Contact metasomatic magnetite ores of the Cornwall type in Pennsylvania occur as replacements of carbonate rocks adjacent to sheets and dikes of Triassic diabase. In addition to large (10 8 tons) magnetite deposits at Cornwall and Morgantown (Grace mine), about 45 smaller occurrences are known in a zone 150 km long. Major gangue minerals include early diopside, phlogopite, and garnet followed by actinolite, ohiorite, talc, serpentine, pyrite, chalcopyrite, and pyrrhotite associated with abundant magnetite. Based on mineral assemblages, temperatures of deposition are inferred to be less than 500 degrees C for most of the magnetite.At Cornwall, 50 samples of magnetite have delta 18 O between 5.2 and 11.0 per mil (SMOW), with the highest values (mean (x) delta 18 O = 9.3ppm, standard deviation (Sigma ) = 0.66ppm, n = 19) adjacent to the diabase and the lowest values (x = 6.6ppm, Sigma = 0.69ppm, n = 14) near replaced marble. Thirty-five magnetite samples from 19 other Cornwall-type deposits fall within a similar range (x = 7.9ppm, Sigma = 1.97ppm). Silicates from the Cornwall deposit also show unusually high delta 18 O (13 actinolites, x = 12.4ppm, Sigma = 0.47ppm; 6 phlogopites, x = 12.3ppm, Sigma = 0.73ppm; 3 diopsides, x = 13.1ppm, Sigma = 0.19ppm). At Cornwall, 36 samples of pyrite and chalcopyrite have delta 34 S (CDT) of 6.3 to 19.9 per mil (x = 10.9ppm, Sigma = 2.91ppm).Calcites from the ore (delta 13 C = -5.9ppm (PDB), Sigma = 0.93ppm, delta 18 O = 17.5ppm, Sigma = 0.45ppm, n = 4) are distinctly depleted in both 18 O and 13 C relative to the unaltered host limestones (delta 13 C = -1.3ppm, Sigma = 0.44ppm, delta 18 O = 20.6ppm, Sigma = 0.68ppm, n = 15), which are similar to other Cambrian limestones in the literature. Marbles adjacent to ore are intermediate in 13 C and 18 O between carbonates in ore and unaltered limestones. Thermally metamorphosed marbles adjacent to diabase but away from ore are depleted only in 13 C relative to unaltered limestones.The magnetite, silicate, and carbonate minerals in the skarn-type ore in all 20 deposits are calculated to have formed in fluids with delta 18 O (sub H 2 O) of 13 to 16 per mil. The very high delta 18 O of the fluid is difficult to explain by conventional ideas on a magmatic origin for skarnforming fluids. The diabase, which has a normal igneous delta 18 O of 7.4 to 7.8 per mil, would have furnished fluids with delta 18 O of 7 to 10 per mil and therefore camlot be the direct source of the ore fluids.The preferred explanation of the 18 O-rich skarn is that an 18 O- and 34 S-enriched ore solution was formed by heating and circulation of meteoric, connate, or possibly magmatic waters in sedimentary rocks near the contact of the diabase. Exchange of 18 O with shales, argillaceous sandstones, or limestones at high temperatures created the 18 O-rich fluid. This fluid, upon encountering carbonate in the contact zone of the diabase, replaced it with magnetite and silicates. The relatively high Cu and Co contents of the Cornwall ore suggest that the ore fluid either contained a magmatic component or interacted with diabase to some extent. Zoning of 18 O away from the contact appears to result from decreasing temperature of deposition of the skarn farther from the contact.An alternative explanation is that the high delta 18 O values of magnetite and silicates were determined by the high delta 18 O values of the replaced carbonate in a diffusion-controlled process. In a diffusion-controlled process, the amount of water flowing through the replacement front might be negligible compared to the flux of Fe and other dissolved species. The presence of delta 18 O-rich alteration within the diabase and the lack of correlation of delta 18 O-rich of ore with delta 18 O of replaced carbonate indicate that this process is probably not the dominant one causing the high delta 18 O, but it might contribute to the range of values observed.