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Geology of the Rittenhouse Gap Iron District, Berks County, Pennsylvania

Orner, R.J., and Friehauf, K.C., Kutztown University, Kutztown, PA 19530 Geological Society of America Abstracts with Programs, Reno, NV, November 14-16, 2000, v. 32, no. x, p. Axx.

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.

November 14-16, 2000 Kurt C. Friehauf