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Gold Bearing Mercury Deposits of the Coast Ranges of California

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Gold Bearing Mercury Deposits of the Coast Ranges of California TERTIARY GOLD BEARING MERCURY DEPOSITS OF THE COAST RANGES OF CALIFORNIA By LARRY M . VREDENBURGH. Geologist Bureau of Land Management Sacramento. California INTRODUCTION bon dioxide (CO,), silica (SiO,), and con­ cury deposits of the Pacific Coast was that taining abundant hydrocarbons, dissolved of Becker (1888). He determined the Recently an unsuspected reserve of the gold and mercury contained in the presence of gold at mercury mines at Wil­ gold, estimated to exceed 3.2 million sedimentary units. At the surface, hydr~ bur Hot Springs, Sulphur Bank Hot ounces, was discovered by Homestake carbons may have played an important Springs, and the Knoxville mine area. He Mining Company at the Knoxville mer­ role in the deposition of the gold, in a also reported the occurrence of gold at the cury mines in Napa County. This is one of manner similar to the Carlin-type depos­ Picacho Mine, located southwest of New several occurrences of gold associated its of Nevada, which closely resemble the ldria. Other worker8 (Crutchfield, 1953; with mercury or antimony in the Coast gold deposits of the Coast Ranges. Mer­ Newmont Exploration, 1965; Bailey and Ranges of California (figure 1) . These cury is presently being deposited by hot Meyers, 1949) have identified gold at the deposits began forming shortly after the springs at the Sulphur Bank mine near Calistoga and Oat Hill mines, and the implacement of volcanic rocks in the Mid­ The Geysers steam field in the Clear Lake Stayton District (table 1) . Tertiary Period and continued to form area of California . through Quaternary time. Connate water GEOLOGY squeezed from sedimentary units was PREVIOUS STUDIES heated and eventually found its way to the The major rock types found in the surface via fracture zones. Presumably The earliest and perhaps one of the Coast Ranges are the Great Valley se­ this hot water, high in chloride (Cl), car- most comprehensive studies of the mer- quence, the Franciscan assemblage, a mafic-ultramafic sequence, and volcanic rocks of Tertiary and Quaternary age. The Great Valley sequence and the Fran­ ciscan assemblage consist predominately of marine sedimentary rocks of Late Ju­ rassic to Late Cretaceous age. The Fran­ ciscan assemblage consists of graywacke, shale, mafic volcanic rock, chert, lime­ stone, some of which have been metamor­ 1. Wilbur Springs District phosed to the zeolite and blueschist facies. 2. Sulphur Bank Mine The Great Valley sequence consists 3. Baker Mine predominately of graywacke, shale. and · 4. Shamrock Mine some conglomerate. The Jurassic section 5. Knoxville District of the Great Valley sequence depositional­ 6. Oat Hill Mine ly rests upon an accumulation of mafic 7. Palisade Mine volcanic rocks, that in turn rest upon ser­ 8. Stayton District pentinized ultramafic rocks. The mafic­ 9. Picacho Mine ultramafic sequence closely resembles present in-situ oceanic crust (Bailey and Jones, 1970) . The Coast Ranges thrust fault separates the Franciscan assemblage from the Great Valley sequence with out­ crops of serpentinite occurring on the up­ per thrust plate. Much of the mercury in the Coast Figure 1. Tertiary gold deposit occurrences in the Coast Ranges Ranges occurs in altered serpentinite. So­ of California associated with mercury or antimony. lutions enriched in silica (SiO,) and car- CALIFORNIA GEOLOGY February 1982 23 bon dioxide (C02), rising along faults, bearing gravels in the Sierras, Graham mine and the Stayton District; however, have replaced serpentinite by silica-car­ (1981) sampled the Great Valley se­ the relationship of gold with other miner­ bonate rock. Silica-carbonate rock typi­ quence (and younger formations), hoping als has not been determined. Where gold cally consists of chalcedony, opal, quartz, to detect the timing of gold emplacement has been reported at mercury mines in the magnesite.and calcite. in the Sierras. He noted a general increase Coast Ranges, it either occurs as free gold in gold with decrease in age. The Knox­ or is contained within the iron sulfides of LOCALIZATION OF ORE ville Formation assayed .001-.002 oz. per pyrite or marcasite. The nature of the ton in gold, and the Eocene formations gold's occurrence has not been deter­ Although over SO per cent of the largest assayed .006-.008 oz. per ton (Steve Gra­ mined at the Shamrock mine, Palisades mercury deposits in the Coast Ranges are ham, personal communication, 1981) . mine and the Stayton District. Investiga­ associated with altered serpentinite, mer­ The high chloride content of the water tions at the Palisades mine (Crutchfield cury also occurs in rocks of the Great may have facilitated in the remobiliza­ 19S3, p. 49, SO) and in the Stayton Dis­ Valley sequence, the Franciscan assem­ tion of gold, since in chloride-bearing hy­ trict (Bailey and Meyers 1949, p. 47) blage, in Tertiary volcanic rocks, and at drothermal solutions, gold is transported have shown that cinnabar was deposited active hot springs. Thus, rock type does as the auriferous chloride complex AuCl 2 after other base metals. Pyrite was depos­ not appear to be the controlling factor in (Radtke and Scheiner, 1970) . ited with both the base metals and cinna­ the deposition of mercury in the Coast bar, but it is not known if the gold is Ranges metallogenic province. As an alternative to this hypothesis for associated with cinnabar or other metals. the source of gold, Eyans ( 1981) suggests The presence of young, high-angle that gold present in ultrabasic rocks may COMPARISON WITH CARLIN­ faults appears to be the most important be chemically dissolved and reprecipitat­ TYPE DEPOSITS factor in the location of mercury deposits ed in placer deposits by the process of within the Coast Ranges. All mercury laterization. If gold is present in the ser­ There are some conspicuous similari­ deposits are associated with faults and pentinites, it could be leached by the con­ ties between mercury-gold deposits in the fracture zones. Averitt (194S) suggested nate water present in the system. Coast Ranges and the Carlin-type gold that clay gouge zones associated with Although this process may contribute to deposits which have become the source of faulting, effectively channeled mineral­ gold present in the Coast Ranges deposits, large quantities of gold being mined in bearing waters, thus controlling mercury it probably is not the only source, since Nevada. Radtke and other workers (see deposition at the Knoxville mine. some mercury-gold deposits are not near Boyle, 1980, for a summary) have done occurrences of serpentinite. the pioneer work on the Carlin-type ORIGIN OF ORE deposits. From their work a checklist of SOURCE OF HEAT geologic, mineralogic, and geochemical The origin of the mercury, gold, hydro­ features common to these types of depos­ carbons, and the water, which acted and During Tertiary and Quaternary time its has been developed. This list includes: still acts as a transporting medium, are all the Clear Lake, Sonoma, Tolay, Berkeley silty carbonate rocks, the presence of hy­ intimately associated with the marine Hills, Quien Sabe, and Neenach-Pinnacles drocarbons, gold associated with mer­ sediments. In a study of thermal and min­ volcanic fields erupted in the Coast cury, arsenic, antimony, tungsten and eral waters in the Clear Lake area, White Ranges, generally decreasing in age thallium, replacement by fine-grained sili­ and others (1973) concluded that the northward, and providing an ample ca and minor pyrite, and mineralization high chloride connate waters found at source of heat for mercury and gold-bear­ related with large displacement normal Wilbur Springs are chemically and ing hydrothermal solutions. Hot volcanic faulting. Fluids at these deposits _have isotopically similar to the oil-field waters rocks at depth in the Clear Lake area still been shown to be of meteoric origin. of the Central Valley of California, but are provide a source of heat for geothermal Work by Radtke and Scheiner (1970) generally enriched with bicarbonates and fluids. The Geysers geothermal area is be­ suggests that the presence of carbona­ boron. This water probably evolved from lieved to be heated by a silicic magma ceous materials (hydrocarbons) may be compaction and loss of pore water in the body centered below 10 km in depth, the critical for the deposition of gold. marine sediments, but is also generated to top of which is within 7 km of the surface some extent by the continuing low-grade (Hearn and others, 1981) . At Wilbur In the Coast Ranges of California, mer­ metamorphism occurring at depth. Hy­ Springs, on the basis of a detailed gravity cury deposits are largely, but not entirely, drocarbons present at Wilbur Springs and study, a series of 100 m-deep temperature restricted to occurrences of serpentinite Knoxville are common in many of the gradient holes, and two deep wells ( 400 m which has been replaced by silica and car­ mercury mines in this region and came to and 1,200 m) , Harrington and Verosub bonate. Hydrocarbons and pyrite are very the surface in the same fracture zones that ( 1981) concluded that the source of heat common in these deposits. The presence hot mineral-laden water did. At Wilbur may be a deep magma intrusion. Frac­ of hydrocarbons supports the findings of Springs, hot-water and hydrocarbons (liq­ tures within the rock provide the conduits White and others (1973) that the water uid and gas) are ascending in the same needed to transfer the heat in the form of which transported the mercury is of con­ fracture system. hot water or steam from the intrusion to nate origin, and that the silty sediments of the surface. the Great Valley sequence are the source Moisseeff (1966) concluded that the of the connate water. This water, there­ mercury at Wilbur Springs probably was PARAGENESIS fore, has a different source than the water concentrated from sediments in "solu­ associated with Carlin-type deposits in tions of metamorphic or connate origin." A paragenetic sequence for minerals Nevada.
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