MYLONITIZATION OF PRE-CAMBRIAN GNEISS AND IRON ORE AT THE RITTENHOUSE GAP MINES, LONGSWAMP TOWNSHIP, BERKS COUNTY, PENNSYLVANIA

While exploring the Rittenhouse Gap Mines in the Grenville age gneisses of the Reading Prong geologic province, I noticed that many samples of magnetite and hematite ore, along with the host gneiss have been tectonically changed to mylonite. Mylonitization varies from shearing, barely noticeable in thin section, through protomylonite to ultramylonite.

The mylonite zones are very narrow, rarely exceeding 20 centimeters in width. Mylonite zones average about nine centimeters thick. No large masses of mylonite exist at the Rittenhouse Gap Mines. There is usually an abrupt change from normal gneiss to mylonite seen here.

Failure of the rock and ore first occurred in the quartz, magnetite ore. The feldspathic gneiss is much more resistant to mylonitization. Thin sections of the gneiss show a progression in deformation from micro faulting and bending of plagioclase feldspar crystals in otherwise normal looking gneiss through ultramylonite. Ribbon quartz is seen in thin sections of samples of ore from the vicinity of the Weiler Mine. Quartz deformed duct ally while magnetite was elongated and pulled apart. Mylonites from all other parts of the mining district are recrystallized as mostly quartz renucleated in response to tectonic conditions. Minerals like hornblende, feldspar and magnetite show little recrystallization and deformed in a more brittle manner than quartz.

Felsite dikes cross cut the gneiss, ore and mylonite. These dikes exhibit brittle deformation compared to the older ductile deformation of the rocks they intrude.

These mylonites show a more ductile deformation than mylonite samples I have seen from the nearby Siesholtzville Mines. The Siesholtzville mylonites are derived from calcareous gneiss and marble. They also appear to have been deformed under lower confining pressures and temperatures than the Rittenhouse Gap mylonites.