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Essay: Keweenaw Peninsula

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The Keweenawan Supergroup located in the Keweenaw Peninsula in the western Upper Peninsula of Michigan has been known to be the host rocks of major native copper and copper sulfide deposits concentrated within a 45 km belt on the western flank of the Mesoproterozoic Midcontinent Rift. The Keweenaw Peninsula native copper district and the Porcupine Mountain sediment-hosted copper district have been studied to identify the mechanisms of these districts that sourced copper-ore fluids, facilitated copper-ore fluid transport, and led to precipitation of native copper and copper sulfides into host rock. An accepted model that provides an explanation for the high volume and concentration of copper to a relatively small area places importance of post-rift compression initiated by the Grenville orogeny. The resultant faulting and fractures of compression provided a heat source for a rising copper-rich thermal plume, a shorter path for fluids to migrate, and the necessary plumbing for fluids to continue to rise towards the surface uninhibited.
Copper deposits in the Keweenaw Peninsula of northern Michigan were formed in the Mesoproterozoic volcanic and sedimentary rocks (Keweenawan Supergroup) that filled the Midcontinent Rift basin (Fig. 1) (Brown, 2008). Many metallic minerals have been identified in the rift-fill, but 99% of these minerals are native copper, making this locale the largest known copper deposit in the world (Bornhorst and Lankton, 2011). A unique feature of the copper deposits of the Keweenaw Peninsula is the presence of copper in native form due to degassing of subaerial basalt flows, whereas most copper deposits on Earth occur as copper sulfides (Bornhorst and Barron, 2011). Most of the rocks in which native copper has been mined were within the Portage Lake Volcanics and consolidated in a 45 km belt that stretched from the city of Calumet to the Copperwood project in the western Upper Peninsula (Fig. 1).
The copper precipitated from a mineralized hydrothermal fluid that formed during Grenville compression 1.06 Ga (Fig. 2) (Bornhorst et al. 1988). The lateral extent of the hydrothermal fluids extended the width of the rift from Ontario to Minnesota evidenced by the presence of native copper across the region (Bornhorst and Lankton, 2011). The mineralizing fluid is thought to have been heated by the buried Portage Lake Volcanics, which were between 300o and 500o C (Bornhorst and Lankton , 2001). Bornhorst and Lankton (2001) calculated that if a few parts per million of copper was dissolved from the voluminous basalt, and if the ore fluids were generated at a depth of 10 km, there would be enough copper in the fluid to precipitate the amount of native copper found near the surface. Deposition of native copper occurred in the pore spaces of basalts, sandstone of the Copper Harbor Conglomerate, and the basal layer of the Nonesuch Shale.
The migration of fluid from depth toward the surface was controlled by specific factors. First, the rift-fill rocks were permeable (Brown, 2006). The upper sections of the basalt flows are brecciated and vesicular (Brown, 2006). The Copper Harbor Conglomerate is primarily a coarse-grained, permeable sandstone. The Nonesuch Shale is permeable in the basal layers (Bornhorst and Lankton, 2011). Together, these units provided a permeable pathway for fluid flow and eventually deposited copper where permeability ceased (Bornhorst and Lankton, 2011). Another factor that influenced flow of ore fluids was the Grenville compression (Brown, 2006). This event resulted in the formation of faults and fractures that permitted fluid flow when flow would have otherwise been inhibited (Brown, 2006). In addition, faulting caused the fluid to concentrate within the relatively more open space and provided a pathway to the more permeable layers of the rift-fill sequences (Brown, 2006). Ore fluids combined with lower temperature, diluted fluids that were present in host rocks where chemical reactions between the ore fluid and rocks resulted in precipitation of native copper into stratiform deposits (Bornhorst and Lankton, 2011). These deposits are focused in the Keweenaw Peninsula Native Copper District and the Porcupine Mountains Sediment-Hosted Copper District (Fig. 1).

Geologic Background
The geology of the Keweenaw Peninsula has been dramatically influenced by the Midcontinent Rift, which occurred 1.15-1.0 Ga (Bornhorst and Barron, 2011). The rift has been filled with 25 km of basalt and 8 km of Oronto Group sedimentary rocks (Bornhorst and Barron, 2011). The exposed section of basalts experienced volatile degassing which resulted in sulfur deficiency (Bornhorst and Lankton, 2011). These basalt flows erupted sub-aerially through vents in the rift axis. Most flows are 10-20 m thick, up to 90 m wide, and occur with vesicular and brecciated flow tops (Bornhorst and Lankton, 2011). Within the basalts are interflow clastic sedimentary layers. These units were sourced from the edges of the rift and occur as fine beds to beds up to 40 km thick. They consist of some shale and sandstone but are primarily red pebble to boulder conglomerates deposited on the flat lying basalt flows (Bornhorst and Lankton, 2001). Towards the cessation of magmatism, the basin began to sag providing accommodation space for of Keweenawan Supergroup sedimentary rocks. The oldest unit of this group is the Copper Harbor Formation. These beds were deposited as alluvial fans evidenced by red conglomerates fining upward to red sandstone. The lowest layers are interbedded with the last basalt flows. The Nonesuch Formation was deposited stratigraphically above the Copper Harbor Formation. This unit was deposited in anoxic lake conditions as dark grey siltstones and shales. Stratigraphically above the Nonesuch Formation is the Freda Sandstone, which is the final rift-fill unit. The Freda Sandstone was deposited in a shallow river environment as red sandstone (Bornhorst and Lankton, 2011). Each of these units provided the necessary porosity and permeability for hydrothermal fluid flow, but only the Copper Harbor Conglomerate and the Nonesuch shale became host rocks for native copper stratiform deposits (Brown, 2006).
Compression from the Grenville orogenic event caused the Midcontinent Rift to fail (Bornhorst and Lankton, 2011). The normal faults associated with rift extension became reverse faults. In addition, new faults and fractures were formed, which became an important factor for the development of a pathway that copper ore fluids would eventually follow (Brown, 2006).

Keweenaw Peninsula Native Copper District
The largest known deposit of native copper on Earth is located in the Keweenaw Peninsula native copper district. Adequate porosity and permeability of these rocks established the necessary pathway for copper-bearing hydrothermal fluids to move through for eventual deposition (Brown, 2006). Most copper deposits in this district occur within brecciated and amygdaloidal flow tops. However, economically exploitable deposits also occur in the interflow units of the Copper Harbor Formation and to a substantially lesser degree, cross vein systems (Bornhorst and Barron, 2011). The space needed for precipitation occurred in the 2-3 cm diameter vesicles of the basalt and in the space between the breccia clasts of the Copper Harbor Conglomerate. Native copper between the breccia clasts occurs as small masses to masses that weigh several tons (Bornhorst and Barron, 2011). Most commonly, native copper was deposited as a fragmental amygdaloid. These deposits occurred above the vesicle-free basalt layer of the equivalent flow and below the vesicle-free basalt layer of the next flow. Deposition occurred as tabular lodes, 3-5 m thick and 1-11 km along strike (Bornhorst and Barron, 2011). Although the interflow sedimentary units accounted for only 5% of the volume of the total volcanic section, these units hosted 40% of mining production in this district. The largest masses of copper mined (400 tons) were located in veins, but copper in veins accounted for the smallest percentage of production (Bornhorst and Barron, 2011).
The conditions for native copper mineralization in this district are unique. Bornhorst and Barron (2011) hypothesize that the copper-rich ore fluid was produced via burial metamorphism of the Portage Lake Volcanics at 300o-500o C and at a depth of 10 km. This depth is a reasonable estimate to account for the volume of copper that has been deposited (Bornhorst and Barron, 2011). The presence of native copper, as opposed to copper sulfides, is the result of sulfur deficient basalts and host rocks. The rocks of the vesicular layers of the Portage Lake Volcanics, interfingering clastic sedimentary layers, Copper Harbor Conglomerate, and Nonesuch Shale, in addition to fractures and faults formed by compression, provided permeable pathways for the buoyant ore fluids to migrate toward the surface uninhibited (Brown, 2006). Specifically, the faults and fractures facilitated flow into permeable layers above (Bornhorst, 1997).
Eventual precipitation of native copper occurred at ~225 oC by three processes. First, ore fluids became mixed with the lower temperature diluted resident fluids that were oxidized. Second, ore fluids reacted with the rocks that were in the route of migration. Third, cooling of the ore fluids led to eventual precipitation (Brown, 2006). These mechanisms in association with the origin of the fluids by burial metamorphism and the plumbing produced by compression make this district unique to other flood basalts that yield copper sulfides rather than native copper (Bornhorst, 1997).

Porcupine Mountains Sediment-Hosted Copper District
Two sediment-hosted, stratiform copper sulfide deposits (chalcocite) are located on the margin of the Porcupine Mountains. These deposits have been mined primarily from the reduced facies of the Nonesuch Formation, but some deposits were mined between the lower conglomerate beds of the Copper Harbor Conglomerate and the upper sandstone layer of the same unit (Bornhorst and Barron, 2011). The grade of the copper was 1.14% per ton at the White Pine mine and 1.65% at the Copperwood mine (Bornhorst and Barron, 2011).
Chalcocite at White Pine mine was deposited fine grain disseminations and was concentrated in the lower 5 m of the Nonesuch Formation. The concentration of copper grades upward from chalcocite and into a fringe zone of djurleite, bornite, and chalcopyrite. Above this zone is pyrite-bearing shale (Bornhorst and Barron, 2011). Cutting through the formation are various faults related to compression and host minor amounts of native copper deposits (Brown, 2009). Overall, ineralization of chalcocite occurred during digenesis at <130 oC (Brown, 2009). The aquifer for the deposits was the Copper Harbor Conglomerate, which also is the source of the copper ore (Bornhorst and Barron, 2011). As with the Keweenaw Peninsula native copper district faults provided plumbing for the fluids, but stratigraphic thinning at the rift-basin margins also focused the fluids to the base of the Nonesuch Formation (Brown, 2009). Continued deposition caused compaction of the Copper Harbor Conglomerate, which discharged copper rich fluids from the formation. Bornhorst and Barron (2011) suggest that the thickness of sediments along strike of the mountain, the extent of faulting of the White Pine fault, and the copper-ore fluid pathway throughout diagenesis were each influenced by the Porcupine Mountain volcanic structure. This model is consistent with other copper deposits on Earth that were influenced by basin-marginal structures and faults like those of the Porcupine Mountains sediment-hosted copper district. These analogous deposits had copper brines that were sourced from red-bed clastic sedimentary rocks with low-temperature oxidizing brines that precipitated chalcocite in reduced-environment shales (Bornhorst and Barron, 2011).

Copper deposits are constrained to a long, narrow district on the Keweenaw Peninsula. Models published to explain the derivation, movement, and precipitation of copper bearing fluids share similar concepts of mechanics. A widely excepted model of ore-fluid transport describes compaction of the Portage Lake Volcanics and Copper Harbor Conglomerate as an important aspect of the source for copper-rich fluids. As the fluids entered the aquifer, hydrologic behavior became topographically influenced. According to Brown (2008), the fluid rose along updip rock layers of the aquifer established by compression. This encouraged the fluid to become focused along a 45 km belt along the Keweenaw fault. A product of compression was the hot, elevated rock of Portage Lake Volcanics, which erupted during rift extension and was quickly buried during post-rift sedimentation. Relative to crustal rock elsewhere proximal to the rift, this heated segment provided a heat source for the less dense hydrothermal fluids, which otherwise would have cooled prematurely. Also, the tilt of the Portage Lake Volcanics supplied a shorter pathway and higher flow for the rising copper-bearing fluid through permeable stratigraphy and fractures/faults (Brown, 2008). Together, these unique features produced a less dense and less inhibited fluid, which became focused on the Keweenaw Peninsula relative to other areas of the Midcontinent Rift basin where faults were not present.

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