Carlin Trend - Geology
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The Carlin Trend gold district is located within the Great Basin geologic province of Nevada, USA. The host rocks of the deposits include sediments ranging in age from Ordovician to Carboniferous (Mississippian), as well as a Jurassic to Cretaceous granodiorite stock and associated dykes.
The deposits of the Carlin Trend include:Ivanhoe, Hollister, Cornucopia, Storm, Dee, Arturo South, Bootstrap, Capstone, Ren, Meikle, Purple Vein, Rodeo, Goldbug, Griffin, Barrel, Screamer, Bazza, Long Lac, Post, Goldstrike, Betze, Pancana, Teamster, Star, Beast, Bobcat, Genesis, Blue Star, SOLD, Payraise, Lantern, Exodus, Turf, Four Corners, Leevilles, Carlin, Perry, Fence, Pete, Mike, Tusc, MAC, Gold Quarry, Rain, SMZ, Gnome, Emigrant. See the images in the Carlin Trend - Mineralisation record for locations, geological setting and tonnage-grade statistics.
During the assembly of the Laurentia section of the Nuna-Columbia supercontinent in the mid Palaeoproterozoic, Palaeoproterozoic terranes were accreted to the Archaean
Wyoming craton. This accretion defined a suture zone in the Cheyenne belt of Wyoming, trending westward into northern Nevada where it forms a broad zone of intermixed
Palaeoproterozoic and Archaean rocks (Lush et al., 1988; Tosdal et al., 2000). Between 1.3 and 1.0 Ga and 0.9 to 0.6 Ga, in the Meso- and Neoproterozoic respectively, rifting and consequent thinning (Karlstrom et al., 1999; Timmons et al., 2001) resulted in a progressively westward-thinning margin of continental crust (Tosdal et al., 2000). A westward-thickening wedge of Neoproterozoic and early Cambrian clastic rocks was deposited above the thinned crystalline basement during the late Neoproterozoic rifting (Stewart, 1972, 1980; Poole et al., 1992).
As the Neoproterozoic rifting phase waned in the early Palaeozoic, a miogeoclinal sequence developed on the continental margin of the North American craton. A westward thickening and deepening wedge of sediments was deposited on the cratonic margin and slope. Lithologies graded westward from continental carbonates on the craton, through interbedded carbonate and shale on the shelf to the east, grading to silty carbonate rocks along the edge of the continental slope to fine grained siliceous clastic and cherty units of the continental slope. Restricted basins developed along the shelf disrupting the regularity of this setting and resulting in important district-scale stratigraphic variability (Christensen, 1993).
Tectonic activity associated with the late Devonian to Early Carboniferous (Mississippian) Antler Orogeny produced large scale eastward thrusting of the western siliceous rocks over generally time equivalent or younger transitional and continental carbonates. This thrusting took place over a package of flat lying thrust planes collectively known as the Roberts Mountains Thrust System. Loading by the allochthon, which was up to several km thick (E.L. Miller et al., 1992), onto the continental margin resulted in an eastward migrating foredeep basin forming in eastern Nevada, ahead of the thrust belt. The leading edge of the over-riding allochthonous plate formed the emergent Antler Highland. Coarse siliceous clastic detritus was eroded from this highland and shed eastward into the foreland basin formed by the orogeny, which produced Lower Carboniferous (early Mississippian) synorogenic and Upper Carboniferous (Pennsylvanian) postorogenic sedimentary sequences (Poole et al., 1992). This resulted in Early Carboniferous (Mississippian) rocks which overlap the pre-Antler Cambrian to Devonian sequences of both the allochthon and the autochthon. The Mississippian rocks are in turn overlain with angular discordance by carbonates and clastic sediments of Pennsylvanian to Permian age, also of the overlap sequence. The three major tectono-stratigraphic sequences are locally termed: i). the eastern or carbonate autochthonous assemblage, ii). the western or siliceous allochthonous assemblage, and iii). the coarser clastic flysch or overlap assemblage (Christensen, 1993). The eastern, or carbonate autochthonous assemblage, includes transition rocks between the two assemblages towards the west, in the Carlin Trend area.
Intermittent shortening and extension in the Late Carboniferous (Pennsylvanian) and through the Permian (e.g., the south-directed Humboldt orogeny of Ketner, 1977; Theodore
et al., 2004) ensued, prior to the next major event (Cline et al., 2005).
The sequence was subjected to another orogenic event in the late Permian to lower Triassic, the Sonoma Orogeny, which resulted in the further eastward transport of western siliceous rocks over both the eastern carbonate assemblage and the post Antler overlap assemblage. This thrusting was over the Golconda Thrust, the main trace of which outcrops some 50 to 75 km to the west of the Roberts Mountains Thrust. The Golconda Thrust is not seen on the Carlin Trend, but is represented in the Cortez-Battle Mountain Trend and Getchell (Stewart, 1980). The associated compression however, will have had some influence in the Carlin District.
By the Middle Triassic, an east-dipping subduction zone had developed along the western margin of the North American craton. This produced a main magmatic arc well to the west of Nevada, represented by the Mesozoic granitic batholiths of the Sierra Nevada Range. Associated magmatism in north-central Nevada is initially represented by Middle Jurassic, backarc volcanic-plutonic complexes and lesser lamprophyre dykes. Accompanying shortening produced a north-trending belt of east-verging folds and thrusts in eastern Nevada (Elko orogeny of Thorman et al., 1991). Plutons evolved from I-type granitoids during the Early Cretaceous to S-type peraluminous granites in the Late Cretaceous, as the crust progressively thickened (Barton, 1990) during the Late Cretaceous Sevier and Laramide orogenies (Burchfiel et al., 1992). By ~65 Ma magmatism had migrated eastward into Colorado and did not resume in Nevada until about 42 Ma (Lipman et al., 1972; Hickey et al., 2003b). The last significant period of exhumation in the northern Carlin trend was during the Late Cretaceous when it had cooled to <60 to 70°C by ~60 Ma (i.e., at a crustal depth of <2- to 3km; Hickey et al., 2003b). The preceding paragraphs are after Cline et al. (2005).
The Carlin-type deposits of Nevada are restricted to the area underlain by Archaean crust (e.g., the northern Carlin trend and Jerritt Canyon) or by the thinned and mixed Palaeoproterozoic and Archean transitional crust (e.g., the Battle Mountain-Eureka trend and Getchell district) occuppying the interval between the rifted edge of the continental margin and dominantly intact crystalline basement rocks. The deposits are also concentrated in areas underlain by thick Neoproterozoic to Early Cambrian rift-related clastic rocks (Seedorff, 1991).
The Carlin-type deposits of north-central Nevada are localised by 330 to 350° and 290 to 310° trending faults and folds, interpreted to be controlled by major basement fault fabrics established during Neoproterozoic rifting (Roberts, 1966; Tosdal et al., 2000; Grauch et al., 2003). Basement-penetrating fault systems are interpreted to have influenced patterns of sedimentation, deformation, magmatism and hydrothermal activity (Cline et al., 2005).
Petroleum maturation and migration commenced following the Antler Orogeny when the organic bearing eastern assemblage was buried to depths locally exceeding 5 km below the Antler (or Roberts Mountains) Allochthon (Kuehn & Rose, 1992).
Christensen, (1993) states that compression between the late Triassic and late Jurassic produced folding within the Palaeozoic sequences resulting in the development of the north plunging, 325° trending, broad, low amplitude Tuscarora Antiform. This antiform has been substantially disrupted by later folding and faulting, and is interpreted from the reconstruction of a series of north plunging anticlines found along the trend. It is possible that the compression which formed the Tuscarora Antiform was related to either the closing stages of the Sonoma Orogeny, or the preliminary stages of the Sevier Orogenic Event, the main phase of which commenced during the late Jurassic, as described below.
District wide granodioritic activity is also recorded during the late Triassic and Jurassic, represented by several granodioritic intrusives, some of which have considerable contact metamorphic aureoles. Early veins of barite±base metals were either associated with, or followed this phase (Kuehn & Rose, 1992).
A major compressive event, or series of events, referred to as the Sevier Orogenic Event, commenced in the late Jurassic. In detail this orogenic period comprised three events, overlapping both geographically and in time. They entail 1). the late Jurassic to early Cretaceous, 163 to `152 Ma Nevadan Orogeny of the Sierra Nevada Ranges in south-western Nevada and California; 2). the succeeding and geographically over-lapping early to late Cretaceous 145 to 75 Ma phase known as the Sevier Orogeny, and 3). the later period from 75 to 45 Ma which has been referred to as the Laramide Orogeny (see section 3.7).
Several periods of igneous and tectonic activity were subsequently focused along the Tuscarora Antiform (Christensen, 1993). Plutonic activity during the late Mesozoic, was accompanied by doming and folding of the sedimentary units along the axis of the Tuscarora Antiform, contributing to local structural highs (Christensen, 1989). These local structural highs in the crest of the antiform are believed to have formed petroleum traps at some time in their history (Christensen, 1993).
During the late Jurassic to early Cretaceous, hydrocarbons were further maturated to pyrobitumen with the production of CH4. Between 42 and 37 Ma, following the main Sevier/Laramide orogenic period, an interval of emergence and erosion is postulated. This was followed from 37 Ma by major extension above flat lying detachments which outcropped further to the east. This extension coincided with a second period of intrusive activity in the Carlin Trend, marked by small plutons, stocks and a district wide series of dykes of intermediate composition (Kuehn & Rose, 1992).
A number of Mesozoic and Cenozoic intrusive bodies outcrop along the Carlin Trend. The oldest is the Goldstrike granodiorite, exposed in the Goldstrike mine area where it is dated at 158 Ma (late Jurassic). Other intrusives along the trend include the Richmond Mountain " quartz-monzonite (adamellite) dated at 106±5 Ma (mid Cretaceous), the Welches Canyon" granodiorite of 37±0.8 Ma (late Eocene) and the Bullion granodiorite at 35 to 37 Ma (Christensen, 1993).
These intrusives are generally more magnetic than the surrounding lithologies. However, in addition, a number of significant magnetic anomalies are distributed along the Carlin Trend. These anomalies have peaks of around 500 nT above background. The highest order anomaly and most extensive is known as the 'Mary's Mountain Intrusive' which is connected via a magnetic spur to a lower order anomaly corresponding to the Goldstrike granodiorite. Two further significant magnetic anomalies have been dubbed the Ivanhoe and the South Bullion intrusives. The latter is found below the Bullion granodiorite mentioned above. The interval over which these occur is of the order of 100 km. Further similar anomalies continue well to the south-east, beyond the limits of known Carlin Trend gold mineralisation. The Mary's Mountain anomaly has dimensions of the order of 30 x 15 km, elongated along the Carlin Trend. The main body of this intrusive is calculated to be at a depth of 6000 m, although intrusives which outcrop over limited areas are believed to be related to it. These magnetic intrusives, all occur along gravity highs (Wright, 1993). The Mary's Mountain intrusive corresponds to a 15 mgal high, with an arm connecting it to the Goldstrike intrusive anomaly (detailed images in Newmonts Gold Quarry office).
According to Wright (1993), modelling of the magnetic susceptibilities of these intrusive bodies generally indicate a mafic to ultramafic composition, which is also consistent with the accompanying gravity highs. The Carlin Trend is sub-parallel to the North Nevada Rift Zone some 35 km to the south-west, whose margin closely sub-parallels the Cortez-Battle Mountain Trend of gold deposits as described below.
The Gold Quarry, Tusc, MAC and Mike deposits are located on the eastern margin of this anomaly, while the Carlin orebody is on its northern rim. The Post/Goldstrike orebodies are on the margins of the Goldstrike granodiorite and the weak magnetic response with which it corresponds. The Genesis, Blue Star, Deep Star, North Star, Bobcat and Lantern deposits are distributed between the two anomalies, while Bootstrap/Capstone and Dee are along trend to the north-west of the Goldstrike granodiorite anomaly. The Hollister deposit is above the Ivanhoe intrusive anomaly, while the Rain group are some 10 km to the north of the South Bullion intrusive anomaly and not on the same trend. No known major Au deposits are associated with the South Bullion intrusive.
Several windows were eroded through the overthrust western allochthonous assemblage exposing the up-folded and up-faulted eastern assemblage rocks of the lower plate. These windows are distributed along the broken, irregular spine of the Tuscarora Antiform, largely corresponding to structural highs in the folded and faulted Roberts Mountains Thrust as described above. The main windows are from north to south, the Bootstrap, Lynn, Carlin and Rain windows. The main gold deposits are located near the margins of these windows (Christensen, 1993), with mineralisation, including the Carlin, Lantern, Blue Star, Genesis and Post deposits, being found along, or near the antiformal crest (Harvey, 1991).
A major generally NE-SW trending continental scale structure and crustal weakness, the Snake River Plains - Humboldt Lineament crosses the Carlin Trend between the Gold Quarry and Rain deposits. This structure is a downwarp zone across which differences in the degree of extension between the Great Basin to the south and both the Columbia Intermontane Region and the Rocky Mountain Basin and Range Province to the north are accommodated. Similarly it divides different segments of the Sevier-Cordilleran Thrust Front. The north-eastern most section of this lineament is defined by the broad elongate band of young volcanics of the Eastern Snake River Plains which probably follows Precambrian structures. Its south-western extrapolation along the Humboldt zone in northern central Nevada may trace a mid Miocene transform boundary (Christiansen and Yeats, 1992).
Major uplift and lesser extension commenced within the region at around 17 Ma, corresponding to the onset of basin and range tectonics. The North Nevada Rift, a major tensional break which closely sub-parallels the Cortez-Battle Mountain Trend to the south-west, was formed at 17 Ma. This rift was accompanied by dykes and extrusives of basaltic and andesitic composition. From 14 Ma a third period of igneous activity ensued, marked by rhyodacitic flows and domes exposed on the western flank of the Tuscarora Range, and basalt, basaltic andesite and andesite extrusives elsewhere in the district. The basin and range uplift and the corresponding erosion contributed to the exposure of the windows, and the subsequent deep burial by Pliocene sediments in the basins (Kuehn & Rose, 1992). The thickness of sediment in these basins may exceed 3500m (Wright, 1993). The main windows of lower plate assemblage rocks appear to have been exposed prior to the emplacement of the extrusives of this third phase of igneous activity which rest directly upon them.
See the separate Carlin Trend - Mineralisation record for a geological map showing the distribution of mineralisation and deposits.
STRATIGRAPHY OF THE CARLIN TREND
The stratigraphy on the Carlin Trend may be summarised as follows, divided into the three main assemblages, and later cover rocks and intrusives.
Western, Siliceous Allochthonous Assemblage from the base:
Ordovician Vinini Formation - interbedded argillites, siliceous mudstone and shale with lesser limestone, fine grained sandstone and chert. There is a general increase in grain size and carbonate content up-section. Stratigraphic thicknesses are not well constrained due to original depositional variability and to tectonic thickening and attenuation (Christensen, 1993). According to Ketner (1990), the Vinini Formation can be subdivided into a lower, middle and upper member. He describes each as follows -
• Lower Vinini Member ,- which is characterised by its extreme heterogeneity. Commonly it is composed of both siliceous and carbonatic arenites, limestones conglomerate, shale, chert, siltstone, dolostone and greenstone. Characteristically it is composed of rocks that are coarse grained and more limy than elsewhere in the sequence. Sand size components form the bulk of the unit and conglomerates are common. Sandstones are heterogeneous and commonly are hard to classify. The sequence of beds is randomly varied in thickness, composition, texture and sedimentary structure. Iron, zinc and lead sulphides occur sporadically in the lower parts of the member.
• Middle Vinini Member - which is much more uniform laterally and stratigraphically than the lower member. Regionally it is a fine grained carbonaceous shale that locally contains thin beds of micritic limestone, mature quartz sandstone or quartzite, and radiolarian bearing chert. Graptolites are the principal fauna. The limestone beds which are concentrated near the top of the member are black micrites composed almost entirely of calcareous spicules and calcic-spheres. Cherts are concentrated near the middle of the unit and contain radiolaria or other siliceous skeletons, although most of the chert is free from any sign of life. The thickness is typically 'several hundred metres'.
• Upper Vinini Member - which is composed mainly of thick bedded, radiolarian-bearing black chert. Chert concretions as much as 0.5 m in diameter are common. Thin micritic limestone strata, similar to those in the upper part of the middle member are present among the chert beds in some exposures. The upper member is commonly only a few metres thick, and rarely a few tens of metres. The uppermost part of the upper member contains pyrite, galena and sphalerite.
Further to the west and north the Vinini Formation becomes the Valmy Formation, characterised by a sequence comprising chert, shale, quartzite, greenstone and minor amounts of limestone. The difference from the Vinini is in the increased content of greenstone and quartzite (Stewart & Carlson, 1978).
Silurian to Devonian - a few minor occurrences of rocks of this age are known in the vicinity of the Carlin Trend (Stewart, 1980). Where present the lower Silurian is represented by a sequence of thin bedded black and strongly pigmented chert and unusually thick bedded white chert. This chert is extensively mineralised with Ag bearing Fe, Pb and Zn sulphides, with intersections of up to 12 m containing up to 4% Pb, 1% Zn and 12 g/t Ag. The lower to middle to early upper Silurian beds are composed of black to mainly pale green, tan weathering siltstone or fine grained sandstone. Most of the rock is made up of quartz clasts although feldspar and muscovite are also abundant. Black to commonly light coloured chert and shale are also present. Graptolites preserved on the few surfaces free from bioturbation indicate middle to early Silurian age. Lower and middle Devonian beds are rare. Upper-most Devonian to lower-most Carboniferous beds are exposed to the east of the Carlin Trend and are characterised by an abundance of clastics and extreme lateral and vertical variations in lithic composition. Common lithic types are coarse, clast supported, chert conglomerate, chert grit nearly free from quartz grains, heterogeneous carbonate cemented sandstone, limy siltstone, silty shale, dark cherty limestone, argillite, dark bedded chert and bedded barite. Scarce small greenstone beds are encountered. Lowermost Carbonaceous beds are commonly dark, siliceous, silty, argillite with sporadic sandy beds, scarce limestone and bedded barite (Ketner, 1990).
Eastern and Transition, Carbonate Autochthonous Assemblage, from the base:
Middle to Late Cambrian Hamburg Dolomite, 250 m thick in the Lynn Window - a blue-grey, thinly to thickly bedded. locally massive, very fine to coarse grained dolomite. Distinctive, but less common textures of the unit include thin ellipsoidal, white dolomite seams and minor intercalated very thinly bedded quartz clast siltstone beds (Craig, 1987).
Early Ordovician Goodwin Limestone, >300 m thick (base not exposed) - a blue-grey to light tan-grey, fine grained, platy, thin-bedded, silty and argillaceous limestone containing abundant chert. Beds in the lower part of the exposed formation to the south have been recrystallised and metamorphosed (Radtke, 1985).
Middle and Early Ordovician Antelope Valley Limestone, 375 m thick - blue-grey, fine grained, medium to thick bedded limestone containing minor chert (Radtke, 1985).
Note: The Goodwin Limestone and Antelope Valley Limestone together constitute the Pogonip Group (Radtke, 1985). According to Daly, etal., (1990), the Pogonip Group consists primarily of dark-grey fossiliferous limestone and interbedded calcareous shale, having lesser amounts of dolomite. Locally black chert nodules and lenses are abundant.
Ordovician Eureka Quartzite, 170 m, up to 300 m, thick - typically a massive, white, thick-bedded quartzite composed of medium grained, sub-rounded quartz in silica cement, but contains a few interbeds of black siltstone which may be as much as 6 m thick. Parts of the sequence include nearly black quartzite. The top 1 to 2 m are friable. The Eureka Quartzite is locally cross-bedded and contains sparse graptolites. It is also partly laminated with intense syn-sedimentary folding. Locally a carbonate sequence separates the quartzite into a lower and an upper member (Daly, etal., 1990). Grey sandy dolomite occurs in minor amounts in the upper 30 m, while 60 to 80 m of light grey to medium grey, fine to medium grained, thin bedded, sandy dolomite is found separating it from the underlying Pogonip Group (Radtke, 1985).
Middle Ordovician to Lower Silurian Hanson Creek Formation, 150 to 180 m thick - which is principally composed of dolomite. In general this unit is a dark grey to black, fine grained, thinly to thickly bedded dolomite. The basal unit contains lenses and seams of black chert. The upper unit contains less chert and is, in general, lighter coloured, more thinly bedded and coarse grained. Also present in the upper unit are intercalated, dark blue-grey, thickly bedded, fossiliferous, dolomitic limestones. The top of the formation is marked by interbedded, thickly bedded, light grey sandy dolomite and dolomitic sandstone (Craig, 1987). According to Radtke (1985), in the Carlin area it comprises a lower 90 to 120 m of medium to dark grey, medium grained, thick bedded dolomite; overlain by 60 m of light to medium grained, thin bedded dolomite containing sandstone lenses; and capped by up to 5m of light grey dolomitic sandstone to sandy dolomite. To the north in the Jerritt Canyon District however it has been sub-divided into 5 members which also include interbeds of black chert, carbonaceous micrite, argillaceous limestone, carbonaceous limestone, calcareous siltstone and silty limestone and crinoidal limestone (Daly, etal, 1990).
Devonian to Silurian Roberts Mountains Formation, 500 to 600 m thick - essentially a medium to thin bedded, platy, grey, silty limestone to dolomitic and calcareous siltstone (Christensen, 1993). The base of the formation is marked by dark grey dolomite and dolomitic limestone, with black swelling and pinching, very thinly to medium bedded chert lenses and pods. The formation consists of a monotonous sequence of laminated to thinly bedded, silty, medium dark grey, fine grained dolomite and dolomitic limestone. The upper part of the formation also contains medium dark grey, thick bedded bioclastic, silty to sandy, fine to medium grained limestone (Craig, 1987). Of particular importance are debris flow beds which form favourable ore hosts. Bedding thins and the silt content increases up-section, with a gradational upper contact with the overlying limestones (Christensen, 1993). Limestones which are medium to dark grey and medium to fine grained are interbedded with dolomitic rocks in the upper 150 to 180 m at Carlin. Cherty, dolomitic limestone makes up the lower 2 to 30 m (Radtke, 1985).
Devonian Carbonates, 50 to 280 m thick (Christensen, 1993), 400 to 460 m thick (Craig, 1987) - found above the Roberts Mountains Formation, this unit is principally composed of medium to thick bedded grey limestone, with variable micritic, sparry and grainy textures, and is locally fossiliferous. It has different names in different parts of northern Nevada, including the Popovich, Bootstrap, Devils Gate, Nevada & Un-named Formations, and the Wenban Formation of the Cortez-Battle Mountain Trend to the south-west (Christensen, 1993). It consists predominantly of intercalated thinly to thickly bedded, fine grained limestone and laminated silty dolomitic limestone and intra-formational breccia. Minor black chert seams occur near the base. The upper part of the formation comprises a thick bedded limestone depositional breccia with large sub-rounded to angular limestone clasts overlain by a grey thick bedded sandy limestone. It ranges from lower to upper Devonian in age (Craig, 1987). In the Lynn Window the Popovich Formation comprises a lower 15 to 60 m thick, dark grey, fine grained limestone with lesser medium grey, fine grained dolomitic limestone; a middle member comprising 14 to 150 m of dark grey, thin bedded dolomitic limestone with lesser blue-grey, fine grained, laminated dolomitic limestone; capped by an upper member composed of 10 to 30 m of locally fossiliferous, blue-grey, coarse grained limestone with lesser limestone breccia and sandy limestone (Radtke, 1985).
Devonian Rodeo Creek Unit - this is a distinctive unit of rhythmically thin bedded grey siltstone, mudstone, chert and argillite which is found structurally above the Devonian carbonates. Contact relationships with the underlying carbonates is often obscured by alteration or bedding parallel shearing, and are debatable. Fossil age determinations are not internally consistent, and the unit may be either allochthonous or autochthonous. However where present in the Carlin Trend it consistently occurs immediately overlying the Devonian Carbonates (Christensen, 1993). The Rodeo Creek Unit is generally bounded above and below by thrust splays of the Roberts Mountains Thrust and may be a member of the 'Transitional Assemblage' described immediately below.
Transitional Assemblage - This sequence appears to represent a transition between the Western and Eastern Assemblages and is generally bounded above and below by splays of the Roberts Mountains Thrust. It most likely includes the Rodeo Creek Unit. In general, within the Tuscarora Mountains, the Transitional Assemblage has been divided into three units by Craig (1987), as follows:
• Limestone unit - composed predominantly of massive, grey, thinly to thickly bedded limestone and sandy limestone. The upper portion contains intercalated sandy, cross-bedded limestone; pebble sized chert and quartz clast, calcite cemented conglomerate; and green to grey, very thinly to thinly bedded chert and dark grey to black shales (Craig, 1987). This most likely correlates with the James Creek Member of the Rodeo Creek Unit seen at Gold Quarry.
• Intercalated unit - a unit of highly variable lithologies, consisting of intercalated siliceous and carbonaceous rocks in thrust contact with the limestone unit. It appears to grade into more typical Vinini Formation lithologies to the west. Lithologies include, green to grey, very thinly to thinly bedded, translucent to argillaceous chert, with or without clay partings; green to grey, laminated fissile shales; light grey to black, thinly to thickly bedded, siltstones and mudstones; and a distinctive limestone sequence (Craig, 1987).
• Conglomerate unit - another allochthonous unit which is in thrust contact with the 'Intercalated Unit'. It is approximately 300 m thick, with the basal 20 m comprising a green translucent chert containing a thin marker bed of medium grey, coarse grained quartzite. Above this there is approximately 34 m of a medium to very thickly bedded, chert, argillite and quartzite clast, chert matrix conglomerate. The next 65 m consists of interbedded argillaceous matrix conglomerate and argillite. The remaining upper portion of the unit consists of interbedded chert, siltstone, argillite and porcellanite (Craig, 1987).
Overlap Assemblage, from the base:
Devonian to Carboniferous (Mississippian), which may locally be up to 3000m thick - occurring as terriginous detrital sediments in the foreland basin to the east of the Antler Orogenic Belt. The sequence consists of mudstone, siltstone, sandstone, chert pebble conglomerate and subordinate impure limestone. In general it consists of the relatively thin upper section of the Pilot Shale at the base, the thin Joana Limestone, the thick Chainman Shale and the overlying lower and middle parts of the Diamond Peak Formation. The oldest unit of the overlap assemblage on the southern end of the Carlin Trend is the Webb Formation, unconformably overlying the Devonian Devils Gate Limestone (Stewart, 1980). The Webb Formation commences with a lower suite of interbedded mudstones and siltstones with minor quartz and chert sandstones and quartzites. Basal fine grained mudstones and siltstones of the Webb Formation grade upwards into coarser grained siltstones, sandstones, conglomerates and quartzites which are transitional with the overlying Chainman Shale (Thoreson, 1990).
Upper Carboniferous (Pennsylvanian) - which represent the lower section of the 'carbonate-terriginous detrital' sequence within the foreland basin to the east of the Antler Orogenic Belt. These are found in the southern half of the Carlin Trend. They include the limestones and minor quartz-sandy and quartz-silty limestone of the Moleen Formation and the interbedded and interfingering limestone, chert-pebble-conglomerate, sandstone and siltstone of the overlying Tomera Formation (Stewart, 1980).
Upper Carboniferous (Pennsylvanian) to Upper Permian, comprising the upper section of the 'carbonate-terriginous detrital' sequence within the foreland basin to the east of the Antler Orogenic Belt. The sequence contains abundant silici-clastic material derived from the Antler Highland to the west, and are only found in the southern half of the Carlin Trend. They are underlain by an unconformity and commence with the limestone and quartz-sandy and quartz-silty limestone of the Strathearn Formation. This formation passes upwards into a sequence of carbonates containing abundant silt and fine sand of quartz, associated with calcareous quartz siltstone and sandstone and chert pebble conglomerate which includes the upper most Strathearn, Buckskin Mountain, Beacon Flat and Carlin Canyon Formations. These are overlain locally by the Garden Valley Formation which is composed of abundant chert-pebble conglomerate as well as limestone, sandstone and shale (Stewart, 1980).
Upper Carboniferous (Pennsylvanian) to Permian Antler Sequence Conglomerate - these belong to the 'conglomerate and carbonate province' over the Antler Highland. They are equivalents of the 'carbonate-terriginous detrital sequence of the foreland basin which are found on their south-eastern margin, and as such are largely a lateral equivalent of the two immediately preceding descriptions. They are found in the north-western half of the Carlin Trend and are characterised by conglomerate, sandy to conglomeratic limestone, limestone, sandstone and calcareous shale. The sequence includes the basal conglomerates and overlying limestones of the Sunflower Formation to the north, while to the south-west the equivalent sequence comprises coarse conglomerates of the basal Battle Formation, the overlying Antler Peak Limestone and the upper sandstone, calcareous shale, limestone and chert-pebble conglomerate of the Edna Mountain Formation.
Younger Cover, from oldest to youngest:
Jurassic - volcanic sandstone, felsic ash-flow tuffs, and rhyolitic and rhyodacitic flows, which only outcrop over limited areas as the Pony Trail Group, mainly in the Cortez Mountains to the south-west of the Carlin Trend.
Tertiary volcanics and shallow intrusives - divided into three lithological groupings, each of which may contain rocks from different periods through the Tertiary. They include:
• Welded and non-welded silicic ash-flow tuffs, and tuffaceous sedimentary rocks (Tt).
• Rhyolitic flows and shallow intrusive rocks (Tr).
• Andesitic flows, and intermediate breccia and flows (Ta).
Tertiary Carlin Formation - thin bedded to poorly bedded tuffaceous and non-tuffaceous sandstone, mudstone and welded tuff. Also includes poorly consolidated conglomerate containing abundant chert pebbles (Radtke, 1985).
Tertiary to Quaternary - comprising:
• Sedimentary rocks, mainly lake bed sediments deposited in the basins between the ranges. From the end of the Sevier Orogeny when the topography was more extreme, the amount of relief was diminished to produce extensive marshes and swamps. This lasted until around 25 Ma when faulting became more pronounced and the onset of listric faulting rotating older sediments. From 17 Ma the onset of the more extreme basin and range normal faulting limited the areal extent of the lake beds which became less extensive with time (Thorman & Christensen, 1991).
• Older alluvial deposits.
Quaternary - alluvial and playa deposits.
Intrusives, from the oldest to youngest:
Jurassic to Cretaceous- a few small diorite outcrops are known in the north to central sections of the Carlin Trend in the vicinity of the Blue Star/Genesis group of deposits. These are largely late Jurassic in age and preceded the mainly Cretaceous to Tertiary quartz-monzonites (adamellites) and granodiorites below (Stewart, 1980). The largest of the dioritic bodies is the North Big Six intrusive which is some 4 km to the north of the Carlin mine. It is a medium grained diorite to quartz-diorite with abundant chlorite formed from the alteration of biotite and hornblende (Radtke, 1985).
Jurassic to Tertiary - granitic rocks, mostly quartz-monzonite and granodiorite. Intrusive dykes of granodiorite, quartz-diorite and diorite occur throughout the Tuscarora Mountains. The fresh rock is light grey to pinkish-grey, fine grained, holo-crystalline and contains plagioclase phenocrysts. In most areas the dykes were emplaced along high-angle faults that trend NW-SE to north-south. Although igneous rocks outcrop in many areas, swarms of dykes are concentrated in three areas, as known in 1985. These are i). some 16 km to the west of the Carlin Mine; ii). near the Big Six Mine, some 1.5 km to the north-east of the Carlin Mine; and iii). in the immediate vicinity of the Carlin and Blue Star Mines (Radtke, 1985).
In addition to these dykes, the larger intrusive bodies of this age on the Carlin Trend are the:
• Goldstrike granodiorite, exposed in the Goldstrike mine area where it has been dated at late Jurassic, 158 Ma (Christensen, 1993). This is a light to medium grey, fine grained granodiorite, although locally its composition approaches quartz-diorite to diorite. It contains 20 to 25% quartz, 10 to 15% orthoclase, 30 to 35% plagioclase 10 to 15% biotite and 5 to 10% hornblende. Limestone and shale adjacent to the intrusive have been recrystallised and bleached, while locally wollastonite, hedenbergite and iron rich garnet are present (Radtke, 1985).
• Richmond Mountain quartz-monzonite (adamellite) dated at mid Cretaceous, 106±5 Ma (Christensen, 1993).
• Welches Canyon granodioriteof 37±0.8 Ma (late Eocene),
• Bullion granodiorite at 35 to 37 Ma (Christensen, 1993). Occurrences of such intrusives are not restricted to the Carlin Trend.
Tertiary mafic to intermediate intrusives - minor mafic to intermediate intrusives are mapped just to the north of Mary's Mountain some 8 km due west of the Gold Quarry ore deposit, and over the centre of the large, deep 'Mary's Mountain Intrusive' magnetic and gravity anomaly. No other details are readily available.
Tertiary rhyolitic intrusives.
ORIGIN AND FORMATION OF MINERALISATION
Field relationships and conventional geochronological studies indicate that the Carlin Trend deposits formed during a narrow time interval from the mid to late Eocene, between ~42 and 36 Ma (Hofstra et al., 1999; Tretbar et al., 2000; Arehart et al., 2003).
The largest deposits within the Carlin and similar trends in Nevada lie in the lower plate to the Devonian to Lower Carboniferous (Mississippian) Roberts Mountain thrust. This structure juxtaposed allochthonous non-reactive, fine-grained siliciclastic rocks with a lower permeability, over more permeable carbonate stratigraphy, acting as a regional aquitard. NNW and WNW trending normal faults cutting basement and Palaeozoic sequences were inverted during post-rift compressional events to produce structural culminations (in the form of domes and anticlines) that localised auriferous fluids during the Eocene.
Where exposed, these culminations are expressed as erosional windows through the siliciclastic rocks of the Antler allochthon. NW to west directed extension during the Eocene reopened appropriately oriented older structures to form strike-slip, oblique-slip, and normal-slips faults. There is little evidence for overpressuring by hydrothermal fluids, significant syn-mineralisation slip or complex multistage vein dilatancy, all suggesting fluid flow and mineral deposition were relatively passive.
Fluid inclusion data and geologic reconstructions suggest the deposits were formed within a few km below the surface. Ore fluids were moderate temperature, at ~240 to 180°C, and of low salinity (~2 to 3 wt.% NaClequiv.), CO2 bearing (<4 mol.%), and CH4 poor (<0.4 mol.%), with sufficient H2S (10
-1 to 10-2 m) to transport Au. The ore fluids are interpreted to have decarbonatised, argillised, and locally silicified the wall rocks, and deposited disseminated pyrite containing submicron Au. The pyrite was formed as Fe was liberated from wall rock to react with reduced S in the ore fluid.
Isotopic studies indicate multiple sources for ore fluids and dissolved components, and suggest interaction with meteoric waters to overwhelm the deep ore-fluids in most districts. Whilst O and H isotope ratios of minerals and fluid inclusions indicate a deep magmatic or metamorphic fluid source at the Getchell deposit; most other studies elsewhere have identified meteoric water. A large range of S isotopes in ore pyrite from all districts suggests derivation from a sedimentary source, although at Getchell and in some cases in the northern Carlin trend these isotopes are consistent with a magmatic S source. Consequently, it has been suggested deposits are related to i). leaching and transport of metals by convecting meteoric water; ii). epizonal intrusions, or iii). deep metamorphic and/or magmatic fluids.
Apart from the S isotopic data, all other observations suggest the Nevada Carlin-type deposits were formed in response to similar geologic processes. Cline et al., (2005) proposed a model in which separation of the Farallon slab below the subduction zone to the west, and related subcrustal delamination and detachment promoted deep crustal melting that promoted prograde metamorphism and devolatilisation, to generate deep, primitive fluids. They suggest these fluids were likely incorporated in deep crustal melts that rose buoyantly and ultimately exsolved hydrothermal fluids, possibly containing Au. Midcrustal metamorphism may also have contributed fluids or solutes, all of which were channelled into basement-penetrating rift faults, where they continued to rise and scavenge various components, evolving to ore fluids. NNW trending palaeo-normal faults and NE-trending palaeo-transform faults, preferentially dilated during Eocene extension, controlled the regional position, orientation and alignment of the deposits. The ore fluids were eventually accumulated in areas of reduced mean effective stress, particularly at the boundaries between older Jurassic and Cretaceous stocks and structural culminations. They interpret ore fluids to have been diluted by exchanged meteoric water as extension increased fault permeability in the upper crust. When within a few km of the surface, fluids were diverted by structural and stratigraphic aquitards into reactive host rocks, where they sulphidised host rock iron and deposited Au.
Black carbonaceous rocks are always present and commonly host some of the ore (Hofstra and Cline, 2000). Some rocks with high concentrations of C in the Carlin trend show evidence of petroleum generation, migration and accumulation in structural culminations (e.g., Kuehn, 1989; Hulen and Collister, 1999). Both indigenous and concentrated organic C are generally overmature relative to petroleum generation In rocks below the Roberts Mountains allochthon, and typically comprise cryptocrystalline graphite (Poole and Claypool, 1984; Hofstra, 1994; Leventhal and Giordano, 2000). Petroleum migration occurred prior to emplacement of Jurassic intrusions (Emsbo et al., 2003), whilst thermal modelling indicates petroleum generation and subsequent catagenesis resulted from emplacement of the Roberts Mountains allochthon (Gize, 1999). The thermal maturity of indigenous and concentrated C is generally higher near igneous intrusions, Carlin-type deposits, and active geothermal areas (Poole and Claypoole, 1984; Ilchik et al., 1986, Hitchborn et al., 1996; Hulen and Collister, 1999). Hofstra and Cline (2000) found no consistent relationship between organic C content and ore grade although it is likely that pre-exiting carbon may have reacted with hot fluids to precipitate ore and in so doing formed secondary products, e.g., CO2, and been removed with the through-going fluids or been deposited as carbonates. At upper crustal levels, organic matter could have been a direct source of H2S, a critical ingredient for thermochemical sulphate reduction (Hunt, 1996). Organic matter also maintained a reduced condition in the fluid, and thus H2S as the predominant form of S in the fluid, enabling the hydrothermal solutions to scavenge and transport Au to the sites of ore formation (Hofstra and Cline,
This section is paraphrased from Cline et al., (2005).
Note: This summary was initially drawn from literature published by 1996, but has been partially updated from more recent papers. For more detail consult the reference(s) listed below.
For details on the mineral deposits and mineralisation on the Carlin Trend, see the Carlin Trend - Mineralisation record.
The most recent source geological information used to prepare this summary was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Bawden T M, Einaudi M T, Bostick B C, Meibom A, Wooden J, Norby J W, Orobona M J T, Chamberlain C P 2003 - Extreme 34S depletion in ZnS at the Mike gold deposit, Carlin Trend, Nevada: Evidence for bacteriogenic supergene sphalerite: in Geology v31 pp 913-916|
Cline J S, Hofstra A H, Muntean J L, Tosdal R M and Hickey K A, 2005 - Carlin-Type Gold Deposits in Nevada: Critical Geologic Characteristics and Viable Models: in Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J. and Richards, J.P. (eds.), Economic Geology, 100th Anniversary Volume Society of Economic Geologists pp. 451–484|
Emsbo P, Groves D I, Hofstra A H and Bierlein F P, 2006 - The giant Carlin gold province: a protracted interplay of orogenic, basinal, and hydrothermal processes above a lithospheric boundary : in Mineralium Deposita v41 pp 517-525|
Hollingsworth, E.R., Ressel, M.W. and Henry, C.D., 2017 - Age and Depth of Carlin-type Gold Deposits in the Southern Carlin Trend: Eocene Mountain Lakes, Big Volcanoes, and Widespread, Shallow Hydrothermal Circulation: in Bedell, R.L., and Ressel, M.W., (eds.), 2017 Shallow Expressions of Carlin-type Systems, Alligator Ridge and Emigrant Mines, Nevada Geological Society of Nevada field trip guidebook, October 13-15, 2017, SP-64, pp. 149-173.|
Leach T M 2004 - Distribution of alteration and mineralisation in the northern Carlin Trend gold deposits, Nevada: in Hi Tech and World Competitive Mineral Success Stories Around the Pacific Rim, Proc. Pacrim 2004 Conference, Adelaide, 19-22 September, 2004, AusIMM, Melbourne, pp 153-159|
Peters S G, 2004 - Syn-deformational features of Carlin-type Au deposits: in J. of Structural Geology v26 pp 1007-1023|
Ramadorai G, Hausen D M and Bucknam C H, 1991 - Metallurgical, analytical and mineralogical features of Carlin refractory ores: in Ore Geology Reviews v6 pp 119-132|
Ressel M W and Henry C D, 2006 - Igneous Geology of the Carlin Trend, Nevada: Development of the Eocene Plutonic Complex and Significance for Carlin-Type Gold Deposits: in Econ. Geol. v101 pp 347-383|
Theodore T G, Kotlyar B B, Singer D A, Berger V I, Abbott E W, Foster A L 2003 - Applied geochemistry, geology and mineralogy of the northernmost Carlin Trend, Nevada: in Econ. Geol. v98 pp 287-316|
Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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