Paleomagnetic, geochemical, and hydrogeological studies of selected precious metal and mercury deposits of the California Coast Ranges

Four papers are presented on precious metal and mercury deposits in the California Coast Ranges. The first three papers present isotopic, fluid inclusion, rock magnetic and paleomagnetic results on vein minerals and Sonoma volcanic host rocks in the Calistoga Mining District. These elucidate aspects of local meteoric-hydrothermal systems over the last 3.0 million years. The δ 18O values of >100 volcanic rocks (+1.2 to +15.9‰) reveal four small (∼1 km2) low-δ18O zones associated with argillic, chloritic and silicic alteration in the uppermost Sonoma volcanic sequence. Two zones contain precious metal mineralization; the others may be related to mercury mineralization. Quartz vein δ18O values (+1.4 to +10.9‰) are moderately depleted compared to host rocks. Quartz from a silicic rib system surrounding the Silverado Mine on Mt. St. Helena was deposited between 3.0 and 2.6 Ma by evolved meteoric-hydrothermal waters (∼8.0‰ < δ18O < +1.5‰). This system formed along dilational shears related to the regional stress pattern. Deposition occurred at ∼240°C to ∼260°C, from slightly-acidic CO₂-rich boiling hydrothermal fluids. The most silicified tuffs were also maghemitized during hydrothermal alteration, causing anomalous increases in magnetization during thermal demagnetization without degradation in primary remanence direction.; The oxygen isotope depletion pattern around the Palisade Mine is small, corresponding with silicification surrounding the ore deposit; it probably formed at ∼1.4 Ma, contemporaneous with nearby mercury mineralization to the northeast, related to early Clear Lake volcanic activity. Oxygen isotope depletion also coincides with a previously unknown silicification zone near the Aetna Mercury Mine, and chloritic/clay alteration zone south of the silicic ribs on Mt. St. Helena.; The last Sonoma volcanic activity may be younger than the Matuyama/Gauss polarity chron boundary at 2.581 Ma. There are no tectonic rotations of the uppermost Sonoma volcanics, despite early Quaternary right-stepping en echelon faulting along the northeast side of the Napa Valley.; The fourth paper compares mercury in sediments 0 to 10 km downstream of the Klau mercury deposit before and after flooding. Mercury concentrations in stream sediments increased up to two orders of magnitude after January 1997 floods, varying between 0.005 to 0.137 weight % mercury.