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An oxygen-isotope study of water-rock interaction in the granite of Cataract Gulch, western San Juan Mountains, Colorado

Larson, Peter B. and Taylor, Hugh P., Jr. (1986) An oxygen-isotope study of water-rock interaction in the granite of Cataract Gulch, western San Juan Mountains, Colorado. Geological Society of America Bulletin, 97 (5). pp. 505-515. ISSN 0016-7606. doi:10.1130/0016-7606(1986)97<505:aosowi>;2.

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Oxygen-isotope analyses of a 37-km2 exposure of Precambrian granite adjacent to the Miocene Lake City caldera are used to document interactions with a 23-m.y.-old meteoric-hydrothermal system established within the caldera. The granite δ18O values range from +0.7 to +9.2, all lower than for the original granite (∼ +9.5), indicating pervasive exchange with a low-18O fluid. Primary muscovite exchanged oxygen with the fluid faster than did quartz, but much more slowly than did K-feldspar, and primary biotites were altered to variable mixtures of low-18O chlorite and sericite. The granite was altered over a wide range of water/rock ratios in two distinct regimes, a sericite alteration regime and a chlorite alteration regime. Granite in the highly faulted Eureka graben exhibits the lowest whole-rock δ8O values and the highest degree of biotite alteration; this zone was a main channel for fluid flow through the granite. Groups of samples (3) are defined relative to the graben axis: (1) a “graben group” with whole-rock δ18O=0 to +4; (2) an intermediate group (5.5 to 9.0 km from the graben axis) with δ18O=+2 to +8 that shows a positive correlation between sample elevation and whole-rock δ18O; (3) a distal group (>9.0 km) with δ18O=+6 to +9 that also shows a positive correlation between elevation and δ18O. These δ18O-elevation trends (∼0.7 per mil/100 m) imply a systematic vertical thermal gradient in the near-surface portions of the granite during hydrothermal activity, as well as a lateral gradient in water/rock ratio outward from the graben. A subgroup of samples within the intermediate group is from a postcaldera structural dome, and the δ18O-elevation trends in the granite are offset by the uplift.

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Additional Information:Financial support for this work was provided by the National Science Foundation, Grant EAR-78-16874. Discussions with L. T. Silver, S. Epstein, K. Hon, J. R. O'Neil, G. C. Solomon, and R. E. Criss were very helpful.
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Caltech Division of Geological and Planetary Sciences4177
Issue or Number:5
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ID Code:116710
Deposited By: Tony Diaz
Deposited On:04 Oct 2022 22:24
Last Modified:04 Oct 2022 22:24

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