Thermal and Mineral Waters of Nonmeteoric Origin, California Coast Ranges

Abstract

Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement. Recent isotope studies show that the waters involved in a variety of geologic processes are dominantly the local meteoric water of each area. In most active geothermal systems, the D/H ratio of the hot water is nearly identical with the local cold meteoric water, but the O18/O16 ratio has been shifted to a more positive value because of subsurface exchange with rocks. The numerous thermal springs of the Wilbur Springs mercury district, although rich in CO2, are otherwise similar in Cl content and isotopic composition to analyzed California oil-field waters. Some of the springs discharge near the tops of ridges. These relations cannot be explained by normal meteoric recharge. Water of isotopic composition similar to that of Wilbur Springs occurs in the Sulphur Bank mercury district 15 mi west of Wilbur Springs, but the Sulphur Bank water is higher in B and NH3 and much lower in Cl than are the Wilbur and most oil-field waters. The Wilbur Springs and Sulphur Bank waters are enriched by ∼40‰ in δD and ∼13‰ in δO18 relative to local meteoric waters of each area, and thus require processes that differ, at least in part, from most previously studied geothermal systems. The D enrichment, chemical composition, and ridge-top discharge are best explained by large proportions of nonmeteoric water. Wilbur Springs and Sulphur Bank may be dominated, respectively, by waters of connate and metamorphic origin, derived from reaction of ancient ocean waters and marine sediments, and now being forced out by pressures that are higher than hydrostatic. Present data indicate that the most saline of each of these types is more restricted in range of δD than are present-day meteoric waters of the same areas; complete flushing by existing or ancient meteoric waters is unlikely. Many springs in the region are chemically intermediate between the high- and low-chloride types and commonly mix near the surface in different proportions with local meteoric water. Many of these springs are associated with mercury deposits and Alpine serpentinites.