Summary:
The Project area would be located at the contact of two major bedrock units, the Giants Range Batholith (GRB) and the Duluth Complex.
The Duluth Complex is composed of mafic to felsic tholeiitic magmas related to the Midcontinent Rift System and makes up much of the bedrock of northeast Minnesota. It is bounded byafootwall of Paleoproterozoic sedimentary rocks and Archean granite-greenstone terranes andahanging wall largely of rift-related flood basalts and hypabyssal intrusions of the Beaver Bay Complex (Miller et al., 2002).
The targeted mineralization of the Maturi deposit is hosted within the basal portion of the South Kawishiwi Intrusion (SKI), known as the BMZ. The SKI is bordered on the southwest by the Partridge River Intrusion, on the northwest by the GRB and Biwabik Iron Formation, the Anorthositic Series to the northeast, and on the southeast by the Bald Eagle Intrusion. Excluding the transmission corridor, lithologic units within the Project area include Mesoproterozoic rocks of the SKI and the Anorthositic Series of the Duluth Complex, as well as basalt xenoliths of the North Shore Volcanic Group.
Surficial geology in the Project area is dominated by glacial deposits associated with the Rainy Lobe that include areas of peat and lake sediment. In some localities along the shoreline of Birch Lake reservoir, the Rainy Lobe Till has been eroded by water, resulting inaless rugged surface expression andapossible surface lag consisting o concentrated coarse-grained clasts. The lake sediment is predominantly silt, clay, and organic material (Jennings and Reynolds, 2005). The thickness of surficial material in the Rainy Lake Watershed is generally less than (<) 50 ft (15.6 m) and is laterally discontinuous. In the vicinity of the plant site, bedrock crops out in five to 20% of the area (Ericson et al., 1976).
The deposit is composed of anorthositic troctolite to troctolites. The mineralogy consists primarily of plagioclase, olivine, pyroxenes, and oxides which make up more than 85% of the total mineralogy. The alteration minerals (e.g., serpentine, chlorite, etc.) typically comprise 1% to 6% of the mineralogy but are locally found in amounts up to 15%. Sulfide content of the ore-bearing geologic units ranges from 1% to 6%, with very local areas having sulfide contents outside of that range.
The main four sulfides present in the deposit include:
• Chalcopyrite;
• Cubanite;
• Pentlandite; and
• Pyrrhotite.
Other copper and nickel sulfides are present in the deposit but occur in minor amounts (<5% total sulfides).
Rock units and mineralization in the BMZ are planar and sub-parallel to the lower contact with an average strike of approximately 60 degrees (°) and dips of 20° to 52° to the southeast. The vertical thickness of the potentially mineable grades varies in width from 49 to over 591 ft (15 to 180 m), averaging from 197 to 328 ft (60 to 100 m). The depth of the potentially mineable grades ranges between 984 to 3,005 ft (300 to 916 m) amsl. The Maturi deposit has not been significantly deformed, but it has been subjected to minor displacements along reactivated basement faults, as well as cross faults. Mapped structures are mostly sub-vertical north–northeasterly striking faults.