2 Isotopic and geochronologic investigation of authigenic, K‐bearing clays in the Appalachian Orogen and Foreland of the northeastern U.S. yields novel insights into the tectonic and fluid history of the area. The ancient mineralizing fluid is characterized by O-H isotopic measurements of Ar-dated illitic clays, and is dominated by surface derived fluid sources that match modeled Pangea meteoric fluids. In addition, we find that Midcontinent mineralizing fluid chemistry changes away from the orogenic front, consistent with a rain shadow effect from the high elevation Appalachian orogen.

Local and far‐field stresses from late Paleozoic continent‐continent collisional orogeny and exhumation created local permeability pathways for surface fluid infiltration, altering the hydrologic architecture of the brittle upper crust. These results also challenge the popular hypothesis of tectonically-forced, lateral fluid flow from the Appalachian orogen into the Midcontinent foreland ("squeegee hypothesis") that was associated with widespread paleomagnetic remagnetization and ore mineralization.