Uranium-series analysis (238U--234U--230Th) of 13 calcite veins from the hydrothermally altered Madera Limestone in the VC-1 drill core was performed to determine the ages of the veins and their relation to the Valles hydrothermal system. Thermal water from VC-1 and two hot springs in San Diego Canyon was analyzed for U and (234U/238U) to help evaluate the constancy of initial (234U/238U). The (230Th/234U) age of one of the veins is ~95 kyr, and those of two other veins are ~230 and ~250 kyr. Five of the veins have near equilibrium (230Th/234U) and are probably older than ~0.3 m.y. Uranium concentrations in the remaining veins are too low for analysis by the α-spectrometry techniques employed in this study. Of the five veins near (230Th/234U) equilibrium, suggesting ages greater than ~1.0 m.y., but one has (234U/238U)=1.15, suggesting an age between ~0.3 and ~1.0 m.y. Calculated initial (234U/238U) of the veins yielding relatively young ages are neither equal to each other nor to (234U/238U) in thermal water from VC-1, indicating inconstancy of initial (234U/238U) tht may be related to variations in groundwater mixing proportions. Three of the four veins that yield relatively young ages consist of coarse, sparry, vuggy calcite, suggesting that this may be the type of calcite vein which forms under conditions resembling those encountered presently in VC-1. The analytical data are consistent with closed-system behavior of U and Th in the VC-1 calcite veins. ¿ American Geophysical Union 1988