Active skarn formation beneath Lascar Volcano, northern Chile: a petrographic and geochemical study of xenoliths in eruption products |
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| Authors: | S J MATTHEWS R A MARQUILLAS A J KEMP F K GRANGE & M C GARDEWEG |
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| Institution: | Department of Geology, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK,;CONICET-Universidad Nacional de Salta, Facultad de Ciencias Naturales, Buenos Aires 177, 4400 Salta, Argentina,;Servicio Nacional de Geologia y Mineria, Avenida Santa Maria 0104, Casilla 10465, Santiago, Chile |
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| Abstract: | Calcsilicate xenoliths occur in large numbers in some lavas and pyroclastic flows of Lascar Volcano. Their whole-rock major element and REE compositions indicate that the protolith was the Upper Cretaceous Yacoraite Formation, which crops out extensively in NW Argentina. The whole-rock major element compositions of the xenoliths fall into specific groups suggesting a strong geochemical zonation in the skarn zone. Three geochemical zones have been identified; (1) an outer metamorphic zone rich in wollastonite; (2) a middle zone rich in pyroxene and garnet; (3) an inner zone rich in pyroxene and magnetite. The two innermost zones have developed from the wollastonite zone by infiltration of metasomatic fluids rich in Fe, Mn, Mg, Ti and Al. Whole-rock REE patterns have not changed significantly during prograde metamorphism and metasomatism, indicating REE immobility in the altering fluids. Retrograde alteration by acid-sulphate fluids produced anhydrite skarns and secondary calcite and wilkeite veins in the wollastonite zone. The carbon and oxygen isotopic compositions of this calcite indicate that it formed by Rayleigh crystallization from a low-temperature (<200 °C) fluid containing dissolved H2CO3. The calculated δ18O of the water in this fluid suggests a magmatic origin whereas the calculated δ13C of the dissolved carbonate is consistent with derivation from rocks of the Yacoraite Formation at 350 °C. It is suggested that the magmatic acid-sulphate fluid was responsible for leaching carbonate from the surrounding carbonate rocks and redepositing it in the skarn zone. REEs were mobilized during the retrograde acid-sulphate and acid-carbonate alteration. A negative Ce anomaly associated with this carbonate and sulphate indicates high oxygen fugacities in the mineralizing fluids. |
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| Keywords: | Argentina Chile Lascar Volcano skarn Yacoraite Formation |
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