Geology of the Rittenhouse Gap Iron
District, Berks County, Pennsylvania
Friehauf, Kurt C.,
Gap Iron District, Berks County,
Pennsylvania [abs]: Geological Society of America Abstracts
Programs, v. 32, p. 83.
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.