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.

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 1.70 to 1.63 Ga Central Plains orogens.

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. The Eastern Granite-Rhyolite Province is overprinted to the east by the 1.3 to 1.0 Ga Grenville Front, 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 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.
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.

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.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
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.
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.

 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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