U-Th Disequilibrium Dating of Authigenic Calcites in the Mazama (MZI-IIA) Geothermal Well, Oregon

Abstract

U-The goechronological techniques have been used to approximate the ages of precipitation of authigenic calcites in nine core samples from the Mazama geothermal well (California Energy Company MZI-IIA). Vein mineralization extracted from cores at five depth intervals is predominately calcite with associated zeolites and quartz. U concentrations in calcite are highest (> 2.0 ppm) is the shallowest sampling interval and are extremely low (<0.005 ppm) in the deepest interval near the bottom of the wellbore. 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. 232 Th concentrations are negligible in vein mineralization and intensive laboratory leaching of zeolite residues yields no measureable Th. 230Th is interpreted as the result of in growth by decay of 234U. Calcite extracted from veins in two depth intervals older than 350 Ka (Ka=1,000). One morphology of 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 MZI-IIA well cores has been precipitated episodically for >200 Ka.