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 km (super 2) 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 (super 18) O 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 (super 18) O 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 (super 18) O 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 (super 18) O values and the highest degree of biotite alteration. Systematic variation in delta (super 18) O 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.