Cornwall-type magnetite deposits, as exemplified by the deposit at Cornwall, Pennsylvania, are found at the contact between diabase and carbonate rocks. Contact temperatures are estimated at 600 degrees to 670 degrees C. Ore emplacement occurred after the diabase was fractured and at lower temperature ( approximately 500 degrees C). We propose a depositional model based on convecting, supercritical aqueous chloride solutions. Acquisition of FeCl 2 is accomplished at depth (hot end of the cell) as a two-stage process. First HCl is formed by conversion of muscovite to K-feldspar and then Fe-minerals such as magnetite or pyrite are dissolved to yield FeCl 2 . The FeCl 2 -rich solutions circulate to the cold end of the cell, where they come in contact with CaCO 3 . Magnetite and pyrite are precipitated and the solutions become enriched in CaCl 2 and CO 2 , some of which may descend with the return flow.Using recent data on the free energies of formation of supercritical aqueous KCl, FeCl 2 , and CaCl 2 , it has been possible to calculate fluid compositions. The main variables are T, f (sub H 2 ) , and X (sub H 2 O) (or X (sub CO 2 ) ). Making reasonable assumptions, the mFeCl 2 levels are appropriately high and there seems to be no difficulty in transporting large amounts of magnetite or pyrite by this process. Support for the model is found in the ubiquitous K-metasomatism.

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