MICROSTRUCTURES RECORDING BRITTLE OVERPRINT ON DUCTILE STRUCTURES IN THE FOOTWALL OF THE CHATHAM FAULT - STRUCTURAL CONTROL ON THE COLES HILL URANIUM DEPOSIT, GRETNA, VA

The Coles Hill uranium deposit, thought to be the largest unmined uranium deposit in North America, lies in complexly deformed orthogneiss in the footwall of the Triassic Chatham fault in southern Virginia. The Chatham normal fault, at the eastern edge of the western Piedmont, separates the Triassic Dan River Basin to the east from crystalline rocks of the Smith River allochthon to the west. The deposit is hosted in fractured and deformed Leatherwood granite and Rich Acres gabbro-diorite. The ore body is bounded above by the Chatham fault and below by unfractured granitic gneiss. The following units are recognized at progressively deeper structural levels in drill core: (1) Triassic sandstone and conglomerate; (2) weakly lithified fault gouge; (3) a silicified zone characterized by pervasive brittle deformation; (4) an ore-bearing zone consisting of altered, sodium-metasomatized, heterogeneously-fractured granitic gneiss and amphibolite; and (5) a barren zone of unfractured, ductilely deformed granitic gneiss and amphibolite. Uranium mineralization includes U-bearing apatite and U silicates and oxides. The ore minerals are concentrated in dilational breccia zones, in two distinct microstructural modes that developed during this main ore-forming phase: (1) irregular linear vein-like features containing both neoblastic minerals and comminuted host mineral grains; and (2) pervasive brecciation on a thin section scale, with mineralization occurring in the breccia interstices. Tectonically-induced stylolites that post-date the main ore phase intersect the zones of dilational deformation and can serve to further concentrate the ore, especially the U oxides and silicates.