Cornwall, Morgantown

Pennsylvania, USA

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The Cornwall and Morgantown magnetite deposits are located in the Reading Prong iron district. Cornwall is ~115 km WNW of Philadelphia and ~35 km east of Harrisburg, in Pennsylvania, USA, and Morgantown is ~60 km further ESE.

For an outline of the regional setting of the Adirondack and Mid-Atlantic (or Reading Prong) iron belts, see the separate Adirondack & Mid-Atlantic (Reading Prong) Iron Belt record.

This belt also contains a variety of other iron deposits, many of which appear to be Mesoproterozoic in age and of a different, IOCG-related origin, in contrast to Cornwall and Morgantown, although the latter may have derived their iron content from the basement iron endowment of the district (Friehauf et al., 2002).

These two large deposits, Cornwall and Morgantown, and about 45 small contact metasomatic iron and copper-iron occurrences of the Cornwall-type, occur as replacements of fault fractured limestone and other carbonates, adjacent to Mesozoic dolerite-gabbro sheets. The host carbonate rocks belong to the non-marine Newark series in the Triassic rift zone of eastern Pennsylvania, which was filled by a sequence that is dominantly composed of fluvial red sandstone and shales, locally accompanied by beds of limestone conglomerate and quartz conglomerate. The youngest rocks of this sequence of early Jurassic in age. These Mesozoic rocks overlie Cambro-Ordovician Palaeozoic limestones, dolomites quartzites, shales, sandstones and cherts, and Proterozoic gneisses, granites, schists and volcanic rocks. The sedimentary sequence was thrust faulted and folded into complex nappes during the mid- to late-Palaeozoic, and much of the post-Ordovician cover was removed prior to the Triassic.

The large and most of the smaller deposits, occur along the faulted margin of the Triassic rift basin, replace calcareous strata, near the fault or unconformity separating the Triassic from basement Palaeozoic or Precambrian formations, in association with intrusive masses of dolerite-gabbro. Replacement is not restricted to any particular stratigraphic unit, with ores in different deposits found in Proterozoic marbles, the lower dolomitic stratigraphic horizons and higher limestone formations of the Cambro-Ordovician, and Triassic limestone conglomerate containing Palaeozoic carbonate clasts.

All of the deposits are found in the vicinity of dolerite sills or dikes, although the relationship of ore to dolerite varies in detail. Most ore, especially in the larger deposits, occurs as a replacement of carbonate rock overlying dolerite e.g., the Cornwall, the Grace (Sims, 1968), French Creek (Smith, 1931), Ruth, Fritz Island and Jones mines, and some deposits near Boyertown (Spencer, 1908). At the Doner, Raudenbush, Grantham and Esterly mines, small occurrences and orebodies occur beneath the dolerite (Spencer, 1908), while at Dillsburg, ores replace limestone conglomerate between two sills. At some deposits, ore is separated from dolerite by tens of metres or more of shale, sandstone gneiss, or other rocks not favourable for replacement, although the ore-bearing horizons can usually be projected downward to intersect the intrusive,

The dolerite sills have sharp, usually chilled margin contacts with either ore or country rocks, typically, small cracks in dolerite adjacent to ore filled with magnetite, pyrite, and a variety of gangue minerals, including quartz, biotite, pyroxene, chlorite, serpentine and calcite (Spencer, 1908; Smith, 1931; Sims, 1968; Lapham and Gray, 1973). These veinlets and associated alteration appear to have formed after consolidation of the dolerite.

The dolerites have a bulk gabbroic (tholeiitic) composition, although many variations are evident (Smith et al., 1975), and are typically 150 to 450 m thick, to a maximum of about 600 m. The grain size increases inward from fine aphanitic margins with sparse phenocrysts, to the interior of the thicker sheets which comprise medium grained (1 to 5 mm) rocks. Layers of ferrogabbro and lenses and schlieren of coarse gabbroic pegmatite and granitic granophyre are locally present, especially in the upper parts of thicker sheets (Hotz, 1950; Smith, 1973), although these generally constitute <1% of the sill.

The ore comprises 30 to 50% Fe mainly as magnetite, with 1 to 2% S, predominantly pyrite and chalcopyrite, with 15% silica as actinolite, diopside, phlogopite, chlorite, serpentine and talc. Minor amounts of calcite, dolomite, and ankerite are locally present. The ores are low in titanium, but with notable copper and cobalt in some. Gold and silver were also recovered from sulphide concentrates.

The Cornwall mine is located at the northern edge of the Triassic basin. The basement Palaeozoic sedimentary rocks are strongly deformed Cambrian sandy, cherty and shaly limestones with interbedded dolomites (Lapham and Gray, 1973). In the vicinity of Cornwall, these rocks are unconformably overlain by the Triassic Hammer Creek Formation continental red sandstones and shales sequence. Thin slices of the Palaeozoic(?) Blue Conglomerate and Mill Hill Slate are locally thrust(?) over the Cambrian carbonates and are unconformably overlain by the Triassic sedimentary rocks.

The Triassic sediments may have been down faulted against the Palaeozoic rocks, in the Cornwall mine area, although the true nature of the contact has been obscured by the dolerite sheet emplaced into the Triassic sedimentary rocks in Late Triassic or Early Jurassic time. This sill was intruded along the northern edge of the basin, locally extending down into the Palaeozoic sequence, truncating a wedge of thinly interlayered sandy limestone and dolomite of the Cambrian Buffalo Springs Formation above its upper margin (Lapham and Gray, 1973).

The magnetite-chalcopyrite ore formed as a replacement of the limestone overlying the dolerite sill. The two major orebodies at Cornwall are separated by a rise in the upper contact of the dolerite sheet. The western body is tabular, with a strike of 1300 m, extending down-dip for 500 m, averaging 30 m in thickness. It was discovered in outcrop in 1732, and mined by open pit and underground from 1732 to 1973. The eastern orebody is about 800 m to the east, and was blind, extending ~700 m along strike and 800 m down-dip, locally up to 80 m thick, averaging 30 m, mined by underground workings.

Both orebodies comprise mixtures of magnetite, chalcopyrite and pyrite in a gangue of diopside, actinolite, phlogopite and chlorite, occurring as near complete replacement of the carbonate host rocks. The silicates and magnetite frequently occur as up to about 1 cm in thick alternating layers. Locally, banded ore can be traced into banded carbonate at the ore margins, where magnetite-rich layers appear to have replaced the calcite-rich limestone, and the silicate-rich layers the more silica-rich laminae. The ore overlies the dolerite along the footwall of both orebodies. The ore is in contact with marble (recrystallised limestone) along a sharp replacement front in the hanging wall of the western orebody. The marble, in turn, grades upwards into less recrystallised limestone a few tens of metres above the ore.

The hanging wall of the eastern orebody is formed by Blue Conglomerate and Mill Hill Slate. A NE-trending fault with ~50 m of offset cuts the dolerite and hanging-wall rocks between the two orebodies. The paragenesis of the orebody commenced with coarse diopside, minor garnet, epidote and tremolite formed early, in part by simple thermal metamorphism (Lapham and Gray, 1973). This assemblage is most common and best developed near the dolerite contact and is preserved in some ore as scattered grains in a halo of marble surrounding the ore. The later metasomatic phase (phlogopite, fine diopside and coarse actinolite) replace the early silicates and are, in turn, replaced by fine actinolite and chlorite which accompany magnetite, pyrite and chalcopyrite of the ore stage. Local minor talc and serpentine occur in marble and ore near their contact.

Minor zeolites and other minerals are postore. Clear consistent age relations are not common within the metasomatic stage, where there is considerable overlap of mineral ages. Mineral zoning within the ore is not well developed and only partly correlates with the age relations. Diopside was most abundant near dolerite in the west orebody, and magnetite was best developed toward the limestone (Hickok, 1933), although diopside, actinolite and magnetite were all apparently present throughout the ore, varying in abundance but without apparent mineral assemblage zoning.

The total exploited resource amounted to ~100 Mt @ 40% Fe, 0.3% Cu, 1.3% S (mainly as pyrite with by-product Co) from the Cornwall and Morgantown (Grace) mines, as well as 45 smaller occurrences distributed over a 150 x 20 km SW-NE trending zone.

Other mines in the belt include: French Creek, Boyertown, Wheatfield, Ruth, Fritz Island, Jones, Doner, Raudenbush, Grantham, Esterly and Dillsburg.

This summary is largely paraphrased from Rose et al., 1985.

The most recent source geological information used to prepare this summary was dated: 1985.     Record last updated: 24/8/2013
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:
Rose A W, Herrick D C, Deines P  1985 - An Oxygen and Sulfur isotope study of Skarn-type Magnetite deposits of the Cornwall type, southeastern Pennsylvania: in    Econ. Geol.   v80 pp 418-443

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