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Mercur |
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Utah, USA |
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Au Ag Hg
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Super Porphyry Cu and Au
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The Mercur gold mine is located some 53nbsp;km to the south-west of Salt Lake City in the Oquirrh Mountains of north-central Utah. It is also about 25 km to the south of the Bingham Canyon porphyry copper mine.
Mining first commenced in the Mercur district in 1870 with the discovery of silver in a massive jasperoid, the 'Silver Chert' bed at Marion Hill, the site of a subsequent gold open pit. Mining of the silver ores only lasted for two years, although accompanying gold was identified by assay. However, due to its micron size, no gold was visible, nor was it exploited. With the discovery and production of cinnabar in 1879 the presence of gold bearing beds was definitely established, although economic gold production did not take place until 1891 and continued with breaks until 1942. Exploration from 1973 resulted in a new operation from 1983.
Published production and reserve figures are as follows:
Production to 1942 - 6 Mt @ 6.5 g/t Au = 39 t Au (Kornze, 1987)
Startup Reserve, 1983 - 15 Mt @ 2.75 g/t Au (Kornze, 1987)
Production, 1983-86 - 9.3 t Au (Kornze, 1987)
Reserve, 1986 - 16 Mt @ 2.27 g/t Au = 36 t Au (Kornze, 1987)
Geology
The Mercur gold deposits are located within the Oquirrh Mountains, a fault-block range near the western margin of the Basin and Range province of the Great Basin. The range is composed of folded and faulted Palaeozoic rocks which have been intruded by felsic igneous rocks of Eocene to Oligocene age. Mercur is located on the eastern flank of the broad Ophir Anticline, about 1.5 km to the east of its axis (Kornze, 1987).
The upper Palaeozoic rocks of the district are exposed as a result of erosion along a transverse fault and in the core of the Ophir Anticline. The sequence is as follows, from the base (Kornze, 1987; Brunker & Dickson, 1987):
Carboniferous (Mississippian),
* Deseret Limestone - a massive bedded limestone.
* Humbug Formation - a sequence of interbedded limestone, sandstone and calcite cemented orthoquartzite.
* Great Blue Limestone Formation, 1050 m thick - composed of,
Lower Great Blue Limestone, 145 m thick - which in turn can be sub-divided into,
- The lower half, characterised by massive, bedded, nearly pure limestone, locally composed of bioclastic micrites, wackestones and sparse siltstone.
- Mercur Mine Series, 75 m thick - the upper half of the Lower Great Blue Limestone, which constitutes the host to the gold mineralisation. It exhibits upwards increasing terriginous quartzose and bioclastic detritus. These rocks are weakly to strongly fossiliferous, thin bedded to fissile, packstones, wackestone and grainstones, with abundant siltstone and some sandstone. Bryazoans, brachiopods and crinoids are common. The gold mineralisation at Mercur is generally confined to the lower and upper thirds of the Mercur Mine Series, which is divided into five beds, namely:
1) Silver Chert - this is principally a jasperoid bed which extends for several kilometres beyond the mineralised area. The jasperoid replaces the first sequence of thin bedded bioclastic limestone, siltstone and sandstone at the base of the mineralised zone. It is brecciated, partly re-cemented with silica, and contains most of the silver found in the district, with associated low gold values within the mine area.
2) Magazine Sandstone - which ranges in composition from a very fine grained siltstone to a fine grained sandstone, interbedded with clay and mudstone. Where un-altered the sands contain a significant amount of lime cement.
3) Barren Limestone - primarily a thick bedded, sericitic, micritic limestone with some mudstone partings in its lower third. It only contains minor gold and forms a barren layer within the otherwise mineralised Mercur Mine Series.
4) Mercur Bed - a unit of mudstones and grainstones with abundant fenestrate bryazoans.
5) Upper Bed - thinly bedded, blocky wackestones and packstones.
Long Trail Shale - carbonaceous shale.
Upper Great Blue Limestone - limestone.
Carboniferous (Mississippian to Pennsylvanian),
* Manning Canyon Shale - composed predominantly of shale, interbedded limestone and quartzite.
Carboniferous (Pennsylvanian) to Permian,
* Oquirrh Formation, 8000 m thick - interbedded sandstones and carbonates. This unit has been intruded by the main Bingham Stock and hosts the Carr Fork and North Ore Shoot skarn orebodies. It also hosts the peripheral Pb/Zn/Ag mineralisation of the Bingham mineralised system.
Tertiary
* Eocene to Oligocene, Porphyry Hill Granodiorite, 36.7 Ma - occurring as two stock like bodies of coarsely porphyritic biotite granodiorite, about 1.5 to 2 km to the north of the deposit. It contains major plagioclase, orthoclase, hornblende and minor biotite with a trace of magnetite and no pyrite. It is not known to be mineralised and exhibits only weak peripheral argillic alteration.
* Oligocene, Eagle Hill Rhyolite, 31.6 Ma - occurring as a laccolith in the southern sections of the district where it spreads between the sedimentary layers and partly up-domes the overlying rocks. It comprises a very fine porphyritic rhyolite with major quartz, sanidine and an orthoclase with minor biotite, magnetite and pyrite. This rhyolite probably predates the mineralisation, although it contains no gold and only exhibits weak argillic alteration. It intrudes sediments in the Sacramento Mine area.
Structure
The main structural features of the district are the NNW trending Ophir Anticline, whose axis is 1.5 km to the west of the mineralised area, and the parallel Pole Canyon Syncline 4.5 km to the east. These folds were apparently developed during and after the Sevier Orogeny thrusting, being active between the middle Cretaceous and the late Oligocene (Kornze, 1987).
There are at least three main periods of faulting at Mercur. The first was during the folding and predominantly comprised normal displacement with minor strike slip and bedding plane movement. The second period was marked by the beginning of the normal Basin and Range faulting that formed the range front fault on the western margin of the range, and has been stratigraphically dated at somewhere between late Oligocene and Pliocene. The third stage of faulting is represented by a large number of small scale normal faults in the vicinity of Mercur, each with a slight displacement of the order of up to 2 to 3 m. This may in part be contemporaneous with the Basin and Range faulting (Kornze, 1987).
Several major ENE trending faults within the mine area are believed to be closely related to the mineralisation. These faults are located in the Marion Hill, Golden Gate, Mercur Hill and Sacramento areas. The rocks between these ENE trending faults are strongly fractured, brecciated and jointed, as well a being mineralised and altered. The localisation of the igneous rocks also suggest a relationship with these same faults (Brunker & Dickson, 1987).
Mineralisation and Alteration
The main mineralisation at Mercur comprises stratabound gold, localised within the beds of the Mercur Mine Series, on the eastern limb of the Ophir Anticline (Brunker & Dickson, 1987). Mineralisation is largely confined to the Silver Chert Bed, Magazine Sandstone, Mercur Bed and Upper Bed, while the thick bedded micritic limestone of the Barren Bed is largely un-mineralised (Kornze, 1987). The main concentrations of ore grade mineralisation are spatially related to ENE trending faults, although there is not an increase in grade towards the faults. The mineralisation is not stratabound in detail, as on a fine scale it crosscuts bedding planes. In a gross sense however, mineralisation is stratabound, being hosted by marls to dirty fossiliferous limestones. Higher gold grades are usually confined to thinly bedded to fissile, silty, fossiliferous packstone and mudstone. The ore at Mercur Hill extends 250 m along strike and up to 300 m down dip with a thickness of up to 30 m, but averaging 9 m. Ore drops off rapidly with depth, although mineralisation has been encountered up to 600 m down dip (Brunker & Dickson, 1987).
Regionally, jasperoids were apparently formed early in the alteration system, although they are not mineralised with gold, except within and near to the orebody. The jasperoid does have anomalous silver in the mine area although this does not correlate with gold (Brunker & Dickson, 1987).
The most prevalent alteration style involves decalcification of the limestones and the introduction of quartz. In addition, lesser volumes of kaolinite and fine grained muscovite/sericite are developed. The porous matrix of the rocks is usually replaced first, but as the intensity of alteration increases even the fossils are silicified. The alteration results in an increase in the porosity of the rocks, which, although the form of some fossils is retained, becomes a semi-indurated mixture of fine quartz, kaolinite and sericite, with associated illite, halloysite and a little alunite. In the case of extreme silicification, which is common in the Mercur Bed, the rock may resemble a jasperoid, although still retaining a high micro-porosity. The grain size of the quartz is generally <20 micron, while the kaolinite and sericite are very fine grained at less than 2 micron (Kornze, 1987; Brunker & Dickson, 1987).
The sulphide mineralogy of the un-oxidised mineralised zones is simple, with pyrite, realgar and orpiment being the most common, followed by marcasite. Cinnabar is rare in all areas except Sacramento where the past mercury production is recorded. Early diagenetic pyrite is present as fine euhedral crystals up to 0.5 mm (500 microns) across. Disseminated pyrite and marcasite were deposited at the same time as the alteration and mineralisation. They rarely form euhedral crystals, and are usually <100 µm to sub-microscopic. Extensive pyrite is developed at the base of the 'capping' Long Trail Shale. Realgar and orpiment form euhedral crystals, whether disseminated or in veins. Late stage realgar and orpiment, and rare native sulphur form euhedral crystals in vugs and calcite veins. Barite is present with the sulphides, but appears to be later as it forms euhedral masses in late stage calcite veins. The overall sulphide content of the un-oxidised rocks at Mercur is 2 to 3%, with local concentrations in excess of 10% (Kornze, 1987; Brunker & Dickson, 1987).
Gold mineralisation always has an association with quartz/silicification, kaolinite, sericite, pyrite, realgar, orpiment, marcasite and barite. This assemblage however, may also occur without any trace of gold. Organic carbon is present in the mineralised zones as both a hydrocarbon and a kerogen fraction. Texturally it is found as diffuse veinlets in the rock, as thin films coating fossil fragments, or as disseminated amorphous material. In some cases it is associated with sulphides, in others it is not. In some areas the organic carbon has been transported into zones forming volumes of black carbon rich rock. There is however no correlation between the amount of total organic carbon and the grade of gold mineralisation, nor is there a correlation between any one fraction of the organic carbon and the gold content. There is however always some organic carbon present if there is gold, although the converse is not true (Kornze, 1987). Within the un-oxidised mineralisation gold is present as very fine, sub-micron sized inclusions in pyrite and marcasite, in organic complexes and as free gold associated with quartz and calcite (Brunker & Dickson, 1987).
Antimony occurs as stibnite in veins which are found mainly within the Silver Chert Bed. The Thallium content of Mercur ore is quite high, occurring as christite (TlH9AsS3) and lorandite (TlAsS2). Considerable amounts of melanterite (FeSO4.2H2O) and other Fe sulphates are found at the top of the mineralised section, just beneath the Long Trail Shale, generally associated with gypsum (Kornze, 1987). Although calcite and barite are present there is no adularia (Brunker & Dickson, 1987).
The majority of the oxidation at Mercur has been classed as hypogene, although there is some obvious supergene influences. Oxidation is most obvious at the base of the deposit. The lower beds, the Silver Chert Bed and Magazine Sandstone, are always found to be oxidised, whereas the un-oxidised zones are concentrated in the Mercur and Upper Beds. Intense hematite development is found just under the Silver Chert Bed and may represent late decalcification and oxidation. The oxidation within the orebody has not influenced the pervasive quartz, kaolinite and sericite alteration, nor the barite. It has however destroyed the sulphides and carbonaceous material. The iron sulphides are oxidised to limonite and hematite and the orpiment and realgar to scorodite. Gold becomes visible as finely disseminated grains ranging from 10 µm to sub-micron sizes. Visually the rock changes from shades of grey to tan, ochre and purple (Kornze, 1987).
The paragenetic sequence within the orebody has been interpreted to commence with:
1). pre-mineral formation of the Silver Chert jasperoid, in which none of the contained gold is encapsulated by silica, nor is there evidence of post mineralisation silica flooding;
2), decalcification of favourable porous zones contemporaneously with the deposition of further quartz, and the formation of kaolinite and sericite; followed closely by;
3), the deposition of sulphides, hydrocarbon remobilisation, and introduction of gold. The gold deposition continued through the waning stages of the pyrite-marcasite mineralisation;
4), Barite began to form later than the early alteration, as veinlets of pure barite and barite with sulphides cut the previously altered rock;
5), Barite, quartz and calcite represent the last stage of mineral deposition of any significance, although;
6), continued hydrothermal activity is interpreted to have introduced oxygenated meteoric water and produced the Fe sulphates and gypsum. As this continued the Fe sulphates were converted to limonite and hematite, while the lowering of the pH allowed further decalcification of the limestone. Although this stage of oxidation was apparently active for a long period it only oxidised 60 to 70% of the altered rocks in the deposit (Kornze, 1987).
For more 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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Selected References:
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Jewell P W, Parry W T 1987 - Geology and hydrothermal alteration of the Mercur Gold deposit, Utah: in Econ. Geol. v82 pp 1958-1966
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Kornze L D 1987 - Geology of the Mercur gold mine: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 381-389
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Wilson P N, Parry W T 1995 - Characterization and dating of argillic alteration in the Mercur Gold district, Utah: in Econ. Geol. v90 pp 1197-1216
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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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