Summary:
REGIONAL GEOLOGIC SETTING
The regional structures are the Niagara Fault Zone, the collision zone between the Wisconsin Magmatic Terrane and the Superior craton (Schneider et al, 2002), and the Great Lakes Tectonic Zone, which forms the boundary between Archean granite-greenstone and gneissic terranes (Sims et al, 1980). In the Marquette Range area, deformation along the Great Lakes Tectonic Zone evolved from extension and deposition (Schneider et al, 2002) to closure and transpressional deformation and basin inversion (Cambray, 2002). The resulting faultbounded shallow west plunging asymmetric syncline contains a series of second-order growth fault basins that define the detailed stratigraphic variations.
The Paleoproterozoic rocks in Michigan are termed the Marquette Range Supergroup (Cannon and Gair, 1970) and consist of three fining upward sequences (Map in Pocket). The lower portion has been correlated with the upper part of the Ontario Huronian (~2.2 Ga) and the upper parts, which contain the 1875 Ma iron formations, with the Mesabi Range of Minnesota. Simplistically, the sequence is from the Chocolay Group shelf facies quartzites and dolomite to the Menominee Group with argillites and the major iron formations to turbidites, greywacke and shale along with minor iron formation in the Baraga Group. Mafic igneous rocks with a continental tholeiite geochemical signature (Schulz, 1983) are present in the Menominee and Baraga Groups. Basal quartzites in each sequence are used as local structural and stratigraphic marker horizons. Metamorphic grades vary from sillimanite in the west to chlorite in the east and at the Tilden Mine (James, 1955).
Negaunee Iron Formation and equivalents hosted the majority of the natural ore deposits and all of the concentrating grade production in Michigan. In the Marquette trough, the Negaunee reaches a thickness of 1300 meters without including the mafic igneous horizons. Due to the lack of correlative iron formation horizons, the igneous rocks, termed sills locally, are used for structural markers. There appears to be a poorly defined change from dominantly carbonate- chert on the north to magnetite-hematite-chert on the south (Waggoner, 2007).
LOCAL GEOLOGY
The Tilden and Empire Mines are located on the southern margin of the trough and are in fault contact with the Archean gneiss terrane. Local structure consists of upright to steeply inclined second order anticlines and synclines with low angle northwest and southwest plunges (Cambray 2002, Webster, 1999). At Tilden, due to the lack of clear marker horizons and rapid facies changes within the iron formation, igneous horizons are used for stratigraphic and structural correlation (Lukey, Johnson and Scott, 2007). At Empire, stratigraphy is determined by the igneous horizons and by the proportions of carbonates, silicates and clastics in the iron formation (Nordstrom, 1997; Han, 1975).
IGNEOUS ROCKS
Two ages of mafic rocks occur in the mine, the synsedimentary sills and associated dikes and a dike series of Keweenawan (~1000 Ma) related to the Midcontinent Rift. The older series vary from fine porphyritic to diabasic/ophitic and typically display chlorite-carbonate alteration assemblages, particularly in deformation zones. The younger series are typically unaltered diabase. The iron formation is variably altered along the intrusive contacts with the type and extent of alteration dependent on the thickness of the intrusive and the composition of the iron formation.
FOLDING/FAULTING
The major structures are the large (100s meters) scale Tilden Main pit anticline and Empire Main pit syncline; the fault that marks the contact of the Southern Complex and the iron formation. Present geometry of these features is related to transpression during basin closure (Webster, 1999). The fault, initially a basin margin listric normal fault, was reactivated and is now a reverse fault that dips about 65° north (Cambray, 2002). At blast pattern level, faults and folds at the 1-20 meter scale tend to follow the trends seen in the larger structures. These features, while of relatively small amplitudes, can be significant in the detail block modeling and ore type boundaries.