CHARACTERIZING THE MINERALIZATION OF A SCHIST-HOSTED IRON DEPOSIT: SUNRISE IRON MINE, EASTERN WYOMING

The historic Sunrise Iron Deposit in the Hartville Uplift near the town of Guernsey, eastern Wyoming hosts high grade iron(>60 wt.% FeO) and copper ore that was mined starting in the mid 1880’s, with peak production occurring during World War II. The mine reopened in 2011 as a small operation following a temporary period of closure. The mine area exposes many structures of the Hartville uplift, a regional deformational event formed from the suture of two continents in the early Proterozoic. This deposit is unique because the main ore is hosted in Archean-Proterozoic ferruginous schist as opposed to the banded iron formation (BIF) stratigraphically below it. The Great Unconformity overlies the ferruginous schist,

obscuring shallower Proterozoic deposits.

The formation of high-grade iron ore in the Sunrise deposit was previously unstudied. Here, we present field observations, bulk rock geochemistry, and thin section observations to address the formation mechanisms of the Sunrise Iron Deposit. We collected samples, structural measurements, and field magnetic susceptibility measurements along transects leading from the BIF into the schist and iron deposit. Field observations and low magnetic susceptibility readings suggest that the underlying BIF has been leached of iron and silicified. Field evidence of infiltration of Fe-rich fluids along fractures in the schist also suggest mobilization of Fe, with the BIF as the likely Fe source. We present major and trace element bulk rock geochemistry data to address how iron could have been mobilized in this deposit. Microtextures and microstructures in silicified BIF thin sections suggest an intimate association between alteration, iron mobility, and deformation.