The Marysvale Natural Analog Study: Preliminary Oxygen Isotope Relations

Shea, M. and Foland, K.A., 1986. The Marysvale Natural Analog Study: Preliminary oxygen isotope relations. In: N.A. Chapman and J.A.T. Smellie (Guest-Editors), Natural Analogues to the Conditions around a Final Repository for High-level Radioactive Waste. Chem. Geol., 55: 281--295. Rocks of and around the Marysvale mining area in central Utah, U.S.A., have experienced massive, subsolidus hydrothermal exchange with heated meteoric waters. Plutonic granitic rocks in the central mining area have 5 lIO-values ranging from +6 .7°o to as low as +0.1%0 . Quartz monzonite samples from surface exposures have, on the average (E = +2.9%o), lower 5180-values than subsurface samples (E = +4.3°o) . Within the limitations of present data, 180 depletions of the intrusive rocks are not obviously correlated to proximity of known major fractures. The 81sO data are consistent with hydrothermal modifications of normal igneous rocks, initially ~ +7%o, by groundwater initially of -14%0 at temperatures on the order of 200 ° C. Preliminary estimates of integrated water/rock ratios range from <0.1 to 1 for plutonic rocks. The hydrothermal solutions responsible for the mineralization were derived from meteoric water. Samples immediately adjacent to a quartz monzonite--granite contact show symmetrical ~aO depletion which may indicate that the contact was a feature allowing greater flow and/or residence time of the hydrothermal solution, and that the hydrothermal event apparently postdates granite emplacement.