RI-46 Hydrothermal Systems in Manganese-Rich Iron-Formation Ofthe Cuyuna North Range, Minnesota: Geochemical and Mineralogical Study of the Gloria Drill Core

The iron-rich Trommald Formation of Early Proterozoic age on the Cuyuna North range in east-central Minnesota is the largest resource of manganese in the United States. To better elucidate the complex history of the manganese oxides and to investigate their compatibility with in situ leaching techniques, the U.S. Bureau of Mines drilled a core near the Gloria mine (sec. 28, T. 47 N., R. 29 W.) to an inclined depth of about 1200 feet. It intersects a complete section of Trommald Formation 553 feet thick, as well as short intervals of the overlying Rabbit Lake and underlying Mahnomen Formations. At the Gloria site the lowermost part of the Trommald Formation consists of chlorite-bearing hematite iron-formation, and contains features indicative of syndepositional reworking-possibly under shallow-water conditions that include granule-rich layers and pebble-size conglomerate. Much of the overlying thin-bedded facies consists of carbonate-silicate iron-formation broken in places by beds of breccia. In the breccia, sulfides--especially pyrite filled interstitial voids or form discordant composite veins along with quartz, manganese oxides, carbonates, and stilpnomelane. Pyrite contains included pyrrhotite, chalcopyrite, and arsenopyrite, and is replaced by magnetite, in turn replaced by martite. The silicate-carbonate iron-formation contains little manganese