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

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 delta 18O values range from +0.7 to +9.2, all lower than for the original granite (approx +9.5), indicating pervasive exchanged with a low delta 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 delta 18O chlorite and sericite. The granite was altered over a wide range of water/rock ratios in two distinct regimes, a sericite and a chlorite regime. Granite in the highly faulted Eureka graben exhibits the lowest whole-rock delta 18O values and the highest degree of biotite alteration. Systematic variation in delta 18O can define its position relative to the graben axis as well as its vertical position, implying a vertical thermal gradient in the near surface portion, as well as a lateral gradient in water/rock ratio during hydrothermal activity.-L.C.H. Div. of Geological & Planetary Sciences, California Inst. of Technology, Pasadena, CA 91125, USA.