ill., maps; Typescript (photocopy); Presented to the Department of Geological Sciences and the Graduate School of the University of Oregon in partial fulfillment of the requirements for the degree of Doctor of Philosophy; Thesis (Ph. D.)--University of Oregon, 1990; Includes bibliographical references (leaves 157-164)
iv An Abstract of the Dissertation of Carter Dean Hull for the degree of Doctor of Philosophy in the Department of Geological Sciences to be taken June 1990 Title: MULTICOMPONENT CHEMICAL EQUILIBRIUM MODELING OF FLUIDS AND U-TH GEOCHRONOLOGY OF AUTHIGENIC MINERALIZATION IN GEOTHERMAL SYSTEMS Approved: Geothermal waters are ascending from the fractured, crystalline basement complex in the range front of San Bernardino Mountains of southern California. Waters of 75-90°C flow from the basement complex along a shear zone associated with the San Andreas fault system at Arrowhead Hot Springs. Multicomponent chemical equilibrium models and empirical geothermometers indicate 100-150°C waters occur at depth. Numerical models, geothermometers, and ternary molality plots of selected chemical components indicate this deeply circulated geothermal water is diluted and cooled by shallow groundwaters in the adjacent San Bernardino Valley. Geothermal wells aligned on the Loma Linda fault zone in this structural basin produce 60-65°C mixed geothermal waters from sedimentary aquifers. Numerical modeling suggests the hot water component is 80-85°C and comprises <30 to 40% of the well productions. Two sites for drilling intermediate V depth geothermal wells have been identified near Loma Linda, California. Geothermal wells drilled to the basement complex just up gradient of the inferred trace of the Loma Linda or adjacent fault zone could produce waters >80°C from a geothermal plume ascending along shear zones. U-Th geochronological techniques have been used to approximate the ages of precipitation of calcites in core samples from the Mazama geothermal well (MZ1-11 A) near Crater Lake, Oregon. Vein mineralization extracted from cores is calcite with associated zeolites and quartz. U concentrations in calcite are highest (>2.0 ppm) in the shallowest sampling interval and are extremely low (< 0.005 ppm) near the bottom of the well bore. Some vein samples contain multiple sequences of mineralization with significantly different U contents and (234U/238U). Vein samples collected adjacent to vugs in two depth intervals have lost U since precipitation. 232Th concentrations are negligible in vein mineralization and intensive laboratory leaching of zeolite residues yields no measurable Th. 230Th is interpreted as the result of ingrowth by decay of 234U. Calcite extracted from veins in two depth intervals is older than 350 Ka (Ka=1,000 years). Calcite within a third depth interval has an approximated age of 140.3 Ka (+22.3 -19.3 Ka). None of the sampled veins have calculated ages < 120-160 Ka. Authigenic mineralization extracted from the MZ1-11A well cores has been precipitated episodically for >200 Ka.
iv An Abstract of the Dissertation of Carter Dean Hull for the degree of Doctor of Philosophy in the Department of Geological Sciences to be taken June 1990 Title: MULTICOMPONENT CHEMICAL EQUILIBRIUM MODELING OF FLUIDS AND U-TH GEOCHRONOLOGY OF AUTHIGENIC MINERALIZATION IN GEOTHERMAL SYSTEMS Approved: Geothermal waters are ascending from the fractured, crystalline basement complex in the range front of San Bernardino Mountains of southern California. Waters of 75-90°C flow from the basement complex along a shear zone associated with the San Andreas fault system at Arrowhead Hot Springs. Multicomponent chemical equilibrium models and empirical geothermometers indicate 100-150°C waters occur at depth. Numerical models, geothermometers, and ternary molality plots of selected chemical components indicate this deeply circulated geothermal water is diluted and cooled by shallow groundwaters in the adjacent San Bernardino Valley. Geothermal wells aligned on the Loma Linda fault zone in this structural basin produce 60-65°C mixed geothermal waters from sedimentary aquifers. Numerical modeling suggests the hot water component is 80-85°C and comprises <30 to 40% of the well productions. Two sites for drilling intermediate V depth geothermal wells have been identified near Loma Linda, California. Geothermal wells drilled to the basement complex just up gradient of the inferred trace of the Loma Linda or adjacent fault zone could produce waters >80°C from a geothermal plume ascending along shear zones. U-Th geochronological techniques have been used to approximate the ages of precipitation of calcites in core samples from the Mazama geothermal well (MZ1-11 A) near Crater Lake, Oregon. Vein mineralization extracted from cores is calcite with associated zeolites and quartz. U concentrations in calcite are highest (>2.0 ppm) in the shallowest sampling interval and are extremely low (< 0.005 ppm) near the bottom of the well bore. Some vein samples contain multiple sequences of mineralization with significantly different U contents and (234U/238U). Vein samples collected adjacent to vugs in two depth intervals have lost U since precipitation. 232Th concentrations are negligible in vein mineralization and intensive laboratory leaching of zeolite residues yields no measurable Th. 230Th is interpreted as the result of ingrowth by decay of 234U. Calcite extracted from veins in two depth intervals is older than 350 Ka (Ka=1,000 years). Calcite within a third depth interval has an approximated age of 140.3 Ka (+22.3 -19.3 Ka). None of the sampled veins have calculated ages < 120-160 Ka. Authigenic mineralization extracted from the MZ1-11A well cores has been precipitated episodically for >200 Ka.