MODEL FOR THE DEPOSITION OF CORNWALL-TYPE MAGNETITE DEPOSITS

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° to 670°C. Ore emplacement occurred after the diabase was fractured and at lower temperature (∼500°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 HC1 is formed by conversion of muscovite to K-feldspar and then Fe-minerals such as magnetite or pyrite are dissolved to yield FeCU- The FeCl 2-rich solutions circulate to the cold end of the cell, where they come in contact with CaCOa. Magnetite and pyrite are precipitated and the solutions become enriched in CaCb and CO 2, some of which may descend with the return flow. Using recent data on the free energies of formation of supercritical aqueous KC1, FeCl 2, and CaCl 2, it has been possible to calculate fluid compositions. The main variables are T, f H2, and f H2O (or f CO2)- Making reasonable assumptions, the m FeCi2 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. © 1979 Society of Economic Geologists, Inc.