Viburnum Trend - Viburnum No. 27, Magmont, Buick, Brushy Creek, West Fork, Fletcher, Sweetwater

Missouri, USA

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The Viburnum Trend in Missouri, USA, includes the Viburnum No. 27, 28, 29 & 35, Casteel, Magmont, Buick, Brushy Creek, West Fork, Fletcher, Sweetwater (Milliken/Ozark Lead) Mines along longitude 91° 7' 15"W, from 37° 42' 48'N at Viburnum, to 37° 21' 35'N at Ozark Lead in the south.

Together, these deposits account for production and resources that totalled:
    total production + resources - >270 Mt @ 5.6% Pb, 1 to 2% Zn, as estimated in 1988, and
    total production + resources - 369 Mt @ 5.4% Pb, 0.8% Zn, 0.12% Cu, 2.7 g/t Ag, as estimated
            in 2012 (USGS website, 2013)
    total production - 275 Mt @ 5.8% Pb, 1.1% Zn, 0.3% Cu, 1960 to 2012 (Schott, Doe Run Co, 2012)

The first recorded discovery of lead mineralisation in SE Missouri was near Fredericktown in 1720, followed by Bonne Terre in the Old Lead Belt in 1864 and Flat River, Deslodge and Leadwood in the same district in 1890. In 1948, Indian Creek was found, 35 km NE the Viburnum Trend, and in 1955, the blind Viburnum deposits were located. Mining commenced at the Viburnum No. 27 mine on the Viburnum Trend in July 1960.

The Viburnum Trend is one of two major sub-districts of carbonate-hosted lead-zinc deposits comprising the Southeast Missouri district, extending for more than 70 km along the western flank of the St. Francois Mountains, a region of Precambrian igneous rock exposed at the structural apex of the Ozark Plateau. Mineralisation is hosted primarily by the Cambrian Bonneterre Dolomite, a regionally distributed limestone formation that has been extensively dolomitised within the Southeast Missouri district.

The host sequence in the Viburnum Trend is as follows, from the base:
Mesoproterozoic basement - the 1.48 to 1.45 Ga Eastern Granite-Rhyolite Province exposed in the St. Francois Mountains - for detail see the separate Southeast Missouri Iron District record. The Cambrian sequence that hosts Pb-Zn mineralisation laps onto, and is draped around this basement high.
Unconformity - above a very irregular basement surface, which locally led to pinchouts in the overlying sedimentary sequence.
Lamotte Sandstone, 0 to 150 m thick - a quartz arenite sequence that grades from an arkosic conglomerate, near its contact with the underlying Precambrian felsic igneous rocks, to a quartz arenite in its middle portions, to a sandy carbonate near its upper contact. A minor lapilli tuff bed is recorded near the end of Lamotte deposition, particularly in the south of the Viburnum Trend;
Bonneterre Dolomite, 0 to >150 m thick - commencing with a 0 to 6 m thick regional basal dolomite, formed by reaction of the limestone and early diagenetic dolomite protoliths with successive brines during their migration through the underlying Lamotte Sandstone (Gregg, 1985; Gregg and Shelton, 1989). This basal unit is overlain by shelf limestones with interbedded shales that represents the bulk of the Bonneterre Dolomite that can be divided into three distinct facies:
i). "back-reef" - composed of dolomitised microbial laminates and partly dolomitised lagoonal mudstones (Gregg and Shelton, 1990);
ii). "reef" - consisting of dolomitised microbial bioherms interbedded with oolitic grainstones (Gregg et al., 1993); and
iii). extensive offshore facies comprising partially dolomitised wackestones and packstones interbedded with green shales (Gregg, 1988).
Davis Formation, 0 to 90 m thick - a transgressive sequence of dolomitised microbial laminates composed of interbedded basinal limestones, dolomites and shales that may have acted as a capping aquitard for mineralising fluids flowing through the more permeable Lamotte Sandstone and Bonneterre Dolomite (Imes, 1990).
Derby-Doe Run Formation, 0 to 70 m thick - cherty dolomite.
Porosi Formation, 0 to 100 m thick - cherty dolomite.
Eminence Formation, 0 to 150 m thick - cherty dolomite.
Lower Ordovician
Gasconade Dolomite, 0 to 150 m thick - commencing with the 0 to 12 m thick Gunther Sandstone Member, overlain by further cherty dolomite.
Roubidoux Formation, 0 to 90 m thick - composed of interbedded cherty dolomite and quartz sandstone.
Jefferson City Dolomite, 0 to 110 m thick - comprising cherty dolomite characterised by gypsum and halite casts towards the top of the unit.
Cotter Dolomite, 0 to 140 m thick - cherty dolomite with gypsum and halite casts towards the top of the unit, and interbedded quartz sandstone.

Low grade Pb dominant mineralisation is exploited at the mines listed above, which are distributed along the 75 km long and 2 to 6 km wide Viburnum Trend, hosted within the carbonate rocks of the Cambrian Bonneterre Formation, overlying the Lamotte Sandstone. Mineralisation is virtually continuous over the entire length of the trend, forming a single deposit with widths, based on economic cut-of grades, varying from 50 to 450 m. Lead dominates, with grades averaging 5.5 to 6%, accompanied by lesser zinc, the highest average grade of which is 2.8% Zn at Magmont West. Significant copper mineralisation, averaging up to 2% Cu, is found in some mines, as at Viburnum No. 25, but is generally <0.15% overall. Note that while the lead grade is low, the strong ground conditions and good metallurgical properties render lower grades economic than in other styles, and hence a much lower cut-off may be applied.

The mineralisation occurs primarily within the Bonneterre Formation and was prevented from forming in stratigraphically higher units by the immediately overlying Davis Formation, an aquitard whose low-permeability shales and carbonates impeded the ascent of mineralising fluids (Appold et al., 2004). The larger ore deposits, composed mainly of galena, sphalerite and chalcopyrite, are hosted in the "reef-grainstone" facies of the Bonneterre Dolomite.

In addition, much of the mineralisation was focused along a prominent linear, north-south trending stromatolitic reef facies in the Bonneterre Formation, which formed in the shallower waters that existed on the flanks of a broad region of elevated igneous basement, and associated local basement 'knobs', immediately to the east of the Viburnum Trend. This narrow reef zone separates the backreef and forereef facies to the east and west respectively and controls the location of the Viburnum Trend.

Fluid inclusions and isotope studies indicate that the deposits and related alteration are the product of sedimentary basinal brines, with salinities of from 22 to 26 wt.% NaCl Equiv.. Fluid inclusions reveal homogenisation temperatures that vary primarily from ~80 to 140°C. Lead is indicated as being derived from a crustal sources, and sulphur derived from both seawater evaporation and halite dissolution. The δ34S values from ore minerals varied through the paragenetic sequence (i.e., pyrite, chalcopyrite-bornite, sphalerite-galena, late cubic galena), possibly due to deposition from multiple, metal-specific fluids (or a single evolving fluid) with distinct sulphur isotope compositions. (Appold et al., 2004 and sources cited therein).

Geochronologic and geochemical evidence links the formation of the ores to Alleghanian (Upper Carboniferous-Permian) tectonism in the Ouachita Mountains ~300 km to the south (Wu and Beales, 1981; Wisniowiecki et al., 1983; Leach and Rowan, 1986). Uplift of these mountains, and of the associated Arkoma foreland basin to the south of the Viburnum Trend in particular, stimulated a regional groundwater-flow regime that led to carbonate hosted Pb-Zn mineralisation, not only in Southeast Missouri but also in the Tri-State, central Missouri, and northern Arkansas districts (Leach and Rowan, 1986; Garven et al., 1993; Leach, 1994; Appold and Garven, 1999). However, Hagni, (2006) shows that the direction of fluid flow does not follow a single regional migrational direction, but varies across the region. Ore fluids that deposited the main orebodies in the Viburnum Trend came predominantly from the NW. Toward the southern portion of the Viburnum Trend the fluids may have come from a more easterly direction. Ore fluids that deposited the orebodies in the Indian Creek subdistrict further north, also came from the NW. The ore fluids that mineralised the Old Lead belt Subdistrict came from the NE as well as from the NW. Locally, the ore fluids flowed in other directions, e.g., in the Viburnum Trend, the ore fluids flowed westward at the Casteel and Magmont-West mines. Regionally, however, the trends shown by Hagni, (2006) all indicate flows towards the broad St. Francois Mountains basement high, indicating domal uplift of that basement block stimulated fluid flow.

Hagni (1989) indicates that the forms of individual ore deposits and their controlling structures in the SE Missouri Lead district are diverse. The most important control on the Viburnum Trend is solution collapse breccias, best developed at the Buick, Magmont and Sweetwater mines. These breccias are generally north-south trending, 40 to 60 m wide, 10 to 20 m thick and 1.5 to 2.5 km long, parallel to the overall Viburnum Trend, with bounding faults showing reverse senses of throw, indicating compression. In all cases the breccia is promoted by the solution thinning of beds low in the Bonneterre Formation, leaving thin grey silty residuum beds and brecciation in the overlying carbonates. Breccia hosts are also evident at Brushy Creek, Fletcher and Viburnum No. 28.

Solution collapse in the Bonneterre Formation also propagates up through the overlying succession, with the following results: i). clasts of Davis Formation shales are found within the upper Bonneterre Formation collapse breccias, as much as 60 m below the source beds; ii). complete dolomitisation and partial dissolution and brecciation of the Davis Formation over widths of 60 to 150 m (compared to <25% dolomitisation outside of this width), accompanied by a thinning from ~50 to ~35 m, containing a zoning of locally heavy to trace sulphide mineralisation (as disseminations, bands, fracture fill and as breccia matrix), from cubic sphalerite at the base, to nickel, predominantly as millerite, to an upper zone of marcasite with lesser pyrite; iii). a zone of bleaching and oxidation over similar widths in the Derby-Doe Run Formation, with local solution collapse brecciation; iv). variable oxidation and bleaching of the Porosi Formation, with local heavy calcite fracture and breccia fill, and selective erosion where the unit is exposed at surface. These characteristics are based on section of the southern Viburnum Trend, between Fletcher and Sweetwater (Schott, Doe Run Co, 2012).

Other orebodies are contained within submarine slide structures, e.g., Viburnum No. 27 and Magmont West, representing pre-consolidation slide breccias on the flanks of basement hills. These slide breccia may be 75 to 150 m wide, ~15 m thick and extend for 600 to 900 m from the basement hill peak. Blanket-shaped orebodies are generally thin (<10 m), and laterally extensive (up to 600 m east-west) and occur to the east of the high breccia ores, perched on tongues of reef rock above the main reef, e.g., Viburnum No. 29, Magmont East and the Buick Blanket Orebody, hosted by coarse calcarenites.

The key deposits of the Viburnum Trend, from north to south, are as follows:

Viburnum No. 27, mine lies at the northern end of the Viburnum Trend near a 4 km long granite high 5 km west, and basinward of, the main ore trend. Mineralisation, which is primarily associated with clastic structures, occurs almost completely within the "digitate stromatolitic reef" horizon of the dolomitic Bonneterre Formation. The two most common ore-bearing positions are i). calcarenite-filled channels between linear algal mounds and ii). calcarenite-filled troughs behind subtidal offshore algal bars (Grundmann, 1977). A mineralised local "white rock" facies is present in an embayment on the eastern flank of a granite peak that rose over 150 m above the Cambrian sea level, and a major unmineralised breccia within the Bonneterre Formation which contains fragments of the Davis Formation as much as 30 m below their normal position. Two fracture systems are prominent in the mine. Both parallel the ancient shoreline and the major region faults. The most abundant sulphide minerals at No. 27 mine include galena, pyrite, marcasite, sphalerite and chalcopyrite. Mineral zonation is well developed with chalcopyrite generally occurring along the basinward flank and at the base of the galena orebodies while sphalerite is usually found near the top of the orebodies. The main deposit is ~2000 m in length in a north-south direction, with orebodies that have widths of 75 to 150 m and thicknesses of ~15 m. An east-west directed arm branches off the main deposit to the west near its centre (Grundmann, 1977). Hagni (1989) suggest the coarse hosts at Viburnum No. 27, in part represent submarine slide structure breccias on the flanks of the basement hill.

Casteel, is located centrally within the Viburnum Trend, 9 km south of Viburnum No. 27. It has a general 'Y' shape, comprising the north-south aligned Casteel North section, extending north for 1200 m towards the Viburnum No. 28 mine, continuation to the south, the Casteel South section, which extends south for 900 m towards Magmont, and the Casteel West section which spreads WNW for 950 m over a broad buried basement hill or "knob" with a local relief of ~250 m, onto which the sequence as high as the Davis Formation progressively laps to the west. The ore zone varies from 100 to as much as 200 m in width. It tends to be thin, with a blanket like morphology, and is characterised by its high copper content. The deposit is mostly >300 m below the surface. The whole of the Bonneterre Formation has been dolomitise in the mine area, where reef and oolitic facies are up to 30 m thick. To the east these grade into the backreef "White rock", a white-to-light bluish-grey or light grey crystalline dolostone found to the east of the reef throughout the Viburnum Trend. In the mine area, the 2.5 to 3.5 m thick "Lower Davis" shale that occurs 7 to 10 m below the Davis Formation is absent. Fracturing is well developed, with a NE set seen throughout the mine, which may be as much as 30 cm wide, with locally up to 1.5 m of vertical movement. Other fracture and fault sets trend east-west, north-south (parallel to the main orebody trend), NW-SE, as well as low angle reverse and bedding plane faults. Bounding faults mark the edges of collapse structures. Numerous varieties of breccia and pseudobreccia fabrics occur throughout the mine, including ore matrix breccias.
  The ore is a mixture of galena, sphalerite and chalcopyrite, with minor bornite accompanying chalcopyrite in the western sections of the mine. Gangue minerals include pyrite and marcasite, with quartz, dolomite, calcite and dickite in decreasing order of abundance. Ore mainly occurs as ore matrix breccia, and stratiform, finely laminated massive sulphide blankets. The latter may represent either an internal (open space fill) sediment or or replacement of shale or another protolith (Dunn et al., 1989).

Magmont, is located centrally within the Viburnum Trend, 3 km south of Casteel, and occurs within the Buick Embayment, a shallow, ENE-trending depression in the Precambrian basement. Economic mineralisation is restricted to the carbonates of Bonneterre Formation, closely associated with solution-induced collapse breccias along associated bounding faults, gravity slumps, a favourable bed sequence called the "Silty Marker", and inter-reef calcarenites. No structural irregularities within the Bonneterre Formation in the mine area can be attributed to underlying Proterozoic basement knobs, which are absent from this part of the Buick embayment. A NE-NW joint system cuts the host sequence, with associated partially open fractures. Bounding, shale-filled subsidence faults mark the north-south trend of highly brecciated ore zones. Three north-south-trending, 40 to 60 m wide and 10 to 20 m thick parallel ore zones comprise the Magmont orebody, with economic mineralisation vertically confined to the interval from below the overlying Davis shale to the base of the lower reef horizon of the Bonneterre Formation. Individual ore zones extend for 1.5 to 2.5 km along trend. Laterally, a roughly concentric mineral zoning is recognised, comprising from the margin inwards of marcasite, marcasite-galena, and galena-chalcopyrite-sphalerite. Gangue minerals, in addition to marcasite, are dolomite, calcite, quartz and minor dickite. Lesser pyrite, siegenite and bornite are present, accompanied by minor amounts of other copper minerals (Sweeney et al., 1977).

Magmont West, is located ~3 km to the west of the Magmont mine, and ~1 km east of the Mesoproterozoic Boss-Bixby magnetite-Cu-Au deposit, which forms a basement high/knob - the West Dome. Digitate algal stromatolites form a reef over and on the flanks of the West Dome and connect to similar reef developments over the larger Central Dome, just over a km to the north, although these reefs may be a westward extension of the main barrier reef just over 3 km to the east. The back reef facies is not present in the mine area. The Bonneterre Formation deposited over the reef contains variable, but very cyclic, shallow water facies carbonate rocks, characterised by minor terrigenous clastic material, particularly black shale. The upper half of the Bonneterre Formation overlying the reef facies includes the lower, silty "Grey Beds, followed by dolostone and calcarenites, and the False Davis shale unit, followed by more calcarenites and dolostone rocks, and then the base of the Davis Formation (Bradley and Krolak, 1989).
  Mineralisation contains galena, sphalerite and chalcopyrite, with mined grades of ~5% Pb, 2% Zn and minor Cu. Approximately 85% of the mineralisation is hosted by the False Davis bed, with lesser mineralisation in the overlying calcarenites and lower sections of the Davis Formation. The remainder is within the calcarenites below the False Davis, occurring as brecciated and replacement ore. The orebody has a curvilinear shape, trending SSE on its southern half, curving to east-west and WSW further to the north and west, with an overall length of ~1500 m. It occurs near the lateral transition from dolomite to limestone within the Bonneterre Formation. The ore occurs as i). linear breccias extending from the False Davis position, up into the Davis Formation, with dimensions of ~10 m thick x 45 to 55 m wide, and Pb-rich western and Cu-rich eastern margins; and as ii). disseminated galena and sphalerite within the False Davis shale. The False Davis may be locally absent where the linear breccia is developed and ore occurs in the upper calcarenite (Bradley and Krolak, 1989). The mineralisation is closely controlled by NNW-trending and NE-SW trending fault or fracture zones (Bradley and Krolak, 1989: Hagni, 2006).

Buick, is located 5 km south of Magmont. The orebody consists of lead, zinc and copper sulphides in solution-induced collapse breccias which occur in dolomitised, shelf-facies calcarenite of the Bonneterre Formation, underlain by digitate algal stromatolites, and overlain by shale of the Davis Formation. In plan view, the breccia bodies are subparallel, sinuous and continuous in a north-south direction, with occasional confluences and divergences, possibly following intratidal drainage channels. Individual breccia bodies are up to 100 m wide and as much as 25 m thick, with a trough-shaped base, and outward-dipping bounding fractures on the sides. Well mineralised, inward-dipping slump breccias are found along the boundaries of some breccias, while in others, undisturbed unmineralised beds occur at the fracture boundary.
    Typically three subparallel breccia bodies occur, each with characteristic sulphide mineral assemblages and patterns that are traceable over the 3.2 km strike of mine workings. The west breccia is the most complex, containing galena-pyrite and galena-sphalerite zones on its west side, a central core of sparse pyrite and galena, and an eastern assemblage of galena, sphalerite, chalcopyrite, pyrite and a suite of cobalt-nickel sulphides. A pyrite halo caps and flanks the west breccia.
    The central breccia comprises a core of galena and sphalerite in varying proportions, with only trace chalcopyrite, flanked by major metal concentrations with higher ratios of zinc to lead. This breccia is capped by a pyrite-galena zone. An unbrecciated, satellite ore shoot of massive replacement sphalerite and galena lies along the western margin of the central breccia.
    The east breccia has a central core of galena-sphalerite-chalcopyrite, surrounded by a galena-sphalerite assemblage. The margins of the breccia contain galena and pyrite, while the breccia is capped by a thick pyrite zone.
    Breccia tops are vuggy and porous, and contain mainly open-space filling mineralisation, while replacement ore is dominant near the base of the breccias. The paragenesis of the ore has been difficult to determine due numerous overgrowths of different sulphide minerals in vugs, leached and etched crystals, prebreccia ore occurring in a matrix of postbreccia ore, and complex zoning of sulphide minerals suggesting multiple mineralising pulses (Rogers and Davis, 1977).

Brushy Creek, is located 7 km south of Buick, near the centre of the Viburnum Trend. The deposit overlies three buried Proterozoic palaeotopographic knobs which cut the basal sediments deposited on their flanks, which have proximal dips of up to 30°. The mine is situated along the south and west sides of the nose of the larger, NE-trending central basement high, which penetrates to within 12 m of the Davis Formation, with a smaller, circular-shaped knob to the SW and the much smaller high to the SE.
    The principal ore minerals at the Brushy Creek mine are galena, sphalerite and chalcopyrite, in the approximate ratios of 15:3:1, respectively. The main gangue minerals are dolomite, calcite, pyrite and marcasite, with minor dickite, occurring in small voids within the grainstone units. Lead mineralization is continuous throughout the 8.5 km north-south extent of the Brushy Creek mine. There is considerable variation in the content and distribution of the ore minerals throughout the orebody. The ore reaches its maximum width of ~900 m where the basement highs penetrate the Bonneterre Formation. No overall mineral zonation is apparent, although sphalerite is always associated with galena, and chalcopyrite may occur independent of either sphalerite or galena.
    The Brushy Creek area does not have the prominent extensive brecciation zones seen at the other mines within the Viburnum Trend, and the principal factor in localising ore is the porosity and permeability of the host Bonneterre Formation, particularly the grainstone units carrying the disseminated lead, zinc and copper ores (Evans, 1977).

West Fork, is located 5 km south of Brushy Creek, 4 km north of Fletcher, and is apparently continuous with both. However, mineralisation is developed ~30 m structurally lower than the neighbouring deposits. In addition, there are no Proterozoic knobs protruding high into the Bonneterre Formation in the mine area. The stratigraphic sequence in the mine area comprises, from the base, i). Proterozoic basement granite and rhyolite; ii). Lamotte Sandstone; iii). Bonneterre Formation, divided into - Transition bed; Lower Reef; White Rock; Upper Reef; Grainy Zone; Brown (algal spotted beds) Zone; Sullivan Silt; Upper Dolomite; iv). Davis Formation limestone or dolomite and shale (Dingess, 1989).
    The lower limit of economic mineralisation is 12 to 15 m below the base of the Sullivan Silt marker, and ~12 m above the top of the reef facies. The sinuous, north-south trending, 150 to 1500 m wide, 7 km long, deposit overlies the eastern half of the main reef, within dolostones, from <1 to 3 km east of the contact with the forereef limestones, and straddling the dolostone-backreef facies boundary in the east. The principal ore minerals are abundant galena, with lesser sphalerite. Zoning within the main ore zone comprises dominant galena in the western half of the ore zone. Sphalerite increases progressively to the east, and may locally dominate. An up to 200 m wide, colloform marcasite/pyrite zone straddles the eastern margin of the main mineralised zone, increasing in content in the sphalerite zone as galena decreases, and persisting into the barren backreef facies to the east. Sphalerite and then marcasite/pyrite eventually disappear and a narrow, 1 to 1.5 m wide, irregularly developed galena zone, occupies the eastern contact between marcasite/pyrite and barren dolomite to the east. Chalcopyrite is locally abundant, concentrated in a multi-level, narrow, vuggy, sparry, dolomite breccia near the western margin of the main mineralised zone with barren dolomite (Dingess, 1989).
    Paragenetic studies suggest ore forming fluids initially migrated through the central zone, depositing colloform sphalerite, subsequently migrating outwards to precipitate colloform marcasite/pyrite and finally galena on the outer margins. A late stages of the zinc phase that overprints the Fe-Zn zone is characterised by yellow wurtzite. The initial fluids were oxidised, sulphur depleted and metal-rich, depositing metals rapidly, while the later fluids depositing galena on the outer margins, were metal-poor, sulphur-enriched, and precipitated galena slowly (Mavrogenes, 1989).
    The galena mineralisation is mainly present as 'bedded ore', within favourable stratigraphic locations, locally totally replaced to produce massive sulphides. Mostly, it occurs as finely and coarsely disseminated sulphide, selectively replacing granular matrix in disaggregated grainstones which contain unreplaced, thin, dark, clasts and broken bands of fine-grained mudstone. Galena coats and fills the interstices between, and cracks within, colloform marcasite aggregates, but also occurs as dilated bedding plane fill, vuggy open-space filling, and low and high angle fracture filling (Mavrogenes, 1989; Dingess, 1989).
    Sphalerite also occurs in favourable stratigraphic units as bedded replacement ore, and is accompanied by galena and marcasite/pyrite (Dingess, 1989).
    The main ore zone underlies a 1 to 2 m thick, dense, even-bedded zone with abundant brown spots and clasts (the Brown Zone), and overlies a dense brown spotted marker bed within the Grainy Zone. The orebody is centred over the reef high to the west side and a parallel structural low to the east, with the main galena and sphalerite-marcasite/pyrite over the reef high and structural low respectively. The best ore is where the interval between the main ore zone and the Sullivan Silt is thinned, and the overlying Upper Dolomite and Davis Formation are disturbed, distorted and brecciated. Where the bedded ores are not totally replaced, they commonly exhibit breccia textures, with rounded to sub-rounded clasts, recrystallised/sanded dolomite matrix, which may be pseudobreccia. Individual bedded breccias are 0.3 to 1 m thick, sandwiched between undisturbed dolostone beds. Along the western side of the deposit, the galena zone fills thicker sparry dolomite cemented bedded breccias (Dingess, 1989).
    High angle faulting is absent, although o the south, low angle and bedding plane faulting is evident, accompanied by shearing and brecciation, but with displacements of <2 m (Dingess, 1989).

Fletcher, is located 4 km south of West Fork, on an arcuate buried basement ridge that stood as a topographic high during Upper Cambrian Bonneterre deposition. Following deposition of the Lamotte Sandstone that surrounds and pinches out against the ridge, algal reefs grew and formed a barrier to open marine shelf deposition, creating offshore, reef and back reef environments. Clastic carbonates of the Bonneterre Formation were then deposited over the area and subsequently became the host for base metal mineralisation. The principal sulphide minerals are galena, sphalerite and chalcopyrite, which follow a pronounced north-south trend within the Fletcher area. The ore zone reaches its widest distribution or ~600 m around the Precambrian ridge that protrudes into the Bonneterre, thinning to 60 m on the southern extension of the deposit and 300 m towards the north of the 8 km mineralised strike length. Or thickness averages ~6 m. Ore distribution is facies related, and local replacement is influenced by porosity, fracturing, brecciation, and the shape of the buried Precambrian high. Much of the ore occurs in the middle of the upper clastic grainstone unit. Where the Sullivan Siltstone in the upper Bonneterre is brecciated, mineralisation extends up into the overlying mudstones and grainstones and into the basal Davis Formation. Two narrow, north-south, linear breccia trends extend the full length of the mine and contain good sulphide mineralisation. The mineralization occurs mainly as open-space and fracture fillings, with minor replacement of dolomite. Dissolution of galena is common and is associated with ore zones that show second-stage enrichment.

Sweetwater/Ozark Lead, is located 8 km south of Fletcher, near the southern extremity of the Viburnum Trend. In contrast to most of the other mines in the Viburnum Trend, much of the orebody lies east-west, at right angles to the overall trend. This divergence is due to localisation of much of the ore along the flank of a NW-trending basement high situated on the upthrown side of a high-angle reverse fault. A second distinctive feature of the orebody is that ore occurs throughout the vertical extent of the Bonneterre Formation within particular horizons, exhibiting both structural and stratigraphically control. The dominant ore minerals are galena and sphalerite, with only relatively minor chalcopyrite, which occurs locally in bedded bodies composed almost exclusively of chalcopyrite with bornite and minor covellite, digenite and djurleite.
    Four broad styles of ore have been outlined: i). bedded and disseminated, localised or controlled by the flanks of highs; ii). breccia, which lies within an interpreted solution collapse breccia that in part parallels the flank of the basement ridge; iii). marginal break ore, which occurs in a complex system of through-going faults or fractures that parallel the breccia but are separated from it by a barren or weakly mineralised zone. This zone is interpreted to result from fracturing and movement promoted by the solution collapse brecciation zone; and iv). fracture stockwork, which cross-cuts the strata and is generally not of economic importance unless the fractures form a well developed stockwork.

Fluid inclusions in sphalerite from the Viburnum Trend indicate 137 to 82°C (black sphalerite probably deposited at 180 to 140°C). Chalcopyrite-bornite exsolution at ~200°C (USGS).

Similar ore to that of the Viburnum Trend is encountered in the same host units to the east, fringing the basement inliers of the St. Francois Mountains in the Old Lead Belt (~50 km east of the Viburnum Trend) and at Fredericktown (~30 km SE of the Old Lead Belt and 60 km east of the Viburnum Trend). Further to the north, at the old Indian Creek mine, 35 km NE the Viburnum mines on the northern extremity of the main Viburnum Trend, a high percentage of the ore was hosted by the underlying siliceous sandstones, with a form not significantly different to that at Laisvall in Sweden. The size of these other deposits and belts were as follows (Schott, Doe Run Co, 2012):

      Old Lead Belt - 240 Mt @ 2.9% Pb, 0.3% Zn (Historic Production 1864 to 1972)
      Fredericktown - 15 Mt @ 3.9% Pb (Historic Production 1720 to 1961)
      Indian Creek - 32 Mt @ 3.6% Pb, 004% Zn, 0.1% Cu (Historic Production 1954 to 1985)

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2011.     Record last updated: 24/8/2013
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
 References to this deposit in the PGC Literature Collection:
Anderson G M  1991 - Organic maturation and ore precipitation in southeast Missouri: in    Econ. Geol.   v86 pp 909-926
Appold M S, Numelin T J, Shepherd T J, Chenery S R,  2004 - Limits on the Metal Content of Fluid Inclusions in Gangue Minerals from the Viburnum Trend, Southeast Missouri, Determined by Laser Ablation ICP-MS: in    Econ. Geol.   v99 pp 185-198
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Marikos M A, Laudon R C, Leventhal J S  1986 - Solid insoluble bitumen in the Magmont west orebody, Southeast Missouri: in    Econ. Geol.   v81 pp 1983-1988
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