Hydrothermal ore deposits record the oxygen isotope composition of meteoric paleo‐waters in the San Juan Volcanic Field, Colorado, USA

The stable isotope ratios of meteoric waters change predictably over orographic barriers. We present a new approach to determine stable isotope ratios in ancient meteoric waters from spatial patterns of hydrothermal alteration in continental volcanic fields. In the San Juan Volcanic Field, Colorado, USA we reconstruct water delta O-18 values feeding hydrothermal systems of -7 to -10 parts per thousand from 35 to 20 Ma, followed by a drop to -17 to -18 parts per thousand between 20 and 12 Ma. This drop is consistent with greater magmatic water input to older hydrothermal systems, similar to 1 km of rock exhumation, similar to 2-3 km of surface uplift, or a combination of all three. Our approach returns water isotope compositions integrated over spatial (kms to 10s kms) and temporal (10(4) - 10(7) years) scales of continental hydrothermal systems. Such length- and time-scales approach those of continental tectonics, potentially alleviating issues with diagenetic space and time limitations associated with other paleo-meteoric water proxies.