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Ducktown - Burra Burra, London, East Tennessee, Mary Polk, Calloway, Eureka, Boyd, Cherokee |
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Tennessee, USA |
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Cu S Zn
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Super Porphyry Cu and Au
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Ducktown is composed of a series of metamorphosed massive sulphide deposits in Tennessee, USA that were mined historically for supergene enriched copper and primary Cu-Zn sulphides. The deposits were subsequently mainly exploited for pyrite. The main deposits are Burra Burra, London, East Tennessee, Mary-Polk, Calloway, Eureka, Boyd and Cherokee (#Location: 38° 59' 0"N, 119° 11' 40"W).
The massive sulphide deposits of the Ducktown district, Tennessee, are hosted within a thick sequence of highly metamorphosed and deformed sedimentary rocks, the Great Smoky Group of the Neoproterozoic Ocoee Supergroup, in the western Blue Ridge of the southern Appalachians.
These Proterozoic rocks are part of an upper plate that has been thrust northwestward over younger, less resistant Lower Ordovician to Lower Carboniferous rocks of the Valley and Ridge province. The two plates are separated by the Great Smoky fault/thrust. The younger, lower plate, is composed of limestones, dolomites, shales, siltstones, sandstones and conglomerates.
The up to 15 000 m thick Ocoee Supergroup unconformably overlies Mesoproterozoic crystalline basement, and comprises a thick wedge of poorly sorted and coarse clastic rocks, derived from a granitic and crystalline basement source area to the northwest. The Ocoee Supergroup sequence includes turbiditic metagreywackes and schists and associated metaconglomerates, quartzites, and calc-silicate hornfelses. This sequence is overlain conformably, or with slight disconformity, by sedimentary rocks of probable Early Cambrian age in the western foothills of the Great Smoky Mountains.
The Ocoee Supergroup can be divided into three groups (USGS):
• Snowbird Group, the basal unit of the Ocoee Supergroup, which rests nonconformably on Mesoproterozoic crystalline rocks basement, comprises a 4000 to 6000 m thickness of sedimentary rocks that includes the Pigeon Siltstone (3000 m of laminated, greenish quartzose and feldspathic siltstone; minor fine-grained grey sandstone); Roaring Fork Sandstone (2000 m of Interbedded massive feldspathic sandstone, greenish siltstone and greenish phyllite); Metcalf Phyllite (pale-green and silvery sericitic and chloritic phyllite with abundant siltstone interbeds); Longarm Quartzite (1500 m of light-coloured, current bedded and crossbedded, feldspathic quartzite and arkose); Wading Branch Formation (500 m of medium- to dark-grey, commonly graded bedding, sandy slate to coarse, pebbly feldspathic sandstone and greywacke, with a basal quartz-sericite phyllite).
• Great Smoky Group which forms the main mass of the Great Smoky Mountains, and can be divided into the following formations:
Copperhill Formation, 600 and 1500 m mostly massive metagreywacke with graded bedding and interbedded dark-grey slate, metaconglomerate, quartzite and meta-arkose, and nodular calc-silicate rock. Metagreywacke (~60%) and mica schist (~30%) are interbedded throughout with ~10% metaconglomerate, mata-arkose, quartzite, chlorite-garnet schist and chlorite-garnet-staurolite schist. The mica schist are generally a few cms to 6 m thick, but occasionally are as much as 60 m, composed of biotite and muscovite in various proportions, frequently grading into and hybrid schist-greywacke containing ~40% quartz. These beds are stratigraphic markers, but only have continuities of a few hundred metres at most, lensing out or changing mineralogical composition along strike. Garnet, staurolite and graphite are present to varying degrees. These schists are conspicuous in the vicinity of the orebodies (Magee, 1968).
Hughes Gap Formation, 122 to 1800 m of garnet-mica schist, staurolite schist, meta-quartz conglomerate, quartzite and pseudo-diorite, with beds of metagreywacke and mica schist near the base. Individual beds range from <1 cm to >15 m, commonly <3 m. It underlies the Hothouse formation and overlies the Copperhill formation with a transition zone of a ~100 m.
Hothouse Formation, 2400 to 3300 m of interbedded metagreywacke, quartzite, with prominent metaconglomerate at the top of the sequence, and mica schist at the base.
Dean Formation, 700 to 1000 m of staurolite-mica schist, biotite schist, meta-quartz-conglomerate, quartzite and pseudo-diorite, interbedded with lesser amounts of grey slate or phyllite and sericite schist. Except for abundance of biotite schist and absence of quartz-kyanite schist at its top, the Dean formation is lithologically indistinguishable form Hughes Gap formation.
• Walden Creek Group - up to 600 m of olive-green and grey, argillaceous, micaceous shale with coarse feldspathic sandstone and quartz-pebble conglomerate of probable Cambrian age.
The Ocoee Supergroup is overlain by the:
• Chilhowee Group - ~1000 m of Lower Cambrian dark-grey, micaceous, sandy shales and sandstones, and greyish white quartzose layers.
• Shady Dolomite - which consists of ~250 m of mainly of bluish-grey limestone with some mottled, grey, blue and white layers, calcite seams and beds and common thin seams of blue and grey shale, and frequent impurities of sand and chert nodules.
The only indication of igneous activity in the Ducktown area is the presence of small amphibolite pods within the immediate mining area, and an amphibolite sill, which varies from a few cms to >100 m in thickness, that can be traced over a length of up to 20 km, passing within 1.5 km to the SE of the mining district, and continuing into Georgia to the SW (Magee, 1968).
Metamorphic grade in the area increases from NW to SE, from chlorite to biotite and garnet to the NW of Ducktown. The rocks at Ducktown belong to the staurolite-almandine and kyanite-almandine subfacies of the almandine-amphibole facies. The first occurrence of massive sulphides coincides approximately with the staurolite isograd. An isolated zone defined by the kyanite isograd is centred on the Ducktown district, while the regional kyanite isograd reappears ~16 km to the east (Magee, 1968; Carpenter, 1970).
Isotopic ages within the mineralised district infer three periods of metamorphic events and deformation (Magee, 1968; Dallmeyer, 1975; Addy and Ypma, 1977):
i). regional metamorphism (M1) occurred during the 480 to 420 Ma Taconic orogeny, during D1, which produced tight isoclinal folds.
ii). peak metamorphism (M2) which reached staurolite grade, during the 430 to 360 Acadian orogeny; probably following the peak of D2, which also also produced isoclinal folds with axial planes differing by ~10 to 20°. The dominant mechanism of F2 folding was differential slip on crenulation cleavage planes.
iii). retrograde metamorphism (M3), during the 300 to 250 Ma Alleghanian orogeny (Holcombe, 1973). This event overlaps D3 to D5. D3, which affected the mineralised area, was not as pervasive as D2. It produced a crenulation cleavage S3 that is observed to transpose earlier structures at places such as the Calloway mine. A late Acadian to Alleghanian time has been assigned to this deformation on the basis of the accompanying low grade metamorphism. The local formation of chevron folds in mica-rich layers and conjugate kink bands in high grade metamorphic minerals like staurolite and kyanite characterise D4. D5 resulted in two sets of wrench faults that displace the previous structures, namely an east-west trending and 60°S dipping dextral set, and a sinistral set striking at 150° and dipping at 60 to 80°NE. The inferred principal stress directions of D3, D4 and D5 are similar at ~120°.
All of the lithologies of the Copperhill Formation occur in the wall rocks of the deposits, although there are rock types that are uniquely associated with ore, namely:
• Sericite schist, composed of a very fine grained white muscovite with up to 20% disseminated pyrite and pyrrhotite, occurring as beds up to 10 m thick, is abundant in both the footwall and hanging wall of the deposits, and as clasts within the massive sulphide margins.
• Biotite schist composed almost completely of pure dark brown to black biotite, altered in part to phlogopite. It is similarly distributed to the sericite schist, but less abundant, but often occurs as in-folds into the sulphides.
• Chlorite schist, is a common wall rock, often separated from the sulphides by a metre or so of biotite-sericite schist. It is in contact with the sulphides over much of the hanging wall of the Burra Burra deposit. It is more abundant in hanging walls, over down dip-extents of 150 to 300 m with thicknesses of up to 150 m. It contains varying amounts of sericite, biotite and quartz, but also porphyroblasts up to 15 mm across of garnet, as well as occasional magnetite and kyanite porphyroblasts.
• Quartzite, which is fine-grained, friable and biotitic (<5%) with rounded, <1 mm diameter quartz grains and up to 30% (usually <10%) sulphides (usually pyrite-pyrrhotite, but locally up to ore grade chalcopyrite (Magee, 1968).
Eight separate orebodies occur in two or possibly three stratigraphic horizons in the lower half of the Copperhill Formation. Two major structures are recognised in the main mineralised district, the Burra anticlinorium and Coletown synclinorium. The Burra anticlinorium is overturned to the NW, so that both limbs dip about 60°SE. The Burra Burra, London and East Tennessee orebodies are located along the northwest limb of the Burra anticlinorium and Mary, Polk and Calloway on the southeast limb. The Eureka, Boyd and Cherokee deposits occur in its centre (Addy and Ypma, 1977).
The eight orebodies are almost identical in many geological aspects, namely they are generally conformable with the enclosing host rocks, with a regional strike of NNE-SSW, and dip SE. They mostly plunge in to the SW, while the major anticlinal structure has a NE plunge (Magee, 1968).
The orebodies have been grouped into two types, on the basis of their shapes; i). Flat tubular, less complex morphology, west of the Burra anticlinal axis - Cherokee, Burra, London and East Tennessee; ii). Folded deposits with sulphide accumulation in the hinges - Calloway, Mary Polk and Eureka, which all occur on the east of the anticlinal axis (Emmons and Laney, 1926).
The Ducktown deposits occur in three zones. In outcrop, they comprise a gossan of limonite with lesser kaolin, quartz and other minerals, which extends to a maximum depth of ~30. Below the gossan there is generally 1 to 1.5 m of sooty chalcocite ore that lies like a "floor" below the gossan. Below the chalcocite blanket, the ore becomes primary yellow sulphide. These primary sulphides are generally mutually intergrown, without any "crustified" banding (Emmons and Laney,1926).
On average, the primary massive ore comprises of 60% pyrrhotite, 30% pyrite, 4% chalcopyrite, 4% sphalerite, 2% magnetite and traces of silver and gold. Wide variations in the relative abundance of these sulphides occurs from deposit to deposit. There are localities where massive, almost pure pyrrhotite occur, and similar zones of nearly pure granular pyrite, whilst locally areas of the deposit can contain up to 20% magnetite or 15% sphalerite. Pyrrhotite is predominantly fine grained and massive. Pyrite is usually found as fine to coarse crystalline aggregates from 3.5 to 12.5 mm across, although masses of almost pure pyrite masses up to 15 m across are found. Pyrite zones usually have low chalcopyrite and sphalerite, but higher magnetite contents. Chalcopyrite is most frequently associated with pyrrhotite, but is also abundant in quartz, calcite and calc-silicates. Chalcopyrite and sphalerite are most commonly found as disseminations and fine veinlets within the massive pyrrhotite, and in the associated gangue. Magnetite is disseminated widely, but is most commonly as fine-grained masses in the pyritic ores. It is sometimes replaced by sulphides and may be an early phase of mineralisation. Minute traces of gold and silver are primarily associated with chalcopyrite and galena (Magee, 1968).
Gangue minerals in the primary mineralisation include actinolite, tremolite, hornblende, quartz, calcite, with occasional well-developed diopside, garnet and zoisite. Actinolite is almost universally intergrown with the ore minerals and is the most abundant gangue mineral of the deposits. The most frequent mineral assemblage observed is pyrrhotite + pyrite ± magnetite ± chalcopyrite + actinolite + calcite ± quartz (Emmons and Laney, 1926; Ross, 1935; Magee, 1968).
Chalcocite is the most abundant secondary copper mineral, occurring as a sooty amorphous material in the secondary zone. It also occurs as solid crystalline masses intergrown with sphalerite, pyrite and other sulphides. It is thus confined to the flat-lying secondary zone and to the veinlets and stringers of secondary ore that locally extend downward a short distance below the secondary blanket. Covellite is a common secondary mineral, though less abundant than chalcocite. Subordinate
Chalcopyrite is found in the secondary ore. Other minerals in the secondary zone include bornite, ducktownite, rahtite, allisonite, harrisite, argentite, sulphur, chalcanthite, melanterite, pisanite, gypsum, cuprite, melaconite, chrysocolla, malachite, azurite, native copper, limonite, turgite, kaolin, chalcedony and jasper, allophane, talc and alum (Emmons and Laney, 1926).
The Burra Burra, London and East Tennessee deposits on the NW flank of the Burra Burra antiform, and the Calloway and Mary-Polk deposits on the SE flank of the same structure, have higher primary copper grades of ~1.6% Cu, while the three central deposits of Eureka, Boyd and Cherokee only contain ~0.7% Cu. Zinc has a similar distribution, with ~1.2% in the outer deposits, and 0.5% in the centre. The central deposit have a higher proportion of sulphide (~70%), with more pyrite (and magnetite at ~3%), while the outer ores only average ~55% sulphide and ~1% magnetite. The Boyd deposit in the centre, has a core of 10% magnetite, which declines outward along strike and dip to <1%, Whilst the core of the Eureka deposit, also in the centre of the field has up to 20% magnetite. A zone of 15% magnetite is evident on the SW end of Cherokee. Magnetite is very low to rare in Calloway and Mary-Polk (Magee, 1968).
Copper was first discovered at Ducktown in 1843 and the first mine opened in 1850. In about 1870 the secondary chalcocite ores were nearing exhaustion and several mills were
built to concentrate the underlying primary sulphide ores, which could be profitably mined from the more copper-rich ore bodies. By 1879 the richer primary ores had been mined out and operations ceased until 1890, when the completion of a railway line to Ducktown and metallurgical advances made possible the economic exploitation of the lower grade primary ores. The Burra Burra mine closed in 1959 after producing ~14 Mt of ore. Production from other mines continued, including the last discovered, Cherokee (exploited from 1960), until July 1987, while the main phase of acid production from the dumps ceased in 2001.
According to Emmons and Laney (1926) the secondary ore averaged 15% Cu, while later remnant mining produced ore in the range of 6 to 12% Cu. Primary ore from the Mary mine is stated as containing 2.45% Cu and 2.79% Zn. Zinc comprised ~4% of the primary ore overall. The maximum widths of the primary orebodies exceeds 100 m locally, but average ~30 m. Primary ore was exploited to a depth of ~840 m. The Cherokee orebody described below is one example of the Ducktown deposits.
Cherokee deposit
The Cherokee orebody is located on the NW limb of the Burra anticlinorium. It is ~2000 m in strike length, persists to 800 m below the surface, with an average thickness is 10 m, although locally thickened by folding to 50 m (Magee, 1968). The deposit strikes at 35° and dips at 45 to 65° SE, parallel to the schistosity of the host rocks. Small, tight to isoclinal folds appear to be common throughout the ore along with minor shears. A NW trending, sinistral cross fault offsets the ore by 80 m between sections (Magee, 1968).
The upper hanging wall of the orebody is composed of sericite-biotite schist, although at lower levels of the mine, the sericite-biotite schist grades laterally into a chlorite-garnet schist containing sericitised pseudomorphs of staurolite. The upper footwall contains metagreywacke and metagreywacke schist, that also grades down-dip into chlorite and sericite schists. Amphibolite units in the immediate mine vicinity (Slater, 1982) possibly represent early sills. Wall-rock alteration includes a decrease in iron content in biotite and chlorite within a 6 to 8 m wide zone enveloping the orebody (Brown, 1961; Magee, 1968) and increases in sericite and epidote relative to the host rocks farther from the orebody. A zoning pattern perpendicular to the orebody of chlorite schist footwall, copper-enriched footwall mineralisation, zinc-enriched hanging wall mineralisation, and sericite schist hanging-wall alteration has been identified at Ducktown (Slater, 1982; Brooker et al., 1987).
The ores at Cherokee have been classified into six major types on the basis of mineralogy and megascopic appearance: i). pyrrhotitic ore, with more than 60% pyrrhotite and no pyrite; ii). porphyroblastic-pyrrhotitic ore, composed of a pyrrhotite matrix and pyrite porphyroblasts; iii). pyritic ore, composed primarily of granular (generally 1 to 3 mm) pyrite; iv). siliceous pyrrhotitic ore containing less than 60% pyrrhotite; v). veined ore consisting of chalcopyrite and pyrrhotite within quartzites and other wall rocks, and; vi). banded magnetite ore. The ore varies from massive to moderately foliated with sheet silicates defining the foliation, which generally parallels that of the enclosing host rocks. Pyrrhotite grains are commonly elongated and aligned with the regional schistosity, indicating that the entire deposit underwent regional metamorphism (Larsen, 1973; Brooker et al., 1987).
Hexagonal pyrrhotite with 47.4 to 47.7% Fe is the dominant sulphide, whilst pyrite is the second most common ore mineral in the deposit, ranging widely in abundance and crystal size. Although only present in small amounts in most ore types, it constitutes 50 to 90% of the rock in the pyritic ore, occurring as subhedral to euhedral crystals 0.01 to 5 mm in size dispersed in a pyrrhotite matrix. In the porphyroblastic-pyrrhotitic ore, the dispersed pyrite crystals are very large and constitute 20 to 50% of the rock. Chalcopyrite is the most abundant base metal sulphide at Cherokee, constituting 1 to 10 modal%, of the ore, averaging ~4%. In pyrrhotitic ore, chalcopyrite occurs as small blebs in the pyrrhotite or as pressure shadows and fracture fillings in gangue. It also occurs in pressure shadows adjacent to large coarse pyrites or within fractures in pyrite in the porphyroblastic ores. In pyritic ore the chalcopyrite occurs as a fine-grained matrix material between pyrite grains. Sphalerite comprises up to 4% of the ore (Magee, 1968), and is generally has an antipathetic abundance relative copper. It predominantly occurs in fractures within and around pyrite grains, whilst inclusions in pyrite are much larger and more abundant than are those of chalcopyrite. Sphalerite grains within the pyrrhotite are commonly elongated in the direction of schistosity (Brooker et al., 1987).
Other deposit
The Burra Burra deposit, the largest in the field, had an almost continuous outcrop of 900 m, striking at 55° and dipping 72°SE, with a width that varies from near 1 m to as much as 60 m (Emmons and Laney,1926). Underground, it is a thin, semi-continuous, warped, tabular lens, with local thickening to 60 m in fold noses. The plunge is almost parallel to the 70°SE dip, with minor folds plunging at 45 to 65°NE. The deposit is overall conformable, but cuts bedding in the footwall rocks by 15° or less. The deposit contained 17.24 Mt of ore, of which 14 Mt was mined, the remainder being in pillars. The ore grade was ~1.6% Cu, and the ore deposit averaged 57% sulphides in a quartz, calcite and calc-silicate gangue (Magee, 1968).
The London deposit, 700 m NE of Burra Burra, strikes it 60°E and dips ~60°SE, parallel to the bedding of the country rock. It is mostly from 7.5 to 12 m wide, but is locally as much as 30 m thick or pinched out to a narrow stringer, with a strike length of over 250 m and down dip extent of >150 m (Emmons and Laney,1926). It produced 1.4 Mt of ore at 1.8% Cu from 50% sulphide ore (Magee, 1968).
The East Tennessee deposit, 600 m NE of London, produced ~0.225 Mt of ore of unrecorded grade, from below a deep leached gossan that extended to 60 m below the surface. (Magee, 1968).
The Eureka deposit is composed of two partially displaced, but overlapping segments, Eureka and Isabella, displaced by ~150 m across an oblique east-west fault. The deposits generally strike at 30° and dip 45°SE. Much of the deposit had been eroded and exposed as a gossan up to 75 m wide and 450 m long, with a narrow attenuated centre across the fault.
The Boyd orebody was concealed at a depth of ~180 m, and is ~720 m long and up to 60 m thick, plunging at 20°SW. The ore is typically massive sulphide, but has an adjoining zone of 10 to 50% disseminated sulphide in biotitic quartzite and biotite-sericite schist (Magee, 1968).
The Calloway deposit is 450 m long, extends downwards for >900 m, and is complexly folded, with a doubly plunging isoclinal anticline and syncline pair in the upper levels. It has an arcuate, elongate tabular shape that steepens to a dip of 70° SE with a plunge reversal to 70°SW. Folding in the lower parts of the deposit has produced local thickening to as much as 120 m (Magee, 1968).
The Mary and Polk deposits have been shown to be a single entity, with a strike length of ~900 m, composed of several ore lenses. Grades in the primary zone averaged 2.5% Cu for the first 100 m or more below the secondary ore, and 1.5% Cu below that (Magee, 1968).
This summary is drawn from the references listed below, and "Magee, M., 1968 - Geology and ore deposits of the Ducktown District, Tennessee; in Ridge, J.D., Graton-Sales Volume, Ore Deposits of the United States, 1933-1967, The American Institute of Mining Metallurgical and Petroleum Engineers, Inc., New York, v. 1, pp. 207-241" and "Emmons, W.H. and Laney, F.B., 1926 - Geology and ore deposits of the Ducktown mining district, Tennessee: U.S. Geological Survey Professional Paper 139, 159p." and "Laurence, R.A., 1965 - Field Trip No.2, Ducktown, Tennessee; for Joint Meeting of the American Crystallographic Association and the Mineralogical Society of America, Gatlinburg. Tennessee June 27 - July 2. 1965, pp. 18 - 47."
The most recent source geological information used to prepare this decription was dated: 1987.
Record last updated: 24/11/2014
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.
Burra Burra
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Selected References:
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Addy S K and Ypma P J M, 1977 - Origin of Massive Sulfide Deposits at Ducktown, Tennessee: An Oxygen, Carbon, and Hydrogen Isotope Study: in Econ. Geol. v.72 pp. 1245-1268
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Brooker D D, Craig J R, Rimstidt J D 1987 - Ore metamorphism and Pyrite Porphyroblast development at the Cherokee Mine, Ducktown, Tennessee: in Econ. Geol. v82 pp 72-86
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LeHuray A P 1984 - Lead and Sulfur isotopes and a model for the origin of the Ducktown deposit, Tennessee: in Econ. Geol. v79 pp 1561-1573
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Nesbitt B E 1982 - Metamorphic sulfide-silicate equilibria in the massive sulfide deposits at Ducktown, Tennessee: in Econ. Geol. v77 pp 364-378
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Nesbitt B E, Kelly W C 1980 - Metamorphic zonation of sulfides, oxides and graphite in and around the orebodies at Ducktown, Tennessee: in Econ. Geol. v75 pp 1010-1021
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Stephens M B, Swinden H S, Slack J F 1984 - Correlation of massive Sulfide deposits in the Appalachian-Caledonian orogen on the basis of Paleotectonic setting: in Econ. Geol. v79 pp 1442-1478
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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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