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Tintic District |
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Utah, USA |
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Au Ag Cu Pb Zn
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Super Porphyry Cu and Au
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The Tintic district in Utah is located near the town of Eureka, ~90 km to the SSW of Salt Lake City in north-central Utah. This places the district some 75 and 45 km generally south of the Bingham Canyon porphyry Cu deposit and the Mercur sediment hosted Au mine respectively.
The greater Tintic district is made up of two parts, the Main Tintic and East Tintic sub-districts, which together have historically produced 106 t Au, 8400 t Ag (Ashley, 1991), 115 000 t Cu, 940 000 t Pb and 77 000 t Zn. To 1966 approximately 15.5 Mt of ore had been extracted from the mines (Shepard, etal., 1968; Morris, 1968). The East Tintic group of mines are centred approximately 4 km to the north-east of the centre of the Main Tintic mines. The Tintic mining district officially covers some 390 sq. km, although essentially all of the ores have come from an area of only 15 sq. km in the Main Tintic sub-district and less than 50 sq. km at East Tintic.
Total production from the Main Tintic sub-district was 12.2 Mt of ore from which 66.9 t Au, 6140 t Ag, 102 500 t Cu, 597 000 t Pb and 64 000 t Zn have been extracted (Morris, 1968).
Total production from the East Tintic sub-district was 3.3 Mt of ore from which 16.7 t Au, 1953 t Ag, 12 800 t Cu, 335 000 t Pb and 13 000 t Zn have been extracted. Of this nearly two thirds came from the Tintic Standard mine, with the balance from eight other properties (Shepard, etal., 1968).
Over 80% of the ore at East Tintic was either i). silver-lead ore with an average grade of 1.2 g/t Au, 960 g/t Ag, 20% Pb and 0.5% Cu; or ii). siliceous silver ore with a recovered grade of 1.5 g/t Au, 685 g/t Ag, 4% Pb and 0.5% Cu (Shepard, etal., 1968).
Geology
The Tintic district lies within the East Tintic Mountains, one of the eastern-most ranges of the Basin and Range terrane of Nevada and Utah. The range is block faulted, trends north-south and has a moderate relief, rising to heights of up to 750 m above the alluvium filled Tintic Valley. The rocks within the district comprise more than 3000 m of lower to middle Palaeozoic marine sediments, including limestone, dolomite, quartzite, shale and argillite (Morris, 1968). These were cut by several sets of faults, before being overlain by up to more than 1500 m of middle Eocene latite [trachy-andesite] and quartz-latite [rhyolite] lavas, tuffs and agglomerates. All are intruded by stocks, plugs, dykes and sills of monzonite and quartz-monzonite porphyry [adamellite] and diabase [dolerite] (Morris, 1968; Shepard, etal., 1968).
The host sequence is as follows, from the base (Morris, 1968; Shepard, etal., 1968):
Upper Proterozoic,
• Big Cottonwood Formation, >510 m thick - grey-green phyllitic shale, greenish-brown quartzite and minor brownish grey dolomite. The base of the unit is not exposed.
Unconformity, representing the removal of a substantial thickness of intervening sequence.
Lower Cambrian,
• Tintic Quartzite, 850 to 975 m thick - buff quartzite, with grey-green phyllite beds near the top of the sequence, and conglomerates near the base. Much of the quartzite in the Eureka district comprises more than 90% silica
Middle Cambrian,
• Ophir Formation, 120 m thick - grey-green shale and blue oolitic limestone. This is mainly a limy shale, but also includes one or more thick beds of limestone that are important hosts to ore at East Tintic, but are non-productive in the Main Tintic workings.
• Reutonic Limestone, 120 m thick - blue shaly limestone with pisolitic zones.
• Dagmar Dolomite, 25 m thick - creamy white laminated dolomite.
• Herkimer Limestone, 130 m thick - blue shaly limestone and green shale.
• Bluebird Dolomite, 60 m thick - dusky grey dolomite with white markings.
• Cole Canyon Dolomite, 260 m thick - dusky grey and creamy white dolomite.
Upper Cambrian,
• Opex Formation, 75 m thick - thin bedded sandy limestone and shale.
• Ajax Dolomite, 170 m thick - dusky grey, cherty dolomite.
Lower Ordovician,
• Opohongo Limestone, 210 to 260 m thick - blue-grey, thin bedded shaly limestone.
Disconformity,
Upper Ordovician,
• Fish Haven Dolomite, 210 to 260 m thick - blue-grey, thin bedded shaly limestone.
Ordovician Silurian and Devonian,
• Bluebell Dolomite, 105 to 180 m thick - dark grey, coarse grained, dolomite with some beds of sub-lithic creamy-white dolomite, containing curly-laminated marker beds near the middle of the sequence.
Upper Devonian,
• Victoria Formation, 85 m thick - grey dolomite and buff quartzite, locally containing some lenses of penecontemporaneous breccia.
Disconformity,
Upper Devonian to lower Carboniferous,
• Pinyon Peak Limestone, 23 m thick - blue-grey shaly limestone, with sandy sections at the base.
Lower Mississippian (Carboniferous),
• Fitchville Formation, 85 m thick - composed of seven distinctive units of limestone and cherty dolomite, with a "curly laminated" bed near the top.
• Gardison Limestone, 150 m thick - blue-grey, distinctly bedded cherty limestone.
Upper Mississippian (Carboniferous),
• Deseret Limestone, 300 m thick - blue-grey chert and coquinoid limestone.
• Humbug Formation, >75 m thick - blue limestone and buff sandstone, the top sections of which have been eroded.
Unconformity,
Tertiary, possibly, Eocene,
• Apex Conglomerate, 0 to >150 m thick - brick red conglomerate and sandy shale. Mapped in places as part of the Packard Quartz-Latite.
Tertiary, Eocene, made up of the remnants of a large composite volcano and caldera complex that essentially buried a mature, structurally complex mountain range. The core of this volcano was invaded by the Silver City Stock and associated plugs, dykes and sills, some of which were the eruptive conduits of the extrusives. The precise relative ages of the different igneous episodes described below is uncertain, although textural indication suggest the following:
• Packard Quartz-latite, 0 to >825 m thick - purplish-grey, contorted flows and white tuff.
• Swansea Quartz-monzonite, granitoid intrusive, chiefly altered and bleached.
• Laguna Springs Latite, 0 to >750 m thick - reddish grey flows, tuffs and agglomerates.
• Sunrise Peak Stock principally a medium to dark grey, coarsely porphyritic monzonite stock and associated hornblende-monzonite porphyry, which is altered near veins.
• Silver City Stock, 46.5 Ma - greenish-grey, granitic to coarsely porphyritic monzonite and associated biotite-monzonite porphyry, which has been altered near veins.
• Quartz-monzonite Porphyry, greenish-grey, coarsely porphyritic dykes and plugs which are commonly included with monzonite of the Silver City Stock.
• Andesite or latite dykes & related intrusions - purple, porphyritic dykes, locally altered to kaolinite, probably contemporaneous with ore emplacement.
Unconformity,
Tertiary, Pliocene,
• Salt Lake Formation - a sequence of marly limestone, bentonitic tuff, sandy silt and gravel, the base of which is concealed.
Unconformity,
Quaternary, Pleistocene,
• Older Alluvium - fanglomerates, colluvium and stream gravels, overlain by the deposits of Lake Bonneville in adjacent areas. The older alluvium forms fans from the gorges of the East Tintic Range, coalescing into the Tintic Valley.
Quaternary, Recent,
• Younger Alluvium, 0 to 30 m thick - Fanglomerate, gravel, sand and silt.
Structure
The Tintic district is located near the centre of the Sevier Orogenic belt, a NNE trending, 150 km wide linear belt, characterised by overlapping, internally deformed thrust plates that extend from Montana to California. Deformation commenced in the late Jurassic, continuing into the Cretaceous, culminating in the eastward transport of great plates of rock across the Tintic district in the Late Cretaceous. By the middle Eocene the orogenic activity had subsided in the Tintic area and the deformed and faulted rocks had been invaded by igneous suites. During the mid-Miocene and later the sequence was subjected to extension and then basin and range deformation (Morris, 1968).
The dominant structural feature of the Main Tintic region is the asymmetric Tintic syncline which occupies most of the main district and sections of the East Tintic sub-district. The syncline is cut by a number of transverse faults. In the Main Tintic district the western limb is steeply east dipping to overturned, while the east limb dips at 20 to 30°W. In the East Tintic area however, the dominant structures are i). the asymmetric Tintic anticline; and ii). the East Tintic Thrust which cuts the eastern limb of the anticline. In the East Tintic sub-district the western limb of the anticline dips uniformly at around 30°W, while the east limb is overturned and sharply crumpled above the East Tintic Thrust. These structures are largely concealed by lavas and are only known from drilling and underground mine workings (Morris, 1968; Shepard, etal., 1968).
The faults that cut the Tintic Syncline are classified as: i). shears formed during folding, occurring as high angle conjugate sets; ii). normal faults that are post folding, but pre-volcanism, strike east-west and dip steeply to the north with displacements of up to 500 m; iii). mineralised fissured and faults related to the period of volcanic activity, occurring as a system of generally north to NNW trending structures that cut the Silver City Stock; and iv). late normal basin and range faults (Morris, 1968). The East Tintic Thrust however, localises the principal orebodies of the Burgin Mine. At East Tintic the youngest pre-basin and range faults are a short NNE trending set of fissures that cut the lavas and intrusive rocks, as well as the Palaeozoic sediments beneath. These structures, which probably correlate with the third group in the Main Tintic district, dip steeply west and have displacements of a few metres to 100 m. They apparently localised the emplacement of monzonite plugs and dykes and the associated pebble dykes and lodes (Shepard, etal., 1968).
The folded sequence of the Tintic district forms part of the upper plate of the Midas Thrust which is exposed to the north.
Mineralisation
The ore deposits of the Tintic district are of two types, namely, i). massive, columnar, irregular and manto [blanket-like] 'replacement' orebodies which have yielded 90% of the ore produced in the district; and ii). veins that cut the larger porphyry masses and the adjacent aureoles of pyro-metasomatically altered sediments (Morris, 1968; Shepard, etal., 1968).
There are local variations in the occurrence of ore in the Main and East Tintic sub-districts which will be described separately in the separate East Tintic and Main Tintic mineralisation records.
For detail also consult the reference(s) listed below.
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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Selected References:
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Hildreth S C, Hannah J L 1996 - Fluid inclusion and sulfur isotope studies of the Tintic Mining District, Utah: Implications for targeting fluid sources: in Econ. Geol. v91 pp 1270-1281
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Morris H T 1987 - Tintic mining district, Utah: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 390-394
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Stavast W J A, Keith J D, Christiansen E H, Dorais M J, Tingey D, Larocque A and Evans N, 2006 - The Fate of Magmatic Sulfides During Intrusion or Eruption, Bingham and Tintic Districts, Utah: in Econ. Geol. v101 pp 329-345
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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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