Summary:
The Tilden iron deposit is an example of a Lake Superior-type BIF deposit. These types of deposits occur worldwide, represent the largest global source of iron ore, and were deposited between 2,700 Ma and 1,800 Ma, formed by chemical precipitation in shallow waters such as continental shelves. Precipitation of iron oxides was due to low atmospheric and ocean oxygen levels resulting in increased iron content in sea water. These deposits are typically characterized by alternating layers of iron oxides and SiO2 -rich material such as cherts or SiO2 -rich sediments.
The Tilden BIF deposit forms the base of the Negaunee IF of the Menominee Group within the MarqueSe Range Super Group. The Tilden BIF is Proterozoic in age and sits on the southern margin of the Marquette trough. It is fault-bounded to the south by Archean gneiss terrane, with the fault contact dipping steeply north and aligning with the south wall of the Main Pit at Tilden.
The Tilden BIF is interbedded with three distinct, syn-sedimentary, mafic intrusive sills: The Summit Mountain Sill, the Suicide Sill, and the Tilden Lake Sill, as well as associated smaller dikes and sills. There is a younger dike series of Keweenawan age (approximately 1.0 Ga) that crosscuts bedding. Alteration is present along all intrusive contacts, with the type and extent of alteration dependent on the thickness of the intrusive and the composition of the iron formation (Lukey et al., 2007). Brittle fractures and late quartz veins cut all units.
Tilden is dominated by a 100 m-scale, northwest-plunging incline. The hinge line of the incline dips steeply north, plunges 30°NW, and runs down the center of the Main Pit. The hinge line of the incline is mapped locally coincident with the Keweenawan Dike. The Summit Mountain Sill, locally termed the Pillar Intrusive, defines the asymmetry and orientation of the incline. Smaller faults and folds, on a scale of one meter to 20 m, are observed in the Main Pit to follow trends of larger, regional-scale structures. These structures tend to reflect ductile deformation in the Main Pit, where folds with sheared limbs are common (Lukey et al., 2007).
To date, there has been no formal subdivision of the Negaunee IF at Tilden, and stratigraphy is discussed in relative terms from bottom to top. A stratigraphic section, without formal names or dates, was prepared by SLR.Stragraphically, the upward mineralogical variation is from (martite)-magnete-carbonate-chlorite (“Carbonate”) to (magnetite)-martite (“Martite”) to (martite)-microplaty hemate-goethite (“Hematite”) and represents a transition upwards from dominantly ferrous iron (Fe ) mineralogy to dominantly ferric iron (Fe ) mineralogy (Lukey et al., 2007). Some BIF units were disrupted during turbidite flows that manifest as discontinuous lenses of clastic material. Clastic lithology is most prevalent along the bottom (southern) contact with the Archean gneiss. All BIF units are ferrous iron-dominant and increase in ferric iron content upward (generally northward and westward).
Mineralization
The Tilden Mine is unique among Cliffs’ operations because the primary ore mineral at Tilden is hematite, with other minerals including martite (oxidized pseudomorph of magnetite), goethite, and siderite (iron carbonate mineral), as opposed to strictly magnetite. Tilden is also unique in the world in that the hemate-dominant ore is mined at a low grade, concentrated using a selective-flocculation desliming and flotation process, and pelletized. Although some now-expended areas at Tilden did mine and magnetically recover magnete-dominant ore prior to 2009, remaining Mineral Resources at Tilden are hematite-dominant.
At Tilden, the Negaunee IF can be divided into five distinct facies:
• Clastic Iron Formation (IFCL) Units: Varying thickness of interbedded slate with laminated chert, iron silicate, and siderite. Clastics have a lower weight recovery (wtrec) due to the presence of interbedded clastic material. They are highly oxidized in the east side of the Main Pit.
• Carbonaceous Iron Formation (IFCB) Units: Alternating thin layers of magnetite, martite (oxidized pseudomorph of magnetite), iron silicate minerals, iron carbonate minerals and chert. Carbonate material is characterized by the presence of siderite (iron carbonate mineral), low phosphorus, and higher wtrec.
• Martite Iron Formation (IFCH) Units: Thicker beds of hematite-martite-chert with intervals of magnete-carbonate. The oxidation level increases in the east and where proximal to intrusive sills.
• Magnetic Iron Formation Units: Magnetite domain consisting of magnete-carbonate and magnete-silicate-chert with variable oxidation. It is defined principally by magnetite content and is generally fresh, with some localized oxidation. At Tilden, it is found within and defines the (now
expended) material of the CDIII Pit.
• Hematite Iron Formation Units: The oxidized equivalent of the Magnetite Iron Formation prominent in both the Empire deposit and in the east side of the Main Pit, is located stratigraphically above the Summit Mountain Sill. It is dominantly composed of hematite and chert interbeds. At Tilden, this unit has locally very high levels of silica and phosphorus in concentrate (consio2 and conphos, respectively).
The iron formation facies at Tilden have also been modified by clay-silicate alteration associated with Keweenawan faults in the east of the Main Pit, as well as varying levels of oxidation throughout.