Metamorphic Aspects of the Magnetite-Hematite Deposit at Benson Mines, New York


The relationships between the iron oxide, sulfide, and silicate minerals were determined in more than 500 polished sections and about 200 thin sections of Benson magnetite-hematite ore and host gneiss. The universal stage was utilized to determine the orientation of those opaque ore minerals that have a unique physical direction and the determined ore fabric was compared with that of biotite and sillimanite in the host gneiss. Rims of garnet, sillimanite, and plagioclase about the iron oxide and some sulfide grains, together with poikiloblastically enclosed iron oxide and sulfide grains in garnet and sillimanite, indicate that some of the opaque minerals were present at the time of formation of those silicate minerals. Similar microtextures show that part of the sulfide minerals, pyrite, pyrrhotite, and chalcopyrite, were present at the time of iron oxide crystallization. Other grains of those sulfide minerals form later veinlets that transgress the silicate, oxide, and earlier sulfide grains. Petrofabric diagrams for the basal cleavage of biotite grains in thin sections of oriented specimens collected from the Benson open pit mine exhibit well the foliation that may be observed in the mine; the c-axes of sillimanite crystals commonly are aligned in a direction of lineation within the biotite foliation plane. Petrofabric diagrams for the long axes of ovate ore grains exhibit more scatter than those of biotite and sillimanite, but the axes of such grains exhibit a tendency to lie within the biotite foliation plane and to be relatively concentrated in the direction of sillimanite lineation. This tendency is greatest in the more strongly foliated and lineated specimens. The writers interpret the ore microscopic, petrographic, and petrofabric results to indicate that metamorphism has played an important role in the development of the present nature of the Benson ores.


Geosciences and Geological and Petroleum Engineering

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New York

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Article - Journal

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© 1969 Society of Economic Geologists, Inc., All rights reserved.

Publication Date

01 Apr 1969