Hydrothermal alteration enthalpy and heat flow in the Roosevelt Hot Springs Thermal Area, Utah

Abstract

Mass abundances of hydrothermal alteration product minerals at the Roosevelt Hot Springs thermal area have been calculated using whole rock chemistry and mineral analyses. These computations together with appropriate formulation of the alteration chemical reactions have been used to estimate total enthalpy produced per unit volume of altered rock. Enthalpy of alteration varies widely depending on the model assumed for sulfate production and the intensity of alteration. Maximum enthalpy production for the most altered rock ranges from 2450 to 7070 J/cm3, depending on whether none or all of the sulfate required to produce alunite is derived from H2S oxidation. The mean enthalpies in the lower 30 m of two drill holes 60 and 66 m deep are 345 and 105 J/cm3. Approximate age and depth‐of‐alteration constraints suggest heat flow due to alteration may be as low as 4 mW/m2 or as great as 852 mW/m2 depending on alteration intensity and sulfur oxidation. Even though chemical reactions could account for a substantial fraction of measured heat flow, comparison of measured temperatures with temperature calculations indicates an insignificant contribution of alteration reactions to present‐day heat flow. The present thermal gradient is explained in terms of convective circulation with a mass flux of water of 1.3×10−6 kg/m2 s estimated from alteration mineral abundance.