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Southeast Missouri Iron District - Iron Mountain, Pea Ridge, Pilot Knob, Boss-Bixby, Kratz Springs

Missouri, USA

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The Southeast Missouri province includes eight known major magnetite and hematite deposits (including Pea Ridge, Bourbon, Kratz Spring, Camels Hump and Lower Pilot Knob), totalling in excess of 1 Gt of ore, and numerous (>30) minor occurrences.

For detail on specific, more significant deposits, see the separate Boss-Bixby, Pilot Knob and Pea Ridge records. Iron Mountain and Kratz Springs, and the Shepherd Mountain vein deposit are described at the end of this record.

These deposits are hosted by the volcanic rocks of the Eastern Granite-Rhyolite Province where it is exposed in the Saint Francois Mountains of Missouri, USA. The 1.48 to 1.45 Ga Eastern Granite-Rhyolite Province (EGRP) is contiguous with the 1.40 to 1.34 Ga Southern Granite-Rhyolite Province (SGRP) that is locally exposed in the Arbuckle Mountains of Oklahoma. Both are ~12 km thick, relatively undeformed, and together cover a 250 to 600 km wide curvilinear belt outboard of the Archaean Superior craton of Canada and north-central USA. They are respectively separated from the craton, by the intervening, fringing, 1.88 to 1.83 Ga Penokean, and 1.76 to 1.60 Ga Central Plains orogens. The Central Plains Orogen is divided into two elongated ENE-WSW strips, the northern Yavapai Province, principally composed of the amalgamation of a series of 1.76 to 1.73 Ma oceanic arc terranes; and the southern 1.70 to 1.60 Ma Mazatzal Province composed of continental margin volcanic arcs and backarc related supracrustal successions (Whitmeyer and Karlstrom, 2007).

The combined Eastern and Southern Granite-Rhyolite Province, extend for over 2500 km from southern Ontario in Canada, to New Mexico (USA). Over this interval, they form part of the stable basement to the Phanerozoic sedimentary basins that cover the interior of the continent. In addition, very extensive 1.45 to 1.35 Ga granitoids of the two Granite-Rhyolite province intrude both the Yavapai and Mazatzal provinces. The Eastern Granite-Rhyolite Province is bounded to the east by the Grenville and Llano provinces which were reworked from 1.3 to 1.0 Ga, and to the west and northwest by the 1.1 Ga Mid-continental Rift. Sm-Nd depleted mantle model ages (TDM) of <1.55 Ga in the south and east of the two Granite-Rhyolite provinces, imply both granite and rhyolite were essentially juvenile, incorporating little crust of appreciably greater age, while to the north and west, ages are considerably greater, suggesting they incorporated significant amounts of Palaeoproterozoic crust. The Grenville and Llano provinces have been interpreted to reflect collision and re-accretion of a pre-1.5 Ga sliver rifted from the North America craton prior to deposition of the Granite-Rhyolite provinces. The collision resulted in imbrication of rocks from deep crustal levels followed by orogenic collapse of overthickened crust and widespread intrusion of a 1.19 to 1.11 Ga anorthosite-mangerite-charnockite-granite plutonic suite (McLelland, 1996).
SE N America
Where exposed in the Saint Francois Mountains, the Eastern Granite-Rhyolite Province is dominantly composed of anorogenic granites, intruding coeval rhyolite, rhyolitic and minor intermediate alkalic rocks, but elsewhere is also intruded by significant gabbroid masses, and may overlie a more mafic layer (Seeger, 2000; Pratt et al., 1992; Atekwana, 1996 Van Schmus et al., 1993; 1996; Rohs and Van Schmus, 2007). More than 1500 m of these rhyolites, dominantly ash flow tuffs, with rare trachytes, are preserved in the Saint Francois Mountains. The rhyolites typically contain perthitic alkali feldspar phenocrysts and iron-rich mafic minerals, including fayalite, ferrosilite and ferrohastingsite, and are characterised by very high SiO
2, K2O:Na2O, Fe:Mg and fluorine (locally up to 20% F), and low CaO, MgO and Al2O3. The trachyte suite includes magnetite trachyte, trachyte, trachyandesite and trachybasalt (Kisvarsanyi and Kisvarsanyi, 1989).

Granitoids found within the district are of three types, i). sub-volcanic massifs, which are comagmatic with the rhyolites, and contain perthitic alkali feldspars, biotite and ubiquitous magnetite, occurring as fine-grained granophyre, grading down into coarse rapakivi granites; ii). ring intrusions, that define the ring structures, and are composed of intermediate to high silica porphyritic trachyandesites, trachytes, syenites and amphibole-biotite granites, and iii). central plutons that form the core of the ring complexes, and are typically high-silica, two-mica granites with accessory fluorite, topaz, apatite, spinel, orthite, titanite and cassiterite. They also characteristically contain high uranium and thorium contents (Kisvarsanyi and Kisvarsanyi, 1989).
SE Missouri
Iron oxide deposits are generally associated with, but not necessarily hosted by, the magnetite trachytes, which may carry up to 10 vol.% magnetite over significant areas. These deposits are generally spatially associated with ring structures, and are considered to be related to the phase in the evolution of the complex that produced the ring plutons, which are comagmatic with the trachytes (Kisvarsanyi and Kisvarsanyi, 1989).

Mineralisation is controlled by structures which provided fluid migration paths and sites of ore localisation. Breccias formed by explosive volcanic eruptions and caldera collapse were especially favourable sites. The ore occurs as dykes, veins, open-space fillings, breccia matrix and replacement deposits of magnetite and hematite, with ores containing 35 to 56% Fe. In most deposits, the contact between ore and wall rocks is sharp, although locally, narrow fringes of partial or total replacement of wall rock by fine grained magnetite is evident. In the massive (often ~100%) magnetite sections of some deposits, unaltered or slightly altered, suspended rhyolite blocks are found within the massive magnetite near the contact, locally forming jigsaw breccias, leading to the suggestion the magnetite may represent an ore-magma injection (Kisvarsanyi and Kisvarsanyi, 1989). Low tonnage accumulations of mainly hematite, may also occur at high levels in the volcanic pile as conformable, bedded or bedding replacement, tuffaceous deposits, representing vapour-phase condensates or fumarolic precipitates in caldera lakes. At Pilot Knob, a 3 to 10 m thick band of conformable, finely banded (2 to 20 mm), steel grey specular hematite, with structure interpreted as mudcracks, ripple marks and rain drip or hail imprints is interbedded with a clay seam and volcanic conglomerate, and overlain by rhyolite. The gangue is principally angular quartz and feldspar (Seeger, 1989; Kisvarsanyi and Kisvarsanyi, 1989).

The most complex deposits occur near intrusive contacts between ring intrusion syenites and late-stage central plutons, at the deepest levels of the terrane, where, in addition to magnetite and hematite, copper sulphides, uranium and thorium minerals, REE and gold are found (e.g., Boss-Bixby). Late stage breccia-pipes and hydrothermal quartz veins have been responsible for the enrichment in uranium, thorium, REE and gold in some of these latter deposits. (Kisvarsanyi and Kisvarsanyi, 1989).

This summary is an extract from Porter (2010) in "Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, v.3, Advances in the Understanding of IOCG Deposits", available from PGC Publishing, Adelaide.

Other deposits not described in more detail in separate records include:

Iron Mountain

This deposit is located in St. Francois County, Missouri, ~8 km north of Pilot Knob. It was mined intermittently between 1844 and 1966, producing about 9 Mt of iron ore concentrates. It had a distinctive inverted shell or dome shape with the bulk of the ore occurring as dykes/veins or as breccia fill (Murphy and Ohle, 1968). On the 654 level the main deposit was ~400m across in both east-west and north-south sections, with thicknesses ranging from ~10 to 100 m. The hosts to most of the mineralisation is trachyte to trachyandesite, cut by numerous thin dolerite dykes (Dudley, 1998). Hematite and lesser magnetite are the principal ore minerals. The most abundant gangue minerals are andradite, actinolite, apatite, quartz, calcite, epidote and chlorite. The deposit is zoned, with the western limb comprising massive hematite accompanied by andradite pseudomorphs after actinolite, whilst the eastern limb is composed of massive magnetite with a gangue of 'phenocryst-like' actinolite crystals, plus quartz. Dudley (1998) and Dudley and Hagni (2000) noted that much of the hematite is martite after primary magnetite. Some of the high-grade hematite is devoid of any trace of martitic texture, has finer grain-sizes near the contacts, and contains very small magnetite inclusions. Trace element contents of magnetite from Iron Mountain (Kisvarsanyi and Proctor, 1967) exhibit low Ti levels, similar to other deposits in the district. Some of the apatite at Iron Mountain is present within the hematite ores as coarse, pink, unidirectional crystals. Very similar textures of lineated apatite within magnetite ores have been reported by Nold (2012) for the Pea Ridge deposit, Missouri, and by Daliran et al. (2010) for the Gasestan deposit in the Bafq district, Iran. Production from Iron Mountain totalled ~9 Mt of iron concentrate.

Kratz Spring

Very little has apparently been published on the Kratz Spring deposit, although Snyder (1969) described it as being predominantly composed of magnetite, conformable to the felsic volcanic host rocks, cut by numerous dolerite dykes, and suggested that it is most likely a 'sill-type injection'. Day et al. (2001) described it as being similar to the Pea Ridge. No production or resources are recorded.

Shepherd Mountain

This is a vein type deposit, located ~1.5 km SW of Pilot Knob. It occurs as two NE-trending, nearly vertically dipping veins that are each 1 to 7 m thick cutting gently dipping host rhyolites. Dudley and Nold (2001) note that the mineralisation is characterised by abundant magnetite and lesser hematite and quartz that fill open spaces, with minor pyrite, chalcopyrite, goethite, chlorite and sericite. Textures observed suggest a complex history of oxidation and reduction. Magnetite typically occurs as euhedral aggregates and less commonly has a colloform habit. Hematite is found as specular laths and as martitic replacements of magnetite. Other characterisitcs of the deposit include partial reduction of specular hematite laths to magnetite and late cross-cutting fluorite veinlets containing chalcopyrite. Although the nature of the iron mineralisation has similarities to Pilot Knob magnetite, Pea Ridge, and Iron Mountain, the gangue at Shepherd Mountain lacks the diversity or complexity observed in those larger deposits (Dudley and Nold, 2001). Production from Shepherd Mountain totalled 0.075 Mt.

The most recent source geological information used to prepare this summary was dated: 2000.     Record last updated: 18/7/2013
This description is a summary from published sources, the chief of which are listed below.
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  References & Additional Information
   Selected References:
Day, W.C., J.F., Slack, Ayuso, R.A. and Seeger, C.M.,  2016 - Regional Geologic and Petrologic Framework for Iron Oxide ± Apatite ± Rare Earth Element and Iron Oxide Copper-Gold Deposits of the Mesoproterozoic St. Francois Mountains Terrane, Southeast Missouri, USA: in    Econ. Geol.   v.111, pp. 1825-1858.
McCafferty, A.E., Phillips, J.D. and Driscoll, R.L.,  2016 - Magnetic and Gravity Gradiometry Framework for Mesoproterozoic Iron Oxide-Apatite and Iron Oxide-Copper-Gold Deposits, Southeast Missouri: in    Econ. Geol.   v.111, pp. 1859-1882.
Mercer, C.N., Watts, K.E. and Gross, J.,  2020 - Apatite trace element geochemistry and cathodoluminescent textures - A comparison between regional magmatism and the Pea Ridge IOAREE and Boss IOCG deposits, southeastern Missouri iron metallogenic province, USA: in    Ore Geology Reviews   v.116, 22p. doi.org/10.1016/j.oregeorev.2019.103129
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.
Porter T M,  2010 - Current Understanding of Iron Oxide Associated-Alkali Altered Mineralised Systems: Part II, A Review: in Porter T M, (Ed),  2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide   v.3 pp. 33-106
Starkey, M.A. and Seeger, C.M.,  2016 - Mining and Exploration History of the Southeast Missouri Iron Metallogenic Province: in    Econ. Geol.   v.111, pp. 1815-1823.
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.

   References in PGC Publishing Books: 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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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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