Isotope dilution analyses for He, U, and Th have been performed on magnetite and pyrite samples from the Cornwall, Pennsylvania magnetite deposits. If the extra-lattice U and Th are removed by leaching, the method gives an age for the ore of 194+ or -4 m.y. in agreement with a K-Ar age of 195+ or -5 m.y. on associated muscovite and 197+ or -5 for biotite from the Palisades diabase. It appears that the He age method can be quantitative and can be useful in dating ore deposits under suitable conditions.

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