Geology of the Piney River-Roseland titanium area, Nelson and Amherst counties, Virginia
The titanium deposits of Nelson and Amherst counties, Virginia, occur in Precambrian (?) rocks that constitute part of the core of the Blue Ridge-Catoctin Mountain anticlinorium. Approximately 72 square miles were mapped in this study. The central part of the mapped area is underlain by a mass of pegmatitic anorthosite, about which other mappable rock units are distributed more or less peripherally. The chief units, listed from oldest to youngest, are: augen gneiss, biotite pencil gneiss, biotite aplitic gneiss, granitic gneiss, feldspathic gneiss, hypersthene granodiorite, pegmatitic anorthosite, and nelsonite.
The pegmatitic anorthosite occurs as sills and dike-like bodies. It originally consisted of coarse-grained, antiperthitic plagioclase (An₃₀). Most of the primary textures in this rock have been obliterated by alteration so that the present rock consists of saussuritized feldspar and minor amounts of altered mafic minerals, plus introduced or recrystallized quartz, rutile, and ilmenite. The mafic minerals include tremolite or anthophyllite, in complete or partial pseudomorphs after coarse-grained pyroxene crystals, and abundant alteration halos of biotite and chlorite.
Most of the rocks have a well developed, generally northeasterly striking, southeasterly dipping gneissosity. The rocks were deformed prior to and after alteration and mineralization. Layered structures in hypersthene granodiorite suggest that the rocks have a domal arrangement. A low angle fault in the northeast part of the mapped area apparently resulted in thrusting of the augen gneiss over part of the pegmatitic anorthosite.
Most of the rock types are believed to be of igneous origin, although the augen gneiss may be all or in part metasedimentary. The pegmatitic anorthosite and the hypersthene bearing rocks are believed to be comagmatic.
Most of the titanium occurs as ilmenite in ilmenite nelsonite bodies and disseminated in highly altered rocks adjacent to the pegmatitic anorthosite. Lesser amounts of rutile occur disseminated in relatively pure but altered pegmatitic anorthosite, in rutile nelsonite end in rutile-bearing quartz veins. The titanium deposits are associated with zones of intense alteration characterized by the development of chlorite, biotite, and amphiboles from mafic minerals in the wall rock, and by saussuritization of the feldspars. Evidence indicates that most or all of the deposits formed by replacement of the wall rock.
Titanium, fluorine, phosphorus, water and minor carbon dioxide were added to the wall rocks during alteration and mineralization. The iron-titanium. ratio increases outwardly from the central pegmatitic anorthosite. The original mineralizing fluids may have acquired iron from alteration of the wall rocks. Although the mineralizing fluids may have been derived by differentiation of the same magma from which the hypersthene granodiorite and pragmatic anorthosite were derived, the mineralization was later than the crystallization of the relatively titanium-rich wall rocks.
The purer pegmatitic anorthosite is quarried and ground principally for use in the glass industry. Reserves are probably large, but the discontinuity of the pure feldspar rock units demands that each prospective quarry site be drilled thoroughly to determine the quality and extent of the feldspar.
A conservative estimate places the reserves of TiO₂ at approximately 12 million tons. Only weathered deposits of ilmenite, at Piney River and the Wood property, are being mined at present, but some of the dike-like ilmenite nelsonite bodies and the disseminated rutile deposits are of present-day ore grade.
Areas of intensely altered rocks near or adjacent to the border of the pegmatitic anorthosite should be investigated further so far as their containing economically recoverable titanium.