The Complex Paragenetic History of Basal Negaunee Iron Formation Iron Ores, Tilden Mine, Marquette District, Upper Michigan

Variable mineral chemistry and textures of basal Negaunee iron ores mined in the Main Tilden Pit have led to metallurgical difficulties. Core-logging and detailed petrography supported by microprobe investigations, identify three upward fining lithofacies within the Main Pit Carbonate and overlying Martite ore domains: 1) Basal Clastics; 2) Medial BIF; and 3) Granular Iron Formation. Growth fault related subsidence controlled deposition of the Basal Clastics, comprised of detrital quartz dispersed in a matrix of chlorite, cemented by ferri-hydrite, chert and Mg-siderite. Subsequent starvation of any clastic input led to cyclic iron-silica precipitation throughout the deposition of Medial BIF. Increasing wave action accompanied marine transgression caused deposition of granular rip-up clasts to form the Upper Granular Iron Formation. These domains are cross-cut by numerous chloritized feeder dykes and are capped by a greenstone Pillar, indicating overlapping mafic magmatism. Low grade regional metamorphism attending the 1850 Ma Penokean arc-continent collision led to magnetite growth, carbonate grain coarsening and chlorite crystallization. Metamorphic fluids facilitated martite replacement of magnetite and deformation led to developing local platy specularite schists. A late retrograde hydrothermal overprint post-dates peak thermal conditions that accompanied the development of the 1750 Ma Republic Metamorphic Node. This is expressed by high-Fe chlorite, Fe-dolomite/ankerite, zoned Mn-rich siderite with associated trace Cu-Fe sulphide and REE bearing fluro-apatite and monazite, diagnostic of an “IOCG” type signature. This complex paragenetic history accounts for the unpredictable geometallurgical response of the basal Negaunee iron ores. Treatment difficulties relate to: 1) variable silica-iron separation occurs due to liberation from detrital quartz versus massive mosaic and granular textured chert; 2) the bulk iron content is expressed not only in iron oxide but in iron carbonate and iron chlorite; and 3) the intense late IOCG overprint proximal to the Signal Hill Fault