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Buckhorn |
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Nevada, USA |
| Main commodities:
Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The Buckhorn gold deposit is located in Eureka County in north-central Nevada, USA. It is approximately 60 km to the south-east of the town of Battle Mountain and 12 km due east of the Cortez Mine.
Although the Buckhorn Mine is within the Battle Mountain - Eureka Trend, it is hosted by Tertiary basaltic-andesite flows which are dated at 16.3±0.9 Ma. This is younger than the other deposits of the trend, although similar in age and host lithology to the Hollister deposit at Ivanhoe on the Carlin Trend, which is also younger than the other deposits on that trend.
Gold was first found at Buckhorn in the winter of 1908-09. A cyanide plant operated from 1914 to 1916, producing a total of 1 t of Au and 7.7 t of Ag from 195 000 t of oxidised ore taken via a glory hole from the South Buckhorn Workings. Exhaustion of the ore that was amenable to cyanide extraction led to the cessation of operations until 1935 when a flotation mill was installed to treat sulphide ore. Mining ceased in 1937 after the recovery of a further 0.225 t Au and 1.85 t Ag. From 1938 to 1978 there was only minor production which came from re-working of dumps. In 1978 an attempt was made to process oxide ore from a small open pit, the Bar Pit, some 400 m to the north of the old workings using a cyanide agitation leach mill. This attempt failed due to the high clay content of the ore which hindered its treatment. During a 1980 drilling program, Bethlehem Copper blocked out a potentially bulk minable orebody in the area of the Bar Pit. When Cominco acquired Bethlehem Copper in 1980 it took over the prospect. A feasibility study was initiated in March 1983, construction began in September and the first gold was poured in June 1984 (Plahuta, 1987).
In August 1985 an exploration drill hole encountered the West Sinter deposit immediately beneath a cap of siliceous sinter and silicified clastic rocks. This deposit is some 3.5 km to the north of the main North Buckhorn open pit. During 1988 and 1989 open pit mining of the West Sinter deposit extracted 350 000 t of leachable ore averaging 2.2 g/t Au at a 1:1 stripping ratio (Jennings, 1990).
Published reserves and production figures include:
3.15 Mt @ 1.22 g/t Au, 19 g/t Ag = 3.8 t Au (Total Resource, Buckhorn Mine, 1987, Plahuta, 1987).
0.35 Mt @ 2.23 g/t Au = 0.8 t Au (Total Productioon West Sinter Mine, Jennings, 1990).
The cut-off for the reserve above was 0.6 g/t Au. Ore was being mined in 1986 at the rate of 750 000 tpa with an average stripping ratio of 1.38:1. A two stage crushing operation and cement lined agglomeration is followed by heap leaching. Gold is recovered by the Merrill-Crowe process and refined to dore bullion at the mine (Plahuta, 1987).
Geology
The Buckhorn orebodies and mineralisation are hosted by the Tertiary volcanic flows and sediments that mask all older rocks in the Buckhorn district. These rocks unconformably overlie two juxtaposed, coeval, successions of Palaeozoic sediments, namely: i). the autochthonous Eastern or Carbonate Assemblage, and ii). the allochthonous Western, or Siliceous Assemblage, which are separated by the Devono-Carboniferous Roberts Mountains Thrust. These assemblages outcrop some 10 km to the west within and adjacent to the Cortez Window where uplift has resulted in the overlying Tertiary sequence being eroded.
The Roberts Mountains Thrust contact is exposed near the Cortez and Horse Canyon Mines to the west and is overlain by the allochthonous Western, Siliceous Assemblage which surrounds the Tertiary volcanic hosts of the Buckhorn Mine to the east, north-west and west. A third Palaeozoic succession, the Overlap Assemblage, which is largely composed of the coarse clastics of the Brock Canyon Formation, was derived by post thrusting erosion of both the autochthonous and allochthonous assemblages. This succession unconformably overlies both of the earlier assemblages and is found to the north and north-east of the mine area (Plahuta, 1987).
Jurassic adamellite (quartz-monzonite) stocks and batholiths intrude the Palaeozoic rocks and crop out several kilometres to the north-east and west of the mine (Plahuta, 1987).
Large areas of the Oligocene Caetano Tuffs, found to the west of Buckhorn, comprise welded and waterlain rhyolitic and lesser andesitic tuffs, sandstone and conglomerate. These have been dated at between 31.0 and 33.6 Ma (Radtke et al., 1978).
The pre-Tertiary rocks are described in the 'Battle Mountain - Eureka Trend Geology' and 'Cortez' records.
The stratigraphy in the Buckhorn Mine area is as follows, from the base:
Early Tertiary, Alluvial Fan Deposits, which locally exceed 250 m in thickness - These range from clast supported pebble conglomerates to clayey siltstone, occurring as repeated upward fining graded to sub-graded sequences. Lithification is generally poor, although well lithified, silicified zones are present along structures controlling the Buckhorn mineralisation. These unconformably overlie rocks of the Western Siliceous Assemblage and the Brock Formation of the Overlap Sequence. Clasts derived from the Western Assemblage dominate the conglomerate, while rare Oligocene Caetano Tuff clasts provide a maximum age of deposition (Plahuta, 1987; Jennings, 1990).
Miocene, Basaltic-Andesite Flows, with a total thickness of 110 m - Up to eight flows overlie the Tertiary Fanglomerate and form a cuesta which covers an area of around 40 km2. No intercalated sediments or interflow weathering horizons have been noted. The unit is dated at 16.3±0.9 Ma. Fresh rocks are dark green-grey, massive to vesicular and locally amygdaloidal. They comprise the dominant outcrop in the Buckhorn area and are the primary host to mineralisation (Plahuta, 1987; Jennings, 1990).
Miocene, Rhyolite Plug, dated at 15.3±0.4 Ma - These occur approximately 6 km to the south-west of the mine where they intrude Palaeozoic Western Assemblage rocks, the Miocene basaltic-andesite flows and the underlying alluvial fan deposits (Plahuta, 1987; Jennings, 1990).
Miocene, Clastic and Chemical Sediments - Three distinct facies are evident, namely:
- Relatively coarse clastics which are compositionally similar to the fanglomerates below the basaltic-andesite. To the north-east of Buckhorn this facies caps up-faulted blocks and comprises pervasively silicified bedded conglomerate, sandstone and siltstone.
- Pelites which occur within the mineralised Buckhorn structural zone. These include maroon to tan claystone and variably silicified, finely banded shales which commonly display soft sediment slump structures. The source of these pelitic sediments is not known, although it has been suggested that they may represent a combined clastic and chemical precipitation process, within a siliceous hot spring, overlying argillically altered basalts.
- Siliceous sinter deposits - present as opaline siliceous sinters which sporadically cap the basalts along fault zones east of the deposit. Exposures are massive to crudely banded and locally contain fossil reeds (Plahuta, 1987; Jennings, 1990).
All three facies display erosional upper surfaces and have highly variable thicknesses, totalling up to approximately 15 m (Plahuta, 1987).
The age of these deposits is constrained by the underlying 16.3±0.9 Ma basaltic-andesite flows, the felsic dykes which cut the basaltic-andesites and are dated at 15.3 Ma, and the Buckhorn hydrothermal event which affected all three facies and has been dated at 14.6±0.4 Ma (Plahuta, 1987).
Some 3.5 km to the north of the main North Buckhorn Pit, in the vicinity of the West Sinter Deposit, the overlying sequence comprises fine grained sediments which unconformably overlie the basaltic andesites. These sediments are moderately calcareous lacustrine rocks up to 100 m thick that outcrop over an area of 7.8 sq. km. Within the West Sinter deposit fine grained sediments which are presumed equivalents, are preserved below a cap of chalcedonic sinter and interbedded, silicified clastics rocks. These are the primary host to the West Sinter orebody (Jennings, 1990).
Structure
Within the mine area the pre-Tertiary rocks and structures are concealed by the host Tertiary fanglomerate and volcanic sheets. The main Devono-Carboniferous Roberts Mountains Thrust passes some 10 km to the west of Buckhorn, in the Cortez-Horse canyon area. At Buckhorn the main structural features are the steep faults of the Basin and Range deformation which are largely responsible for the topography of the district. The more prominent examples of these faults have offsets of the order of 600 to 3000 m within the district and were active during the extrusion of the basaltic andesites. Late stage regional south-east tilting of the area by 5 to 10° has preserved the basaltic andesite flows of the mine area as a cuesta (Plahuta, 1987).
A system of parallel faults with west-down displacements of generally less than about 100 m trend across the area in a direction of about 350°. These control the mineralisation at Buckhorn. The Buckhorn orebodies are all located along one of these fault zones. The Aspen mineralisation occurs along another such NNW striking parallel zone 1200 m to the west, while the West Sinter deposit is along a third parallel fault which passes 1200 m to the east of Buckhorn. Movement on these faults can be demonstrated to have occurred as late as during the deposition of the sediments that overlie the basaltic-andesites (Plahuta, 1987). Field evidence exposed during mining also suggests that the first movement on this fault set was prior to the deposition of the lavas (Jennings, 1990). Hence it has been concluded that movement may have continued during the extrusion of the volcanics, as suggested by feeder dykes paralleling this trend to the north of the mines (Plahuta, 1987).
A second, younger fault set is represented by the Crescent Fault, the major north-east to ENE trending range front normal fault that bounds the Cortez Mountains on the west and north-west. The initial displacement on the Crescent Fault post-dated the extrusion of the basaltic-andesites, with movement such as the offset of recent alluvial fans, suggesting activity continuing to the present. Drilling and mapping has indicated that the ENE Crescent set normal faults cut the NNW striking normal faults and serve to localise mineralisation at structural intersections. It has been suggested that reactivation of the earlier NNW striking faults by post volcanic ENE Crescent fault related movement was critical to the localisation of the Buckhorn district mineralisation (Jennings, 1990).
The Buckhorn deposits are localised within a several kilometre long structural zone which trends at about 350°, or NNW, cutting the gently south-east dipping basaltic-andesites. In the immediate mine area two stages of block faulting and four major faults are evident. The contact between the fanglomerate and overlying basaltic-andesites define three easterly tilted fault blocks forming a horst structure in the first stage of block faulting. Movement along individual faults is thought to have been of the order of 25 to 30 m. The medial block has been rotated, bringing the pre-basalt fanglomerate to the surface north of the mine. Post-basalt sediments are un-effected by this stage of faulting. The basalts are thinner over the horst, which may be either erosional or a thinning over a pre-existing horst (Plahuta, 1987).
The second stage of block faulting reactivated the earlier faults and formed grabens in the basalt. These faults dip at 60° or greater and have generally <15 m of displacement. Antithetic and sympathetic joints appear to have controlled mineralisation and alteration. The post basaltic-andesite Miocene graben fill sediments are present as a fault bounded open syncline. Slump structures and brecciation along bounding faults indicate movement during, and possibly after, sedimentation. Late stage quartz-breccia fillings and plugs cutting both the basalt and overlying sediments are preferentially emplaced along major fault zones (Plahuta, 1987).
Alteration and Mineralisation
The main ore deposits at Buckhorn are distributed along the NNW (350°) trending Buckhorn Structure, a 30 to 300 m wide zone of normal faulting which passes through the mine area. The parallel Aspen Zone 1200 m to the west is also mineralised. The West Sinter deposit occurs within the 345° trending West Sinter fault zone which passes some 1200 m to the east of the Buckhorn Structure (Plahuta, 1987; Jennings, 1990).
Within the Buckhorn Zone, minable deposits occur over a 1000 m strike length of the structure, extending from the old South Buckhorn Glory Hole (Workings), through the Junkyard Pit to the main North Buckhorn Pit. Mineralised areas are separated by altered but barren zones, generally containing <50 ppb Au. Mineralisation within an ore pod is irregularly distributed, both laterally and vertically. Precious metal values occur within a zone extending from the surface to 60 m in depth. The upper two thirds of the mineralised zone is oxidised, whereas the lower third is pyritic and was not amenable to treatment in 1987. Both the base of mineralisation and redox boundary occur at lower elevations along major structures (Plahuta, 1987). At West Sinter the redox boundary is from 20 to 25 m below the surface (Jennings, 1990).
Extensive argillic alteration characterises the Buckhorn Structure. It is most noticeable in the basaltic-andesites, while the underlying fanglomerates are silicified with argillised igneous clasts. The overlying pelites are locally bleached and silicified in this zone. Exposures of altered basaltic rocks from the oxidised zone consist of blocks of siliceous clay ranging in colour from light purple to pink, to dull green and yellow, to pure white. They are cut by a network of dark quartz veins with limonitic selvages. There is a colour zonation from white near veins to purple in the centre of blocks. Individual blocks vary from a few cm's to 3 m across, while the veins may be up to a metre thick, although more commonly they average 5 cm. Dark quartz also occurs as irregular shaped pods throughout the clay and altered basalt, and as a 20 m diameter funnel shaped plug cutting the ore zone along a major north-south fault. This plug appears to pinch out at a depth of 30 m (Plahuta, 1987).
The altered volcanics at Buckhorn are composed of quartz, kaolinite, montmorillonite, adularia, jarosite, goethite, pyrite and trace amounts of sericite, calcite, gypsum and plagioclase. The main difference between the oxidised and reduced zones is in the lighter colour of the former. The reduced zone contains around 6% pyrite, while the oxidised interval has approximately 20% limonites (mainly jarosite with lesser goethite). Both contain around 7% adularia, while the oxide zone has nearly 45% silica, twice that of the reduced zone. Total clay is higher in the reduced zone, around 45%, occurring as both montmorillonite and kaolinite, while the oxidised zone has only 20%, mainly kaolinite (Plahuta, 1987).
In addition to this vertical zonation, there is lateral variation at surface in the oxidised zone, ranging from white kaolinite rich clays in the centre of the system to purple montmorillonite rocks on the peripheries, before grading out into un-altered basaltic-andesite. Montmorillonite appears to increase at the expense of kaolinite, while the surficial weathering products such as chlorite, calcite and zeolite increase as the hypogene quartz, adularia and limonite decrease. At Buckhorn, dark crypto-crystalline quartz pods, stockwork veins, plugs and breccia matrices are emplaced along structures which are spatially associated with alteration and mineralisation. Two stages of quartz are recognised, i). light-grey to black, milky to vitreous, sulphide-poor, pods and veins; and ii). black, vitreous to dull, sulphide-bearing, plugs and breccia fillings. The second stage incorporates fragments of altered basalt, Miocene pelites and sinter, and first stage dark quartz. The dark quartz is typically poorly mineralised (Plahuta, 1987).
Hydrothermal alteration at Buckhorn has been dated at 14.6±0.4 Ma (Plahuta, 1987).
Silica at West Sinter occurs as siliceous sinter that caps the orebody; in lenticular replacement bodies in laminated lacustrine sediments; as massive breccia plugs in hydrothermal vents; as massive black to brown pods that disrupt sedimentary structures; and with iron oxides in occasional veins that commonly cut basaltic-andesite. Limited sampling of each silica type has revealed a wide range of Au values. The siliceous sinter carries between 7 and 1160 ppb Au (8 samples), while in the replaced bodies values match those in the adjacent unreplaced rocks. Gold was not detected in samples from two massive silica pods. One sample of brecciated silica at the core of a vent assayed 11 ppm Au (Jennings, 1990).
At West Sinter the alteration of sedimentary rocks is dominated by pervasive cristobalite and alunite associated with smectite. In argillised basaltic-andesite this assemblage occurs with minor kaolinite. The moderately calcareous lacustrine sediments above the basaltic-andesite are also much lower in calcite in the ore zone, implying decalcification. It has been suggested that the inferred acid sulphate media which have created the cristobalite-alunite assemblage, also leached calcite from the sediments. The oxidation above the redox boundary has produced limonite from pyrite and has also de-carbonised carbonaceous shales (Jennings, 1990).
There is a good correlation between gold and limonite in the oxidised portions of the deposit. Microscopic studies have revealed very fine grained free gold with skeletal limonite after globular pyrite. The gold:silver ratio at North Buckhorn is about 1:15, and both are enriched immediately below the redox boundary (Plahuta, 1987). At West Sinter the Au:Ag ratio is 1:1. In the reduced section at North Buckhorn gold occurs both within pyrite grains and as free particles up to 18 µm in diameter. A similar situation applies at West Sinter. Visible gold is rarely seen in either deposit (Jennings, 1990). Although there is a good correlation between Au and Ag, the form of the silver is not known. As, Hg, W, Tl and Sb are enriched in the deposit. Cu and Zn values are slightly higher in the sulphide zone (Plahuta, 1987).
At West Sinter pyrite is the dominant sulphide in the reduced zone, occurring as disseminated grains and narrow veins in argillised basaltic-andesite. Carbonaceous sedimentary rocks exposed in the pits often contain framboidal pyrite. Limonite occurs above the redox boundary as disseminated grains, clots and fracture fillings in the lacustrine sediments and oxidised, argillised basaltic-andesite. Goethite and minor hematite are apparent in hand specimens. Gold and silver occur in both the lacustrine sediments and the underlying altered basaltic-andesites at West Sinter. The mineralisation at this deposit is controlled by both structure and lithology, with the highest grades being in the more permeable rocks in the hangingwall of the main West Sinter Fault (Jennings, 1990).
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.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Plahuta J T, 1987 - Geology of the Buckhorn mine, Eureka County, Nevada: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 333-337
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