GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 100-9
Presentation Time: 9:00 AM-6:30 PM


COUTRE, Cory, Department of Geology, St. Lawrence University, 23 Romoda Drive, Canton, NY 13617, WALSER, Sandra L., Department of Geology, St Lawrence University, 23 Romoda Dr, Canton, NY 13617, LUPULESCU, Marian, New York State Geological Survey, New York State Museum, 3140 CEC, Madison Ave., Albany, NY 12230, HANCHAR, John M., Dept of Earth Sciences, Memorial University of Newfoundland, 9 Arctic Ave., Room 4063, Alexander Murray Building, St John's, NF A1B 3X5, Canada and CHIARENZELLI, Jeffrey R., Department of Geology, St. Lawrence University, 147 Brown Hall, Canton, NY 13617

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.