Chromite in Lower Pennsylvanian Nuttall Sandstone from West Virginia--Possible Blue Ridge Province Source

J. T. O'Connor

Detrital chromite grains from heavy mineral separates of three samples of the Lower Pennsylvanian Nuttall Sandstone Member of the New River Formation were examined by use of a reflected-light petrographic microscope, a scanning electron microscope, and an electron microprobe. Some textures observed microscopically in the chromite grains are similar to those in chromite grains in dunite from the Blue Ridge province of North Carolina that has undergone regional metamorphism.

Additional exsolution features are also observed in the chromites of this study. The major-oxide compositions of the detrital chromite grains vary more widely than those of the chromites of the Blue Ridge, possibly due to a larger sampling base. The detrital grain compositions [Cr/(Cr + Al), Mg/(Mg + Fe⁺²)] plot near the fields of the Blue Ridge chromites, although the appearance of both more aluminum-rich and magnesium-poor compositions possibly indicate a higher facies of metamorphism than the Blue Ridge samples studied to date.

Paleocurrent directions in the Nuttall Sandstone Member in the area of sample collection indicate that the source area for the sandstone could include part of the Blue Ridge province. Trace-mineral identification in the heavy mineral suite of the sandstone includes some of the accessory minerals (tourmaline, zircon, kyanite, garnet) of the rocks surrounding the chromite-bearing Blue Ridge ultramafic bodies. The writer suggests that the source of the Nuttall Sandstone Member included, among others, rocks equivalent to the dunites from the Blue Ridge province. The absence of feldspar and other major components of the surrounding Blue Ridge host rocks in the Nuttall Sandstone Member implies that the weathering regime at the time of deposition, leaving only the most resistant detritus to e supplied to the Appalachian basin in original form, was very pervasive.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.