February 2021 LIP of the Month
Mineral deposits of the Mesoproterozoic Midcontinent Rift System in the Lake Superior region – Metallogeny of the prolifically mineralized Keweenawan LIP
Laurel G Woodruff1, Klaus J. Schulz2, Suzanne W. Nicholson2, and Connie L. Dicken2
1U.S. Geological Survey, St. Paul, Minnesota: email: woodruff@usgs.gov
2U.S. Geological Survey, Reston, Virginia
Figures and text modified from Woodruff et al. (2020a)
General
The Keweenawan large igneous province (LIP) of the Midcontinent Rift System (MRS) of North America is perhaps the most prolifically and diversely mineralized LIP known on Earth (Nicholson et al., 1992). The MRS is an approximately 2,200 km curvilinear continental rift that stretches from Kansas northeast to the Lake Superior region where it turns southeast and extends through lower Michigan (Fig. 1). Rocks of the MRS host a varied suite of magmatic and hydrothermal mineral deposits in the Lake Superior region of the United States and Canada where rift rocks are exposed at or near the surface. Historically, hydrothermal deposits, such as Michigan’s native Cu deposits and the White Pine sediment-hosted stratiform Cu deposit, were major MRS metal producers. On-going exploration for and potential development of Cu-Ni sulfide deposits hosted by the Duluth Complex of Minnesota and the opening of the Eagle Ni mine in Michigan indicate an expanding interest in MRS magmatic deposits. Many of the MRS hydrothermal and magmatic mineral deposits are significant past, present, and likely future providers of critical minerals. We have placed these deposits into a space and time metallogenic framework (Woodruff et al., 2020a) that is summarized here.
Figure 1. Distribution of volcanic, sedimentary, and intrusive rocks related to the 1.1 Ga Midcontinent Rift System (MRS). Coverage is taken from an MRS Geographic Information System (GIS) compilation by the U.S. Geological Survey, with assistance from a consortium of organizations, including the Ontario Geological Survey, the Minnesota Geological Survey, University of Minnesota-Duluth, and Macalester College. Precambrian bedrock older and younger than the 1.1 Ga MRS is shaded gray. Extent of the Paleozoic cover shown in diagonal pattern. Map is in Lambert conic conformal projection.
Introduction
The MRS represents a major thermal-tectonic rifting event ca. 1.1 Ga marked by a huge volume of igneous rock emplaced in a relatively short time span of ~30 million years, followed by deposition of a thick package of clastic sediments with subsequent faulting (Wold and Hinze, 1982). Seismic reflection profiles from the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) across the Lake Superior basin show that during rifting Archean and Paleoproterozoic crustal rocks were thinned to less than half of their pre-rift thickness of ~50 km. Sag basins imaged under the modern lake are filled by ~20 to 25 km of basaltic lava overlain by up to 5 to 7 km of mainly clastic sedimentary rock (Behrendt et al., 1988; Cannon et al., 1989; Hutchinson et al., 1990). Conservative estimates of the volume of rift-related erupted basalt range from 1.5 up to 2 million km3, possibly with an equal volume of intruded rock (Hutchinson et al., 1990; Cannon, 1992). The large volume of MRS mafic igneous rocks as well as their geochemical and isotopic characteristics support the existence of a mantle plume as a source of the magmas that helped create the MRS (Hutchinson et al., 1990; Nicholson and Shirey, 1990; Nicholson et al., 1997).
The overall distribution of mineral deposit types and their metal endowment developed from a recent compilation of MRS mineral deposits (Woodruff et al., 2020b) reflect changing magmatic compositions and styles during development of the Keweenaw LIP and subsequent hydrothermal events. Like many other LIPs around the world, the MRS hosts a variety of mineral deposit types including magmatic and related hydrothermal deposits. At the mineral system scale, MRS mineral deposits fit many of the criteria of LIP-related metallogeny, such as magmas acting as metal sources for Ni-Cu-PGE sulfide deposits, igneous activity providing thermal energy for hydrothermal deposits, and sills and dikes acting as barriers to fluid flow (Ernst and Jowitt, 2013). One critical element of mineral systems is preservation of primary depositional zones (McCuaig and Hronsky, 2014). The MRS in the Lake Superior region is remarkably well preserved despite its great age, and erosion over millennia has exposed many mineral deposits at or near the surface (Woodruff et al., 2020a; Fig. 2). There are, however, differences at the deposit scale that are distinctive to a specific time interval or geological setting of the MRS, and to specific magmatic or hydrothermal processes.
Figure 2. Map of the Lake Superior region showing the distribution of magmatic units of MRS Plateau, Rift, and Late-Rift stages, sedimentary units of Late-Rift and Post-Rift stages, and MRS-related mineral deposits, classified by deposit type. The sedimentary Oronto Group includes, in stratigraphic order, the Copper Harbor Conglomerate, Nonesuch Formation, and Freda Sandstone. Early Paleoproterozoic Sibley and Animikie Groups sedimentary rocks had an important role for some MRS-related hydrothermal vein deposits. Other Precambrian bedrock, both older and younger than the 1.1 Ga MRS, is shaded gray, and Paleozoic rocks are shown in the cross-hatched pattern. MRS mineral deposit types and locations are from Woodruff et al. (2020b).
MRS Metallogeny
MRS development in the Lake Superior region is divided into three distinct time/style magmatic stages with characteristic mineral deposits (Fig. 2).
(1) The Plateau stage (~1112 to ~1105 Ma) was a time of spatially extensive basalt eruptions from multiple centers. During this early stage, hundreds of subaerial basalt flows erupted from fissures and low-angle shield volcanoes across a broad area, forming a ≤ 10-km-thick volcanic plateau, mostly preserved in the Lake Superior region (Green, 1989). Within some magma conduits that fed eruptive centers are now found small, rich conduit-type Ni-Cu-PGE sulfide deposits (Schulz et al., 2014; Barnes et al., 2016). Conduit-type sulfide deposits are hosted by MgO-rich, and thus also originally Ni-rich, mafic intrusions that typify this magmatic stage and only have been found away from the central MRS rift basins. MRS conduit-type deposits include the Eagle mine in Michigan (Ding et al., 2010), the Tamarack deposit in Minnesota (Taranovic et al., 2015), and the Thunder Bay North (Current Lake) deposit in Ontario (Thomas et al., 2011). The Eagle mine has been the only operating primary Ni mine in the United States since it began production in 2014. Other mineral deposit types related to magmatic rocks of the Plateau stage include layered Fe-Ti oxides in mafic intrusions of the Duluth Complex (Grout, 1950) and alkalic-hosted U-Nb±REEs deposits in the Coldwell Complex, Ontario (Scott, 1987), as well as scattered occurrences of Ni-Cu sulfide mineralization throughout the region.
(2) The Rift stage (~1102 to 1090 Ma) was characterized by voluminous high Al subaerial flood basalt eruptions that filled rapidly subsiding basins along the central rift axis in what now is the modern Lake Superior basin. Eruption rates were rapid, with individual lava flows commonly occurring one after the other, keeping pace with subsidence such that successive flows erupted onto relatively flat surfaces (White, 1960). Breaks in volcanism are indicated by the presence of multiple laterally-extensive sedimentary interflow conglomerate and sandstone units that are consistent time markers and important host rock to hydrothermal native Cu deposits in Michigan (Butler and Burbank, 1929). Emplacement of the multiple intrusions of the Duluth Complex in Minnesota into sulfur-rich Paleoproterozoic sedimentary rocks created the large, mainly disseminated contact-type Cu-Ni-PGE sulfide deposits along the basal contact (Miller et al., 2002). Listerud and Meineke (1977) estimated that the Duluth Complex contains about 4.4 billion metric tons of ore with average grades of 0.66% Cu and 0.2% Ni, at a 0.5% Cu cut-off. Contact-type sulfide deposits typically have Cu>Ni, whereas conduit-type sulfide deposits have Ni>Cu, a distinction controlled by evolving magma chemistry from Mg-rich compositions during the Plateau stage to more Al-rich during the Rift stage (Ripley, 2014). The Duluth Complex also is host to small Fe-Ti oxide, plug-like discordant intrusions (Severson and Hauck, 1990) and has potential for reef-type PGE deposits (Miller, 1999). Exploration of the Duluth Complex is ongoing, although no sulfide mines have been developed.
Magmatic-hydrothermal mineralization related to felsic intrusive activity in the Mamainse Point area near the eastern shore of Lake Superior in Ontario includes cogenetic Cu-rich polymetallic veins, porphyry-style Cu-Mo mineralization, and related Cu-Mo breccia pipes (Norman and Sawkins, 1985; Richards and Spooner, 1989). The differing styles of mineralization are thought to reflect different levels of exposure of small hydrothermal systems related to local porphyritic intrusions (Richards and Spooner, 1989; Perelló et al., 2020). Although Woodruff et al. (2020a, b) originally placed the timing of these eastern Ontario intrusions in the Late-Rift stage, new Re-Os dating of molybdenite is consistent with the MRS Rift stage (Perelló et al., 2020). Norman and Sawkins (1985) and Perelló et al. (2020) recognize the unusual occurrence of porphyry copper mineralization in a continental rift setting rather than a more typical subduction-related magmatic arc setting.
(3) The Late-Rift stage (~1090 to 1083 Ma) had waning, sporadic mafic and felsic volcanism around Lake Superior accompanied by increasing sedimentation into central rift basins from highland shoulders along basin margins. With the end of major magmatism, MRS metallogeny became increasingly dominated by hydrothermal mineralization. It is postulated that a diverse yet related suite of polymetallic veins in northern Ontario with varying settings, metal assemblages, and gangue mineralogy formed from regional and localized hydrothermal systems (e.g., Franklin, 1978; Smyk and Franklin, 2007), perhaps driven by residual magmatic heat. Hydrothermal fluids for this vein mineralization are consistently described as mobilized Na-Ca-Cl-rich basinal brines expelled from underlying pre-MRS sedimentary rocks (Franklin and Mitchell, 1977; Franklin, 1978; Kissin and Sherlock, 1989). Scott (1990) suggested that vein systems with overlapping ore minerals fit into a general metallogenic pattern. Many of the veins carry native Ag along with a combination of galena, sphalerite, amethyst, calcite, fluorite, and barite and rare pitchblende, with spatially connected overlapping commodities. The 5-element Ag-Bi-Co-Ni-As vein exploited by the Silver Islet mine produced ~180,000 pounds of Ag in the mid- to late-1800s (Franklin et al., 1986).
The end of the Keweenawan LIP event was followed by two additional time intervals critical to hydrothermal mineralization:
(4) During a Post-Rift stage (~1083 to ~1060 Ma) increasingly mature clastic sedimentation continued to fill thermally subsiding central rift basins. Within this many-kilometer-thick Post-Rift stage section of mainly conglomerate and sandstone is a relatively thin interval of marine shale and siltstone (Daniels, 1982; Jones et al., 2020). The only mineral deposit type that falls within this MRS stage is sediment-hosted stratiform Cu mineralization on the south shore of Lake Superior. Mineralization in the lowermost gray/black shale unit (Nonesuch Formation) is attributed to chalcocite replacement of pyrite during diagenesis by compaction-driven Cu-bearing hydrothermal fluids moving through red beds of the underlying Copper Harbor Formation (Ensign et al., 1968; Mauk et al., 1992; Swenson et al., 2004; Bornhorst and Williams, 2013). The White Pine mine produced ~2Mt of Cu from 1953 to 1996, and the near-by Copperwood deposit (Bornhorst and Williams, 2013) is undergoing evaluation.
(5) A final Compressional stage (~1060 to ~1040 Ma) began when northwest-southeast compressional stresses, thought to be related to Grenville orogenesis, reached the Lake Superior region (Cannon, 1994). During this final stage, Cu-rich hydrothermal fluids generated by burial metamorphism and dehydration of the thick buried basalt sections in the central rift basins migrated upwards along pathways created by faulting and minor folding, depositing mostly trace amounts of native Cu throughout the Lake Superior region (Stoiber and Davidson, 1959; White, 1968; Jolly, 1974; Bornhorst, 1997). Economic native Cu deposits, however, were concentrated in permeable basalt flow tops and interflow sedimentary units of the Portage Lake Volcanics (PLV) within a relatively small area with a strike length of 45 km along the Keweenaw Peninsula in Michigan (White, 1968; Fig. 2). Michigan mines produced a total of about 5.3 million metric tons of native Cu and a small but significant quantity of native Ag between 1845 and 1968. About 40% of this total came from a single mineralized conglomerate horizon called the Calumet & Hecla conglomerate lode (White, 1968). Cu-rich hydrothermal fluids released by this same tectonic event also created localized native Cu along fault planes in the White Pine sediment-hosted stratiform Cu deposit (Mauk at al., 1992), a number of small, localized chalcocite deposits hosted in the lower PLV section, (Bouajila and Sirois, 2014), and possible U mineralization in veins at contacts between MRS diabase dikes and Archean rocks in Ontario (Robertson and Gould, 1983).
Summary
The space and time framework for MRS mineral deposits is illustrated in Fig. 2 and summarized in Fig. 3. Well-characterized deposits, such as disseminated contact-type Cu-Ni-PGE sulfide deposits of the Duluth Complex and sediment-hosted stratiform Cu mineralization at Copperwood and White Pine, continue to be evaluated. Recent recognition of the occurrence of rich, but small and elusive conduit-type Ni-Cu-PGE has intensified exploration of MRS rocks beyond their current mapped extent. Influential factors for magmatic deposits that controlled development of MRS magmatic mineral deposits in time and space include magma compositions that controlled metal concentrations, incorporation of external sulfur into metal-rich magma, and differing intrusion styles that created localized massive to disseminated sulfide mineralization. For hydrothermal deposits, influential factors include the generation of metal-rich hydrothermal fluids in differing environments, movement of hydrothermal fluids along permeable fluid flow paths (in the case of native copper tens of millions of years after major rift-related events), and fluid focusing into favorable depositional zones. The MRS has a long 175-year history of exploration and mining and remains a focus of active exploration and mining because of its immense metal endowment.
Figure 3. Summary of the tectonic and mineralization history of the ~1.1 Ga MRS. Tectonic stages of the MRS indicate periodic magmatism during Plateau and Rift stages, followed by minor magmatic activity during the Late-Rift stage. During the Late-Rift and Post-Rift stages, following major magmatism, clastic and minor marine sediments were deposited into thermally subsiding central rift basins. During a final tectonic event (Compressional stage) reverse faulting created the geometry of the MRS seen today, with older rocks overlying younger rocks along rift basin margins. For MRS mineral deposits, numbers refer to the name of the deposit types described in the text and arrows represent the estimated duration of mineralization. For each deposit type, a specific deposit or mine is given. Details for this space and time framework are available in Woodruff et al. (2020a, b).
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