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Pilot Knob |
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Missouri, USA |
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Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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Pilot Knob comprises two Proterozoic volcanic hosted, iron deposits in the Saint Francois Mountains district of South-east Missouri, ~112 km SSW of Saint Louis, ~40 km SE of Pea Ridge and 110 km ESE of Rolla (#Location: 37° 37' 15"N, 90° 38' 00"W).
For details of the regional to local scale setting and geology of the Saint Francois Mountains district see the separate Southeast Missouri Iron District record.
Iron mining in Missouri began in 1815, with production from a vein deposit, just over 1 km SW of Pilot Knob. Mining at the separate Pilot Knob commenced on outcropping stratabound hematite ores in 1835, and continued until 1890, when underground workings were abandoned. Sporadic working took place in 1910 and the 1920s. Total production to that stage was ~1.45 Mt @ ~50% Fe. Aeromagnetic surveys during the 1950 revealed another, separate, concealed magnetite body, just to the west and production commenced in 1967 and continued until the mine closed in 1980 after having procuced ~18 Mt of ore for 9.2 Mt of pellets. Production has been quoted at 20 Mt @ 35 to 40% Fe pellets (Nold et al., 2014).
Pilot Knob is a 180 m high, near circular, conical hill of Mesoproterozoic volcanic rocks, with a diameter of ~1.2 km, protruding through Cambrian sedimentary rock cover which has an average thickness of ~100 m above the main concealed deposit (Nold et al., 2014).
Of the two main iron deposits at Pilot Knob, the first comprises the Pilot Knob Hematite (PKH) deposit, which crops out on the top of Pilot Knob, while the second is the Pilot Knob Magnetite deposit (PKM) that subcrops beneath the Cambrian sedimentary rocks near the northwest base of the mountain, ~1 km due west of the hematite ores.
Pilot Knob is composed of a series of felsite flows, a laminated hematite-bearing tuffaceous unit and volcanic agglomerates, dipping at 10 to 30°SW. The bedded volcanic units are folded and form a shallow syncline, the axial trend of which plunges at ~20°SW (Ryan, 1981; Panno and Hood, 1983), with an interlimb angle of ~120°. The bedded hematite deposit is 4.8 to 18 m in thickness, composed of finely laminated specular hematite that preserves ripple marks, rain-drop impressions, mudcracks, salt hoppers, and other features of sedimentary origin. It is ~240 m stratigraphically above the magnetite deposit.
The concealed Pilot Knob magnetite deposit is located near the western base of the mountain, and is roughly tabular, with the exception of a few minor veins, bounded above and below by two different and distinct volcanic units. It has a known strike length of ~1000 m and in plan view is crescent shaped, with the limbs dipping to the SW at ~45 to 55°. A fault is indicated between the two deposits, although, if the magnetite body is projected up-dip, it is aligned with the shallower hematite deposit (Panno and Hood, 1983).
The hosts stratigraphic succession, from the footwall sequence upward is as follows:
Footwall units - made up of two flow units, which, when unmineralised, are megascopically similar, comprising a lower, >10 m thick, silicic, red, mottled and welded glassy rhyolitic ash-flow tuff, cut by veins and veinlets, occasionally up to 2 m, thick of coarse-grained magnetite. The upper footwall flow unit is ~20 m thick lithic tuff, which consists of red to red-brown, subrounded to rounded and poorly sorted, lapilli-sized rock fragments in a groundmass of red felsite, or less commonly, magnetite. Where magnetite is the matrix to the lapilli, it typically becomes more abundant stratigraphically higher, closer to the main ore zone. Veins and veinlets of coarse-grained magnetite also cut across this upper footwall unit.
Main deposit - the bulk of the magnetite deposit is hosted in a rock interpreted to have originally been a single ash-flow tuff. This tuff unit was originally 37 m thick and was composed of three main zones: i). zones without welding, at the top and bottom of the unit; ii). zones of partial welding, interior to the zones without welding; and iii). a zone of intense welding, near the middle of the ash flow. The base of the ash flow is a 2 to 3 m thick autobreccia composed of closely packed, subangular to subrounded clasts of various sizes and lithologies set in a magnetite matrix. The clasts range from a few mms to 45 m in diameter, and mainly consist of a red, commonly porphyritic felsite, or a brown ash-flow tuff.
Generally, the autobreccia is immediately followed, above a sharp contact, by a zone of dull black, coarse-grained high grade magnetite 'matrix ore'. This is the usual stratigraphic position of the main ore, which may, occur higher in the sequence, within the disseminated ore zone, in some localities. Petrographically, it is comprises massive magnetite with subordinate quartz and minor feldspar, and lenses of secondary calcite, quartz, orthoclase and/or barite, from several mms to 70 cm thick. It also often contains angular blocks, up to 2 m in diameter, derived from the adjacent wall rocks. The upper contact with the disseminated ore is frequently gradational, comprising a framework breccia of disseminated ore fragments in a stockwork of magnetite ore, with veinlets dying out upwards. This transitional zone is referred to as the 'mixture ore'.
The 'mixture ore' is followed by ~37 m of 'disseminated ore', a medium- to fine-grained mixture of magnetite and other minerals composed of irregularly distributed grains and crystal aggregates of magnetite, in a microcrystalline mixture of quartz and plagioclase feldspar, with accompanying minor amounts of chlorite and fluorite. This ore typically consists of 50 to 60% magnetite. Near the stratigraphic centre of the 'disseminated ore' zone, there is an irregular, internal, up to 11 m thick unmineralised section, with an ~1 m thick gradational boundary. This zone is termed the 'internal rock', and is grey to grey-brown, vitreous, often mottled brown or medium brown near the ore-rock contact. It is often peppered with euhedral to subhedral, I to 2 mm diameter magnetite crystals and less frequently contains discontinuous bands of magnetite, and small veins and veinlets of coarse-grained magnetite are common.
Bedded ore and felsite - there is a distinct bedding or layering in the upper 1 to 1.5 m of the disseminated ore, the result of magnetite-rich layers in parallel alignment with thin, lenticular fragments of gray-brown and red felsites up to 2 mm in length. Just above the layered ore zone, there is a similarly bedded or layered grey to grey-brown felsite unit that is 1 to 1.5 m in thickness. This felsite has a vitreous luster and texture and contains no visible phenocrysts. It is layered with individual layers ranging from 5 to 8 mm in thickness.
Hanging-wall 'felsite' - a red to red-brown andesitic ash-flow tuff that is between 122 and 131 m thick and is a distinctive volcanic unit that overlies the grey bedded tuff and comprises the main hanging wall of the deposit Pilot Knob Magnetite deposit.
Pilot Knob Felsites - the hanging-wall felsite is stratigraphically overlain by a series of felsites, agglomerates, and tuffs, collectively known as the Pilot Knob Felsites with a total thickness of >150 m. They comprise, in ascending order:
i). Lower Red Rhyolite, which is 85 to 90 m thick; ii). purple rhyolite, 30 to 35 m thick and includes an upper agglomerate sub-unit; iii). a lower laminated tuff that represents the host to the Pilot Knob Hematite deposit; v). upper purple volcanic agglomerate that is up to 30 m thick, vi). grey-brown to red rhyolite, and vii). upper laminated tuff. This sequence, or minor variations of it, is always found above the hanging-wall felsite of the subsurface Pilot Knob Magnetite deposit. The Pilot Knob Felsites are in turn overlain by the Shepherd Mountain flows.
The Pilot Knob Hematite deposit (PKH) is located ~1 km east of the town of Pilot Knob. It is hosted within the lower laminated tuff of the Pilot Knob Felsites, and is ~240 m stratigraphically above the banded grey tuff and banded mineralisation at the top of the subsurface Pilot Knob Magnetite deposit. The host unit is 4.8 to 18 m thick and exhibits sedimentary features in its lower parts, including graded bedding, ripple marks, mud cracks, salt casts and questionable hail prints. The immediately overlying upper purple volcanic agglomerate is in gradational contact with the underlying laminated tuff. The matrix of this volcanic agglomerate contains fine-grained siliceous hematite (Ryan, 1981; Seeger et al., 1989). At surface, the hematite of the main mineralised beds is mainly composed of hematite, with lesser maghemite and goethite in a gangue suite of quartz, silicates of the host-rock, tourmaline and barite. Where banded/laminated, the hematite mineralisation varies from bands of nearly pure hematite to almost barren interlayers. It is interpreted to represent extensive replacement of a porous and permeable host rock. The laminated hematite mineralised beds are divided into an lower and upper package of beds, separated by an up to 1 m thick clay seam (Ryan, 1981). The lower mineralised beds are dominated by fine-grained, hard, dense, steel-grey, thinly laminated specular hematite with an average grade of 58% Fe (Ryan, 1981; Seeger et al., 1989). The thickness of each lamina varies from 2.5 mm up to 2.5 cm (Seeger et al., 1989). The exposed upper mineralised beds strike at between 200 and 220° and dips between 21 and 25°. They are also composed of thinly laminated, fine-grained, bluish-grey hematite with a grade of between 40 and 50% Fe (Ryan, 1981; Seeger et al., 1989). Similar to the lower mineralised hematite beds, the upper beds also contains ripple marks and mudcracks (Seeger et al., 1989). They are characterised by alternating red and dark laminae that vary between 2 and 30 mm in thickness with contain minor 0.1 to 0.5 cm thick clay interbands (Seeger et al., 1989). The lower portion of the upper mineralised beds contains sparse 0.5 to 2 cm rhyolitic clasts and thin, up to 15 cm thick uniform bands of quartz and feldspar phenocrysts parallel to bedding (Seeger et al., 1989). Proceeding up-sequence, the size and abundance of rhyolite clasts increase, up to 15 cm long and up to 25 modal % respectively (Seeger et al., 1989). The upper purple volcanic agglomerate that overlies the bedded mineralisation contains angular to sub-rounded clasts of rhyolite porphyry that are up to 30 cm long, set in a fine-grained siliceous hematite matrix with a grade of 20 to 23% Fe (Seeger et al., 1989). Some of the clasts have their long dimensions parallel to bedding (Ryan, 1981). The abundance of clasts and their size in the agglomerate also both increase stratigraphically upward (Ryan, 1981; Seeger et al., 1989). No structural overprinting produced a penetrative planar fabric such as cleavage or schistosity have been observed, implying that the stratigraphic section was never deeply buried nor had experienced regional metamorphism (Day et al.,
2016).
The Pilot Knob Magnetite deposit (PKM) comprises a series of tabular, sill-like bodies that strike NW and dip at ~45°SW, and are concordant to layering within the host sequence of pink to grey rhyolitic pyroclastic rocks (Wracher, 1976; Panno and Hood, 1983; Nold et al., 2013). The deposit has a strike length of ~500 m and persists down a 45° dip for ~700 m, with a thickness of ~100 m (Nold et al., 2013). Its upper part was exposed on the Precambrian unconformity, partially altered to hematite, and is unconformably overlain by the Upper Cambrian Lamotte Sandstone (Nold et al., 2013). It is composed of two main mineralisation types, one consisting of relatively homogeneous, higher grade, black euhedral magnetite that forms the bulk of the deposit, and a relatively heterogeneous, lower grade magnetite-cemented breccia that forms an envelope around the higher grade ores. The higher grade ore contains fine- to medium-grained magnetite interspersed with granular silicate minerals, mainly albitic plagioclase, accompanied by minerals such as K feldspar, quartz and chlorite. The high-grade magnetite ore contains 0.02 to 0.68 wt.% TiO2, whilst the lower grade ore comprises predominantly of larger magnetite grains, which typically display optically discernible zonation (Nold et al., 2013, 2014). The cores of low TiO2 magnetite grains contain small inclusions of silicates, carbonates, sulphates, halides, and sulphides, whereas the rims are relatively free of inclusions (Nold et al., 2013). The rims of these zoned magnetite grains are slightly enriched in Fe and depleted in Al and Si, compared to the cores (Nold et al., 2014). The deposit is crosscut by the 125 m thick near-horizontal Shepherd Mountain Gabbro at a depth of 500 m. This gabbro was dated at 1333 ±56 Ma using bulk rock Sm-Nd geochronology (Lowell and Rämö, 1999). This age has been interpreted to represent the younger age limit of mineralisation (Nold et al., 2013). The style of deformation in the deposit is brittle fracturing and brecciation, with no penetrative structural fabrics observed (Day et al., 2016). Tunnell et al. (2021) have undertaken precise U-Pb age data on apatite intergrown with massive magnetite in the PKM deposit which yielded an age of 1437.7 ±5.8 Ma.
Petrographic observations of PKH hematite mineralisation, bulk rock compositions, and the mineral chemistry of hematite, which contains up to 2.7% Ti, led Tunnell et al. (2021) to suggest that the hematite in the PKH deposit crystallised from acidic and hypersaline hydrothermal fluids at a temperature between 200 and 250°C. The Fe isotopic composition of 9 bedded (δ56Fe = 0.05 to 0.30‰, average 0.13‰) and 3 brecciated hematite samples (δ56Fe = -0.19 to 0.01permil;, average -0.06‰) from the PKH deposit are slightly lighter than the published δ56Fe results of magnetite from the PKM deposit (δ56Fe = 0.06 to 0.27‰, average 0.17‰). They noted however, that all isotopic signatures fall within the magmatic range, indicating that iron in both deposits was originally sourced from a magma. Because of the hydrothermal origin of the PKH deposit, the iron isotopic compositions of the PKM and PKH ores that imply a shared/similar iron source, and the spatial proximity of both deposits, Tunnell et al. (2021) argue that the PKM and PKH deposits are genetically related and represent two end members of a high to low temperature magmatic-hydrothermal continuum. In this scenario, ore fluids exsolved from the magma that facilitated the formation of the PKM deposit at temperatures between 500 and 200°C migrated upwards, infiltrated existing sedimentary structures in porous tuffs and agglomerates at shallower depths, and precipitated hydrothermal hematite ore at lower temperatures while preserving the original bedded and brecciated structures.
Tunnell et al. (2021) note that the geochemical signatures of the rhyolites/rhyodacites that host the PKM deposit imply that these rocks are A2-type felsic rocks that were emplaced in an extensional setting. They also note that bulk silicate Earth normalised patterns of the PKM deposit and wall rocks display a negative slope from Cs to Lu with negative Nb and Ta anomalies, indicating a hydrous source for the rhyolites and rhyodacites, possibly an earlier subduction-modified subcontinental lithospheric mantle.
The most recent source geological information used to prepare this decription was dated: 2012.
Record last updated: 3/12/2021
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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Childress, T.M., Simon, A.C., Day, W.C., Lundstrom, C.C. and Bindeman, I.N., 2016 - Iron Oxygen Isotope Signatures of the Pea Ridge and Pilot Knob Magnetite-Apatite Deposits, Southeast Missouri, USA: in Econ. Geol. v.111, pp. 2033-2044.
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Nold, J.L., Davidson, P. and Dudley, M.A., 2013 - The pilot knob magnetite deposit in the Proterozoic St. Francois Mountains Terrane, southeast Missouri, USA: A magmatic and hydrothermal replacement iron deposit: in Ore Geology Reviews v.53, pp. 446-469.
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Nold, J.L., Dudley, M.A. and Davidson, P., 2014 - The Southeast Missouri (USA) Proterozoic iron metallogenic province - Types of deposits and genetic relationships to magnetite-apatite and iron oxide-copper-gold deposits: in Ore Geology Reviews v.57, pp. 154-171.
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Panno S V, Hood W C 1983 - Volcanic stratigraphy of the Pilot Knob Iron deposits, Iron County, Missouri: in Econ. Geol. v78 pp 972-982
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Seeger C M, Marikos M A, Nuelle L M 1989 - The Pilot Knob hematite deposit: in Brown V M, Kisvarsanyi E, Hagni R (Ed.s), Olympic Dam Type Deposits and Geology of Middle Proterozoic Rocks in the St Francois Mountains Terrane, Missouri Soc. of Econ. Geol. Guidebook no. 4 pp 55-68
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Tunnell, B.M., Locmelis, M., Seeger, C., Mathur, R., Dunkl, I., Sullivan, B. and Lori, L., 2021 - The Pilot Knob iron ore deposits in southeast Missouri, USA: A high-to-low temperature magmatic-hydrothermal continuum: in Ore Geology Reviews v.131, 21p. doi.org/10.1016/j.oregeorev.2020.103973.
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