Mississippian limestone wallrocks surrounding a low-temperature ore body in the Hicks dome area of southern Illinois were dolomitized, recrystallized, and silicified, and their delta O 18 and delta C 13 values were lowered during ore deposition.The rocks studied are adjacent either to a major body of slump breccia or to fissures and vugs. The effects of hydrothermal alteration are megascopically inconspicuous, even though the rocks were greatly modified. Alteration occurred during two episodes of hydrothermal activity, each of which is correlated with structural events and with ore deposition. The earlier alteration is associated with a late stage of slumping that was due to dissolution of limestone and with the earliest stage of ore deposition; and during this time limestone was converted to dolomite, and the delta O 18 values of the rocks were uniformly reduced from about 26ppm to about 21ppm (SMOW), while delta C 13 values remained constant at 2ppm (PDB).The later alteration is associated with a later stage of ore deposition during which additional limestone was recrystallized and silicified and the delta O 18 and delta C 13 values of the rock were reduced to values as low as 18ppm and --4ppm, respectively. The degree of recrystallization and silicification and the amount of isotopic exchange increase toward fissures or other channelways that were conduits for hydrothermal solutions. In one section of the ore body the delta O 18 values of gently dipping beds of limestone, adjacent to fissures, decrease along the probable direction of movement of the hydrothermal fluids through the ore body.Isotopic exchange between limestone and the altering fluid probably occurred during recrystallization. The extent of the isotope halo appears to have been limited by the extent of recrystallization. The minimum amount of water required to produce the observed changes in the delta O 18 values in a prism with a cross-sectional area of 1 cm 2 extending through the alteration halo in a selected bed is 46 moles, or 830 grams, corresponding to a volumetric water-to-rock ratio of about 0.8. However, considerations based on the amount of introduced quartz and the amount of C 13 exchanged in the wallrock indicate that a much larger amount of fluid actually passed through the wallrocks. The temperature of the hydrothermal fluid during alteration is estimated to have been 130 degrees C, as indicated by filling temperature measurements of fluid inclusions in associated fluorite. The delta O 18 of the hydrothermal pore fluid is calculated at 4.7, 2.9, and 14ppm from delta O 18 values for calcite, dolomite, and quartz, respectively, in the most altered parts of the wallrock.O 18 halos in limestone could be the result of (1) a temperature gradient, or (2) a gradient in the isotopic composition of the pore fluid in the wallrock, or (3) partial exchange between the pore fluid and the wallrocks. Gradients of temperature and isotopic composition may have had some effect on the formation of the isotopic patterns in the Hill mine, but the dominant effect was related to partial exchange.