Pilot Knob

Missouri, USA

Main commodities: Fe
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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 Pilot Knob commenced on surface 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 the concealed magnetite body, just to the west and rpoduction commenced in 1967 and continued until the mine closed in 1980 after having procuced ~18 Mt of ore for 9.2 Mt of pellets.

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 iron deposits at Pilot Knob, the first comprises hematite, which crops out on the top of Pilot Knob, while the second is a magnetite orebody 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 to the SW and an interlimb angle of ~120°. The bedded hematite orebody is 5.8 to 8.8 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.

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 sequnce 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 orebody - 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 - the grey bedded tuff is overlain by a distinctive volcanic unit that comprises the main hanging wall of the deposit. This is a red to red-brown andesitic ash-flow tuff that is between 122 and 131 m thick.
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). undifferentiated felsites, ii). purple felsite, iii). purple agglomerate, iv). laminated tuff, v). upper purple agglomerate, vi). grey-brown felsite, and vii). upper laminated tuff. This sequence, or minor variations of it, is always found above the hanging-wall felsite of the subsurface orebody. The Pilot Knob Felsites are in turn overlain by the Shepherd Mountain flows.

The hematite orebody is in the lower laminated tuff and is ~240 m stratigraphically above the banded grey tuff and banded ore at the top of the subsurface deposit. The host unit is 5.8 to 8.8 m thick and exhibits sedimentary features in its lower parts, including graded bedding, ripple marks, mud cracks, salt casts and questionable hail prints. The upper purple agglomerate is in gradational contact with the underlying laminated tuff. The hematite ore at surface 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, the hematite ore 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 magnetite ore is composed of two main ore types, one consisting of relatively homogeneous, higher grade, black euhedral magnetite that forms the bulk of the orebody, 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. in 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 minimum age of the deposit is constrained by the 120 m thick crosscutting Shepherd Mountain Gabbro, which has an Sm-Nd isochron age of 1333±56 Ma (Lowell and Rämö, 1999).

The most recent source geological information used to prepare this summary was dated: 2016.     Record last updated: 20/11/2016
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:
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.
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
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

 References to this deposit in PGC Publications: Want any of our books ? Pricelist
Seeger C M, 2000 - Southeast Missouri Iron Metallogenic Province: Characteristics and General Chemistry,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v1  pp 237-248
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