GARNET GRAVELS AND THEIR SOURCE ROCKS: A COMPARATIVE STUDY OF GARNET RESOURCES OF THE EMERALD CREEK AREA, IDAHO

The garnet gravels of the Emerald Creek area are located within the St. Joe National Forest, which currently allows recreational mining for star garnets and garnet sands. This resource has the potential to be mined for industrial applications (e.g., packing sands and abrasives), which prompted this initial characterization of these garnets. The source rock is saprolitized, garnet-bearing metapelitic schist of the middle Proterozoic Wallace Formation, a member of the Belt Supergroup. Microprobe analysis provided quantitative chemical characterization of garnets from twelve sites within the Emerald Creek area, including garnets from Fe-hydroxide-cemented, garnet-rich gravels and from the garnetiferous metapelitic schists. The garnets are light pink to deep red, almandine-rich (~80 mole %), and have higher Mn in their cores and higher Fe and Mg in their rims. Numerous fluid and mineral inclusions (quartz, ilmenite, apatite, tourmaline, and monazite) are concentrated in growth zones. The metapelitic schists contain a mix of deformed, partially dissolved, and fractured garnets, together with unbroken, euhedral garnets. Fewer fractured garnets are observed in the gravels compared to the schists. Garnets in gravels are rimmed with secondary minerals, primarily Fe-oxide/hydroxide minerals zoned between layers in Al-content. Fe-oxide/hydroxide minerals also fill fractures within the garnets. In general, the garnet gravels are larger than the garnets within the schists, having an average diameter of 2.60 mm (range: 1.4 to 6 mm) compared to 1.96 mm (range: 0.35 to 5.75 mm). Garnets from the metapelitic schists show subtle variations in mole fractions between aluminous end-members, almandine, pyrope, grossular, and spessartine. The mean mole fractions for garnets in the metapelitic schists range between 81% and 84% for almandine, 11.5% and 13% for pyrope, 2% and 6% for grossular, and 1% and 4% for spessartine. In general, garnets in the gravels have slightly broader ranges for mole fractions (75.5% to 85% almandine, 1.4% to 9.6% grossular, and 0.9% to 9.1% spessartine) except for the pyrope end-member (8.9% to 13.1%). Physical and chemical characterization of these garnets is the first step in determining the future of this resource and its uses.