GEOCHEMICAL CHANGES DUE TO PROPYLITIZATION IN THE EOCENE SAWTOOTH RING ZONE, IDAHO BATHOLITH, USA

Large annular zones of hydrothermal alteration are centered on shallow Eocene granites emplaced into the Idaho batholith. Meteoric-hydrothermal systems driven by the plutons have propylitically altered the batholith, where typical products are chlorite, illite, calcite, and albite. It has recently been suggested that these altered rocks, which have oxygen isotope ratios near or less than 0 per mil, serve as sources for the voluminous Miocene and younger low-¹⁸O rhyolites in the Snake River Plain/Yellowstone (SRP/Y) volcanic province. A suite of samples has been collected across the Sawtooth ring zone in the batholith’s Atlanta Lobe to test this hypothesis. The sample traverse comprises a 30km section across the altered zone and went from fresh, through altered, and back into fresh rock. Major and trace element concentrations, oxygen and hydrogen isotope ratios, Sr, Nd, and Pb isotope ratios, and mineral electron microprobe compositions have been measured for up to 10 samples. Many of these parameters vary across the ring zone due to hydrothermal processes.

Quartz-feldspar oxygen isotope fractionations increase with increasing mineralogic alteration, where quartz stays nearly constant and feldspar is lowered. The fractionations monitor the degree of hydrothermal exchange. Loss on Ignition increases with alteration because chlorite and illite are common hydrothermal products, but major element concentrations do not change significantly. The major element ratios K₂O/(K₂O+MgO), CaO/(CaO+Na₂O), and MgO/(MgO+FeO) show no changes due to alteration. Sr, Nd, and Pb isotope ratios also show no significant changes across the zone. Ba and Sr are slightly elevated in the altered rocks relative to the unaltered batholith, and are the only elements other than the O ratios, increased hydration, and lower D/H ratios that show consistent changes due to the hydrothermal water/rock exchange. Cu, Pb, and Zn show only minor and inconsistent variations in concentration across the zone. The most pronounced geochemical signature in the altered rocks that correlates with the geochemistry of the SRP/Y rhyolites is the lowering of the ¹⁸O/¹⁶O ratios.