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 Virginia Blue Ridge Complex generally composed of Grenville-age gneisses and granitoid intrusive rocks. Chemical variation in the Old Rag Granite indicates simple orthomagmatic crystallization from the borders to the core of the pluton. Fractional crystallization of plagioclase, oxides, 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 crystallization of monazite could contribute to the observed REE patterns. It could not, however, account for Rb depletion and Sr enrichment, and no clear relationship between REE data and Th/U ratios are observed. The 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 Bokan Mountain, included the Precambrian basement of central and northern Virginia in a list of proposed target areas. Geological, mineralogical and whole-rock chemical characteristics show a close association between 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, requiring an explanation beyond simple removal of U by weathering. Rayleigh fractionation modeling shows that both U and Th, although probably distributed originally by closed-system fractionation, have been subsequently mobilized. Uranium can be lost in an oxidized form by secondary processes such as weathering or hydrothermal alteration. Thorium, however, win be affected only by hydrothermal processes. It is concluded that although primary fractionation was orthomagmatic, the Old Rag Granite was affected by late hydrothermal alteration whereby U was lost from the body and Th was redistributed. Three primary avenues for U migration are possible: (1) the rocks of the Saddleback Mountain Intrusive Suite which may have been contemporaneous with the Old Rag Granite; (2) Paleozoic fault zones that cut the Old Rag pluton; and (3) quartz veins, pegmatites, cupolas and roof pendants of the country-rock Nellysford gneiss that have been lost to erosion. The last option is preferred on geological and geochemical grounds. It is possible that the sedimentary uranium deposits of the adjacent Triassic Culpepper Basin were derived from eroded upper levels of the Old Rag Granite and like intrusives of the Virginia Blue Ridge Complex.