The Geysers-Clear Lake area, California; thermal waters, mineralization, volcanism, and geothermal potential

Abstract

Manifestations of a major thermal anomaly in the Geysers-Clear Lake area of northernCalifornia include the late Pliocene to Holocene Clear Lake Volcanics, The Geysers geothermal field, abundant thermal springs, and epithermal mercury and gold mineralization, The epithermal mineralization and thermal springs typically occur along high-angle faults within the broad San Andreas transform fault system that forms the western boundary of the North American plate in this area. The young volcanic rocks overlie Mesozoic marine rocks of the Great Valley sequence which have been thrust above the coeval Franciscan Complex and penecontemporaneously dropped back down along low-angle detachment faults.Many of the waters of the region are nonmeteoric as defined by their isotopic signature. One type of isotopically shifted water emerges from or near Great Valley sequence rocks and is the most chloride rich. It is interpreted to be evolved cormate in origin. A second type, evolved meteoric water, has moderate chloride contents, high boron contents, and high B/CI ratios and is found locally in Franciscan rocks, notably at the Sulphur Bank mercury mine, where it probably results from near-closed-system, repeated boiling of meteoric water in host rocks that also contribute organic components to the water. At the Sulphur Bank mine fracturing of otherwise impermeable Franciscan rocks by faulting has created a localized zone of permeability in which thermal water boils repeatedly with limited venting to the surface. Boron-rich fluids were apparently present at depth in The Geysers geothermal field when intrusion of silicic magma occurred, because the concealed intrusion of felsite is surrounded by a halo of tourmaline-bearing hornfels. The volume of this poorly dated early to middle Quaternary intrusive body probably exceeds the 100 km a of erupted Clear Lake Volcanics. Similar intrusions may have occurred in the eastern part of the area at Wilbur Springs and theMcLaughlin mine, where gold deposition and evidence of hydrothermal phenomena suggest more magmatic activity than is indicated by the small exposed bodies of early Quaternary basaltic lava. The Clear Lake Volcanics are the present locus of volcanism in the northern Coast Ranges and progressively older volcanic centers occur to the south. Geophysical data suggest that a large silicic magma body may be centered north of The Geysers steam field, providing the heat for the geothermal field.Geothermal power production has peaked at The Geysers and pressure declines indicate significant depletion of the fluid resource. The vapor-dominated field evolved from a preexisting hydrothermal system within fractured, otherwise impermeable Franciscan metamorphic rocks. A deep water table of saline fluid has been postulated to be present under the steam field, but no chloride-rich water has been found at drillable depth. We propose that recently discovered, isotopically shifted steam in the northwest Geysers area indicates the presence not of deep connate water but xather of boiled-down, boron-r...

Mercury and methylmercury concentrations and loads in the Cache Creek watershed, California

“…Boron and lithium were elevated at the Cache Creek at Rumsey, relative to the concentrations in either Clear Lake outflow or the North Fork of Cache Creek, but it is unclear whether the source is from geothermal or the mine sites. Similar relations were found for boron and chloride (Donnelly-Nolan et al, 1993;Goff et al, 1993a,b).…”

Midcrustal, Long-period Earthquakes beneath Northern California Volcanic Areas

“…This volcanic field has developed with multiple eruptions over the past 1 Ma. The most recent eruptions of basaltic lava occurred roughly 11,000 ybp from vents near Mount Konocti along the southwestern shore of Clear Lake (Donnelly-Nolan et al, 1993;Sims et al, 1988).…”

Regional 18O-depletion of Neoproterozoic igneous rocks from Avalonia, Cape Breton Island and southern New Brunswick, Canada

“…The present geothermal fi elds located throughout California and extending into Mexico are associated with extensive transform fault systems activated at the time of cessation of subduction with transition to a transtensional rifting environment at ca. 3 Ma (Donnelly-Nolan et al, 1993;Portugal et al, 2000). We suggest that this behavior is analogous to the processes that resulted in the initial rifting of the Mira and Caledonia terranes from the Gondwanan margin at 560-550 Ma.…”