The Mid-Atlantic Iron Oxide Belt - Geological Background

Kurt Friehauf - Kutztown University

Four major geologic events occurred in the Mid-Atlantic region that would have been conducive to the formation of geothermal waters:  formation of granitic magmas related to the Grenville mountain-forming event (1.1 billion years ago), the Catoctin rifting event that broke apart the Supercontinent (roughly 700 million years ago), the Roberts Mountain rifting event (525 million years ago), and/or the Jurassic/Triassic rifting event that broke apart the Supercontinent of Pangea, forming the Atlantic ocean (200 million years ago) (see also my summary of the geologic history of Pennsylvania).  While each of these geological events is capable of creating geothermal waters that could form the iron deposits, each event would have created a very distinctly different geological environment and so form iron deposits with different characteristics.

Foose-McLelland modelThere are several potential sources of hot waters capable of forming iron deposits within a given geologic environment.  Many geothermal waters bubble directly off bodies of molten rock (magma), flowing out into fractures in the surrounding rocks and depositing dissolved metals.  Granites are particularly well known for doing this (e.g., the giant rare-earth-gold-iron deposit at Olympic Dam, Australia) and if the Pennsylvania iron ores are related to a similar granite, there would be potential for the occurrence of other valuable elements such as the rare earth elements and gold in the old mines.  Some workers studying the iron ores in limestones at Cornwall have proposed that iron-rich magmas (basalts) may have been the source of the hot, mineralizing waters.

Rose, Barton, and Johnson modelAlternatively, Barton and Johnson argue that hot, salty, iron-rich geothermal waters can form by magmatic heating of seawater, pore waters trapped within sedimentary rocks, or evaporative brines.  Such a water source is currently observed deep within the ocean at sea floor hot springs, such as those in the middle of the Dead Sea.  Any of the three rifting events could have created a similar geological environment.

 The age of mineral deposition bears strongly on the potential validity of each model.  This project proposes to use geologic mapping to document characteristics of the alteration and constrain the age of iron metasomatism within the Reading Prong and adjacent basins.  From this knowledge we can evaluate the three principle hypotheses for the origins of the metasomatic components.

Lapham modelLapham (1968) suggested exsolution of a hydrothermal fluid during the latest stages of differentiation of the Triassic diabase intrusions introduced the metasomatic components at Cornwall, Pennsylvania.  Magnetite ore and associated hydrothermal alteration cutting the diabase at the Cornwall and Grace (Pennsylvania) mines demonstrate a Triassic or younger age.  Rose et al. (1985) argued that the known volumes of diabase could not account for the mass of fluids and iron introduced at Cornwall, nor the stable isotope compositions of sulfur- and oxygen-bearing minerals in the deposit.  They proposed that circulation of connate brines heated by diabase intrusions may have leached metals from large volumes of rock and precipitated them in favorable geochemical environments.  Barton and Johnson (1996) added that involvement of evaporites or saline waters derived from arid, restricted rift basins of the Triassic would create higher salinity brines, thereby increasing the efficiency of leaching and transportation.

The ages of iron deposits in Proterozoic rocks of the Reading Prong such as the Rittenhouse Gap deposit, however, are unconstrained, making their origins more difficult to determine.  Foose and McLelland (1995) noted similarities in the mineralogy, metal association, age and composition of granitic host rocks, and style of metasomatism between deposits in New York and New Jersey and Olympic Dam-type deposits elsewhere in the world.  They concluded that the mineralizing fluids may have been related to the granites, forming deposits over a wide range of crustal levels in the late Proterozoic during and slightly post-dating peak metamorphism.  Robert Smith of the Pennsylvania Geological Survey suggests these deposits may be related to the 700 Ma rifting event (i.e., Mount Rogers sequence) or the 570 Ma rifting event (i.e., Catoctin sequence) (Pers. Comm., 1998).  

The Rittenhouse Gap iron deposit in eastern Berks County occurs within Proterozoic gneiss and a cross-cutting, undated, albite-rich, quartz-phyric felsic dike.  Smith reports sodic pyroxene in the felsic dike.  Although Smith interprets this mineralogy to represent the primary mode of the dike, sodium metasomatism forming aegirine and albite in less exotic igneous rocks is common in the Triassic Cornwall-type deposits.  Alteration of the gneiss remains undocumented.  The occurrence of the Rittenhouse Gap deposit within the well-defined belt of magnetite deposits that includes the Triassic Grace and Cornwall districts, the nearby association of sulfide- and arsenide-bearing deposits, and the coincidence of a parallel belt of small limonite (oxidized sulfide?) deposits in the Cambrian-Ordovician sedimentary rocks to the northwest suggests the deposit may be related to the Triassic rift basin system.  Evidence of a r elationship between gneiss-hosted magnetite deposits and the limonite deposits in nearby Paleozoic rocks would preclude the Proterozoic model for iron metasomatism in the Reading Prong and support a Triassic-only model.  If this is true, then the absence of Triassic diabase in the Reading Prong deposits would  discount the Lapham et al. model calling on diabase as a source of hydrothermal fluids in the belt and support the Rose et al./Barton and Johnson models.

Finally, the great number of small deposits in the region begs the question of whether the deposits are related to one another by a common process or common very large-scale event.

Mapping and microscopic analysis of the rocks in this project hopes to help answer this question.
By comparing the hydrothermal alteration, mineralogy, and whole-rock geochemistry of magnetite deposits in Precambrian granites with iron oxide deposits in the adjacent Paleozoic sedimentary rocks, we hope to evaluate which geologic event formed the iron deposits.  

An integrated geologic and geochemical examination of granite-hosted hydrothermal iron (-Cu-Au±Co±U±REE) deposits in Pennsylvania, New Jersey, and New York will help constrain models of hydrothermal activity responsible for their formation.

on to Phase 1 - Rittenhouse Characterization

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Barton, M.D., and Johnson, D.A., 1996, Evaporite-source model for igneous-related Fe oxide-(REE-Cu-Au-U) mineralization: Geology, v. 24, no. 3, p. 259-262.

Foose, M.P., and McLelland, J.M., 1995, Proterozoic low-Ti iron-oxide deposits in New York and New Jersey – Relation to Fe-oxide (Cu-U-Au-rare earth element) deposits and tectonic implications:  Geology, v. 23, no. 7, p. 665-668.

Friehauf, Kurt C., Smith, Robert C., II, and Volkert, Richard A., 2001, Regional Trends in the U.S. Mid-Atlantic Proterozoic Iron Oxide Belt [abs]: Geological Society of America Abstracts with Programs, vol. 33, no. 6, p. 4.

Hitzman, M.W., Oreskes, N., and Einaudi, M.T., 1992, Geological characteristics and tectonic setting of Proterozoic iron oxide (Cu-U-Au-REE) deposits:  Precambrian Research, v. 58, p. 241-287.

Lapham, D.M., and Gray, C., 1972, Geology and origin of the Triassic magnetite deposit and diabase at Cornwall, Pennsylvania:  Pennsylvania Topographic and Geologic Survey Mineral Resources Report M56, 343 pp.

Rose, A.W., Herrick, D.C., and Deines, P., 1985, An oxygen and sulfur isotope study of skarn-type magnetite deposits of the Cornwall type, Southeastern Pennsylvania: Economic Geology, v. 80, no. 2, p. 418-443.