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Major strata-bound and minor vein deposits of magnetite and hematite are found within the Iron Springs district of south-western Utah, USA, 20 to 30 km west of Cedar City in Iron County, and 350 km to the SSW of Salt Lake City (#Location: 37° 42'N, 113&Deg; 16'W).
The Iron Springs district was an important strategic source of iron during World War II, but continued production at a rate of around 3 Mt pa until the 1960's (Mackin, 1968).
The Iron Springs deposits occur as pod-like replacement bodies within Jurassic limestones around the margins of three Miocene quartz-monzonite intrusives which are dated at 24 to 22 Ma (Mackin, 1968). All three intrusives, the Three Peaks, Granite Mountain and Iron Mountain plutons, are laccolithic, emplaced along the same stratigraphic position, concordant with the updomed basal siltstone member of the Jurassic Carmel Formation, following a Laramide décollement surface. These intrusions have oval shaped exposures in plan, are 5 to 8 km in diameter, and define a NE trend through the district over a length of ~25 km and width of ~6 km, aligned along a northeast-trending Laramide anticlinal flexure which is locally overthrust. The margins of two of the intrusions are complexly folded and faulted. Other intrusions of similar composition continue the trend and are found within 50 km to the NE, SW and also off the trend to the south, but cut different stratigraphy and do not host iron mineralisation (Bullock, 1973; Barker, 1995).
The three quartz monzonite porphyry intrusions possess a zonation of three phases:
i). a peripheral shell, developed along the sides and across the top of the intrusions as a fine-grained, chilled margin with a variable thickness that usually ranges from 30 to 60 m.
ii). a zone of selvage joints within the quartz monzonite porphyry. The selvage joints are composed of generally parallel ribs of highly resistant rock, caused by bleaching and hardening of the porphyry on either side of any individual joint over a width ranging from <1 to >30 cm. Differential weathering produces a ribbed topography of resistant linear selvages, which pass outward into soft crumbly, altered quartz monzonite characteristic of the interior phase; and
iii). an interior phase, which is coarse-grained quartz monzonite porphyry, with a chemical composition and mineralogy substantially identical to the peripheral shell. Alteration of mafic minerals in the interior phase promotes weathering to form low crumbly knobs or flat barren areas in outcrops, with a coarse granular, friable texture.
The host Homestake Limestone Member of the Carmel Formation, is a grey, blue and black, massive, to thin bedded limestone, that is locally shaly or carbonaceous, ranging from 65 to 90 m in thickness. It is underlain by the Basal Siltstone Member of the Carmel Formation, a thin bedded light to dark greenish-grey siltstone which averages ~10 m in thickness. The siltstone is in turn underlain by the regional Navajo Sandstone. The regional décollement surface that has controlled the intrusive emplacement is at the upper contact of the Navajo Sandstone (Mackin, 1968).
The host limestone grades upwards via approximately a metre of ripple marked limy mudstone, into a sandstone unit. This Upper Jurassic sandstone unit, equivalent to the Entrada Sandstone of the Colorado Plateau, is composed of interbedded grey to maroon sandstone and shale which is 0 to 65 m thick (Bullock, 1973; Barker, 1995).
A local talus-fanglomerate, the Cretaceous Marshall Creek Breccia, is found along the margins of the Iron Springs intrusions, at the disconformable contact between Entrada Sandstone and the overlying Iron Springs Formation. It occurs as irregularly lenses, varying in thickness to 30 m within short distances along the strike, and largely consists of angular blocks of Homestake Limestone up to 2 m in diameter, with smaller clasts of Entrada Sandstone. The matrix commonly is maroon calcareous siltstone, which may or may not contain limestone fragments. The Entrada Sandstone and Marshall Creek Breccia are unconformably overlain by the ~900 m thick Cretaceous Iron Springs Formation, composed of 120 m of basal conglomerate, overlain by a sequence of shale, sandstone and conglomerate with local thin beds of limestone and coal (Mackin, 1968).
Ore occurs in three main styles:
i). Replacement-type bodies in limestone. These represent the most important ores and are found in the lower sections of the Homestake Limestone Member, ranging from a metre thick to replacement of the full thickness of the unit up to a maximum of 75 m. Individual deposits range from a few metres to >300 m in length along strike and down-dip. The ore bodies are generally tabular and lenticular, occurring in limestone, close to and dipping away from the intrusive contact zone. The footwall of the ore generally follows the stratigraphic base of the Homestake Limestone Member, and is separated from the porphyry by 5 to 25 m of the Basal Siltstone Member, which has been metamorphosed to an assemblage of plagioclase + phlogopite + tremolite ± chloritoid ± scapolite, with relict quartz and alkali feldspar. The overlying shale, siltstone and siltstone above the limestone has not been replaced. Where the limestone is not mineralised with iron in the proximity of intrusive contact, it is converted to an assemblage of wollastonite + hydrogrossular ± clinopyroxene ± calcite ± forsterite (Bullock, 1973; Barker, 1995).
ii). Breccia filling and replacement along faults and in pipe-like structures - which occur in the Carmel Formation, Entrada sandstone, Iron Springs Formation and the quartz monzonite porphyry, contain iron mineralisation as breccia fillings and breccia replacements. Brecciated zones may occur within Homestake Limestone replacement ore bodies or above, along the margins of, or below these ore bodies, or independent of these relationships. Some of these ore bodies resemble breccia pipe structures and have crude cylindrical morphologies. They lie on the periphery of intrusives along major faults or fault intersections and mainly occur in the quartz monzonite where they may contain large angular to rounded blocks and fragments of porphyry or sedimentary rocks completely surrounded by ore. The principal ore mineral of the breccia pipes is magnetite. Tonnages of ore associated with breccia fillings and replacements are some of the largest in the district (Bullock, 1973).
iii). Fissure vein fillings in quartz monzonite porphyry intrusions. Most of these veins or fissure fillings, are too small to be of economic importance. They occur in numerous selvage joints which cut intrusions and are radial, concentric, and oblique. Radial joints, which are continuations of the joint system from the interior zone, strike normal to intrusive contacts and have vertical dips. Some 30 to 50% of the veins are mineralised, containing an assemblage comprising variable amounts of clinopyroxene, magnetite, apatite, aragonite, calcite, stilbite, chalcedony and quartz (Bullock, 1973; Barker, 1995).
In addition mineralisation also occurs as detrital alluvial ores, concentrated from weathered products of intrusive rock and iron-ore deposits. As weathering progressed, magnetite-hematite debris moved downslope and concentrated in alluvium on pediment and scree slopes. This mineralisation has been exploited and average ~10% Fe, with very low Ti and high magnetite content (Bullock, 1973). These deposits have been worked extensively in the past using a mobile concentrator (Klemic, 1970; Mackin, 1968).
The ores are characterised by magnetite and hematite, with lesser local specular hematite and martite, and minor goethite, limonite and turgite. Gangue minerals include (in order of decreasing abundance) calcite, phlogopite, apatite (fluorapatite), quartz and chalcedony, pyrite, marcasite, diopside, azurite, malachite, chrysocolla, chalcopyrite, bornite, galena, magnesite, gypsum, barite, epidote, garnet, vesuvianite, scapolite, tourmaline, chlorite, tremolite, actinolite and wollastonite. Ores range from hard pure magnetite to soft friable varieties of hematite (Barker, 1995).
The mineral assemblage in the strata-bound ore differs from that in the veins in that it includes hematite, dolomite and phlogopite. Magnetite has accompanying, wider and more abundant hematite lamellae in the strata-bound bodies and is more depleted in TiO2 than the veins (Barker, 1995).
Studies of stratigraphic thickness indicate that there was no bulk change in volume of the limestone unit associated with the emplacement of ore. For every million tonnes of Fe, there were corresponding additions of 40 000 t SiO2, 20 000 t Mg and 10 000 t of Al. The ore averages 0.2% P; 0.03 to 0.05% S; 680 ppm F; 3% CaO (Mackin, 1968).
More than 55 separate deposits have been delineated and many mined. Individual deposits range from small pods of ore with a few thousand tonnes, to a significant number with 0.5 to 10 Mt. The larger deposits include, A and B (Replacement; 30 to 40 Mt), Black Hawk (Replacement; 20 Mt), Blowout (Breccia pipe, Replacement; 10 Mt), Burke (Replacement; 16 Mt), Comstock (Replacement; 50 to 60 Mt), Desert Mound (Replacement; 20 Mt), Lindsay (Replacement; 20 Mt), McCahill (Replacement; 30 Mt), Mountain Lion (Replacement; 15 to 20 Mt), Pinto (Replacement; 10 Mt), Section 2 (Replacement: 10 Mt), Section 9 (Replacement: 35 Mt) and Rex (Breccia, Replacement; 100 Mt) of ore, including low grade material that requires upgrading (Bullock, 1973). The ore has predominantly had direct shipping grades which have averaged 53% Fe, although the ores vary between 40 and 60% Fe. They are soft and readily crushed. The small, low grade alluvial Fe deposits contain 10 to 20% Fe (Klemic, 1970; Mackin, 1968).
Resource and production figures include:
Reserve, 1960 - 250 Mt @ 40 to 60% Fe (Klemic, 1970).
Production, 1923-65 - 72 Mt @ 53% Fe (Mackin, 1968).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1995.
Record last updated: 30/7/2013
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 to this deposit in the PGC Literature Collection:
Barker D S 1995 - Crystallization and alteration of Quartz Monzonite, Iron Springs Mining District, Utah: relation to associated Iron deposits: in Econ. Geol. v 90 pp 2197-2217|
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