Computer Science
Scientific paper
Oct 1979
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979gecoa..43.1633r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 43, Issue 10, pp.1633-1643
Computer Science
2
Scientific paper
The volcano-sedimentary sequence at the Raul mine, central Peru, consists of andesitic volcanics, graywackes, and siltstones, and has been metamorphosed to the upper greenschist-lower amphibolite facies at temperatures of 400-500°C. Isotopic data (O and H) have been collected from: (a) quartz and magnetite from stratiform ores, (b) amphiboles from amphibolite units that host stratiform ores, (c) calcite from late veins, (d) detrital quartz from graywackes, and (e) whole rocks. Interunit differences in quartz and magnetite 18 O values suggest that these minerals have resisted isotopic exchange during metamorphism, and that quartz-magnetite isotopic temperatures (380-414°C) represent primary formational temperatures. Calculated 18 O values of water in equilibrium with quartz and magnetite range from 9.1 to 12.6%.. Amphibole 18 O and D values show no interunit differences and suggest that the amphiboles have exchanged isotopes with a large metamorphic fluid reservoir. Calculated 18 O H 2 O and D H 2 O values range from 8 to 12%. and -3 to +42%., respectively. 18 O H 2 O values calculated from 18 O calcite and fluid inclusion filling temperatures range from 7.5 to 10%.. Water extracted from fluid inclusions in calcite has a D value of -20%.. 18 O values of metamorphosed graywackes and volcanic sediments are not atypical, but andesitic lavas are 18 O-rich (8-10%.) compared to normal andesites. Waters involved in ore deposition, metamorphism, and late vein formation at Raul are all thought to have a common source, principally seawater. The 18 O H 2 O and D H 2 O values could be produced by evaporation of seawater, shale ultrafiltration, and isotopic exchange with host rocks during deep circulation through the volcano-sedimentary pile. A model is proposed whereby coastal ocean water is restricted from the open sea by volcanic island arcs, and subsequently undergoes evaporation. Circulation of this water is initiated by heat associated with seafloor volcanism. 18 O-enrichment in andesites may be produced by isotopic exchange with high 18 O waters at elevated temperatures and sufficiently high water/rock ratios.
Ohmoto Hiroshi
Ripley Edward M.
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