /sup 208/Pb//sup 204/Pb and /sup 206/Pb//sup 204/Pb variation are indicate of variable interaction with a U-depleted source, such as lower crust. Strontium isotopic ratios also support this model.
- Research Organization:
- California Univ., Santa Barbara (USA)
- OSTI ID:
- 5503355
- Resource Relation:
- Other Information: Thesis (Ph. D.)
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ARIZONA
VOLCANIC REGIONS
GEOLOGIC HISTORY
VOLCANIC ROCKS
GEOCHEMISTRY
PETROLOGY
AGE ESTIMATION
COPPER ORES
CRETACEOUS PERIOD
DIKES
GEOLOGIC STRUCTURES
ISOTOPE DATING
ISOTOPE RATIO
LEAD ISOTOPES
MINERALIZATION
MINERALOGY
MINERALS
QUARTZ
SEDIMENTARY ROCKS
STRONTIUM ISOTOPES
ALKALINE EARTH ISOTOPES
CHALCOGENIDES
CHEMISTRY
FEDERAL REGION IX
GEOLOGIC AGES
GEOLOGY
IGNEOUS ROCKS
ISOTOPES
MESOZOIC ERA
NORTH AMERICA
ORES
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
ROCKS
SILICON COMPOUNDS
SILICON OXIDES
USA
580202* - Geophysics- Volcanology- (1980-1989)
580300 - Mineralogy
Petrology
& Rock Mechanics- (-1989)
580400 - Geochemistry- (-1989)