Geochemistry and origin of Neoproterozoic ironstone deposits in the
New Jersey Highlands and implications for the eastern Laurentian rifted
margin in the north-central Appalachians, USA
Volkert, R.A.,
Monteverde, D.H., Friehauf,
K.C., Gates, A.E., Dalton, R.F., and Smith,
R.C., II, 2010, Geochemistry and origin of Neoproterozoic ironstone
deposits in the New Jersey Highlands and implications for the eastern
Laurentian rifted margin in the north-central Appalachians, USA, in
Tollo, R.P., Bartholomew ,
M.J., Hibbard, J.P., and Karabinos, P.M.,
eds., From Rodinia to Pangea: The
Lithotectonic Record of the
Appalachian Region: Geological Society of America Memoir 206, p.
283–306.
Abstract
Small deposits of
Neoproterozoic ironstone in the New Jersey Highlands are hosted by the
Chestnut Hill Formation, a terrestrial sequence of siliciclastic rocks,
sparsely preserved felsic and mafic volcanic and tuffaceous rocks, and
thin limestone metamorphosed at greenschist-facies conditions.
Sediments of the Chestnut Hill Formation were deposited in alluvial,
fluvial, and lacustrine environments in a series of fault-bounded
subbasins along the Iapetan eastern Laurentian margin. Ironstone occurs
mainly in the upper part of the sequence in sandstones, quartzites,
fine-grained tuffs, tuffaceous sediments, and carbonate-bearing beds.
Ore is massive to banded and contains the assemblage hematite ±
magnetite, which is locally associated with tourmaline and Fe-silicates
+ sericite + calcite + chlorite ± quartz. Ironstone alternates
with clastic bands, and sedimentary structures in ore bands and clastic
bands are consistent with alternating chemical and clastic
sedimentation deposited synchronously.
Chestnut Hill rocks exhibit geochemical compositions that are
dissimilar to typical sedimentary and volcanic rocks. They display
evidence for two stages of postdiagenetic alteration. The first stage
involved widespread potassium metasomatism, which produced increased
values of K, Ba, and Rb that are not correlated with increased Fe or
other hydrothermal elements. The metasomatizing fluid may have been
basinal water heated during emplacement of Chestnut Hill volcanic
rocks. The second stage produced alteration of Chestnut Hill rocks, and
also Mesoproterozoic rocks along the footwall contact of the deposits,
by hydrothermal fluids likely from a volcanogenic source.
The ironstone deposits were formed by hydrothermal processes related to
extension during formation of continental rift subbasins in the New
Jersey Highlands. Iron was sourced from Fe-rich Mesoproterozoic rocks
at depth, where it was leached by hydrothermal fluids that migrated
upward along extensional faults. Iron and other metals were
precipitated in permeable basin sediments and chemically favorable
volcanic rocks, as well as precipitated directly as chemical sediment.
