GEOCHEMICAL EVOLUTION OF THE PRECAMBRIAN OLD RAG GRANITE, VIRGINIA, USA - TESTING A U-TH EXPLORATION MODEL

The Old Rag Granite is one of several recognized units within the Virg inia Blue Ridge Complex generally composed of Grenville-age gneisses a nd granitoid intrusive rocks. Chemical variation in the Old Rag Granit e indicates simple orthomagmatic crystallization from the borders to t he core of the pluton. Fractional crystallization of plagioclase, oxid es, apatite and zircon accounts for much of the variation. Some trace elements, however, show anomalous behavior in the central part of the pluton. Y and Sr (and possibly Ba and Nb) are enriched whereas Rb and total REE are depleted in the central part of the pluton. Late crystal lization of monazite could contribute to the observed REE patterns. It could not, however, account for Rb depletion and Sr enrichment, and n o clear relationship between REE data and Th/U ratios are observed. Th e geochemical patterns indicate overall orthomagmatic crystallization overlain by a late-stage hydrothermal event. An exploration model for uraniferous granites, based upon the uranium deposits at Rossing and B okan Mountain, included the Precambrian basement of central and northe rn Virginia in a list of proposed target areas. Geological, mineralogi cal and whole-rock chemical characteristics show a close association b etween the Old Rag Granite and the Rossing-type deposits. The uranium (mean 6.9 ppm, range 0.1-19.8 ppm) and thorium (mean 43 ppm, range 3.5 -114 ppm) values are anomalous. Th/U ratios average 7.5 and are widely variable, indicating a decoupling of uranium from thorium. Neither U nor Th are lognormally distributed within the Old Rag Granite, requiri ng an explanation beyond simple removal of U by weathering. Rayleigh f ractionation modeling shows that both U and Th, although probably dist ributed originally by closed-system fractionation, have been subsequen tly mobilized. Uranium can be lost in an oxidized form by secondary pr ocesses such as weathering or hydrothermal alteration. Thorium, howeve r, win be affected only by hydrothermal processes. It is concluded tha t although primary fractionation was orthomagmatic, the Old Rag Granit e was affected by late hydrothermal alteration whereby U was lost from the body and Th was redistributed. Three primary avenues for U migrat ion are possible: (1) the rocks of the Saddleback Mountain Intrusive S uite which may have been contemporaneous with the Old Rag Granite; (2) Paleozoic fault zones that cut the Old Rag pluton; and (3) quartz vei ns, pegmatites, cupolas and roof pendants of the country-rock Nellysfo rd gneiss that have been lost to erosion. The last option is preferred on geological and geochemical grounds. It is possible that the sedime ntary uranium deposits of the adjacent Triassic Culpepper Basin were d erived from eroded upper levels of the Old Rag Granite and like intrus ives of the Virginia Blue Ridge Complex.