Rocks of early Miocene age in the Breitenbush Hot Springs area have been affected by at least two major episodes of hydrothermal alteration, one of which had temperatures in excess of 200??C. Alteration minerals in younger Tertiary rocks are characteristic of temperatures below 100??C. The most important factor in controlling alteration is fracture permeability. The best guide to interpreting alteration is secondary mineralogy of volcaniclastic and pyroclastic rocks, flow breccia, and vesiculated or fractured lavas; alteration effects in interbedded massive lava flows are much more subtle.
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Hydrothermal alteration patterns in the Breitenbush Hot Springs area, Cascade Range, Oregon