100 D2. Recent Advances in Economic Geology (Posters)

Monday, 23 September 2019
9:00 AM - 6:30 PM
Hall AB, North Building

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

Paper number: 100-9
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-340567

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