The Mid-Atlantic Iron Oxide Belt -
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.
There 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.
and Johnson argue that hot, salty, iron-rich geothermal waters can
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
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 (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
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
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
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.
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.
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.