McLaughlin Mine

The McLaughlin Mine is a silver and gold mine located in Napa county, California at an elevation of 1,959 feet.

About the MRDS Data:

All mine locations were obtained from the USGS Mineral Resources Data System. The locations and other information in this database have not been verified for accuracy. It should be assumed that all mines are on private property.

Mine Info

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Site Name

Primary: McLaughlin Mine
Secondary: Manhattan

Commodity

Primary: Silver
Primary: Gold
Secondary: Mercury
Tertiary: Lead
Tertiary: Iron
Tertiary: Copper
Tertiary: Thallium
Tertiary: Arsenic
Tertiary: Antimony
Tertiary: Zinc

Location

State: California
County: Napa
District: Knoxville District

Land Status

Land ownership: BLM Administrative Area
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.
Administrative Organization: BLM Ukiah District

Holdings

Not available

Workings

Not available

Ownership

Owner Name: Homestake Mining Co.

Production

Not available

Deposit

Record Type: Site
Operation Category: Producer
Deposit Type: Hot spring; hydrothermal vein
Operation Type: Surface
Discovery Year: 1978
Years of Production:
Organization:
Significant: Y
Deposit Size: L

Physiography

Not available

Mineral Deposit Model

Model Name: Hot-spring Au-Ag

Orebody

Form: Wedge

Structure

Type: L
Description: Stony Creek Fault OR (alternatively) The segment of the Stony Creek Fault associated with the ore deposit dips moderately (30o-45o) to the northeast, although it is near vertical along strike to the northwest and southeast of the deposit. Tosdal and others (1996) believed that the precious-metal-bearing veins associated with the fault formed during two distinct stages, which resulted from a major change in the regional stress field. The first stage produced a local pressure shadow in the fault zone, which was invaded by hydrothermal fluids to form the high-grade sheeted vein complex. The second stage produced a larger but lower-grade set of veins spread over a 6,000-foot segment of the fault zone. The metalliferous veins are mostly within the tectonic melange in the immediate footwall side of the fault, although some mineralization is present in hydrothermally Knoxville Formation on the immediate hanging wall side of the fault. Width of the deposit appears to be restricted to the southwest by a large mass of serpentinite melange in the footwall and to the northeast by non-altered Knoxville Formation in the hanging wall. Vein thicknesses range from less than an inch to about two feet, but typically are no more than six inches. In general, stage 1 veins strike northeasterly, while stage 2 veins strike southeasterly.

Type: L
Description: Changes from barren rock to high-grade ore were notably very abrupt. Ore grade was observed to correlate more with density of veins rather than type of veins; the sheeted vein complex was notable for its multiple episodes of cross-cutting (at least five have been recognized). The main ore body extended to about 1,000 feet depth, although Sherlock and others (1995) show detected gold down to at least 1,600 feet.

Type: R
Description: Stony Creek Fault

Alterations

Alteration Type: L
Alteration Text: Early Phase: Silica-Carbonate; quartz, chalcedony, calcite, dolomite Coeval Phase: Silicic; quartz, chalcedony, opal Potassic; adularia Sericitic; sericite Argillic; montmorillonite Advanced argillic; alunite Late Phase: Argillic; kaolinite, marcasite, barite

Rocks

Name: Mixed Clastic/Volcanic Rock
Role: Host
Description: volcaniclastic rock
Age Type: Host Rock
Age Young: Pleistocene
Age Old: Late Pliocene

Name: Mixed Clastic/Volcanic Rock
Role: Host
Description: volcaniclastic rock
Age Type: Host Rock
Age in Years: 2.200000+-
Dating Method: K-Ar
Age Young: Late Pliocene

Name: Andesite
Role: Host
Description: basaltic
Age Type: Host Rock
Age in Years: 2.200000+-
Dating Method: K-Ar
Age Young: Late Pliocene

Name: Andesite
Role: Host
Description: basaltic
Age Type: Host Rock
Age Young: Pleistocene
Age Old: Late Pliocene

Name: Basalt
Role: Host
Description: plagioclase
Age Type: Host Rock
Age in Years: 2.200000+-
Dating Method: K-Ar
Age Young: Late Pliocene

Name: Basalt
Role: Host
Description: plagioclase
Age Type: Host Rock
Age Young: Pleistocene
Age Old: Late Pliocene

Name: Tectonic Melange
Role: Host
Description: polymictic melange
Age Type: Host Rock
Age Young: Mesozoic
Age Old: Mesozoic

Name: Mudstone
Role: Host
Age Type: Host Rock
Age Young: Early Cretaceous
Age Old: Late Jurassic

Name: Serpentinite
Role: Host
Age Type: Host Rock
Age Young: Mesozoic
Age Old: Mesozoic

Analytical Data

Not available

Materials

Ore: Gold
Ore: Alunite
Ore: Pyrite
Ore: Stibnite
Ore: Pyrite
Ore: Arsenic
Ore: Realgar
Ore: Orpiment
Ore: Arsenopyrite
Ore: Sphalerite
Ore: Chalcopyrite
Ore: Adularia
Ore: Quartz
Ore: Chalcedony
Ore: Pyrite
Ore: Miargyrite
Ore: Freibergite
Ore: Polybasite
Ore: Electrum
Ore: Pyrargyrite
Ore: Cinnabar
Ore: Metacinnabar
Ore: Mercury
Ore: Galena
Gangue: Opal

Comments

Comment (Location): Deposit occupies parts of three contiguous sections. Location point selected as mine symbol on USGS 7.5-minute quadrangle map, which approximately represents center of south pit of open-pit mine. Mine is accessible by paved road from Lower Lake or Lake Berryessa.

Comment (Geology): Regarding development of transform structures and magmatism in the northern Coast Ranges during the Cenozoic, a spreading ridge initially separated the Farallon and Pacific plates. While the Farallon Plate progressively subducted under the North American Plate, the Pacific Plate and intervening ridge approached the North America continent. The ridge system was locally offset and generally oblique to the subduction zone. Because of the geometry and motion between the plates, a proximal portion of the ridge moved into the subduction zone. At this location, subduction ceased and the North American and Pacific plates made contact. This event marked the birth of a triple junction. This contact essentially bisected the Farallon Plate into two smaller plates, the Juan de Fuca and Cocos Plates. The new triple junction marked the point where the two new plates and the Pacific Plate met. However, it was short lived. As subduction continued, the area of contact between the Pacific and North American Plates lengthened. What was a single triple junction split into two, joined by an incipient transform fault, the proto-San Andreas Fault. With time, the transform lengthened and the triple junctions separated farther. The growth of this proto-San Andreas Fault created a gap, or window, behind the subducting slab as it descended beneath the North American Plate. This window was increasingly enlarged and represented a region on the North American Plate-side of the proto-San Andreas Fault where the process of subduction was no longer operable. The path of the northward-migrating triple junction (Mendocino Triple Junction) is delineated by the San Andreas Fault (Dickinson, 1981, 1997; Atwater, 1970, 1989). A northward-younging sequence of Neogene-Holocene volcanic fields is thought to represent the surficial expression of a progressive upwelling of asthenosphere into the enlarging slab window. In the southern Coast Ranges, the volcanic fields are located along the San Andreas proper. However, in the north, starting just south of San Francisco Bay, the San Andreas Fault splays out into three codominant active strands. The western strand, the main San Andreas Fault, extends directly to the current position of the Mendocino Triple Junction. The volcanism shifted inboard along the eastern splays of the San Andreas system, which include the Bartlett Springs and Collayomi faults. It then extended northward to its current position just north of the Clear Lake volcanic field (Thorkelson and Taylor, 1989; Dickinson, 1981, 1997; Griscom and others, 1993). Debate exists concerning the interaction of the San Andreas Fault and the triple junction and is beyond the scope of this discussion. As the Mendocino Triple Junction and the track of volcanism migrated northward, localized transtension due to wrench faulting accompanied the strike-slip tectonics and produced a N-S string of pull-apart basins. This Neogene faulting as well as inherited Mesozoic structures controlled the locus of shallow magmatism and hydrothermal activity associated with the slab window. The most recent volcanic rocks, the Clear Lake Volcanics and Sonoma Volcanics, and their associated intrusions and hot- spring deposits, are localized within active pull-apart basins along the Collayomi and Bartlett Springs faults (Griscom and others, 1993).

Comment (Geology): Associated with the three main lithologic units described above are deposits of sedimentary serpentine. Such deposits are present in the Wilbur Springs quadrangle, near the Cherry Hill deposit. There, lenses of sedimentary serpentinite are found interbedded in the upper Knoxville Formation of the Great Valley Sequence. The origin of these enigmatic masses has been controversial. They may represent brecciated ophiolite, landslide deposits, or diapirs or protrusions from serpentine mud volcanoes. Recent studies of active mud volcanism in the Mariana subduction complex may provide clues to relationships in the Coast Ranges. The mud volcanoes erupt slab-derived fluids, serpentinite mud, and blocks of blueschist (Fryer, 1992; Fryer and others, 1999; Carlson, 1981a, b; 1984a, b; McLaughlin and others, 1980; Bailey and others, 1964; Phipps, 1992). Interpretations vary regarding the process by which the fault-bounded portions of Coast Range Ophiolite came to reside between the two sedimentary units. Seismic reflection profiles reveal a stack of detached ophiolite-like slabs encased in fault-bounded wedges that underlie most of the eastern margin of the Coast Ranges. The uppermost, subsurface ophiolitic slab produces a prominent magnetic anomaly. It is shaped like an airfoil and measures about 600 km along strike, 10-20 km wide, and about 4 km deep (Griscom and others, 1993). A 3-km-deep geothermal exploration well near the Manzanita Mine in the Sulphur Creek District corroborates the seismic data. It revealed two layers of ophiolite separated by melange and overlain by Great Valley Sequence (McLaughlin and others, 1990). Over the past decade, two structural models have been used to explain the above relationships of the ophiolite with the surrounding sedimentary units. One model suggests that imbricate thrusting produced the stack of wedges and ophiolite slabs. This process inserted Coast Range Ophiolite between the Great Valley Sequence and Franciscan Complex. The other model proposes that the ophiolite slid beneath both units, as the leading edge of the subducting slab of oceanic crust, until it locked up. Eventually the locked ophiolite broke off the oceanic slab, and subduction resumed at a lower level. The wedges are interpreted as abandoned accretionary prisms. The continued subduction drove outboard Franciscan sediments beneath the ophiolite. This process would indicate a cycle of ophiolite generation and wedge abandonment. Later extensional faulting, as mentioned above, exhumed the slab of ophiolite. The first model requires that the bounding faults are thrusts. Conversely, the second model requires them to be normal faults. The kinematics and significance of these faults remains unresolved. Field evidence seems ambiguous and may represent a more complicated history. Tectonic blocks of blueschist, which are thought to have formed deep in the subduction zone, crop out in the Franciscan Complex. Proponents of extensional dynamics have suggested that these blocks were brought up from depth by normal faulting (Platt, 1986; Jayko and others 1987). To the south, Harms and Jayko (1992) determined the timing of such extension in the Diablo Range to about 60-70 Ma. Conversely, Ring and Brandon (1994) suggested that exhumation could be accomplished by both out-of-sequence faulting in the upper plate and erosion. Their model negates the need for extension. Namson and Davis (1988), who conducted structural studies in the southern Coast Ranges, concluded that the Franciscan Complex was thrust eastward over itself, Coast Range Ophiolite, and lower Great Valley Sequence, which developed a series of east-dipping backthrusts that form the tectonic wedge geometry.

Comment (Development): The McLaughlin Mine occupies the site of the former Manhattan Mine, which was operated from the 1860's to 1978 for mercury as part of the highly productive Knoxville District. Although not produced, gold in association with pyrite was historically reported in this district (Averitt, 1945). Analysis of surface samples collected at the Manhattan Mine by Homestake Mining Company during reconnaissance exploration in 1978 also revealed gold. The site was chosen because of its similarities to the Cherry Hill deposit in the Sulphur Creek District to the north. Negotiations for land acquisition were begun in October of that year. Detailed surface exploration by Homestake began in 1978, with exploration drilling conducted from 1979 to 1982. Over 500 holes were drilled, up to a maximum depth of about 2,000 feet. These revealed deposits of microscopic gold confined mainly to depths of less than 1,000 feet. Public announcement of the discovery was made in 1980. Construction of the mine began in 1983, and the first gold was produced in 1985. Mining was completed in 1996. Two identified deposits were mined as open pits, the South Pit and the North Pit. The South Pit was mined first. The two pits eventually merged at the Zodiac sill. The ultimate pit dimensions reached 6.360 feet by 2,530 feet with a depth of 640 feet. Total exploration and development costs were $14,300,000 plus land acquisition costs (Gustafson, 1991). Processing of the remaining stockpile of ore is expected to last until 2002 (Dean Enderlin, Homestake Mining Company, 1999, personal communication). The ore is crushed at the mine, mixed with water, and piped as slurry to the mill, which is about 5 miles to the northwest. Until 1996, the ore was pretreated in autoclaves before treatment with cyanide. Since then, ore has been directly treated with cyanide in a vat process at the mill. The autoclave operated at 320?F at 260 psi with 98% pure oxygen and sulfuric acid. The autoclaving was done to dissolve iron sulfides that contained gold. Following the autoclave, the slurry was washed with water to remove the acid and dissolved metals and to cool the ore. Eventually, autoclaving was considered unnecessary and given up. In current processing at the mill, quicklime and cyanide are added to the slurry. The cyanide leachate is filtered over activated charcoal in a series of tanks called the Carbon-in-Pulp curcuit. Gold is stripped from the carbon using a hot caustic/cyanide solution. The dissolved gold is then electroplated to form a sludge of precious metals, which is smelted with fluxing agents and cast into bars. The mine is being reclaimed for use as a environmental research station for studies under the direction of the University of California. Homestake Mining Company intends to retain ownership of and ultimate responsibility for the land. The open pits are filling with water, and the adjacent mining facilities area, ore stockpile area, and waste rock dumps have been, or will be, recontoured and revegetated to serve as wildlife habitat and watershed. Most surface facilities will be removed. The mill tailings impoundment will be covered with topsoil and revegetated. The mine reservoir will be maintained for water supply and wildlife habitat.

Comment (Economic Factors): Total ore mined was about 38 million tons at an average grade of 0.110 ounces gold/ton. Waste rock mined was about 115 million tons. Ultimate production of gold is expected to be just under 4 million ounces. About 25% of this total was produced from the sheeted vein complex. About 12 flasks of mercury have been produced as a by-product of the gold mining. Gold is still present in the deposit, but the largest observed concentrations were at shallow depths (<700 feet).

Comment (Workings): Two large open pits, the North Pit and the South Pit, merge to create one north-northwest-trending pit, which in plan view is about 6,400 feet long and 2,500 feet wide. The expected bottom elevation of the North Pit was about 1,420 feet, while in the South Pit, it was about 1,270 feet. The maximum depth of mining reached approximately 640 feet below the original ground surface.

Comment (Geology): Introduction In the Coast Ranges, mercury, gold, and silver have been mined since at least the 1880's. A few gold deposits are known to be closely associated with mercury mines. The McLaughlin Mine represents the only known large-volume, world-class gold deposit in the Coast Ranges. It and the Cherry Hill deposit, in the Sulphur Creek District 14 miles to the north, originated in hot-spring environments within the Clear Lake volcanic field. This field is the northernmost and most recent of a NNW-trending chain of Neogene-Holocene volcanic fields that follow and cut across a regional fold and thrust belt better known for petroleum and mercury resources than for gold. The McLaughlin, Cherry Hill, and other hydrothermal systems within the Clear Lake and adjoining Sonoma volcanic fields formed by extensional and compressional tectonics, high heat flow, and intermediate-silicic magmatism (Griscom and others, 1993). The McLaughlin deposit has been well studied. Detailed structural studies by Tosdal and others (1996), regional geophysics by Griscom and others (1993), and geochemical analyses reported by Rytuba (1993) and Sherlock and others (1995) revealed that it formed from a complex combination of Mesozoic and Cenozoic ground preparation, Cenozoic magmatism, and chemical interactions with hydrocarbon-bearing hydrothermal fluids. The deposit consists of two main ore bodies aligned along a NNW-trending fault. Two distinct stages of mineralization, which represent discordant strain fields, are reported by Tosdal and others (1996). Regional Tectonics and Structure The northern Coast Ranges have experienced several deformational episodes (i.e., Mesozoic compression and Neogene-Holocene intermittent dextral translation, transtension and transpression, and volcanism). The resulting complicated structure of the northern Coast Ranges is defined primarily by a broad region of numerous, closely spaced, generally north-northwest-trending faults, folds, ridges, and pull-apart basins (Hearn and others, 1988; Namson and Davis, 1988). A secondary system of short (less than a quarter mile) faults trends east-west (Jennings, 1994). In the Clear Lake region, the NNW-trending structures exceed one mile in length; some represent active components of the San Andreas Fault system. Since the Mesozoic, the development of the northern Coast Ranges has been dominated by the tectonic interaction of three major plates: the North American, Farallon, and Pacific. During this time, the Farallon Plate has been subducting beneath the North American Plate (Thorkelson and Taylor, 1989). Seafloor sediments, exotic terranes, and ophiolites that failed to subduct were accreted to the continental margin. The accreted material formed several packages of north-trending lithotectonic belts. The Coast Ranges are composed of a package of the three westernmost belts. From east to west, they are Great Valley Sequence (forearc basin sedimentary rocks), Coast Range Ophiolite, and Franciscan Complex (thick accretionary prism of sedimentary and volcanic rocks). The Coast Range Ophiolite is an assemblage of serpentinized ultramafic rocks, mafic intrusions, and submarine volcanic rocks. The ophiolite is tectonically dismembered. In some localities, it occurs as a serpentinite-matrix melange (Hopson and others, 1981). In a few areas, large lenses of sedimentary serpentinite, presumably derived from subjacent ophiolite, are interbedded with lower Cretaceous and Miocene sediments. The Franciscan Complex and Great Valley Sequence are coeval and formed on opposite sides of the subduction zone. With time, these marine deposits were progressively compressed and uplifted in a fold and thrust belt. Once uplifted, this over-thickened package may have become gravitationally unstable leading to collapse through extensional faulting (Platt, 1986).

Comment (Geology): In the Clear Lake area, several lines of geophysical, geochemical, and geologic evidence suggest that a NE-trending zone of extension cuts across NW-trending structures. The zone extends from the Collayomi Fault in the west to at least the Bartlett Springs Fault in the east. Farther east, NE-trending structures in the Sulphur Creek District seem to be related to the zone. The Clear Lake and Sonoma volcanics occur within and may be genetically related to this zone also (Stanley and others, 1997). Overall, the zone of extension corresponds with a NE-trending basement structure proposed by Griscom and others (1993), which trends N70E and is defined by the alignment of magnetic and gravity anomalies and the alignment of the Geysers geothermal field, the Sonoma and Clear Lake volcanics, and the Sutter Buttes volcano. The gold deposits at McLaughlin and Sulphur Creek lie above the intersection of the proposed structure and the contact between Coast Range Ophiolite and Great Valley Sequence. Also, above this intersection is a possible local window in the ophiolite slab, indicated by an anomalous magnetic low. These deep structures may have been important in the development of the deposits (Griscom and others, 1993). Hot Springs-Type Mineral Deposits Several hot-springs-type mineral deposits are present within the Clear Lake and Sonoma volcanic fields. Some are still hydrothermally active. Hydrothermal activity at the McLaughlin Mine persisted from about 1.0 to 0.5 Ma (Dean Enderlin, Homestake Mining Company, personal communication, 1999). Sulphur Bank Mine, at Clear Lake, is in an active vapor-dominated hydrothermal system, which continues to deposit mercury, but not precious metals. Precious-metal deposition is restricted to water-dominated hydrothermal systems, such as at Sulphur Creek District, where hot springs are actively depositing gold and mercury (Rytuba, 1993). Local Geology The McLaughlin deposit was hosted in both Coast Range Ophiolite and Knoxville Formation. In the area of the deposit, Coast Range Ophiolite is a tectonic melange divided into two fault-bounded packages, which strike NW and dip moderately NE. Internal stratigraphy, faults, and fabrics within the two packages are subparallel to the strike of the major rock units in the area. The lowest structural package of the tectonic melange is derived from mafic and ultramafic rock of the ophiolite. The upper package is composed of serpentinite and a polymictic melange, which consists of blocks of greenstone and sedimentary rocks within a serpentinite matrix. The sedimentary rocks were derived from the Knoxville Formation. Blocks range in size from meters to kilometers in length, and are commonly tens of meters thick. The Upper Jurassic Knoxville Formation is a flysch unit of the Great Valley Sequence composed of mudstone, siltstone, graywacke, and minor conglomerate. The deposit consists of two main ore bodies, which were situated along a fault considered by Tosdal and others (1996) to be a southern continuation of the Stony Creek Fault. The fault generally strikes northwesterly and dips moderately (30o-45o) northeast in the deposit, but is near vertical immediately to the northeast and southwest of the deposit. The fault juxtaposes a footwall of the tectonic melange against a hanging wall of the Knoxville Formation. Tosdal and others (1996) believed that slip along the fault during mineralization was minimal.

Comment (Environment): Rugged, narrow valleys and NW-trending ridges with peaks up to several thousand feet in elevation parallel the regional structure of this area. At the mine site, the original elevation ranged from 1,160-2,800 feet. Numerous deep seated landslides occur within the ophiolitic melange. The Mediterranean climate delivers approximately 30 inches of rain per year, 90% of which falls between October and April. The three creeks in the deposit area are intermittent, either dry or nearly dry in the summer. Vegetation types and densities vary depending on the underlying lithologies: ultramafic-serpentinite areas generally support chaparral and juniper with some scrub pine, whereas Knoxville Formation and volcanic rock generally support a mixed grassland and low-density broadleaf woodland.

Comment (Deposit): The ore deposit developed in a shallow epithermal, hot-spring environment, centered around a sheeted vein complex, which was immediately below a siliceous sinter terrace. Hydrothermal explosion breccias, chalecedony veins, and maar deposits indicate periodically explosive fluid flow. The geochemistry of the ore fluids is similar to that observed at Cherry Hill, an active gold-depositing hot-spring, located about 14 miles to the north. Oxygen and hydrogen isotope studies and fluid inclusions studies show that the ore fluids developed as a boiling, low-salinity (~2.4 weight % NaCl equivalent), low-CO2 mixture of three distinct fluids: evolved, isotopically heavy, petroleum- and methane-rich connate water, magmatic fluids, and meteroic water. The gold, found in the hydrocarbon-bearing opal occurs 1) as a coating in large pores containing fluid inclusions, 2) as small crystals, which coalesce to form dendrites, and 3) within syneresis cracks that cut the vein banding (Rytuba, 1993). Gold and silver mineralization are confined largely to less than about 1,100 feet depth below the original ground surface of the deposit as represented by the sinter terrace. In the sheeted vein complex, gold was locally more abundant than silver only in the upper 700 feet; from 700 feet to 1,100 feet silver was dominant with minor gold. Below this depth, base metals are dominant. Shallow conditions of ore formation are also indicated by geochemical evidence of a system dominated by meteoric fluid and a boiling phase as well as open textures in veins and the presence of opal and chalcedony. Boiling of the hydrothermal system is interpreted to be the dominant control on gold mineralization and the vertical metal zoning in the deposit. Temperature of formation of the deposit, based on fluid inclusion studies, ranged from 121o-263oC.

Comment (Commodity): Ore Materials: Electrum, pyrargyrite, native gold, pyrite, miargyrite, freibergite, polybasite, cinnabar, metacinnabar, native mercury.

Comment (Commodity): Gangue Materials: Siliceous sinter, opal, chalcedony, quartz, adularia, alunite, pyrite, stibnite, arsenian pyrite, native arsenic, realgar, orpiment, arsenopyrite, sphalerite, chalcopyrite, galena.

Comment (Geology): The two ore bodies, when considered together, formed a wedge-shaped deposit about 1.5km long, 200m wide at the surface that tapered downward. The transition from high-grade ore to barren rock was abrupt. Mineralization was largely restricted to the upper 350 meters of the fault zone, although Sherlock and others (1995) report gold down to 560 meters. Mineralization was more intense in the footwall (melange) than in the hanging wall (Knoxville Formation). Veins, both as discrete stringers and swarms, were widely distributed along the fault and typically contained abundant carbonates, including magnesite. The veins ranged in thickness from a few to tens of centimeters, averaging about 15 centimeters. A stockwork of discrete veins characterize both ore bodies. However, the south ore body, the site of the former Manhattan Mine, additionally contained a sheeted vein complex, which represented an earlier phase of vein development. The sheeted veins strike N52E, discordant to the regional fabric, and occur in the melange adjacent to a large, entrained block of competent basalt. The second stage of mineralization occurred after the end of the first stage. The stage 2 veins cut stage 1 veins and are concordant with regional structures, indicating a 90? rotation of the strain field. Ore grades were highest where vein density was high, regardless of the stage. For example, at least five sets of crosscutting veins were found in the high-grade sheeted vein complex. To explain the apparent rotation in the stress field, Tosdal and others (1996) suggested that a deeper and more extensive ramp and thrust system underlies the area. First, they postulated that 1) movement along the deeper thrust fault rotated the basaltic block and 2) the sheeted veins formed in a pressure shadow behind the block. Then, stage 1 vein development shut off, and stage 2 veins developed as the upper plate bent as it moved over a ramp. The veins filled fractures that developed in the bend for 2 km along strike. It is noted, however, that orientation of the stage 2 veins does not correspond to any recognized regional strain field. The sheeted vein complex (also described as a pipe-like feature) was immediately below the sinter and consisted of banded crustiform/colloform veins of several phases of silica: white-clear opal, amber-brown opal, clear-milky white quartz, and chalcedony. The amber-brown opal, which resulted from interactions with hydrocarbons, contained the highest gold-grades. Outside of the sheeted vein complex, however, that relationship did not hold true. Also localized along the Stony Creek Fault at the deposit are four basaltic-andesitic intrusions (2.2 Ma), which may have provided at least some of the heat that drove local hydrothermal activity. At the ground surface, hydrothermal activity during ore deposition produced opaline (now chalcedonic) sinter interbedded with explosion breccia, which was still preserved when the deposit was discovered in 1978. A previous mining operation in the deposit, the Manhattan Mine, produced at least 17,000 flasks of mercury from veins that crosscut the sinter and surrounding country rock. Wall-rock alteration spatially associated with gold mineralization consisted of pervasive silicification and adularization. Locally, there were zones of intense argillic alteration near the surface. These zones included pockets of ammonium-bearing alunite and buddingtonite.

Comment (Identification): McLaughlin Mine is a gold/silver mine operating at the former site of the Manhattan Mine (mercury).

Comment (Commodity): Commodity Info: Although small amounts of native gold are visible to the eye, most gold is present in microscopic size. The gold is present in veins only; there is no reported disseminated gold. Sulfide content in the remaining stockpile is about 1-2%. The silver/gold ratio increased with depth in the deposit.

References

Reference (Deposit): Griscom, A. and others, 1993, Regional geophysical setting of gold deposits in the Clear Lake region, California, in Rytuba, J.J., editor, Active geothermal systems and gold-mercury deposits in the Sonoma-Clear Lake volcanic fields, California: Society of Economic Geologists Guidebook Series, v. 16, p. 289-310.

Reference (Deposit): Gustafson, D.L., 1991, Anatomy of a discovery: The McLaughlin gold mine, Napa, Yolo, and Lake counties, California: Economic Geology Monograph 8, p. 350-359.

Reference (Deposit): Harms, T.A. and others, 1992, Kinematic evidence for extensional unroofing of the Franciscan Complex along the Coast Range Fault, northern Diablo Range, California: Tectonics, v. 11, no. 2, p. 228-241.

Reference (Deposit): Hearn, B.C., Jr., and others, 1988, Tectonic framework of the Clear Lake basin, California: Geological Society of America Special Paper 214, p. 9-20.

Reference (Deposit): Hopson, C.A. and others, 1981, Coast Range ophiolite, western California, in Ernst, W.G., editor, The geotectonic development of California: Prentice-Hall, Englewood Cliffs, New Jersey, p. 418-510.

Reference (Deposit): Jayko, A.S. and others, 1987, Attenuation of the Coast Range Ophiolite by extensional faulting, and nature of the Coast Range ?Thrust?, California: Tectonics, v. 6, no. 4, p. 475-488.

Reference (Deposit): Jennings, C. W., 1994, Fault activity map of California and adjacent areas with locations and ages of recent volcanic eruptions: California Division of Mines and Geology, Geologic Data Map No. 6, scale 1:750,000.

Reference (Deposit): McLaughlin, R. J. and others, 1980, Structure of Late Mesozoic rocks in the core of the Wilbur Springs Antiform, northern Coast Ranges, California: Geological Society of America Abstracts with Programs, v. 12, no. 3 , p. 119.

Reference (Deposit): McLaughlin, R. J. and others, 1990, Geologic map and structure sections of the Little Indian Valley-Wilbur Springs geothermal area, northern Coast Ranges, California: U. S. Geological Survey Miscellaneous Investigations Series Map I-1706, scale 1:24,000.

Reference (Deposit): Namson, J.S. and Davis, T.L., 1988, Seismically active fold and thrust belt in the San Joaquin Valley, California: Geological Society of America Bulletin, v. 100, no. 2, p. 257-273.

Reference (Deposit): Peters, E.K., 1991, Gold-bearing hot spring systems of the northern Coast Ranges, California: Economic Geology, v. 86, p. 1519-1528.

Reference (Deposit): Nelson, C.E., 1987, Gold deposits in the hot springs environment, in Schafer, R.W. and others, editors, Bulk mineable precious metal deposits of the western United States: Symposium Proceedings of the Geological Society of Nevada, p. 417-432.

Reference (Deposit): Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North America: Geologic Society of America Bulletin, v. 81, no. 12, p. 3513-3536.

Reference (Deposit): Atwater, T., 1989, Plate tectonic history of the northeast Pacific and western North America, in Winterer, E.L. and others, editors, The eastern Pacific Ocean and Hawaii: Geological Society of America, The Geology of North America, Vol. N, p. 21-72.

Reference (Deposit): Averitt, P., 1945, Quicksilver deposits of the Knoxville District, Napa, Yolo, and Lake counties, California: California Journal of Mines and Geology, v. 41, no. 2, p. 65-89.

Reference (Deposit): Bailey, E.H. and others, 1964, Franciscan and related rocks and their significance in the geology of western California: California Division of Mines and Geology Bulletin 183, 177 p.

Reference (Deposit): Berger, B.R., 1986, Descriptive model of hot-spring Au-Ag, in Cox, D.P. and Singer, D.A., editors, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 143-144.

Reference (Deposit): Carlson, C., 1981a, Sedimentary serpentinites of the Wilbur Springs area -a possible Early Cretaceous structural and stratigraphic link between the Franciscan Complex and the Great Valley Sequence: Master's thesis, Stanford University, 105p.

Reference (Deposit): Phipps, S.P., 1992, Late Cenozoic wedging and blind thrusting beneath the Sacramento Valley and eastern Coast Ranges, in Erskine, M.C. and others, editors, Field guide to the tectonics of the boundary between the California Coast Ranges and the Great Valley of California: American Association of Petroleum Geologists, Pacific Section, p. 63-84.

Reference (Deposit): Sherlock, R.L. and others, 1995, Origin of the McLaughlin Mine sheeted vein complex: Metal zoning, fluid inclusion, and isotopic evidence: Economic Geology, v. 90, p. 2156-2181.

Reference (Deposit): Stanley, W. D. and others, 1997, Tectonic controls on magmatism and geothermal resources in the Geyers-Clear Lake region, California: Integration of new geologic, earthquake tomography, seismicity, gravity, and magnetotelluric data: U. S. Geological Survey Open File Report 97-95, 40p.

Reference (Deposit): Thorkelson, D.J. and Taylor, R.P., 1989, Cordilleran slab windows: Geology, v. 17, no. 9, p. 833-836.

Reference (Deposit): Tosdal, R.M. and others, 1995, Structural evolution of the McLaughlin precious metal deposit, northern California, in Geology and ore deposits of the American Cordillera; a symposium: Geological Society of Nevada, United States, 76 p.

Reference (Deposit): Tosdal, R.M. and others, 1996, Precious metal mineralization in a fold and thrust belt: The McLaughlin hot spring deposit, northern California, in Coyner, A.R. and Fahey, P.L., editors, Geology and ore deposits of the American Cordillera: Geological Society of Nevada Symposium Proceedings, Reno/Sparks, Nevada, April 1995, p. 839-854.

Reference (Deposit): Wakabayashi, J. and Unruh, J.R., 1995, Tectonic wedging, blueschist metamorphism, and exposure of blueschists: Are they compatible?: Geology, v. 23, no. 1, p 85-88.

Reference (Deposit): D?Appolonia Consulting Engineers, Inc., 1983, Project description and environmental assessment report, prepared for Homestake Mining Company: unpublished report (CDMG Library, Sacramento).

Reference (Deposit): Phipps, S.P. and Unruh, J.R., 1992, Crustal-scale wedging beneath an imbricate roof-thrust system: Geology of a transect across the western Sacramento Valley and northern Coast Ranges, California, in Erskine, M.C. and others, editors, Field guide to the tectonics of the boundary between the California Coast Ranges and the Great Valley of California: American Association of Petroleum Geologists, Pacific Section, p. 117-140.

Reference (Deposit): Platt, J.P., 1986, Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks: Geological Society of America Bulletin, v. 97, no. 9, p. 1037-1053.

Reference (Deposit): Ring, U. and Brandon, M.T., 1994, Kinematic data for the Coast Range Fault and implications for exhumation of the Franciscan subduction complex: Geology, v. 22, no. 8, p. 735-738.

Reference (Deposit): Rytuba, J.J., 1993, Epithermal precious-metal and mercury deposits in the Sonoma and Clear Lake volcanic fields, California, in Rytuba, J.J., editor, Active geothermal systems and gold-mercury deposits in the Sonoma-Clear Lake volcanic fields, California: Society of Economic Geologists Guidebook Series, v. 16, p. 38-51.

Reference (Deposit): Sherlock, R.L. and Lehrman, N.J., 1995, Occurrences of dendritic gold at the McLaughlin Mine hot-spring gold deposit: Mineralium Deposita, v. 30, p. 323-327.

Reference (Deposit): Donnelly-Nolan, J.M. and others, 1993, The Geysers-Clear Lake area, California: Thermal waters, mineralization, volcanism, and geothermal potential: Economic Geology, v. 88, p. 301-316.

Reference (Deposit): Enderlin, D.A., 1993, Epithermal precious metal deposits of the Calistoga mining district, Napa County, California, in Rytuba, J.J., editor, Active geothermal systems and gold-mercury deposits in the Sonoma-Clear Lake volcanic fields, California: Society of Economic Geologists Guidebook Series, v. 16, p. 52-76.

Reference (Deposit): Forstner, W., 1903, The quicksilver resources of California: California State Mining Bureau Bulletin 27, p. 81-89.

Reference (Deposit): Fox, K.F., Jr., 1983, Tectonic setting of Late Miocene, Pliocene, and Pleistocene rocks in part of the Coast Ranges north of San Francisco, California: U.S. Geological Survey Professional Paper 1239, 33 p.

Reference (Deposit): Fryer, P., 1992, Volcanoes of the Marianas: Scientific American, v. 266, no. 2, p. 46-52.

Reference (Deposit): Fryer, P. and others, 1999, Mariana blueschist mud volcanism: Implications for conditions within the subduction zone: Geology, v. 27, p. 103-106.

Reference (Deposit): Carlson, C., 1981b, Upwardly mobile melanges, serpentinite protrusions, and transport of tectonic blocks in accretionary prisms: Geological Society of America Abstracts with Programs, v. 13, no. 2, p. 48.

Reference (Deposit): Carlson, C., 1984a, Depositional environments and sedimentary facies of foliate serpentinite breccias, Wilbur Springs, in Carlson, C., editor, Depositional facies of sedimentary serpentinite: Selected examples from the Coast Ranges, California: Society of Economic Paleontologists and Mineralogists Field Trip Guidebook No. 3, Tulsa, Oklahoma, p. 113-116.

Reference (Deposit): Carlson, C., 1984b, Stratigraphic and structural significance of foliate serpentinite breccias, Wilbur Springs, in Carlson, C., editor, Depositional facies of sedimentary serpentinite: Selected examples from the Coast Ranges, California: Society of Economic Paleontologists and Mineralogists Field Trip Guidebook No. 3, Tulsa, Oklahoma, p. 108-112.

Reference (Deposit): Chapman, R.H. and others, 1982, Gravity, structure, and geothermal resources of the Calistoga area, Napa and Sonoma counties: California Geology, v. 35, no. 8, p. 175-183.

Reference (Deposit): Dickinson, W.R., 1981, Plate tectonics and the continental margin of California, in Ernst, W.G., editor, The geotectonic development of California (Rubey volume 1), Prentice-Hall, Englewood Cliffs, New Jersey, p. 1-28.

Reference (Deposit): Dickinson, W. R., 1997, Tectonic implications of Cenozoic volcanism in coastal California: Geological Society of America Bulletin, v. 109, p. 936-954.

California Gold

"Where to Find Gold in California" looks at the density of modern placer mining claims along with historical gold mining locations and mining district descriptions to determine areas of high gold discovery potential in California. Read more: Where to Find Gold in California.