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The McLaughlin gold mine is located at Knoxville in Napa and Yolo Counties within central California, USA, ~100 km north of San Francisco. It falls within a wide fault zone which marks the terrane boundary between the Coast Range and the Great Valley. This fault is a major east dipping thrust which has subsequently undergone large scale dextral transcurrent movement related to the San Andreas Fault (#Location: 38° 50' 8"N, 122° 21' 33"W).
The McLaughlin orebody was discovered in 1978-79 by the Homestake Mining Company, who mined the deposit between 1985 and 2002 to produce ~105 tonnes of gold.
Intermittent small scale mercury mining had been undertaken in the surrounding district from 1879 to 1974. Gold had also been recorded in the area in 1880, although there had been no follow-up on that report. One of the more significant mercury operations had been the Manhattan Quicksilver Mine which operated from 1960 until 1978 and produced 70 000 to 80 000 flasks of Hg. Other mines included the Reed Mine to the north-west and the Knoxville Mine to the south-east which produced 27 000 and 125 000 flasks respectively. The three deposits are located within the same major fault zone and are distributed over a 7.5 km interval of that fault (Brunker and Dickson, 1987; Lehrman, 1987).
Published reserve figures included:
Pre-mine Resource, 1984 - 17.5 Mt @ 5.2 g/t Au = 91 t Au (Bonham, 1988)
Reserve, 1982 - 18.1 Mt @ 5.5 g/t Au (Gustafson, 1990)
Geological Mineable Inventory, 1982 - 23.8 Mt @ 4.8 g/t Au (Gustafson, 1990)
Geological Inventory, 1982 - 35.6 Mt @ 4.1 g/t Au (Gustafson, 1990)
Production 1985-87 - 5.15 Mt (I M Clementson & others, 1987)
Reserve 1987 - 20 Mt @ 4.15 g/t Au (I M Clementson & others, 1987)
Reserve, 1993 - 7.3 Mt @ 3.1 g/t Au (American Mines Handbook, 1995)
Proven + Probable Reserve, 1994 - 20 Mt @ 2.9 g/t Au (AME, 1995).
The McLaughlin orebody was mined by open cut. In 1986-87 the stripping ratio was 10:1, waste:ore, although an overall average of 6:1 was anticipated for the total mine life. The ultimate pit was expected to be 1650 x 600 m in plan area and have a maximum depth of 270 m, but averaging 140 m. The total waste to be removed was expected to be more than 120 Mt (Lehrman, 1987; I M Clementson and others, Mine visit, 1987).
The McLaughlin gold deposit is localised within a broad fault zone that varies from 100 to 250 m in width and separates the Coast Range and Great Valley terranes of western California. This fault trends in a north-westerly direction, has a substantial strike length and is part of a major structure, the Stony Creek Fault, that is hundreds of kilometres in length. This fault was originally an east dipping thrust structure across which the Great Valley Sequence was overthrust onto the Franciscan Complex of the Coast Range Terrane. The thrust has subsequently been rotated and steepened, ranging from a dip of 40°NE in the mine area, to near vertical further to the north-west (Lehrman, 1987). The fault also steepens with depth. Subsequent to the thrust movement, this fault zone has been part of the San Andreas dextral, transcurrent, fault system. In detail the McLaughlin orebody, as well as the adjacent Reed and Knoxville Hg mines, are developed within jogs in the Stony Creek fault (Brunker and Dickson, 1987). Movement on the main San Andreas fault system has mainly been post 29 Ma.
The Franciscan Complex, which belongs to the Outer Super-Terrane and is upper Jurassic to Eocene in age, includes sandstone-mudstone turbidites, with locally interbedded radiolarian ribbon chert, and minor basaltic flows and tuffs. It has a central longitudinal band comprising a tectonic mélange consisting of blocks of sedimentary, volcanic, ultramafic and metamorphic rocks in a matrix of argillite, greywacke and tuff, ranging from late Jurassic to mid Cretaceous in age. Fault bounded bands of 160 Ma, Middle Jurassic, ophiolite are found within the mélange. The Franciscan Complex, which has been metamorphosed to blue-schist facies, appears to comprise lithologically similar, but more deformed, equivalents of the Great Valley sequence, which is immediately to the east, separating it from the Sierra Nevada Terrane of the Eastern Super-Terrane (Cowan and Bruhn, 1992; Oldow, et al., 1989). Within the mine area, the Franciscan Complex is represented by ophiolitic mélange which dominantly consists of serpentinite with scattered large knockers of greenstone and greywacke (Lehrman, 1990).
The rocks of the Great Valley Terrane are from upper Jurassic to Recent in age. The sequence commenced with upper Jurassic strata which are dominantly mudstones, locally interbedded with thin sandstones, and cut by conglomerate filled channels. Ophiolitic basement supplied sporadic serpentine rich debris flow deposits and sandstones containing clasts of basalt and gabbro. Overlying lower Cretaceous sediments were primarily sandstones and mudstones, supplied by the Cretaceous Sierra Nevada volcanic arc to the east, with an increasing proportion of feldspar and quartz in sandstones of progressively younger age (Cowan and Bruhn, 1992; Miller, et al., 1992; Christiansen and Yeats, 1992).
During the Cainozoic, the granitic Salinian Terrane to the south-west moved progressively to the north-west, along the NW-SE trending proto-San Andreas transverse dextral fault, to form a western margin to the basin in the south, while in the north it remained open to the ocean to the west. The northern segment was filled with a cyclic succession of up to 2500 m thick of bathyal shales and sandstones, punctuated by shallow shelf to deltaic sandstones, corresponding to periods of low sea level. During the Oligocene, volcanic detritus again contributed to the sequence from rivers originating in the Great Basin to the north-east. In the Miocene, volcanic derived marine sedimentation is interpreted in the northern segment of the Great Valley, although by the Pliocene it had become a terrestrial lowland, with the formation of the Coast Ranges to the west, and was filled with terrestrial sediments derived from both the east and the west. This transition corresponded to a change in the composition of sandstones, from volcanic to granite derived detritus (Cowan and Bruhn, 1992; Miller, et al., 1992; Christiansen and Yeats, 1992). Within the mine area the Great Valley Succession is represented by mudstone, siltstone and minor conglomerate (Lehrman, 1990).
The Stony Creek Fault zone within the mine area is occupied by 100 to 250 m of cataclasites derived from both the hangingwall and footwall lithologies, although those of the footwall ophiolite mélange predominate. Included within the cataclasites are relatively coherent, rootless lithons of pillowed sea floor basalt with maximum dimensions of up to a kilometre (Lehrman, 1990; Brunker and Dickson, 1987).
A cluster of small olivine rich andesitic basalt plugs was emplaced within and near the fault zone at about 2.2 Ma. These plugs have a mushroom structure and become more acid as they approach the surface. The initial manifestations of this magmatic activity in the mine area was a period of phreato-magmatic eruptions that excavated three known maar craters that were a minimum of 60 m deep and 300 to 350 m across at their highest mapped extent. These were the Manhattan, Gail and Johnstown maars, on the northern extremity, centre and southern limit respectively of the ore deposit. The maars were filled with agglomerates representing the phreatomagmatic explosion debris. Only one of the three craters, the Johnstown maar, was intruded by a basaltic plug, which had a mushroom-shaped appearance in cross-section and upper plan dimensions of ~120 x 220 m (Lanier 1983). Ejecta blankets surrounding the maar complex contain angular clasts of all underlying rocks and of basalt, with minor pumice. A few gold bearing chalcedonic vein fragments are found as clasts. Gold mineralisation is spatially coincident with the diatreme complexes, and the basalts are mineralised (Lehrman, 1990).
A large sill was encountered intruding the Stony Creek Fault in a decline just north of the Gail maar towards the centre of the deposit (Enderlin, 1995). This may run the length of the ore deposit, on its NE side, and was probably the heat source for the hot springs that formed the ore deposit (Jeff Wilson pers. comm., 2017).
Two or more sinter terraces were formed at the surface. These are composed of porcelaneous, porous, white chalcedony, showing features of surface deposition. The preserved sinter pile in the main central terrace (the San Quentin sinter) was about 120 m in diameter, and locally at least 30 m thick. Interbedded with the sinter are numerous hydrothermal explosion breccias containing blocks of sinter up to 5 m in diameter, accompanied by clasts of underlying rock types (Lehrman, 1990).
Mineralisation and Alteration
The mineralisation at McLaughlin comprises an Ag-Au, stock work, hot springs deposit (Lanier and Wilson 1983). It is syn-tectonic, controlled by movement along the associated fault, but is also associated with the maar complexes and emplacement of the basaltic plugs. The presence of mineralised chalcedony within the maar ejecta implies that mineralisation commenced early in the volcanic phase, prior to the emplacement of the basalt pugs. However as the plugs are mineralised, it continued until after they were in place (Lehrman, 1990).
The most intense hydrothermal activity occurred following the lithification of the extrusive parts of the olivine-pyroxene basalt 'mushrooms'. Hypogene alunite found within the 2.2 Ma basalts and underlying agglomerates yields K-Ar dates of 0.75 Ma. The agglomerate is typically silica flooded, while auriferous veins cut both the silicified agglomerates and the overlying basalts. These are in turn overlain by the sinters described above. The sinters are generally not anomalous with respect to 'epithermal elements', with the exception of mercury, which is present as diffuse cinnabar pigmentation pervading the chalcedony, and as cinnabar paint on cross-fractures. Most of the historical production from the Manhattan Mine was also derived from this style of mineralisation, which was located within 20 m of the surface (Lehrman, 1990).
The mineralised zone below the sinter consists of multiple stage chalcedony-quartz vein stockwork with accompanying pervasive silica flooded cataclasite, agglomerate and breccia lithologies. The veining within these rocks is believed to be controlled in part by hydro-fracturing associated with the hydrothermal processes. Each stage of the siliceous sinters are interpreted to have plugged the vents, which were then fractured and mineralised by the subsequent pulse of hydrothermal activity and sinter out-pouring. The veins strike north-east, transverse to the overall south-east orebody trend which is generally conformable with the strike of the fault. The configuration of the ladder veins suggests continued dilation by strike-slip reactivation of the thrust. Sheeted veins, which may locally be up to 30 m wide, and are made up of 2 to 60 cm wide, bilaterally symmetrical vein components. This is taken to reinforce the suggestion that the fault zone was being repeatedly opened and re-opened by episodic movement. The chalcedonic veins commonly exhibit multiple irregular banding. Gold mineralisation is closely associated with silicification, and particularly to multiple stage veining. All rocks types in the deposit area locally host ore grade mineralisation, although the rheological properties of each rock type influences the vein size, geometry and abundance (Lehrman, 1990).
Regionally there is a temperature gradient outwards from the McLaughlin Mine. Within the orebody there is also a temperature gradient, increasing from footwall to hangingwall, indicated by the development of opal, through chalcedony to quartz (Brunker and Dickson, 1987).
The orebody is wedge shaped in cross-section and dips to the north-east in conformity with the host fault. Significant mineralisation occurs over a vertical interval of ~345 m, between 660 and 315 m elevations, and appears to die out below this depth. In plan view the orebody trends NW-SE at ~145° and dips at 40°NE. It and persists over a minimum strike length of 1800 m and has a maximum continuous ore width of 180 m near the surface, thinning with depth (Lanier and Wilson 1983; Lehrman, 1990).
Alteration, which is no more extensive than the gold mineralisation, consists of siliceous and argillic to advanced argillic, with alunite. It is a very irregular hybrid system with much overprinting due to repeated pulses of hydrothermal activity. In addition, the variations in protolith results in a range of alteration products. A surficial advanced argillic zone with alunite nodules is sporadically present. Most of the deposit is characterised by a green tint due to a montmorillonite-chlorite-celadonite assemblage. Within the mineralised hangingwall mudstone of the Great Valley sediments, pervasive microscopic adularia is common, with spotty advanced argillic alteration. Within the footwall an irregular zone of silica-carbonate alteration parallels the precious metal distribution and extends for about 1.5 km off either end of the system. This silica-carbonate alteration is early, and may be pre-mineralisation. The major alteration minerals are celadonite, chlorite, montmorillonite and silica ranging from opal through chalcedony to quartz. Spotty argillic alteration occurs in the footwall, while there is a trend from celadonite-chlorite-montmorillonite in the footwall to chalcedonic silica in the hangingwall (Lehrman, 1990; Brunker and Dickson, 1987).
Mineralisation is associated with siliceous veining, with temperatures of formation estimated at between 130° and 150°C (Lehrman, 1990). Within the siliceous matter the gold occurs as fine native flakes, wires and dendritic aggregates and electrum, typically around 760 fine. Grains have been measured (Dr. R. M. Honea) and range in size from <1 to 300 µm, although it is estimated that more than 99% of the Au is <45 µm in length (Lanier and Wilson 1983). Coarser gold becomes more prevalent with depth. Silver is present as electrum and as sulpho-salts, the most prominent being pyrargyrite. The Ag:Au ratio increases from 0.1 at the surface to 100 or more at depth, averaging 3.2 overall. The gold content also fades out with depth, having become un-economic by ~350 m below the surface. A similar trend is apparent laterally. Au and Ag are the only economic minerals, with the best grades generally being associated with low sulphide zones (Brunker and Dickson, 1987; Lehrman, 1990).
There is also an apparent strong relationship between gold and hydrocarbons within the orebody (Brunker and Dickson, 1987).
Sb is abundant as stibnite and as sulpho-salts. The grade of gold increases slightly with stibnite. Barite is common near the surface, while calcite and dolomite become prominent in the gangue assemblage outwards and downwards from the main ore zone. From 1 to 2% non-auriferous pyrite and marcasite are typically present, although locally these may be up to 7%. These Fe sulphides do not correlate positively with gold mineralisation. Other elements typically associated are As, B, Tl, W and Hg. A prominent Ni-Co-Cr signature may be related to the presence of serpentinites. Base metals and Mn are lacking, as are Se, Te, F, U and Mo (Lehrman, 1990; Brunker and Dickson, 1987). Lanier and Wilson (1983) note that a particularly strong anomaly in Sb which is >20 000 times background was encountered at the deposit.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1995.
Record last updated: 19/1/2017
This description is a summary from published sources, the chief of which are listed below.
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References to this deposit in the PGC Literature Collection:
Lehrman N J, Homestake Mining Co. 1987 - The McLaughlin mine, Napa and Yolo Counties, California: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 197-201|
Sherlock R L 2005 - The relationship between the McLaughlin gold–mercury deposit and active hydrothermal systems in the Geysers–Clear Lake area, northern Coast Ranges, California: in Ore Geology Reviews v26 pp 349-382|
Sherlock R L, Tosdal R M, Lehrman N J, Graney J R, Losh S, Jowett E C, Kesler S E 1995 - Origin of the McLaughlin mine sheeted vein complex: metal zoning, fluid inclusion, and isotopic evidence: in Econ. Geol. v 90 pp 2156-2181|
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