A petrofabric study of the iron ore of Benson Mines, Star Lake, New York

The petrofabrics of the host gneiss silicates and Ovate ore grains were studied in thin sections of oriented specimens collected from widely scattered positions in the iron ore deposit at Benson Mines, New York. Ten oriented specimens selected for petrofabrics study were comprised primarily of feldspar, quartz, garnet and ores, with lesser quantities of biotite and sillimanite. Petrofabric diagrams were prepared for the orientation fabrics of magnetite and biotite in eight of the ten specimens, but sillimanite and quartz were sufficiently abundant for fabric determination in only three specimens. Duplicate thin section and perpendicular sections were utilized to check the resultant orientation patterns. A total of 28 petrofabric diagrams were prepared. The petrofabric diagrams for the basal cleavage of biotite grains reveals foliation which is much more distinct than that observed rnegascopically. The c-axes of sillimanite are distinctly aligned in a direction of lineation which lies within the biotite foliation plane. The petrofabric diagrams for the long axes of ovate ore grains exhibit more scatter than those of biotite and sillimanite, but those axes exhibit a tendency to lie within the biotite foliation plane and to be relatively concentrated in the direction of sillimanite lineation. This tendency is strongest in the more strongly foliated and lineated specimens respectively. The correspondence between host gneiss silicate petrofabrics and orientation of ovate ore grains suggests that the iron oxide grains have developed preferred orientation during the metamorphism which developed the foliation in the host rock gneiss and that the iron was introduced or simply present in the rock prior to the last stages of metamorphism --Abstract, pages ii-iii