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Coeur d Alene - Sunshine, Lucky Friday, Galena, Kellog, Bunker Hill, Hecla, Hercules, Morning Star |
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Idaho, USA |
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Pb Zn Ag Cu
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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SUMMARY
The Coeur d'Alene district has produced several tens of millions of tonnes of ore from a large number of mines spread along the length of a WNW-ESE trending corridor. The hosts are Middle Proterozoic clastics of the Belt Super Group, the US continuation of the Purcell Super Group, that includes the Aldridge Formation, host to the Sullivan Pb-Zn deposit in Canada. The individual deposits are rich, narrow, transgressive veins, or vein swarms with extensive lateral dimensions of hundreds to thousands of metres, but thicknesses of <1 to a few metres. Grades are of the order of 400 to 1000 g/t Ag with associated <1 to 10% Pb, <0.1 to 0.6% Cu, <0.1 to 5% Zn and traces of Au, eg. the Bunker Hill mine which had reserves of 10 Mt @ 4.6% Zn, 2.3% Pb, 41 g/t Ag in 1991. In 1996 the proven reserves at Coeur-Galena totalled 1.63 Mt @ 450 g/t Ag. Total production from the field to 1984 was 31 100 t Ag, 7.60 mt Pb, 2.9 mt Zn, 0.15 mt Cu, 15.9 t Au. One of the main remaining mines (1998), the Coeur-Galena Complex is operated by Silver Valley Resources, a joint venture between Coeur d'Alene Mines Corp and ASARCO Inc.
DETAIL
The Coeur d'Alene district and mines are located within northern Idaho, USA. The district includes the Sunshine, Lucky Friday, Galena, Kellog, Bunker Hill, Hecla, Hercules, Standard-Mammoth, Tamarack, Tiger-Poorman, Frisco, Star, Morning & other mines.
The Production of Ag-Pb-Zn ore was first recorded in 1884. The approximate total historic production from the Coeur d'Alene district is as follows:
22 000 t Ag, 6.25 Mt Pb, 2.2 Mt Zn, 0.1 Mt Cu (Prod. to 1965, Hobbs & Fryklund, 1968),
31 100 t Ag, 7.60 Mt Pb, 2.9 Mt Zn, 0.15 Mt Cu, 15.9 t Au (Prod. to 1984, Ashley, 1991).
Mining continues to the present day, with output from the Lucky Friday mine alone in 1985 being 147.4 t Ag, 484 t Cu , 31 553 t Pb, 3552 t Zn and 0.075 t Au (Noske, 1988).
Geology
The ore deposits of the Coeur d'Alene district are predominantly hosted by lithologies of the Belt/Purcell Supergroup. This package of rocks is part of an extensive thick Mesoproterozoic clastic sediments, which were laid down on the western margins of North American craton, mainly within the area now occupied by the eastern margin of the Cordilleran Orogen. These sediments were deposited between ~1470 and ~1400 Ma (Anderson and Davis, 1995; Sears et al., 1998; Evans et al., 2000).
The Belt/Purcell Supergroup in the north-western United States and south-western Canada are 15 to 20 km thick and appears to have been deposited in an elongate intra-cratonic basin (Oldow, et al., 1989) with both a distinct NE and SW margin. The sequence commenced with the Lower Belt Pritchard/Aldridge Formations, a southwest-derived deep-water turbidite wedge with locally abundant mafic sills, comprising >7000 m of turbiditic argillaceous quartzite, siltite, argillite, carbonaceous black and grey argillite and pyritic/pyrrhotitic argillite. Local basal conglomerates are found on the basin margins to the east. Geophysical data indicates 7 to 8 km of sills, probably mafic, developed in the concealed lower sections of the Pritchard. The Lower Belt is overlain by the Ravalli Group (Burke, Revett, St. Regis Formations) which is around 2000 m thick and comprises mainly quartzite with interbedded siltite, grading eastward into siltstone and shale, representing a largely sub-aerial, southwest-derived fluvial-deltaic complex. These are overlain in turn by the Middle Belt carbonate Piegan Group (Helena, Wallace, Kitchener Formations), an up to 6000 m thick cyclic carbonate and siliciclastic sequence of either lacustrine or marine setting, capped by a sequence of mafic to felsic lavas, including limestone, dolomite, calcareous mudstone and argillite, thick argillite and siltstone packages, dolomitic quartzite, etc.. These are capped by the Missoula/Lemhi Group which is up to 5500 m thick, a south- and east-derived fluvial succession, composed of carbonaceous argillite, siltite and quartzite, with some limestone dolomite and dolomitic siltstone. The Missoula locally includes an 1100 Ma andesitic to basaltic lava near it base. This sequence is overlain by the widespread Swauger Quartzite.
The ore deposits of the Coeur d'Alene district are found within a zone of structural disturbance known as the Great Coeur d'Alene Mineral Belt which extends for more than 150 km in a generally ESE direction across northern Idaho into north-western Montana. This mineral belt is an integral part of the parallel continental scale Lewis and Clark Line, which is 15 to 60 km wide. The main Coeur d'Alene district covers an area of 40 km east-west, by 10 to 15 km across. The mineral belt is characterised by numerous faults, steeply dipping to overturned sediments, sharply bent fold axes, several generations of dykes, a group of small stocks, zones of shearing and alteration and at least six periods of vein formation.
In particular the main mineralisation is localised at the intersection of i) the ESE trending Lewis and Clark Line, of which the Osburn Fault is the dominant structure, and ii) a major north-south elongated anticlinal uplift which extends from the northern margin of the Idaho Batholith in the south, to the Kimberley (Sullivan) district of British Columbia in the north. The main mineral belt is bisected by, and parallels the Osburn Fault which has experienced predominantly dextral strike-slip displacement of around 25 km. South of the Osburn Fault the structural trends are approximately WNW to NW, paralleling the fault, while to the north they are predominantly NW to north. The ore bearing veins are found within subsidiary fractures related to the main disturbed zone, but along which there has only been slight displacement relative to the major faults (Hobbs & Fryklund, 1968; Ashley, 1991).
Mineralisation
Ore occurs in a series of steeply dipping, tabular replacement veins of relatively simple mineralogy. The principal ore minerals are medium to fine grained galena, fine (mostly <1 mm) sphalerite, and tetrahedrite, with lesser chalcopyrite, pyrrhotite, pyrite, arsenopyrite and magnetite, and minor hematite, stibnite, uraninite, gold, gersdorffite, scheelite and other minerals. The gangue may include barite, quartz, siderite, ferroan dolomite, ankerite, grunerite, chlorite, hornblende, biotite and minor garnet. Six periods of mineralisation are recognised, ranging from Proterozoic to Tertiary in age. These are found within three widely spaced periods, the late Proterozoic, late Cretaceous and Tertiary. Within each period there may be one or more discrete stage of vein formation. In some localities only one period or stage may be found, while in others two or more can be superimposed in one vein system or shoot. In some veins only the economically important minerals are present, in which case the ore shoot occupies the entire vein which pinches out into barren country rock at its extremes. In other veins, the ore sulphides define an ore shoot within gangue minerals such as siderite or pyrrhotite that may continue as barren veins for long distances (Hobbs & Fryklund, 1968).
In many places the ore shoots are controlled by extensive fractures or shear zones, that even if not every-where mineralised, provide continuity along a zone of economic potential wherein ore shoots may be found. Local control on the position of ore shoots within the veins systems includes the brittle character of the host rock, the attitude of bedding relative to the mineralised fracture, and the amount and direction of movement on faults. Depth appears to exert little influence on the occurrence or type of ore (Hobbs & Fryklund, 1968).
The main productive galena-sphalerite and tetrahedrite veins are localised within a series of 12 sub-parallel, straight, mineral belts that trend at approximately 295°C, and that are apparently controlled by deep fractures in the basement. These belts range in size from <300 m to more than 1 km in width, and in many places may be wider. Their length varies from a little over 1.5 km, to near 25 km. These belts together define the larger composite Coeur d'Alene mineral belt that covers a width of 25 km and length of 50 km embracing more than 35 significant mines and prospects. The Coeur d'Alene mineral belt is divided into two groups separated by 25 km of post mineralisation movement on the Osburn Fault, connected by an intervening narrower belt along the Osburn Fault (Hobbs & Fryklund, 1968).
Mineralisation within the Coeur d'Alene district is principally hosted by the Middle Proterozoic Belt Supergroup, in a zone corresponding to the thickest section of the Ravalli Group. It occurs within fractures cutting slates and quartzites of the mid Prichard Group (5% of production), at the transition from the Prichard Group to the Burke (19% of production), and the Revett to the St Regis (75% of production) Formations, all of the Ravalli Group (Hobbs & Fryklund, 1968).
The individual ore deposits are known to extend over a strike length of a few metres, to more than 1200 m. They are generally rich, narrow, transgressive veins, or vein swarms with extensive lateral dimensions of hundreds, to thousands, of metres, but thicknesses of a few cmÕs to a 3 m, but locally several times this amount. In a few places, closely spaced stringers or narrow veins may be of economic over a width of up to 30 m. Grades are of the order of 500 to 1000 g/t Ag with associated <1 to 10% Pb, <0.1 to 0.6% Cu, <0.1 to 5% Zn and traces of Au. Some have lower Ag grades of around 100 g/t, but higher Pb and Zn. Reserve tonnages are usually low, being less than 1 to several million tonnes only. The vertical axes of these veins are generally 1.5 to 2 times the horizontal dimensions, influenced by the stratigraphy cut, ie. flat bedded country rock means lower ratios, while steep dips results in high vertical to horizontal ratios. The ore shoots are elongated along and follow stratigraphic contacts. The largest shoot, mined in the Morning and Star mines, measures 1200 m in length, but extends over a vertical interval of more than 2 km from its outcrop to the lowest level mined (Hobbs & Fryklund, 1968).
Hydrothermal alteration that is zonally and spatially related to the main stage sulphide veins is not conspicuous at Coeur d'Alene. Recent fluid inclusion and isotope studies have indicated that the ores of the Coeur d'Alene district were deposited from metamorphic media generated during regional scale greenschist-facies metamorphism. The fluid inclusions suggest emplacement at a fluid temperature of 350°C, pressure of 1 to 2 kb and a low salinity, just after the peak of metamorphism when the strain regime progressed from plastic to brittle. Meagre geochronological data indicates that vein formation took place in late Proterozoic time, with most of the Pb being of that age. Geochemical dispersion patterns and contact metamorphic minerals in veins around stocks indicate some remobilisation of ore elements during the Cretaceous thermal event associated with monzonitic stocks (Ashley, 1991).
The Coeur d'Alene Ag-Pb-Zn-Cu vein deposits are approximately 50 km to the SSW of the stratabound Spar Lake and Rock Lake Cu-Ag orebodies, also hosted by the Revett Formation of the Ravalli Group in the Belt Supergroup.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2010.
Record last updated: 28/12/2012
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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Beaudoin G 1997 - Proterozoic Pb isotope evolution in the Belt-Purcell Basin: constraints from syngenetic and epigenetic Sulfide deposits: in Econ. Geol. v92 pp 343-350
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Bennett E H, Venkatakrishnan R 1982 - A palinspastic reconstruction of the Coeur d Alene Mining District based on ore deposits and structural data: in Econ. Geol. v77 pp 1851-1866
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Eaton G F, Criss R E, Fleck R J, Bond W D, Cleland R W, Wavra C S 1995 - Oxygen, Carbon, and Strontium isotope geochemistry of the Sunshine Mine, Coeur d Alene Mining District, Idaho: in Econ. Geol. v 90 pp 2274-2286
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Fleck R J, Criss R E, Eaton G F, Cleland R W, Wavra C S, Bond W D 2002 - Age and origin of base and precious metal veins of the Coeur d Alene Mining District, Idaho: in Econ. Geol. v97 pp 23-42
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Harris R H, Lange I M, Krouse H R 1981 - Major element and sulfur isotopic variations in the Lower Chester vein, Sunshine Mine, Idaho: in Econ. Geol. v76 pp 706-715
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Hobbs S W, Frykland Jr V C 1968 - The Coeur d Alene district, Idaho: in Ore Deposits of the United States Amer. Inst. of Min. Eng., the Graton Sales volumes v2 pp 1417-1435
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Mauk J L, White B G, 2004 - Stratigraphy of the Proterozoic Revett Formation and Its Control on Ag-Pb-Zn Vein Mineralization in the Coeur d Alene District, Idaho: in Econ. Geol. v99 pp 295-312
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Ramos F C and Rosenberg P E, 2012 - Age and Origin Of Quartz-Carbonate Veins Associated with the Coeur D’alene Mining District, Idaho and Western Montana: Insights from Isotopes and Rare-Earth Elements: in Econ. Geol. v.107 pp. 1321-1339
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Reid R R, Wavra C S, Bond W D 1995 - Constriction fracture flow: a mechanism for fault and vein formation in the Coeur d Alene district, Idaho: in Econ. Geol. v90 pp 81-87
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Sack R O, Fredericks R, Hardy L S and Ebel D S 2005 - Origin of high-Ag fahlores from the Galena Mine, Wallace, Idaho, U.S.A. : in American Mineralogist v90 pp 1000-1007
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Ventakatrishnan, Bennett E H 1998 - Structural controls of the Coeur d Alene ore veins, Shoshone County, Idaho: in Kisvarsanyi G, Grant S K North American Conference on Tectonic Control of Ore deposits and the Vertical and Horizontal Extent of Ore Systems Proceedings Volume, Univ. Missouri-Rolla, Rolla, Missouri pp 125-133
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Wavra C S, Bond W D, Reid R R 1994 - Evidence from the Sunshine Mine for dip-slip movement during Coeur d Alene district mineralization: in Econ. Geol. v 89 pp 515-527
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White B G 1998 - New tricks for an old elephant: Revising concepts of Coeur d Alene geology: in Soc for Min., Met., & Explor. Inc., SME, Littleton Colorado preprint 98-87 10p
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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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