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Park City

Utah, USA

Main commodities: Ag Au Cu Pb Zn
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The Park City district lies some 40 km to the ESE of Salt Lake City in north-central Utah, 110 km to the north-east of the Tintic silver-lead district, 65 km to the ENE of the Bingham Canyon porphyry copper deposit and 80 km to the north-east of the Mercur gold mine.

Production from the Park City district between 1875 and 1964 totalled:
  14.6 Mt of ore from which 33 t Au, 8145 t Ag, 1.18 Mt Pb, 0.61 Mt Zn and 0.05 Mt Cu were recovered.

This would correspond to:
 14.6 Mt @ 500 g/t Ag, 2 g/t Au, 0.3% Cu, 7.7% Pb, 4% Zn (Prod., 1875 to 1964, Barnes & Simos, 1968)

The first discovery of mineralisation at Park City was on the Walker and Webster claim in 1869. The Ontario Silver Mining Company was organised in 1872 with the location and opening of operations on the western sections of the Ontario vein. Additional mines commenced operations on extensions of the same vein system in 1885, namely the Daly Mining Co. and the Daly Judge Mining Co., while in 1892 the Daly West Mining Co. started-up. In 1892 The Silver King Co. began mining a lode to the north of the Ontario-Daly vein set. In 1920 the Park Utah Mining Co. was formed, with the discovery of new ore along the Hawkeye fault to the west of the older mines. The New Park Co was incorporated in 1932, independently of the other operations along a strong vein system in the south-east of the district (Barnes & Simos, 1968).

In 1953 the Park Utah Consolidated Mines Co., which controlled the Ontario, Daly West and Daly Judge mines and had a large interest in the Daly Mining Co., consolidated with the Silver King Coalition Mining Co. to form the United Park City Mines Co. By the late 1960s, approximately 90% of the of the mines in the district were controlled by two companies, the United Park City Mines Co. and the New Park Mining Co.. The remainder were owned by a number of smaller companies (Barnes & Simos, 1968).

In mid 1970 the Anaconda Company [60%] and ASARCO Inc. [40%] jointly leased the properties of the United Park City Mines Co.. New ore reserves were developed and a new mining facility installed. Operations under the new structure commenced in 1975, with an annual capacity of 17 500 t Pb, 28 000 t Zn, and 37 t Ag (Skillings, 1978). No significant operations are currently recorded.

Geology

The bedded sequence of the Park City district ranges in age from lower Carboniferous to early Jurassic, with a cover of younger volcanics and unconsolidated Quaternary. Igneous rocks make up nearly one third of the area of the Park City district, being concentrated mainly in the southern section. Intrusive rocks are spatially closely associated with many of the orebodies (Barnes & Simos, 1968).

The stratigraphic succession in the Park City district is as follows, from the base:

Lower Carboniferous, Mississippian,
Madison Limestone, >35 m thick - the upper 35 m of the sequence that is exposed in the district is composed of interbedded thin to medium bedded dolomitic limestone, shaly limestone and white dolomite, underlain by light grey to white dolomite with occassional beds of white and black limestone, and a few irregular beds of chert (Barnes & Simos, 1968).
Deseret Limestone, 230 m thick - apparently conformably overlies the Madison Formation, and is conformably followed by the Humbug Formation. Nearly all exposures of the unit are in close proximity of intrusive bodies and hence are contact metamorphosed to varying degrees. The basal 45 m of the unit is made up of black dolomitic limestone, thin pale grey chert bands and a narrow zone of light grey, vuggy dolomite, underlain by a metre or so of black shaly limestone. The middle 105 m is a massive white, sugary dolomite with some chert, while the upper 80 m is composed of massive bedded, white to light grey, "marbleised" limestone with some dolomite and sandy limestone (Barnes & Simos, 1968).
Humbug Formation, 105 m thick - in the Ontario mine area this unit has been sub-divided into a lower 75 m thick member known locally as the Quartz-Sandstone Series that is made up of interbedded quartzite, limestone and sandstone members which individually range from 1 to 12 m in thickness. This is overlain by a 30 m thick upper member composed of white to grey crystalline limestone with shaly and muddy partings and thin, interspersed siltstone or mudstone lenses. Regionally it is characteristically lenticular with thickening, thinning and lensing out of units (Barnes & Simos, 1968).
Doughnut Formation, 150 to 165 m thick - comprising black to dark grey, hard, thin bedded, siliceous and silicated limestones, with some thin interbedded argillites. The basal 45 m is marked by a prominent argillite bed, the bottom of which is tentatively assigned to the Doughnut-Humbug contact zone (Barnes & Simos, 1968).

Upper Carboniferous, Pennsylvanian,
Morgan Formation, 85 to 95 m thick - consisting predominantly of grey, fine to coarse grained, well bedded, crystalline limestone, with irregular chert nodules and lenses scattered throughout. It is generally homogeneous, with only minor changes in the lithology and thickness of individual beds. Irregularly spaced nodules and lenses of white, grey, orange and pinkish cherts are characteristic of the entire formation (Barnes & Simos, 1968).
Weber Quartzite, 375 to 410 m thick - composed predominantly of un-fossiliferous quartzite, ranging from ortho-quartzite, through sandstone, calcareous sandstone, argillaceous sandstone, sandy limestone, argillaceous limestone and a few thin pure limestone and dolomite beds. It can be divided into:
 - a 100 m thick lower member composed of thin-bedded, alternating quartzite, sandstone and thin argillaceous limestone beds;
 - a middle member which is 170 to 175 m thick and comprises clean, massive, fine grained orthoquartzite with only two or three thin limestone horizons; and
 - a 110 m thick upper member that consists of five or six rather thin limestone and dolomite beds alternating rhythmically with thicker quartzite intervals (Barnes & Simos, 1968).

Late Carboniferous to Permian,
Park City Formation, 205 m thick - this unit is the host to much of the rich ore found early in the districts history. Although it covers a considerable area in the field, there is only poor and irregular, exposure. The unit is characterised by heterogeneous lithologies, when compared with the units above and below. The lowest member is an arenaceous limestone and/or calcareous sandstone that changes to a quartzite at the base of the unit. For the most part it is composed of limestone, but also contains sandstone and thin shale bands, as well as a narrow phosphatic bed. The limestones range from massive, grey and finely crystalline, to thin bedded, black and fossiliferous with very thin shaly partings. The limestones are often separated by thin bands of sandstone, quartzite and sandy limestone. There is a sharp break between the upper massive limestones of the Park City Formation and the thin shales of the Woodside Shale (Barnes & Simos, 1968).
One particular unit within the the Park City Formation hosts a number of the individual ore deposits. This is the Jenny Bed, which is composed of 5.5 to 6.5 m of cherty limestone sandwiched between two quartzite members some 30 m above the Weber Quartzite (Barnes & Simos, 1968).

Triassic,
Woodside Shale, 200 to 210 m thick - a single lithologic unit composed of uniformly, thin-bedded, fine grained, dark to red shale. A few intervals show increased coarseness due to sandy material, and colour variations of buff, brown, grey and olive (Barnes & Simos, 1968).
Thaynes Formation, 350 m thick - is one of the most extensively exposed units in the district, forming ridges and canyon walls, particularly in the north-west. It is conformable with both the overlying and underlying units, and is composed of thin bedded limestone with abundantly interbedded shale and other coarser detrital material. It is divided into an upper and lower member by the Mid-Red Shale Member. The upper member is more calcareous than the lower, and though true limestones are few and narrow, much of the total section is calcareous. The pure limestones in both the upper and lower members have proved to be susceptible to mineralisation at a number of localities in the district (Barnes & Simos, 1968).
Ankareh Shale, 375 m thick - commencing with the massive, fine grained, basal Nugget Sandstone, which is conformably overlain by the main Ankareh Shale that comprises massive red, yellow and tan shales with interbedded fine grained sandstone. These are the youngest consolidated sediments in the district (Barnes & Simos, 1968).

The igneous rocks of the district in general comprise three main petrographic types. These are:

Diorite - which forms the most rugged topography in the Park City region, including the highest peak. It is found mainly in the extreme south-west corner of the district and is a dark grey, fine textured granular rock that contains little quartz, but such a large orthoclase content to verge on being a monzonite. Mafic minerals are biotite, hornblende and augite, accompanied in places by hypersthene (Barnes & Simos, 1968).
Diorite porphyry - is the most extensive of the igneous lithologies, occurring mainly as stocks in the eastern and southern parts of the district, but also in small isolated bodies in the west and north-east. In the mineralised areas it takes the form of irregular tongues and dykes emanating from the stocks. It generally has a dark green groundmass which is an extremely fine crystalline mass of poorly formed minerals. The phenocrysts are large white feldspars, with smaller crystals of hornblende, biotite and some augite (Barnes & Simos, 1968).
Andesitic extrusives - these are represented as a small area of the Keetley Volcanic Series found in the extreme north of the district, part of a larger sheet developed to the east. The andesite is a pinkish or greenish rock, composed of a fine grained groundmass with scattered phenocrysts of feldspar, hornblende and mica (Barnes & Simos, 1968).

The diorites are late Cretaceous to Tertiary in age, while the volcanics were laid down in the Tertiary. Dating of presumed related intrusives near the district has yielded Eocene ages of 46 to 54 Ma, with one 25 Ma, Oligocene value (Barnes & Simos, 1968).

Structure

The predominant structure in the district is the north-south trending Park City anticline which plunges gently to the north, with a gentle, parallel synform to the west. The eastern flank of the antiform dips under the andesites and is buried by it. These broad gentle folds are severely modified by the intrusives and by strong east-west faulting normal to the regional strike (Barnes & Simos, 1968).

The oldest faulting in the district is a north-south trending thrust which may be synchronous with the folding. It predates all of the other faulting in the district. Intrusion and normal faulting appear to be closely related and are younger than either the folding or thrusting. The east-west normal faults have offsets of up to 200 to 250 m. They converge and coalesce, and occur as en echelon sets with horse-tail ends. These faults dip both to the north and to the south, and with the associated horse-tail structures, are the host locality for much of the ore. Smaller related fractures are found associated with the main faults. They are usually steeper than the main fracture, with little or no movement indicated. Many of the main faults have evidence of pre-ore movement although many appear also to have had post-mineralisation re-activation (Barnes & Simos, 1968).

Mineralisation

The Park City district includes many orebodies distributed over an area of approximately 5 x 8 km and a vertical interval of more than a kilometre. Each has its own peculiarities, although in general most may be classified as either  i). a lode/vein type,  ii). a bedded replacement deposit, or  iii). an irregular replacement body. Each of these various ore types differs in continuity and distribution from mine to mine, depending on the structural controls of mineralisation and host rocks. Only in the Mayflower and Ontario mines does the mineralisation persist over the maximum vertical interval as a single ore shoot. Each of the ore types may be summarised as follows, with appropriate examples:

Replacement Deposits - these deposits vary considerably in character and form. The following is a brief resume of a number of the different examples:

• In the Daly West and Judge mines, four principal replacement deposits have been exploited, all of which are hosted by the Park City Formation as it warps into a gentle synform on the western limb of the Park City antiform. They lie between the Daly Fault to the north and the irregular intrusive contact to the south. In this area the Park City Formation has been metamorphosed to a greater degree, mainly by bleaching and discolouration and as hardening and recrystallisation, with minor creation of calc-silicate minerals. The four ore zones are:
- the Roll Vein - which occurs along a steep fault that has a displacement of 15 m. Ore is confined to the intersection of this fissure zone and one particular bed of the Park City Formation, the Jenny Bed, which is composed of 5.5 to 6.5 m of cherty limestone sandwiched between two quartzite members some 30 m above the Weber Quartzite. This is the same host bed as that of the Silver King deposit. The replacement ore is around 60 m in width, and has a rake of over 600 m from the surface to a depth of 365 m. The ore is composed primarily of argentiferous galena with some tetrahedrite and sphalerite. Approximately 0.125 Mt were mined.
- the  Middle Vein - where ore occurs as bedding replacements of two specific beds, one of which belongs to the Jenny Bed, where it occurs along a zone of fissuring and diorite porphyry dykes. Ore was exploited over a total strike length of 1800 m, was up to 45 m wide and 7.5 to 30 m thick. The second orebody is found where the same Middle Vein Zone intersects a further limestone bed, the 920 Horizon, near the top of the Park City Formation. Both orebodies are found near the bottom of the structural trough formed by the Park City Syncline and are similar in grade and mineralogy. The ore minerals are the same as those of the Roll Vein, and are contained within a gangue of pyrite and quartz. The western limbs of both become more pyritic before being terminated by the fissure system. Approximately 2.59 Mt of ore was mined, at a grade of around 785 g/t Ag, 0.8 g/t Au, 14% Pb and 12% Zn.
- the Back Vein, which occurs along a well defined fault with a displacement of up to 75 m. This fault almost parallels the Middle Vein Fissure, both in strike and dip, and lies around 90 m to the south. Ore occurs in definite shoots separated by barren zones, both within the fissure and in adjacent bedding plane replacements. The best ore again is associated with the Jenny Bed, although replacement ore has been found in at least three other beds. No significant mineralisation is found within the Weber Quartzite, although it re-develops within the Morgan Limestone below that quartzite. Approximately 0.663 Mt of ore was extracted at a grade similar to that of the Middle Vein.
- the 1240 Vein - which occurs along a small fissure zone that intersects the western part of the Back Vein. The ore is present entirely as a replacement within the 920 Horizon and was mined with the Back Vein deposit. Some 0.1 Mt of ore was mined at grades comparable to the Middle Vein system (Barnes & Simos, 1968).

• The Silver King replacement orebodies are predominantly a series of long, narrow, high grade, manto type, bedded deposits in the lower Park City Formation, formed along the north-west flanks of the Park City Anticline. The orebodies follow a monoclinal structure. They are confined almost wholly to the Jenny Bed and have been mined over a vertical interval of nearly 600 m, along five "vein" systems. In this area the Park City Formation is fresh and unaltered, even in close proximity to ore, with no bleaching or metamorphic effects. The only igneous rocks in the area are represented by a diorite porphyry dyke which is apparently un-related to ore (Barnes & Simos, 1968).

Early, minor, thrusts which follow the strike of the beds and cut them at a low angle with little or no displacement, have produced ground preparation breaking and crushing. Later normal faulting has superimposed a series of steep, east-west fissures and normal faults which have shattered the underlying quartzites, but die out in the overlying shales. In the extreme north-west, one of the early thrust zones has brecciated and crushed the Park City Formation such that it provides a locus of mineralisation within the thrust rather than the Jenny Bed. None of the ore runs however pass through the upper quartzite to the top of the Jenny Bed (Barnes & Simos, 1968).

The primary mineralisation comprises galena, tetrahedrite, sphalerite and pyrite. The galena is present as both coarse and finely cleaved masses in a gangue of quartz and calcite. The bedded deposits of the Silver King Mine account for 73% of the tonnage, and 77% of the value from that mine, and totalled 3.42 Mt of ore. The average ore of the mine, including other types, was 570 g/t Ag, 0.75 g/t Au, 13.5% Pb, 0.5% Cu and 10.7% Zn (Barnes & Simos, 1968).

Much of the remainder of the ores of the Silver King mine have come from the Crescent Zone, which is made up of three distinct geological features, namely  i). the Crescent Fault which has a vertical displacement of 400 to 425 m, persists for at least a 1000 m vertically and has been traced over a lateral distance of 3000 m;  ii). the discontinuous Crescent Dyke which is composed of diorite porphyry and occurs in either the footwall or hangingwall of the Crescent Fault; and  iii). the Crescent Ore Zone which comprises both small fissure type deposits, on and parallel to the fissure and dyke, and larger bedded replacement orebodies in the vicinity of the fissure. The bedded replacements occur in both the Thaynes and Park City Formation, especially in the footwall of the Crescent Fault, up to 50 m from that fissure. At one locality up to 12 different beds have been mineralised. These ore beds are usually quite pure limestones sandwiched between sandstones or more impure limestones. The Jenny Bed is however still the most extensively mineralised. The mineralised beds of the Park City and Thaynes Formations are apparently quite different in character. The beds of the latter are lower grade with a higher percentage of sphalerite, iron and silica. In addition, both units in the Crescent Zone are of lower grade and have a higher iron and silica content than the other replacement deposits of the district (Barnes & Simos, 1968).

• The replacement ores of the Ontario mine are largely hosted by the lower Carboniferous Humbug Formation in the nose zone of the Park City Anticline, where it is truncated on both the north-western and southern sides by faults. Between these two faults there are two distinct ore shoots, on the north-east and north-west sides of the anticlinal axis respectively. The two shoots come together near the crest of the structure, just below the Doughnut Argillite which forms a cap to mineralisation in the replacement ores (Barnes & Simos, 1968).

The ore minerals are aligned along bedding planes, although the ore is transgressive along the fissure plane. Replacement is present throughout almost the entire thickness of the Humbug Formation, in a particular zone of fracturing. The ore however, is not a single massive replacement, but a series of bedded replacements, separated by barren beds, controlled by a zone of fracturing. The west flank orebody may be followed down plunge to terminate abruptly against the Hawkeye Fault which forms the contact with the bounding Deseret Formation. The top of the eastern flank orebody only replaced a single bed at the contact of the upper limestone unit and the quartzite-sandstone unit. This ore shoot continued down plunge for 250 m, with an average width of 50 m and thickness of 2 to 6 m, and has little or no apparent fissure control. Further down plunge however, more beds are replaced, with a broader zone of mineralisation and a more pronounced fracture control (Barnes & Simos, 1968).

The character and mineralogy are similar to those described above. There is a marked increase in sphalerite and pyrite at depth, with a slightly decreased silver:lead ratio in the galena. Approximately 0.98 Mt of ore have been produced at an average grade of 180 g/t Ag, 0.8 g/t Au, 6.4% Pb. 0.14% Cu and 8.7% Zn (Barnes & Simos, 1968).

Lode Deposits - many of these deposits, although comprising mineralisation that may be described as being predominantly "lode type", also contain bedding replacement ore at certain stratigraphic levels. The principal lode deposits of the district are the Ontario, Mayflower, Daly and Hawkeye which comprise rich ore shoots that persist over vertical ranges of a hundred metres, up to 500 m or more (Barnes & Simos, 1968).

• In the Ontario and Daly lodes high grade silver ore persisted over a vertical interval of around 400 m. Where the lodes widened, the mineralisation became more spotty and of lower grade, and was accompanied by sphalerite and pyrite. The main Ontario fissure has been mined over a lateral interval of 1400 m in a curvi-linear, but overall east-west direction. On its eastern end this lode is terminated by a series of north-east trending transverse faults, although it is roughly aligned with the Hawkeye Fault and lode system to the east. On its western end however, it merges into, and follows three prominent south-west trending cross spurs to the parallel east-west to south-east Daly Fault where ore was exploited over a strike length of a further 750 m. The Daly and Ontario Faults are around 400 m apart. To the west the Daly Lode feathers out along the Daly Fault which is then barren for around 300 m before the Daly West Lode develops. This lode has a lateral extent of around 750 m in the Daly West mine and a further 600 m in the contiguous Daly Judge mine. The average vertical interval of mineralisation in these mines was around 200 m. The ore of the Daly West and Daly Judge mines was not as rich nor as persistent as the main shoots to the east, although adjacent bedding replacement deposits were both extensive and high grade (Barnes & Simos, 1968).

The upper portions of the Ontario and Daly lodes were developed between faulted walls of highly shattered and brecciated Weber Quartzite. Below this quartzite, at a depth of 400 m, the lodes pass into the Morgan and Doughnut limestones. Within these units the fracture zone becomes less distinct, narrows and contains less mineralisation, thus marking the lower limit of the higher grade, more continuous shoots. The ore contains argentiferous galena and sphalerite ores, and locally tetrahedrite-tennantite within a quartz and pyrite gangue. The high silver generally occurs with the tetrahedrite, which in turn is usually intergrown with galena and sphalerite. Early mining (1875-1902) at the Ontario mine exploited oxide ores of cerussite, anglesite and iron oxides with bindheimite, massicot, argentite, horn silver, malachite, azurite and chrysocolla, with 0.665 Mt of ore averaging 1800 g/t Ag, 1 g/t Au and from 1 to 3% Pb. Subsequently, sulphide ores with grades of the order of 775 to 925 g/t Ag were mined, accompanied by 0.8 g/t Au, and around 3% each of Pb and Zn. Overall more than 1.2 Mt has been mined from these operations (Barnes & Simos, 1968).

• The Mayflower vein system is composed of four easterly to north-easterly striking, predominantly lead-zinc veins, with important amounts of gold, silver and copper. The vein system is mainly developed within a fault block composed of Weber Quartzite, bounded on three sides by Tertiary diorite stocks. Although all four veins are inter-connected, there is a considerable mineralogical difference between the main Mayflower and the other three veins of the system, while all differ texturally and structurally. The Mayflower Vein is composed of quartz, the principal gangue mineral, with some calcite, rhodochrosite and rhodonite, and a sulphide assemblage of pyrite, sphalerite, galena and minor chalcopyrite. The other three veins have a gangue of friable quartz, nearly always accompanied by hematite, with occasional rhodochrosite and rhodonite. The sulphides are pyrite, sphalerite, galena and chalcopyrite. The Mayflower Vein has been mined from the surface to a depth of 600 m, with the most productive interval being from 200 to 530 m. Small ore-shoots on the western end of the system contain enargite, argentiferous tetrahedrite, chalcopyrite and chalcocite, with minor galena and sphalerite. The majority of the ore from all of the veins has diorite wall rocks, although significant ore has also come from the sediments. Limestones of the Morgan, Doughnut, Humbug and upper Deseret Formations host replacement ore along the western extremities of the lode.

For detail consult the reference(s) listed below.

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.


  References & Additional Information
   Selected References:
Borrok D, Kesler S E  1999 - Sulfide minerals in intrusive and volcanic rocks of the Bingham-Park City belt, Utah: in    Econ. Geol.   v94 pp 1213-1230


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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