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

Abstract

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 <2 wt. percent MnO), but the breccias are marked by elevated values of Mn, as well as Cu, As, Sb, S, Sr, Y, Ca, and P. Within the thin-bedded facies, a transitional interval of about 15 feet separates silicate-carbonate iron-formation below from oxide iron-formation above. Isocon analyses show that oxidized strata can be derived from unoxidized strata by a variety of decomposition reactions, all essentially removing Mg, K, Ca, Na, P, and resulting in a mass loss of 40 to 50 percent. Manganese oxides appear in oxidized strata as discordant veins and as concordant, massive layers and lenses, where they are admixed with goethite and hematite. Massive layers are partly enveloped by brecciated oxides of several kinds, have maximum MnO values of 12 wt. percent, and are enriched in Ba, Sr, Ag, and U. Manganese values increase markedly just above the contact between the thin-bedded facies and the overlying thick-bedded facies. Manganese oxide-rich layers just above the contact have brecciated or "gnarled ore" textures and are conformably interlayered with intervals of decomposed oxide iron-formation containing abundant secondary limonite. The manganese oxide-rich layers are intercalated with thick beds of admixed chert and hematite; they have a relict micronodular texture and discordant textural features, involving manganese minerals such as manganite, cryptomelane, and pyrolusite. Whole rock assays show that they can contain as much as 50 wt. percent MnO. They also have elevated Ca, Mg (attributed to secondary carbonates), and P values and are enriched in Ba (as much as 1.8 percent), Pb, Sr, Ag, As, and the LREE. The uppermost part of the thick-bedded facies consists of oxide iron-formation where primary hematite is abundant but where manganese oxides are lacking. The overlying Rabbit Lake Formation is an epiclastic unit that contains thick beds of carbonaceous shale, thin layers of tuffaceous material, and intercalated beds of sulfide (pyrite) and oxide (hematite) iron-formation. The sulfide iron-formation (35.7 wt. percent Fe203, 26 percent S) contains elevated values of Au (31 ppb), As, Cu, Co, Ni, Pb, Sr, V, Mo, and Se (230 ppm). Many of the textural and mineralogical features observed in the Gloria core are consistent with hydrothermal fumerolic processes that started in late Mahnomen time and persisted into early Rabbit Lake time.