AGE AND ORIGIN OF THE BENSON MINES IRON ORE DEPOSIT, NORTHWESTERN ADIRONDACK HIGHLANDS

The Benson Mines iron deposit occurs in granulite-grade metasedimentary and metaigneous rocks of the western Adirondack Highlands and has been attributed to many origins including metamorphism of iron-rich sedimentary rocks, skarn-type mineralization, magmatism, or high-temperature metasomatic processes. A thick sequence of Grenville metasedimentary rocks and ultramafic rocks, were intruded by granitic rocks and folded in a tight synform. Metasedimentary rocks range from fine to medium-grained and strongly foliated, to coarse-grained and lack foliation where partially melted. Much of the ore, magnetite and/or martite, is hosted in pelitic lithologies with garnet ± sillimanite; however, ore also occurs within granitic rocks. Study of drill core show K-metasomatism in both metasedimentary and metaigneous rocks and the dominance of potassium feldspar. Metaigneous rocks show strong positive correlations between K₂O and Al₂O₃ with SiO₂, and negative correlations between all three and iron. Metasedimentary rocks show similar, but less pronounced trends due to original compositional differences. Zircons from migmatitic calc-silicate gneiss yield Shawinigan cores (1169±11 Ma) and Ottawan rims (1040.5±9.5 Ma) but lack detrital zircon. This data is consistent with the crystallization of anatectic zircons during the Shawinigan event and subsequent Ottawan rim growth. A sample of non-foliated, sillimanite-garnet-quartz-magnetite pegmatite yielded zircons with an age of 1056.1±4.3 Ma, interpreted as the timing of crystallization. Ore formation was a multi-stage process including early K-metasomatism and iron mobilization during intrusion of Shawinigan granitoids followed by extensive remobilization of iron during Ottawan intrusive and metamorphic events. Both metasedimentary and A-type metaigneous rocks likely served as the source of iron.