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Barneys Canyon, Melco |
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Utah, USA |
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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.
Available as Full Text for direct download or on request. |
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The Barneys Canyon Project is located in the northern Oquirrh Mountains, to the south-west of Salt Lake City in Utah, USA, and approximately 8 km to the north of the Bingham Canyon porphyry copper mine. The project consists of two main sediment hosted gold deposits, Barneys Canyon and Melco, both of which have been mined. They are located approximately 2.5 km apart along a north-east trending mineralised corridor. A number of other prospects with similar mineralisation are distributed along the same corridor (Gibson & Hammitt, 1991).
The Oquirrh Mountains have been prospected and mined since 1863 when lead and silver were first discovered. The historic gold production has been approximately 500 t, mostly as a by-product of base metal mining at Bingham Canyon and Carr Fork, although the 39 t from Mercur to the south is also included in this figure (Gunter, 1991).
Published reserves are as follows:
Barneys Canyon Pre-mine Reserve, 1989 - 8.6 Mt @ 1.64 g/t Au = 14 t Au (Gunter, 1991)
Melco Pre-mine Reserve, 1989 - 3.5 Mt @ 2.57 g/t Au = 9 t Au (Gunter, 1991)
Mining commenced at Barneys Canyon in February 1989 and at Melco in November of the same year. The first dore was poured in September 1989. Both mines are open pit operations. Barneys Canyon has a planned stripping ration of 3.1:1, waste:ore and Melco 4.9:1. The entire Barneys Canyon deposit and the oxidised portions of Melco are amenable to cyanide heap leaching. Gold extraction had exceeded 85% between the commencement of leaching in 1989 and 1991. The project was initially scheduled to extract some 19.8 t Au over a six year period. The ore contains no appreciable Ag (Gunter, 1991).
Geology
The Barneys Canyon and Melco gold deposits are hosted by late Palaeozoic shallow water carbonates and clastics of the Eastern, or Carbonate Assemblage that have been involved in Sevier Orogeny crustal shortening episodes during the Mesozoic and Tertiary. This orogenic movement has incorporated the sequence into a stack of imbricated thrust slices that has juxtaposed a variety of facies which were originally separated. During the late Eocene to Oligocene the area has undergone extension, followed by uplift from the Miocene onwards, producing the Basin and Range topography. Barneys Canyon and Melco both lie close to the eastern margin of the Basin and Range Province of the Great Basin. The imbricated Palaeozoic sediments have also been intruded by late Eocene to Oligocene felsic intrusives (Gunter, 1991).
The southern Oquirrh Mountains, which embrace the Mercur gold deposit (described above), are dominated by a NNW structural trend. In contrast the northern Oquirrh Mountains, in which the Barneys Canyon and Melco deposits are located, have been deformed by a north-east trending set of folds. The two segments of the Oquirrh Mountains are separated by the long lived, east-west trending Uinta Lineament on which the Bingham Canyon Mine lies. The north-east trending folds of the North Oquirrh Mountains deform both the upper and lower plates of the major south-east directed Sevier Orogeny North Oquirrh Thrust which is mapped through the area and is found above the ore deposits (Gunter, 1991).
The stratigraphy within the Barneys Canyon project area is as follows, from the structurally lowest:
LOWER PLATE OF THE NORTH OQUIRRH THRUST
Cambrian to Lower Carboniferous
* A sequence of carbonates and clastics which include the lower Carboniferous (Mississippian) Deseret Limestone and Humbug Formation limestone, sandstone and orthoquartzite described in the footwall of the Mercur deposit above (Kornze, 1987).
Lower Carboniferous (Upper Mississippian)
* Great Blue Limestone - composed of a lower section of massive, bedded, nearly pure limestone, locally composed of bioclastic micrites, wackestones and sparse siltstone, overlain by limestones which include upwardly increasing quantities of terriginous quartzose and bioclastic detritus. The rocks of the upper half of this unit host all of the gold ore at Mercur, and are weakly to strongly fossiliferous, thin bedded to fissile, packstones, wackestone and grainstones, with abundant siltstone and some sandstone (Kornze, 1987).
* Manning Canyon Shale; - composed predominantly of shale, interbedded limestone and quartzite (Kornze, 1987).
Upper Carboniferous (Pennsylvanian) to Permian
* Oquirrh Group, which includes the:
Butterfield Peaks Formation, >1500 m thick - Tan to light grey, calcareous quartzite and platy calcareous sandstone, cyclically interbedded with medium to dark grey sandy bioclastic limestone containing thin bedded lenses of black chert (Lanier, etal., 1978).
Bingham Mine Formation - which comprises,
- Lower Member, approx. 1000 m thick - Tan to light grey orthoquartzite, calcareous quartzite and calcareous sandstone, locally containing fine banding and crossbedding. Thin calcareous, fine grained sandstones and sandy limestones are interbedded. Thick cherty limestones are also interbedded near the base. These lower limestones, principally the Jordan at the base, and the Commercial Limestone above it, have been altered to skarn adjacent to the Bingham Stock where they host the Carr Fork and North Ore Shoot Cu-Au orebodies and the peripheral Pb-Zn-Ag deposits. The calcite matrix of the calcareous quartzites and sandstones has also been altered to calc-silicates within the skarn zones (Lanier, etal., 1978).
- Upper Member, approx. 500 m thick - Tan to light grey in colour with the same lithologies as the upper part of the Lower Member, but with fewer limestones, and more orthoquartzite and calcareous quartzite (Lanier, etal., 1978).
* Curry Peak Formation, approx. 750 m thick - lower quartzite, grading upwards into sandstones with lesser interbedded shale (Gunter, 1991).
* Freeman Peak Formation, approx. 750 m thick - predominantly orthoquartzite with minor interbedded shale. The Melco mineralisation is localised within a discordant brecciated and altered zone at the base of the Kirkman-Diamond Creek Formation and the upper Freeman Peak Formation. At the Melco mine the Freeman Peak Formation is composed of blocky fractured, pale grey, coarse to rhythmically bedded orthoquartzite with thin, locally carbonaceous, shale and sand interbeds (Gunter, 1991).
* Kirkman-Diamond Creek Formation, 600 m thick - Fine grained grey to white bleached sandstone with locally interbedded quartzites. Thin discontinuous limestone beds occur near the top. In the Barneys Canyon Project area the uppermost 30 m is principally composed of calcareous sandstone which is pale tan to grey, massive to thin bedded and feldspathic, and contains limestone and calcareous dolomite lenses with intra-formational brecciation and intercalations of cross-bedded orthoquartzite. In the Melco Mine the un-altered hangingwall lithologies are calcareous, irregularly fractured, massive to bedded, tan to buff coloured feldspathic sandstones with thin conglomerates and thick depositional breccias near the basal contact. Kaolinite clasts are locally present in the conglomerate. Beds of carbonate and quartzite up to 6 m thick are found in the lower portions of the formation (Gunter, 1991).
* Park City Formation - This unit is the main host at Barneys Canyon where it has been subdivided into:
Lower Host Dolomite, 20 m thick - tan coloured sandy dolomite and dolomitic sandstone which is thick bedded, and has brown oxide spotting. Thin chert beds occur in the upper section, while the upper 2 m are bleached and weakly silicified. It grades to dolomitic sandstone down section and is a significant host to mineralisation in the Barneys Canyon Mine area (Gunter, 1991).
Gouge Zone, 1 m thick - A zone of bedding parallel slip. It is ochre to tan in colour and is brecciated to streaky (Gunter, 1991).
Host Dolomite, 26 m thick - Grey to red-brown when fresh, but pale grey to white when altered, with local brown oxide spotting. It is a sparry and sandy dolomite with interbedded chert and is the dominant host to ore. It grades down section into a sandy dolomite, while a cherty basal unit has minor jasperoid pods (Gunter, 1991).
Dolomitic-wacke, 2 m thick - A medium green-grey rock with distinctive tan oxidation, which is massive, with local alternating pale to dark green-grey, concentric pseudo-leisegang bands (Gunter, 1991).
Dolomitic Siltstone, 17.5 m thick - Medium grey, thin bedded, dolomitic siltstone which is carbonaceous and pyritic down-dip and outside of the mine area. It is varicoloured, with oxidation and Mn oxide dendrites. A distinctive 6 cm thick sandy to phosphatic layer in the middle of the unit provides a marker unit throughout the mine area (Gunter, 1991).
Oolitic Dolomite, 5 m thick - Pale purple to grey, massive, bio-clastic, oolitic dolomite which has crinoid fragments, and is fetid when broken (Gunter, 1991).
Chert Pod Dolomite, >12 m thick - Medium grey, tan weathering, thin to medium bedded dolomite with thin interbeds of silty dolomite and minor chert. There is extensive bedding parallel shearing. Within the unit there is an interbed of pale purple dolomite with 2 to 3 cm vuggy, siliceous brachiopod clasts.
UPPER PLATE OF THE NORTH OQUIRRH THRUST
Upper Carboniferous (Pennsylvanian) to Permian - which are substantially thinner than the autochthonous equivalent sediments.
* Lake Point and Erda Formations - The Lake Point Formation is predominantly a sandy carbonate while the overlying Erda Formation is composed of interbedded grey cherty limestone, tan quartzite, dolomite, siltstone and chert.
Tertiary
* Conglomerates - rusty brown silica-cemented conglomerate.
* Intrusives - dark grey hornblende-biotite-quartz dacite porphyry of probable late Eocene to Oligocene age (Presnell, 1991).
* Volcanics - trachy-andesite to trachy-basalt (latite), composed of grey-green flows, tuffs and breccias. The larger areas of volcanics to the east of Bingham Canyon are from 39 to 31 Ma in age (Presnell, 1991).
Quaternary
* Cover - alluvium and valley fill.
Structure
The Barneys Canyon Mine and three satellite deposits, Water Tunnel, North BCS and South BCS, lie along a north-east trending corridor of low angle thrust faults, high angle normal faults and alteration. The corridor is about 750 m wide and is bounded by the West Fault and the north-south trending, overturned Copperton Anticline. The Bingham Stock and porphyry copper orebody, and the Carr Fork and North Ore Shoot deposits are localised within the Copperton Anticline (Gunter, 1991).
The earliest structural event in the corridor is the Barneys Canyon Thrust, a probable lower plate imbricate of the North Oquirrh Thrust zone which is a prominent structure approximately 1 km to the north-west. The Barneys Canyon thrust passes through the upper sections of the Barneys Canyon orebody and repeats the host sequence of the Park City Formation, with movement of the upper plate towards the south-east. The fault appears as a focused, highly contorted zone of north-east trending, shallow north-east plunging, overturned drag folds. A broad area of bedding parallel slip surrounds the contorted zone and locally contains ore grade gold mineralisation (Gunter, 1991).
High angle faulting off-sets older thrust faults and mineralisation in the Barneys Canyon Mine area. Further north the high angle, normal West Fault offsets Tertiary conglomerates, establishing the age of the latest movement on the structure. A significant amount of the Barneys Canyon orebody lies in a graben structure bounded by the north-east trending West and East Faults. The West Fault separates a high angle, normal fault dominated terrane to the east from an asymmetrical fold terrane to the west. The West Fault dips steeply to the east, strikes north-south and is up to 30 m wide. It contains bleached, calcite flooded, contorted, fractured and faulted sandstone, and embraces pods of gold mineralisation. The trace of the fault parallels the corridor of alteration and mineralisation. Displacement is of the order of hundreds of metres vertically with the possibility of strike slip movement also. The uplifted Park City Formation has not been found to the west of this structure (Gunter, 1991).
The East Fault strikes NNE and dips at 50° WNW. Displacement is around 15 m in the south, but diminishes to the north. It clearly offsets mineralisation. Down-stepping north and north-east striking faults within the graben also each off-set mineralisation by 1 to 6 m and are filled with calcite cemented breccia. Minor east-west normal faulting is also noted. Additional high angle normal faults are known under cover to the east. One of these offsets the mineralisation in the Water Tunnel prospect (Gunter, 1991).
At Melco the ore deposit lies within an asymmetrical to overturned, fold dominated terrane located to the west of the West Fault. Tight isoclinal drag folding occurs in shale and sandstone interbeds between competent orthoquartzites. Larger flexures contain crackle breccias. The north-east plunging, north-west dipping overturned Melco Syncline trends through the Melco deposit. Melco appears to lie along, or near, the axial plane of this structure which has superimposed a brittle deformation upon a sequence of interbedded sandstone, sedimentary breccia, laminated sediments and orthoquartzite. This structural preparation created a corridor for subsequent alteration and gold mineralisation (Gunter, 1991).
At Melco gold occurs within a discordant, 30° trending, 70° NW dipping zone of crackle and matrix supported breccia that is extensively bleached, altered and contorted. Bedding attitudes of laminated silts and layered conglomerates within the contorted zone vary from steeply dipping to sub-horizontal. Crackle breccias are mono-lithologic, clast supported and display minor fragment rotation. Matrix supported types are mono-lithologic to poly-lithologic, have a sandy matrix and sometimes appear along fault traces. Breccia clasts comprise sandstone, quartzite and siltstone, with no igneous fragments. Other breccias contain massive blocks of pure white sedimentary kaolinite. Kaolinite veins and minor alunite are locally present in the altered zones (Gunter, 1991).
Alteration
The alteration patterns and textures observed at Barneys Canyon are relatively subtle. The most pronounced are:
* Decalcification - which is preferentially developed within sparry bioclastic dolomite containing thin chert interbeds, principally the Host Dolomite, and along high angle faults and fractures. The rocks are sometimes bleached a pale grey to white, are friable and contain highly fractured and veined chert. The only carbonate identified in the decalcified ore is dolomite. The decalcification produces a 'sanded' friable rock. Thin sections reveal sparry dolomite rhombs and sand sized dolomite grains with 5 to 10% pore space. This texture is well developed beneath the relatively impermeable Dolomitic Siltstone unit in both the upper and lower plates of the Barneys Canyon Thrust. The structurally lower zone contains a significant proportion of the Barneys Canyon orebody, while the upper zone is weakly anomalous to barren. This alteration of the Host Dolomite apparently preceded gold deposition and is more extensive than the mineralisation. The up-dip extension of the Host Dolomite in the mine area becomes more massive and takes on a red colour. Greatly diminished gold values accompany porosity loss in this area. Some drag folds related to the Barneys Canyon Thrust display decalcification in their cores. Barren decalcified dolomite is found in the upper plate of the thrust (Gunter, 1991).
* Argillic Alteration - occurs dominantly in the sandstone portion of the orebody, and to a much lesser extent in the carbonates. Kaolinite and illite are the major clays, with subordinate sericite. Within sandstone hosts the clays are iron bearing. Argillisation appears to be coeval with the decalcification (Gunter, 1991).
* Silicification - Massive silicification is not seen near the orebody. Only weak silicification is noted. Macroscopic silicification is not apparently related to gold mineralisation in the mine, although in thin section there appears to be a relationship between native gold, fine grained quartz and iron oxides along micro-fractures. Silicification also occurs along the trace of high angle faults, while other diagenetic build-ups of silica have been noted. The "discovery outcrop" of 'auriferous jasperoid breccia' displays moderate silicification at the surface which does not extend to depth (Gunter, 1991).
* Calcite Mobilisation - Coarse calcite crystal lined vugs up to 0.25 m or more across are observed in dolomitic strata within, peripheral to, and overlying gold mineralisation. Calcite cemented fault breccia is found along high angle structures that offset mineralisation and imply that calcite mobilisation followed gold deposition. Remobilised calcite forms a halo to the gold mineralisation (Gunter, 1991).
* Oxide Spotting - Dark brown millimetre sized spots commonly occur peripheral to alteration and gold mineralisation and are well developed in carbonates and sandstones. They are interpreted to represent migration of iron and manganese to the perimeter of the hydrothermal system (Gunter, 1991).
* Carbon Removal - The host dolomitic siltstone is black, pyritic and carbonaceous outside of the mine area. Within the mine it is pale green-grey and contains traces of partially oxidised pyrite, but no carbon. The base of the dolomitic siltstone characteristically weathers to a tan colour in pit exposures with light to dark green 'pseudo-leisegang banding' on fresh broken surfaces which may be related to carbon migration and/or removal (Gunter, 1991).
At Melco the alteration is reflected as moderately argillised, weakly silicified rocks which are characteristically bleached pale grey to white and contain amorphous iron oxide pods from 1 to 5 cm in size. Green arsenate stains and occasional pods of sulphur crystals are present. The most intense bleaching immediately underlies mineralisation (Gunter, 1991).
Mineralisation
At Barneys Canyon, bedding, alteration, relative permeability and structures appear to control the distribution and intensity of gold mineralisation. The orebody, which strikes north-west and dips moderately to the north-east, is approximately 430 x 370 m in plan area and is up to 90 m thick. Approximately 90% of the ore is confined beneath the impermeable Dolomitic Siltstone in the lower plate sequence, below the Barneys Canyon Thrust. Gold grades increase from trace to ore grade at the contact between the Dolomitic Siltstone and the underlying Host Dolomite. Gold values locally continue downwards into underlying sandstones and display excellent vertical and lateral continuity. In the south-west, or up-dip part of the mine, mineralisation is confined to altered sandstone beneath un-altered dolomite (Gunter, 1991).
Discontinuous gold mineralisation is found above the lower plate Dolomitic Siltstone within a broad zone of bedding plane shear and gentle ramping associated with the Barneys Canyon Thrust. This shearing cuts the Dolomitic Siltstone, thus connecting it to the main orebody. Mineralised rocks have a weak, pervasive orange iron oxide tinged stain (Gunter, 1991).
Gold occurs as extremely fine native metal with no impurities such as Ag or Cu. It is typically <2 microns in diameter and is most closely associated with fine grained quartz and iron oxides along micro-fractures (Gunter, 1991).
Several stages of sulphide mineralisation are evident at Barneys Canyon. The first is diagenetic pyrite and carbon which appear up-dip from the ore within dolomitic siltstones. Where these pass into the ore zone they are usually strongly oxidised and barren. However where they are not oxidised they may contain good gold values. Two types of hydrothermal pyrite are noted in the deposit. The most widely distributed is a finely disseminated barren pyrite which occurs as sulphide grains rimmed by limonite in the deposit. The second type is characterised within the oxidised section of the orebody by irregular fracture fillings, veins and pods of amorphous orange-brown and red-brown oxides. Rare limonite cubes are noted. These oxides are anomalous in a broad suite of elements relative to the trace element geochemistry of the host strata. Pod shaped iron oxide masses may be the oxidised equivalents of small orpiment rich pods encountered during exploration drilling, or may have been deposited as gold bearing oxide masses (Gunter, 1991).
The Barneys Canyon 'discovery jasperoid breccia' is a highly anomalous iron oxide rich stringer 100 m above the main orebody in the upper plate of the Barneys Canyon Thrust. It is a 20 cm wide, discontinuous, vertical vein. The silica content decreases rapidly with depth where it becomes an anastomosing to podiform gossanous vein with fragments of chert. Samples from the jasperoid assayed up to 35 g/t Au, although the adjacent dolomite was barren (Gunter, 1991).
Geochemical studies indicate an association of As, Sb, Hg, Tl and Ba, but not Ag, within the Barneys Canyon orebody (Presnell, 1991a).
The Melco deposit is a high sulphur system which is anomalous in Au, As, Sb, Hg, Tl, Ba, U, Sr and Se. Sulphide and carbon are spatially related to the mineralisation, with two types of sulphide being recognised. The first is a pale grey disseminated type that averages 3 to 5% sulphide, but may locally be semi-massive. The second is a dark grey to black flooding of carbon and sulphide that commonly occurs along faults and breccia zones. Pods of realgar and orpiment are often also present. The shape and distribution of the second type indicates it was superimposed on the first. An analysis of a sulphide sample (which of the two is not specified), indicated an early barren phase of pyrite rimmed by a later auriferous, arsenic rich, disordered pyrite phase. Trace amounts of marcasite and other sulphides are also present. Disseminated and massive barite are found within and outside of the deposit (Gunter, 1991).
The Melco orebody is a tabular mass 300 m long, 60 m wide and extending to an unknown depth. It plunges moderately to the north-east along the axis of the Melco Syncline. The orebody has a distinct high grade core exceeding 3.4 g/t Au, spatially related to a central zone of brecciation, and flanked by lower grade material. Extensive oxidised portions of the orebody contain free gold grains that range from sub-micron size to 100 microns. The distribution of gold is enigmatic and complex. There is a gold-sulphide relationship, although not all sulphide zones are auriferous. Carbonaceous zones do not show significant gold enrichment, and in fact reflect the gold values of the surrounding rocks that they invade. Dark grey to black sulphide and carbonaceous gold bearing rocks are in sharp contact with oxidised gold ore, while gold grade contours cross the oxide-sulphide boundaries. The highest gold values in the deposit occurred in a crackle brecciated orthoquartzite layer that contained amorphous iron oxide pods. The orthoquartzite lies between two carbonaceous 'dykes' that intrude the fault breccia. Gold values in the crackle breccia commonly exceeded 35 g/t Au in the upper level of the mine. It has been suggested that the mineralisation observed would be consistent with superimposed oxidised and sulphide hydrothermal systems. Supergene oxidation is also evident near the surface (Gunter, 1991).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1995.
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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Presnell R D, Parry W T 1996 - Geology and geochemistry of the Barneys Canyon Gold deposit, Utah: in Econ. Geol. v91 pp 273-288
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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