Battle Mountain - Eureka Gold Trend - Geology
The Battle Mountain - Eureka Trend is located within the Great Basin geologic province of Nevada.
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The host rocks of the deposits within the trend include sediments ranging in age from upper Cambrian sandstones, cherts and shales, to Siluro-Devonian laminated carbonatic siltstones to Devonian limestones as well as Mesozoic and Tertiary igneous rocks (Bagby & Berger, 1985).
As with the Carlin Trend, the Battle Mountain - Eureka Trend (previously also known as the Cortez - Battle Mountain trend) contains an alignment of mineral deposits, windows through the Roberts Mountains Allochthon and Mesozoic and Tertiary intrusive rocks. The host lithologies and geologic history are little different from that of the Carlin Trend (Bagby & Berger, 1985) - see the 'Carlin Trend - Geology' and 'Carlin Trend Mineralisation' records.
The principal successions within the trend comprise the, i). Eastern, or Carbonate Assemblage of the autochthon, generally exposed in windows through the allochthonous upper plate. These rocks were deposited on the cratonic western margin of North America and include Cambrian to upper Devonian carbonates and lesser shallow water clastics; ii). Transition Assemblage which is recognised further to the west, representing more siliceous and deeper water lithologies than the main Eastern Assemblage; iii). Western, or Siliceous Assemblage of the allochthon which has been thrust east over the major Roberts Mountains Thrust of the Devono-Carboniferous Antler Orogeny. These are middle Cambrian to upper Devonian in age and were deposited within the deeper continental margin to the west of the main craton; iv). Overlap Assemblage of Devono-Carboniferous to Permian rocks, mainly clastics and carbonates, which unconformably overlie the Eastern, Transition and Western Assemblages, following the Antler Orogeny and associated movement on the Roberts Mountains Thrust. The assemblage was deposited to the east of the Antler Highland which was formed by the Roberts Mountain Allochthon. Foreland basin sequences of upper Palaeozoic age in the same area are part of this overlap assemblage; v). Havallah Sequence of silici-clastics and interbedded mafics which were thrust eastwards over the Golconda Thrust during the Permo-Triassic Sonoma Orogeny to form the Golconda Allochthon. This sequence was deposited during the same time interval as the Overlap Assemblage, but to the west of the Antler Highland. Thrusting over the Golconda Thrust has juxtaposed the time equivalent, but lithologically dissimilar, Havallah and Overlap Assemblages; and vi). Mesozoic and Tertiary shelf and terriginous strata, and intrusives and volcanics (Madrid & Roberts, 1991).
The Gold Acres, Cortez, Pipeline and Horse Canyon orebodies are all hosted by lower plate Eastern Carbonate Assemblage rocks within windows through the Roberts Mountains Allochthon. Tonkin Springs occurs within allochthonous upper plate rocks, although a possible lower plate, but un-mineralised Devonian limestone is present (Bagby & Berger, 1985). The Gold Bar deposit is hosted by Devonian fossiliferous and argillaceous limestones of the autochthonous Eastern Succession (Masinter, 1990), while the Marigold and Lone Tree orebodies are found within both Western Succession, Siliceous Assemblage rocks above the Roberts Mountains Thrust, but below the Golconda Thrust, and by Post Antler Orogeny Overlap Succession silici-clastics (Graney & McGibbon, 1990). The Battle Mountain - Phoenix orebodies are hosted by carbonates and silici-clastics of the Overlap Assemblage, by Western Assemblage silici-clastics of the Roberts Mountains Allochthon and by Havallah Sequence cherts and argillites of the Golconda Allochthon, all surrounding an Oligocene felsic plug (Wotruba, etal., 1988).
Other mineralisation in the Cortez-Battle Mountain Trend includes i). the Cortez silver deposits within the Cambrian dolomites, occurring as replacement deposits of manto's and fissure veins; ii). the Buckhorn gold deposit in Pliocene volcanics, which are interpreted to be hot springs related veins; and iii). bedded barites in the Western Assemblage silici-clastics (Bagby & Berger, 1985).
The stratigraphy on the Cortez-Battle Mountain Trend may be summarised as follows, divided into the main assemblages, and later cover rocks and intrusives. Many of these units are also found within the Carlin Trend. For more detail of the units common to both trends, see - see the 'Carlin Trend - Geology' record.
Western, Siliceous Assemblage of the Roberts Mountains Allochthon. This assemblage was thrust eastward above the Roberts Mountains Thrust during the Antler Orogeny to rest on the Eastern, or Carbonate Assemblage described below. The main units recognised are as follows, from the base:
• Cambrian, Shwin Formation - The oldest rocks of the Roberts Mountains allochthon within the Cortez-Battle Mountain Trend belong to the middle Cambrian Shwin Formation, a sequence of shale, thin bedded limestone, greenstone and chert. This unit is a lateral equivalent of the Transition Assemblage Preble Formation described below. Within the Cortez-Battle Mountain trend the Scotts Canyon Formation which is described as comprising chert, shale, greenstone and sparse limestone and quartzite has been assigned a Cambrian age. More recent work has shown this unit to be part of the Devonian Slaven Chert on the basis of radiolaria, although it includes tectonic slivers of lower to middle Cambrian fossil bearing limestone, the source of its earlier 'incorrect' dating (Stewart & Carlson, 1978; Madrid & Roberts, 1991).
• Cambrian, Harmony Formation - of upper Cambrian age, composed of feldspathic and micaceous arkosic sandstone, with interbedded shale, minor limestone and chert. It is everywhere fault bounded, with an age based on four trilobites. In the Battle Mountain district it comprises interbedded sandstone, feldspathic sandstone, arkose, granule and pebbly sandstone, shale and limestone (Theodore & Blake, 1975; Madrid & Roberts, 1991).
• Ordovician, Valmy and Vinini Formations - These two units are well represented on the Cortez-Battle Mountain Trend. The Vinini Formation is characterised by chert and shale, while the Valmy Formation is also composed of chert and shale, but has interbedded thick units of quartzite and some greenstone. The quartzites of the Valmy Formation differ from those of the Eureka Formation in the Eastern Assemblage in that they are a darker grey and coarser, not so well sorted and typically have a conspicuous 'seriate' texture, ranging from fine to coarse. In the Cortez Mountains these two units appear to be transitional with each other. The more carbonate and shale rich rocks of the Vinini Formation grade north-west into the more quartzite and greenstone rich Valmy Formation. These sediments are equivalents in part of the upper Cambrian to lower Ordovician Comus Formation of the Transition Assemblage. The thickness of these units is difficult to measure due to frequent structural repetition. Values of from 800 to 7500 m have been estimated, although the former is favoured (Stewart, 1980; Madrid & Roberts, 1991).
The principal constituents of the Vinini Formation are siltstone, shale, siliceous-shale, sandstone, chert and quartzite. A few thin layers of greenstone are sometimes present. Much more than half of the unit is composed of fine grained clastic rocks, either sandy-siltstone, siltstone or poorly fissile shale. Most of these are notably carbonaceous and very dark grey to black on fresh fractures, but weather to light grey hues beneath a surficial film of iron staining. Most rocks are composed predominantly of very fine angular quartz, with only subordinate muscovite. Despite the fine grain size, current bedding is obvious, emphasised by carbonaceous material and by minute rhombs of dolomite. Some of the siltstones have up to 40% dolomite rhombs, though most are free from carbonate. These rocks resemble argillites, although they are in fact fine siltstones. The sequence is interrupted by interbedded, sandstone, chert and quartzite, generally in sequences a few metres to ten metres thick. The sandstones are nearly all in beds a few cm's thick made up of chert and quartz grains. None are greywackes as they lack a clay matrix. There are a few thin beds of conglomerate. Cherts are black or dark grey due to contained carbonaceous matter and are generally from 1 to 10 cm's thick, but may be up to a metre or more. Rare quartzite beds may be up to 6 m thick (Gilluly & Masursky, 1965).
• Silurian, Elder Sandstone 600 to 1200 m thick - of lower to middle Silurian age, composed mainly of feldspar rich and quartzose sandstones, which are finer than those of the Harmony Formation from which they are presumed to have been derived. It predominantly comprises light coloured, fine grained, commonly silty sandstone, containing 70 to 80% quartz, 15 to 25% potassium feldspar, 5% muscovite and a little albite. Siltstone, shale and chert are present in minor amounts. Some siltstone beds contain ghosts of pumice shards and some sandstones contain grains which appear to be devitrified glass. Cherts, argillite, shale and siltstone of the equivalent Fourmile Canyon Formation are found within the district, but to the west of the Elder Sandstone. This latter unit also contains a few thin beds of fine grained sandstone, while dark volcanic flows with interpreted pillow structures have also been identified (Stewart, 1980; Madrid & Roberts, 1991).
• Devonian, Slaven Chert, 1200 m thick - predominantly a thin to thick-bedded black radiolarian chert with very minor amounts of sandstone, shale, feldspathic siltstone, bedded barite and limestone. Sections of the Scott's Canyon Formation, previously mapped as Cambrian in age, have been shown to contain Devonian fossils and are now included as part of the Slaven Chert. Recent work has also shown that some sections previously mapped as Ordovician Valmy Formation, are Devonian in age, and are now included in the Slaven Chert (Stewart, 1980; Madrid & Roberts, 1991).
Although this assemblage is regarded as being siliceous, many units carry appreciable associated carbonates. The greenstone bearing sequences of the allochthon are interbedded with calcareous turbiditic sandstones and sporadic volcanogenic debris flows. Some of the more feldspathic sandstones, of all ages, carry up to 15% carbonate matrix, mainly dolomite, while the upper parts of the Valmy Formation contains calcareous shales and interbedded cherts, some of which have up to 5% carbonate. Many parts of the Slaven Chert are carbonate rich with abundant turbiditic calcareous sandstones and siltstones (Madrid & Roberts, 1991).
Transition Assemblage, largely autochthonous. To the west of the main carbonate dominated autochthonous assemblage a transitional sequence has been mapped that includes elements of both the Western and Eastern Assemblages. This sequence is as follows, from the oldest:
• Upper Proterozoic to Lower Cambrian Osgood Mountains Quartzite - which is similar to the Prospect Mountain Quartzite found in the south-eastern section of the Cortez-Battle Mountain Trend. The unit is mainly composed of quartzite with minor amounts of conglomerate, phyllitic siltstone, limestone and dolomite (Stewart & Carlson, 1978; Stewart, 1980; Madrid & Roberts, 1991),
• Upper Cambrian Preble Formation - shale and thinly bedded or laminated limestone, but also in part comprising thinly interbedded limestone and chert (Stewart & Carlson, 1978; Stewart, 1980; Madrid & Roberts, 1991),
• Upper Cambrian to Lower Ordovician Comus Formation - this unit has a variable mix of lithologies, comprising shale and chert with minor amounts of quartzite, greenstones/mafics and limestone. On the Cortez-Battle Mountain Trend it is a sequence of thin bedded limestone, shale and chert. In places, thin bedded shaly limestones of the Comus Formation may be found in the upper plate allochthonous sediments. To the west this unit is overlain in the autochthon by the carbonaceous shale and minor quartzites of the Western Assemblage Vinini Formation (Stewart & Carlson, 1978; Stewart, 1980; Madrid & Roberts, 1991).
To the north-west, in the Getchell Trend, the Preble and Comus Formations are the hosts to mineralisation. These rocks have been folded and subjected to lower greenschist to greenschist facies metamorphism that is of pre-middle Pennsylvanian (late Carboniferous) age, and is ascribed to the Antler Orogeny. The greenschist facies metamorphism has been overprinted by contact metamorphism associated with the emplacement of the Cretaceous Osgood Mountains Pluton, which in turn predates alteration associated with gold mineralisation (Madrid & Roberts, 1991).
Eastern, or Carbonate Autochthonous Assemblage. In the north-eastern corner of Nevada, extensive exposures of shallow, shelf sediments are found. West of the Roberts Mountains however, exposures are restricted to windows eroded through the Roberts Mountains Allochthon. Proterozoic and Cambrian strata are generally silici-clastic, while the overlying Cambrian to Devonian rocks are predominantly carbonates. The sequence is as follows, from the base:
• Lower Cambrian, Prospect Mountains Quartzite - which is similar to the Transition Assemblage Osgood Mountains Quartzite described above.
• Upper Cambrian, Hamburg Dolomite, 365 m thick - regionally this unit is composed mainly of limestone and dolomite with local thick sequences of shale and siltstone. Equivalents and constituent members include the Eldorado Dolomite and Pioche Shale (Stewart, 1980). Generally it is a thick, uniform sequence of grey, parallel bedded dolomite and sandstones. The beds range from 0.15 to 3 m in thickness. Colours are dark, medium or light grey in alternating beds. Sedimentary structures are abundant, with cross-lamination, slumps and mottles of medium sand size in a matrix of silt to very fine sand. The darker bands are fetid in fresh breaks and contain abundant blebs of organic material and pyrite. The dark beds consist of fine to medium, sand sized angular to sub-rounded dolomite fragments, while the light coloured beds contain very fine to fine sand size angular clasts of dolomite (Gilluly & Masursky, 1965).
• Cambro-Ordovician Pogonip Group - a sequence of limestone, dolomite and minor shale (Madrid & Roberts, 1991).
Unconformity - a shallow regional feature, with the overlying Eureka Quartzite being locally absent over arches (Madrid & Roberts, 1991; Stewart, 1980).
• Ordovician, Eureka Quartzite, approx. 120 m thick - a regionally distinctive unit of quartzite (Madrid & Roberts, 1991; Stewart, 1980). In the Cortez area it is composed of three stepped cliff forming units, a lower brown, discoloured quartzite and two upper very white to grey quartzites which have limy sandstone interbeds. Each of the three units is separated by 1 to 3 m thick softer dolomite cemented bands. The staining of the lower unit, which is around 17 m thick, is due to iron oxide derived from weathered pyrite. Coarse cross-bedding is abundant throughout (Gilluly & Masursky, 1965).
• Ordovician, Hanson Creek Formation, 150 m thick - which is principally composed of dolomite (Craig, 1987). The upper part of the unit consists of dolomite and a regionally persistent sandstone horizon (Madrid & Roberts, 1991). The lower black dolomite member of the Hanson Creek Formation is a radical contrast to the underlying white Eureka Quartzite. A sandy bed up to 3 m thick is often found at the base, while black elongate nodules of chert are abundant in the dolomite of the member. A chert zone in the middle of this member is tectonically brecciated. Medium grey dolomite sandstone beds, also near the middle, are cross laminated. The middle member is a thin bedded, dark blue-grey limestone that weathers pale orange and breaks into chips and plates. This limestone is about 40 m thick, is very fossiliferous, contains nodular black chert, irregular nodules of limestone, and has shaly partings. It is finer grained than the dolomites of the Hanson Creek Formation. The upper member is a massive, cliff forming, light to medium grey dolomite, near 30 m thick. It is capped by a 1.5 m thick black dolomite marking the contact with the overlying Roberts Mountains Formation (Gilluly & Masursky, 1965).
• Siluro-Devonian, Roberts Mountains Formation, 300 m thick - generally a black, laminated, silty, graptolitic, limestone which is the most homogeneous unit in the stratigraphic succession. The main variations within the unit are slight differences in the thickness of laminae, the size of the plates and chips into which the limestone breaks and the abundance and coarseness of siltstone interbeds which increase upwards. It is composed of 80% calcite, 15% angular quartz fragments, 5% K-feldspar and <1% muscovite flakes. All grains are silt size. Abundant organic carbon helps to define the bedding and authigenic pyrite crystals as much as 5 mm across occur throughout the rock. It contains as much as 3% organic carbon which is extremely finely distributed (Gilluly & Masursky, 1965). In detail however it has been sub-divided into three units, namely a lower laminated silty limestone that has been variably replaced by chert; a middle platy, silty limestone with minor fossil hash beds that increase in thickness and grain size up-section; and an upper section dominated by coarse fossil hash beds that attain thicknesses of up to 2 m. The lower and middle sections are Silurian in age, while the upper is lower Devonian. Clasts of the middle and upper sections of the Roberts Mountains Formation are frequently incorporated in the younger depositional units that overlie it (Madrid & Roberts, 1991).
• Devonian, Wenban Limestone, up to 550 m thick - which is an equivalent of the Popovich Formation of the Lynn Window on the Carlin Trend. In the Cortez-Battle Mountain Trend it is composed of dark grey, thick bedded bioclastic limestone, interbedded with thin bedded, argillaceous, grey to yellow-grey weathering slabby limestone. The beds become lighter grey, finer grained and more massive upwards (Gilluly & Masursky, 1965). It has been subdivided into three units. The lower units include extensive mass transport beds which contain clasts of coarse fossil-hash limestone, and platy and silty limestone derived from the underlying Roberts Mountains Formation. The middle parts of the sequence are dominated by silty limestone and fossil hash beds that also include clasts of the underlying Roberts Mountains Formation. In places the upper sections of these carbonates contain replacement cherts which superficially resemble the ribbon cherts of the Western Siliceous Assemblage of the allochthon (Madrid & Roberts, 1991).
Some papers on deposits within the Cortez-Battle Mountain Trend refer to the Devils Gate Limestone, which is an equivalent to this unit, while the Nevada Formation of the same references may be equivalent to either the Lower Wenban Limestone or upper Roberts Mountains Formation.
• Devonian, Horse Canyon Formation, ~60 m thick - laminated calcareous siltstone, mudstones with interbedded chert and silicified siltstones.
• Devono-Carboniferous, Pilot Shale - a unit of shales and siltstones that may locally be found overlying the Wenban Limestone (Madrid & Roberts, 1991; Stewart, 1980).
Overlap Assemblages. These were deposited following the Antler Orogeny and associated eastward thrusting over the Roberts Mountains Thrust. They unconformably overlie both the Eastern and Western Assemblages and are principally terriginous conglomerates and shallow marine carbonates. The main units are as follows, from the base:
• Upper Carboniferous (Pennsylvanian), Battle Formation, 220 m thick - which is composed of terriginous and marine strata, contains locally derived clasts as big as boulders and is laterally variable in thickness. It was deposited on a surface of considerable topographic relief and rests unconformably on older folded Palaeozoic siliceous rocks. At Battle Mountain it comprises three units, a lower reddish brown calcareous chert-pebble conglomerate, a middle variously coloured shale member and an upper drab quartzite- and chert-pebble conglomerate (Madrid & Roberts, 1991; Stewart, 1980; Theodore & Blake, 1975).
• Late Carboniferous to Early Permian, Antler Peak Limestone and Highway Limestone, 190 m thick - The Antler Peak Limestone at Battle Mountain, is a resistant, well bedded, dark grey limestone. Some of the limestone contains well rounded chert granules. The similar Highway Limestone is partly equivalent to the upper Battle Formation and to the Antler Peak Limestone (Madrid & Roberts, 1991; Stewart, 1980; Theodore & Blake, 1975).
• Upper Permian, Edna Mountain Formation, 120 m thick - predominantly a brown calcareous sandstone. At Battle Mountain three interbedded units are recognised, namely 1). a brown to buff, slope forming calcareous sandstone with minor limy shale; 2). a resistant white to grey massive limestone; and 3). a resistant, fairly well sorted, drab, chert pebble conglomerate (Madrid & Roberts, 1991; Theodore & Blake, 1975).
The clastic rocks of this sequence consist predominantly of detritus derived from the rocks of the Roberts Mountains Allochthon Western, Siliceous Assemblage, although locally derived limestone detritus from the autochthon is also found in these sequences along the eastern-most exposures of the allochthon (Madrid & Roberts, 1991).
Upper Palaeozoic Overlap Assemblage sediments of the lower Carboniferous, such as the Chainman and Diamond Peak Formations and the upper Carboniferous Moleen and Tomera Formations are mapped in the Carlin Trend, but are not significantly represented in the Cortez-Battle Mountain Trend (see section 5.2 for descriptions).
Upper Palaeozoic Sequences of the Golconda Allochthon. Rocks of the Havallah Sequence were thrust east-ward over the Golconda Thrust during the Permo-Triassic Sonoma Orogeny to form the Golconda Allochthon. This sequence comprises a silici-clastic and pelagic assemblage of chert, argillite, sandstone, greenstone and carbonate turbidites of lower Carboniferous to Permian age. The Havallah Sequence was deposited within the main oceanic basin to the west of the Antler Highland which had resulted from the over-thrusting of the Roberts Mountains Allochthon during the Antler Orogeny. As such the sequence is equivalent to the Overlap Assemblage to the East of the same highland, upon which it is partially juxtaposed above the Golconda Thrust (Madrid & Roberts, 1991).
On the Cortez-Battle Mountain Trend the sequence comprises the Pumpernickel Formation, and the probable time equivalent Havallah Formation which are both now thought to be of Permian age, possibly with some late Carboniferous component. The sequence has been subjected to strong thrusting with interleaved slivers from different parts of the sequence. In the Battle Mountain District the Pumpernickel Formation is chiefly made up of chert and argillite with sparse greenstone (altered andesite or basalt), limy shale, siltstone, sandstone and poorly sorted chert-pebble conglomerate. This sequence is thought to be of the order of 1500 m in thickness (Stewart, 1980; Madrid & Roberts, 1991; Theodore & Blake, 1975).
Younger Cover. This includes:
• Mesozoic Rocks - comprising primarily shallow water silici-clastics and carbonate strata with minor volcanic and volcani-clastic rocks. They include Triassic and Jurassic marine strata and minor Cretaceous terriginous rocks (Madrid & Roberts, 1991).
• Tertiary Rocks - generally ranging from Oligocene to Pliocene in age. The Oligocene and Miocene are represented by rhyolitic ash-flow tuffs. Later Miocene rhyolitic ash-flow tuffs are slightly less abundant. Basaltic flows and dykes of early to middle Miocene age occur in a narrow band following the North-Nevada Rift Zone, while younger basalt flows are also found locally (Madrid & Roberts, 1991). The north Nevada Rift Zone is sub parallel to and closely coincident with the Cortez-Battle Mountain Trend.
The regionally extensive Oligocene Caetano Tuff consists principally of several cooling units of ash-flow tuff that were deposited over most of north-central Nevada. Regionally these tuffs are composed of welded and waterlain rhyolitic and lesser andesitic tuffs, sandstone and conglomerate. The tuff is not mineralised near ore deposits, even where mineralisation passes below it, and cuts slightly older Tertiary dykes. However in some cases, where Miocene basalt flows are mineralised, the Caetano Tuff is also, as at Buckhorn (Madrid & Roberts, 1991).
Rytuba (1985) describes an oval shaped collapsed caldera with a diameter of 16 km which was the source of the Caetano Tuff. Collapse of the caldera was concurrent with the eruption of ash-flow tuff and the infill of the caldera by 1500 m of tuff and interstratified collapse mega-breccia. The Caetano Tuff is a calc-alkaline rhyolite with very low, Au (1 to 1.4 ppb), and As values which are normal for this lithology.
Intrusives. Exposed intrusives in north-central Nevada fall into three general age groupings, as follows:
• Late Jurassic, 168 to 143 Ma - mostly plutons of adamellite (quartz-monzonite) to granodiorite in composition which form a north-westerly belt in the vicinity of the Cortez-Battle Mountain Trend. These bodies cut north-west trending folds (Madrid & Roberts, 1991; Stewart & Carlson, 1978).
• Cretaceous, 128 to 90, and 71 to 68 Ma - plutons of this age, also generally of adamellite (quartz-monzonite) to granodiorite in composition, are less abundant, but occur along the same trending belt as those of Jurassic age (Madrid & Roberts, 1991).
• Oligocene, 40 to 30 Ma - occurring as small bodies, trending in a north-westward direction to Battle Mountain, and including the quartz-diorite of the Battle Mountain mineralised system (Madrid & Roberts, 1991).
In most of the ranges of the Cortez-Battle Mountain Trend, gold mineralisation is spatially associated with plutons ranging in age from Jurassic to Oligocene. Regionally the gold mineralisation cuts the late Jurassic and Cretaceous plutons, but is overlapped by early Oligocene ash flow tuffs (Madrid & Roberts, 1991).
The Cortez, Pipeline , Horse Canyon and Gold Acres deposits all have associated Oligocene porphyry dykes and are parallel to and within 4 to 6 km of the margins of the Oligocene Cortez Caldera, the source of the interpreted co-magmatic Caetano Tuffs. It has been suggested that this caldera and its related igneous activity has been the heat source for the formation of the deposits. The very low gold content of the dykes and tuffs (1 to 1.5 ppb Au) however, indicate they were not the source of the gold (Rytuba, 1985).
For information on the mineralisation of the belt, refer to the individual records for each deposit.
The most recent source geological information used to prepare this decription was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
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