49-10 Hydrothermal ore deposits record the oxygen isotope composition of paleometeoric water, examples from the western US and the San Juan Mountains, CO, USA

Session: Exploring Feedbacks Between Tectonics and Climate on Lithospheric Evolution Using Multidisciplinary Approaches

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Abstract:

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. This approach builds on a tracer mass balance inversion technique, and estimates original fluid O isotope ratios from relatively large sections of hydrothermal activity. First used on submarine rocks, we show that the same approach can estimate meteoric water isotope ratios. Our approach returns water isotope compositions integrated over spatial (kms to 10s kms) and temporal (10⁴ − 10⁷ 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.

As an example, we applied this approach to a series of calderas in the San Juan Volcanic Field, Colorado, USA. Our work suggests that water δ¹⁸O values feeding hydrothermal systems evolved from  −7 to −10‰ from 35 to 20 Ma, followed by a drop to −17 to −18‰ between 20 and 12 Ma. This drop is consistent with greater magmatic water input to older hydrothermal systems, ∼1 km of rock exhumation, ∼2–3 km of surface uplift, or a combination of all three. We posit that surface uplift is the most likely cause of isotopic shifts, consistent with suggestions from complementary isotope proxies (e.g., paleosol carbonates, smectite).

We have also applied the approach to the Yerington intrusion in western NV. This unit is Jurassic in age, and was likely emplaced near the coastline during this time. We estimate that the oxygen isotope value of the original alteration fluid was about 1.5‰, consistent with alteration by coastal precipitation. These "bookend" examples broadly outline the uplift history of the western US, with areas near sea level in the middle Mesozoic and high elevation in the Cenozoic. Further work will fill in the record across space and time. 

More broadly, our approach can, if combined with constraints on magmatic water, help untangle the effects of hydroclimate change and erosion/unroofing. Sites of hydrothermal alteration are common in active orogenic settings, and this approach may provide estimates of paleoaltiemtery where traditional sedimentary proxies are missing, especially useful in high mountain regions with a limited depositional record.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9164

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