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Oxygen Isotope Composition of Magnetite Deposits at El. Laco, Chile: Evidence of Formation from Isotopically Heavy Fluids

By
Amy Larson Rhodes
Amy Larson Rhodes
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Naomi Oreskes
Naomi Oreskes
Department of Earth Sciences, Dartmouth College
,
Hanover, New Hampshire 03755
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Published:
January 01, 1999

Abstract

Oxygen isotope analyses of iron oxide at El Laco, Chile, were conducted in order to test whether magnetite formed either by a combination of magmatic and hydrothermal metasomatic processes or by a single hydrothermal-metasomatic process alone. If magnetite formed from two distinct sources—one magmatic, the other hydrothermal—then a contrast in oxygen isotope compositions between magnetite that crystallized in a magma and magnetite that precipitated from a hydrothermal fluid should be expected. In fact, δ18O values in magnetite at El Laco show no significant variability between textural types (mean = 4.1 ± 0.49‰). Perhaps more important, δ18O in wall-rock andesite shows a distinct increase in oxygen isotope values—from 7.2 to 24.2 per mil—with increasing degrees of hydrothermal alteration. This observation strongly suggests that oxygen exchange occurred with an isotopically heavy fluid that was distinct from what might have been generated from a magma. Diopside separated from altered andesite, apatite separated from magnetite, and quartz separated from hydrothermal magnetite-quartz (± apatite) veins all have heavy δ18O values (7.1–8.9‰, 7.8–8.0‰, and 7.1–27.9‰, respectively) relative to values typical of igneous rocks. The quartz values are among the heaviest reported in the literature and are exceedingly variable both between and within individual samples. In contrast to the values for magnetite, apatite, diopside, and quartz, δ18O values of hematite-bearing iron oxide samples have much lower δ18O values, down to a minimum of −8.9 per mil, and the δ18O value decreases as the hematite content increases.

These results strongly support the theory that the bulk of the magnetite at El Laco formed by metasomatic replacement and did not form by direct consolidation from a magma. The hydrothermal fluids that reacted to form magnetite were isotopically heavy in oxygen composition. These 180-rich fluids were also responsible for hydrothermal alteration of wall-rock andesite and the formation of quartz veins. Heated, closed-basin water that experienced significant evaporation, or deep-seated fluids (possibly magmatic) that interacted with buried evaporite deposits, may be the source for these isotopically heavy hydrothermal fluids. The extremely high δ18O values and isotopic variability of the quartz suggests that quartz veins formed as the hydrothermal fluids boiled, perhaps losing large quantities of volatile components. The much lower δ18O values of hematite-rich samples suggest that isotopically lighter meteoric fluids reacted with, and oxidized portions of, the magnetite deposits at temperatures ranging from approximately 65° to 150°C in the near-surface or surficial environment. However, these fluids were volumetrically minor.

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Contents

Special Publications of the Society of Economic Geologists

Geology and Ore Deposits of the Central Andes

Brian J. Skinner
Brian J. Skinner
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Society of Economic Geologists
Volume
7
ISBN electronic:
9781629490311
Publication date:
January 01, 1999

GeoRef

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