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Prime Meridian's nickel-copper exploration program is focused along the margins of the 1,100 Ma Keweenawan Midcontinent Rift System (MRS). The Midcontinent rift has an inverted "U" shape that is clearly expressed as a gravity and magnetic anomaly that can be traced over a 2000 km distance northeast from Oklahoma and Kansas, through Iowa, Wisconsin and Minnesota, then eastward through the Lake Superior region of Michigan and Ontario, and then southward through the length of Michigan.
Rifting and related igneous activity occurred during a 32 million year period from 1109 to 1087 Ma when the rise of a new mantle plume produced crustal fracturing and extension and large volumes of basaltic magma were emplaced as lava flows and related mafic intrusions.
The location of the Midcontinent Rift System can be seen in the following diagram:
Midcontinent Rift |
Gravity Map of Central USA Showing Rift |
Prime Meridian's Michigan project areas are located in the Superior Section of the MRS. In the Lake Superior region the arcuate path of the Midcontinent rift crosses several geologic provinces. The northern extent cuts basement Archean gneiss (3.4 Ga) and Archean greenstone-granite (2.8-2.6 Ga) at the southern margin of the Superior Province. South of the Superior Province the rift crosses an extensive sequence of Early Proterozoic metasedimentary rocks (2.2-1.8 Ga) deposited at the continental margin of the Superior craton and a suture zone consisting of an island arc assemblage of volcanics, intrusives and immature sedimentary rocks.
In the Superior Section, rift-related rocks consist of three major components that are collectively known as the Keweenawan Supergroup. Rock types include 1) a thick edifice of subaerial lava flows, 2) local concentrations of intrusive rocks, and 3) an upper sequence of sedimentary rocks.
The following diagram shows a schematic cross section of the MRS through the Upper Peninsula of Michigan to Northeastern Minnesota, across the Lake Superior basin |
In Minnesota the project areas occur in the Archean-age Wabigoon greenstone belt (subprovince). The Wabigoon greenstone belt is an Archean-age (2.725 to 2.685Ga.) terrane made up of deformed supracrustal volcanic, volcanoclastic and sedimentary rocks that have undergone greenschist facies regional metamorphism and locally amphibolite facies near intrusion contacts. Bedrock lithologies consist of mafic to intermediate metavolcanic rocks and associated tuffaceous metasediments (Wimv), felsic to intermediate metavolcanic rocks and associated tuffaceous metasediments (Wfi) and banded iron formation (Wif; oxide, sulfide facies, rare silicate facies). Volcanism is bimodal and includes a predominantly mafic (tholeiitic composition) metavolcanic suite (both depleted low TiO2 and enriched high TiO2 varieties; Day, 1990) and felsic (tholeiitic to calc-alkaline composition) metavolcanic suite (dacite, rhyodacite, rhyolite and rare andesite). In the project area the volcanic sequence is dominated by bedded and graded metasediments (metagraywacke) interlayered with lesser amounts of mafic to felsic metavolcanic rocks and iron formation. The metasediments are interpreted as volcanogenic turbidities deposited in tectonically active (rifting), fault-bounded back-arc basins. Regionally extensive iron formations in the belt and the project area may be associated with bounding fault structures and mark the edges of the rifted depositional basins.
The metasedimentary-meta volcanic sequence has been intruded by coeval, pretectonic gabbro and associated anorthosite, tonalite and trondhjemite dikes, sills and medium-sized stocks. Ultramafic intrusions (peridotite/pyroxenite) have been identified but are not common.
The metavolcanic-metasedimentary sequence in Minnesota has undergone at least three episodes of deformation.
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