Skarn-type magnetite deposits of the Cornwall-type replace limestone, dolomite, and marble adjacent to York Haven Diabase (gabbro) sheets of Jurassic age in southeastern PA. The deposit at Cornwall, PA produced 106 x 10⁶ tons of ore containing about 40% Fe, 0.3% Cu, 1.3% S, 0.001 oz/T Au, and pyrite with 1.4% Co. A second large deposit near Morgantown produced similar ore containing less Cu and Co, but more pyrrhotite, sphalerite, and galena. Typical gangue is early diopside, phlogopite and garnet followed by actinolite, chlorite, talc, serpentine, pyrite, chalcopyrite and abundant magnetite. Depositional temperatures for most magnetite are estimated at 400-500 C based on phase stability. About 45 smaller deposits are recorded in PA, but none in adjacent states. The deposits occur in a segment of Triassic rifts that developed in early Paleozoic basins, as contrasted to rifts in crystalline rocks elsewhere along the eastern US zone. Most deposits overlie diabase, but a few small deposits underlie diabase. Very high ¹⁸O values for magnetite and actinolite indicate a fluid with much higher ¹⁸O than a simple magmatic hydrothermal fluid from the diabase. Sulfur isotopes are also markedly heavier than magmatic sulfides. High content of Cl in hydrous alteration silicates suggests high-Cl in the hydrothermal fluid. Formation was from an originally sedimentary brine, heated locally in sediments along the diabase contact to above 400 C. This brine may have been pre-heated regionally by a thermal pulse not directly associated with diabase to more than 275 C. Metals are believed to have been acquired from both the diabase and Triassic sediments.