Geology
of the Rittenhouse Gap Iron District, Berks County,
Pennsylvania
Orner,
Richard, and Friehauf, Kurt C.,
2000, Geology
of
the
Rittenhouse
Gap
Iron District, Berks County, Pennsylvania [abs]: Geological
Society of America Abstracts with Programs, v. 32, p. 83.
The
Rittenhouse Gap District occurs within the belt of magnetite
deposits that includes the Cornwall, Grace, and French Creek
districts of Pennsylvania. Unlike the larger Triassic sediment- and
diabase-hosted deposits, ore at Rittenhouse Gap occurs within
Precambrian granite and gneiss of the Reading Prong. Field mapping
at Rittenhouse Gap documents magnetite ore occurring as a series of
20-40 cm wide veins along a steeply dipping, N35E trending contact
between granite and granitic gneiss. Individual vein segments have
strike lengths over 110 m. Host rocks are intensely albitized and
lack biotite and accessory magnetite. Magnetite mineralization is
associated with quartz, biotite, and pyroxene, both within the veins
and as disseminations within one meter of the vein, suggesting
K-metasomatism related to magnetite deposition postdated
albitization at this level of exposure. Several basalt and felsite
dikes crosscut magnetite mineralization. Although they postdate
magnetite mineralization, the basalt is intensely chloritized and
minor fluorite, molybdenite and specular hematite veinlets cut the
felsite, suggesting possible continued hydrothermal activity.
Age constraints on mineralization at Rittenhouse Gap remain
unresolved at this time. The similar association of mineralization
with alkali exchange metasomatism and the occurrence of the deposit
within the Cornwall-Grace belt suggests the Rittenhouse Gap deposit
may be related to the Triassic rifting and mineralization event. If
so, the absence of Yorktown-type diabase at Rittenhouse Gap would
disfavor a genetic requirement of diabase for the formation of
Cornwall-type magnetite deposits. In spite of these similarities
with the Triassic system, a Precambrian origin of the deposits
cannot be ruled out.