| Abstract |
Oxygen isotope data from unaltered 'to intensely altered rocks indicate that substantial 180- exchange precedes the visible effects of hydrothermal alteration. The cumulative water to rock mass ratios calculated from the 180-data are generally less than 1; values more than 1 are Less sensitive to 180-exchange-. These ratios correspond to tines when rock and fluid attained 5 180 compositions close to their current values, a situation that probably occurs during the early part of the history of most geothermal systems. Water/rock ratios which are calculated on the basis of leaching and addition of some common and rare alkalies during hydrothermal alteration, however, are in comparison higher by two orders of magnitude. This is because these latter ratios relate to much slower processes and are affected by subsequent fluid throughput during the entire history of the system. |