An Emerging Major Copper-Nickel-Platinum-Palladium-Gold Camp
Geology of the Duluth Complex
The Duluth Complex is a large, composite mafic intrusion in northeastern Minnesota that extends about 240 km northeast from Duluth Minnesota to the Canadian border. The Duluth Complex lies within Precambrian shield rocks of the Superior Province which include Archaean mafic to felsic volcanic rocks, greywackes, granitic intrusives and older ortho- and paragneisses. The Duluth Complex was intruded beneath a similar age volcanic rocks during the formation of the Midcontinent Rift System which developed approximately 1.2 to 1.1 billion years ago. The fill associated with the active stages of rift development consists mainly of tholeiitic basalt that was erupted under subaerial conditions, together with petrologically related sills, dikes, and large layered intrusions that cooled beneath or within the cogenetic volcanic pile. The largest and most important of the layered intrusions is the Duluth Complex, a composite intrusion of troctolite and gabbro derived from periodic tapping of an evolving magma source. In the waning stages of rifting, the principal rock types deposited in the rift shifted gradually from magmatic to sedimentary; among the sedimentary sequences are those for which alluvial-fan, fluvial braid-plain, aeolian, and lacustrine depositional environments may be inferred. Associated gabbroic intrusion resulted in the composite, arcuate, NE-SW elongated, 450 x 100 km Duluth Complex, which formed from up to 40 separate sheet like and cone shaped sub-intrusions and covers an area of approximately 6500 square kilometers.
The copper -- nickel mineralization in the Duluth Complex rocks occurs in magmatic sulphide deposits. Magmatic sulphide deposits are sulphide mineral concentrations in mafic and ultramafic rocks formed from sulphide liquids, exsolved from the magma. Once formed, the droplets of sulphide liquid settle through less dense magma. The sulphide liquid acts as a "collector" for copper, nickel, cobalt and platinum group elements (PGEs) because these elements prefer the sulphide liquid to the magma by a factor of 10 to 100,000 times more. The combination of physically concentrating dense sulphide liquids and chemically concentrating elements in the sulphide liquid is responsible for forming most economically mineable magmatic nickel-copper sulphide deposits, like those found in the the Sudbury Complex in Canada, the Stillwater Complex in the U.S. and the Noril'sk-Talnakh Complex in Russia.
The Duluth Complex hosts three distinct types of magmatic sulphide copper -- nickel mineral deposits. The deposit types include (1) large, low-to-medium grade, disseminated Ni-Cu concentrations, some of which contain local zones enriched in platinum-group elements (PGEs); (2) localized high-grade lenses and zones of massive Ni-Cu sulfides, some of which are moderately enriched in PGEs; and (3) stratabound PGE-enriched "reefs" associated with specific types of phase-layer transitions. Deposit types (1) and (2) occur only at or very near the basal contact of the Complex, whereas type (3) occurs in the basal zone and also at higher levels. At this time, large resources of low-medium grade copper-nickel sulfide mineralization that locally contain anomalous PGE concentrations are well documented by drilling in the basal zones of the Partridge River and South Kawishiwi intrusions. At least nine deposits have been delineated in the basal 100 to 300 meters of both intrusions. The mineralization consists predominantly of disseminated sulfides that collectively constitute over 4.0 billion tonnes (4.4 billion tons) of material averaging 0.66% Cu and 0.20% Ni.
Summarized from MGS, UM (Miller et. al., 1998, Miller et. al, 2002, and Severson, 2005) and NRRI Information (Jirsa et. al.)
The Importance of Metal in the Duluth Complex
The Mesoproterozoic Duluth Complex and associated intrusions of the Midcontinent Rift System in northeastern Minnesota constitute one of the largest, semi-continuous, mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa. These rocks cover an arcuate area of over 5,000 square kilometers and give rise to two strong gravity anomalies (+50 & +70 mgal) that imply intrusive roots to more than 13 km depth. The geometry of three large mafic intrusions within the Duluth Complex have been modeled by the integration of field mapping and drill hole data with maps of gravity and magnetic anomalies. The igneous bodies include the South Kawishiwi, Partridge River, and Bald Eagle intrusions that collectively outcrop on the western edge of an area of greater than 800 square kilometers. The South Kawishiwi intrusion (in the north) and Partridge River intrusion (in the south) host several billion tons (each) of low-to-medium grade Cu-Ni-PGE mineralization near their base, with deposits in the South Kawishiwi hosting the highest grade deposits in the Duluth Complex.
The Duluth Complex hosts one of the world's largest undeveloped repositories of copper, nickel and PGMs. In comparison to world wide deposits, the Duluth Complex is:
From MGS & UM (Miller et. al., 2002) estimated that there were 13 known Cu-Ni deposits (to date) in the Duluth Complex that combined could host a theoretical resource* of 4.4 billion tons of copper-nickel ore averaging 0.66% Cu and 0.20% Ni using a cut-off grade of 0.50% Cu equivalent.
- the world's third largest accumulation of nickel sulphides
- the world's second largest accumulation of polymetallic copper and platinum group metals.
*These "resources" do not conform to NI43-101 standards.
Deposits situated in the Duluth Complex
According to the Minnesota Department of Natural Resources, from 1951 to the present, more than 1,900 diamond drill holes totaling over more than 230,000 m (754,591 ft) have been drilled to explore the western margin and base of the Duluth Complex for copper and nickel deposits. This work has outlined at least nine disseminated copper-nickel deposits, over a 22 mi. (35 km) strike-length, which have a combined estimated resource of 3.63 billion tonnes (4.4 billion tons) grading 0.66% Cu and 0.20% Ni (unclassified historical resource, not 43-101 compliant).
There is extensive interest in the Duluth Complex because of the massive volume of contained copper and nickel, its platinum-palladium-gold content, advances in metallurgical processing and its North American location.