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.

Operations consist of an open pit truck and shovel mine.

Operations consist of a single stage crushing, AG mills.

The crude ore is hauled to the primary gyratory crusher, where it is reduced to chunks less than 10 inchesin size. From the crusher, the ore is conveyed to a covered ore storage building.

Liberating the iron mineral requires that the crude ore be ground to the consistency of face powder. This process begins in the same way for both magnetite (metallic gray, below) and hematite (red-brown, below) as crude ore and water are fed into large primary autogenous mills. The term autogenous means that grinding media like the steel balls and rods used in some mills are not required. Instead, the tumbling action of the ore in the rotating mills is sufficient to reduce it to a consistency of beach sand. Tilden has twelve primary mills that are 27 feet in diameter and 14 ½ feet long.

Further grinding occurs in grinders and pebble mills which also operate autogenously. Grinders and pebble mills are, put quite simply, cylinders that continuously roll and turn. Inside them are rocks (ore) of various sizes.

The larger pieces of ore essentially act as grinders, crushing the smaller pieces into powder. As the larger pieces of ore ("pebbles") get comminuted in size, more large pieces are added. In the pebble mills, pebbles about 2 inches in size which are screened from the primary mill are used as grinding media. In grinders, larger rocks are used. Thus, grinders preceed pebble mills in the comminution process. The Tilden concentrator has twenty-four 15 ½ foot diameter pebble mills, each about 30 feet long.

Tilden Concentrator
Operations consist of a concentrator that utilizes single stage crushing, AG mills, magnetite separation and floatation to produce hematite and magnetite concentrates that are then supplied to the on-site pellet plant. From the site, pellets are transported by our LS&I rail to a ship loading port at Marquette, Michigan, operated by LS&I.

Concentrating Magnetite
First ground to a fine powder, the ore is next concentrated using magnetic separation and flotation to 60 to 65 % iron, then rolled into 3/8" dia pellets that are purplish-grey when they emerge, steaming, from the plants.

Turning low-grade crude ore into high-grade concentrate requires a means of separating the iron particles from the waste rock. In the magnetite operation, magnetic separators known as cobbers and finishers help perform that function.

The mixture of finely ground crude ore and water enters the separator tanks where stainless steel drums with powerf ........