Negaunee Iron Formation Research Articles

Multiple episodes of hematite mineralization indicated by U-Pb dating of iron-ore deposits, Marquette Range, Michigan, USA

Abstract High-grade hematite ore (so-called “hard ore”) was first mined in the Lake Superior region of North America in A.D. 1848. The development of ore deposit models from the region has strongly influenced research on the origin of this deposit type worldwide. The inability to date high-grade hematite ores has hindered the testing of competing genetic models. In the Marquette Range, northern Michigan (USA), high-grade hematite ore is hosted by the 1.9–1.85 Ga Negaunee Iron Formation and unconformably overlying Goodrich Quartzite. Field relations and new in situ U-Pb geochronology of monazite and xenotime intergrown with hematite from six mines across the Marquette Range, Michigan, indicate that the formation of high-grade hematite ore bodies involved up to three events. The presence of pebbles of hematite ore in basal conglomerate of the Goodrich Quartzite indicates that the first episode of iron-oxide mineralization occurred after deposition and deformation of the Negaunee Iron Formation, but before the ca. 1.85 Ga Sudbury impact. Monazite in iron-ore deposits within the hinges of Penokean-aged folds yields an age of ca. 1.8 Ga, which is interpreted to record a second phase of hematite enrichment. Monazite and xenotime in iron-ore deposits along the limbs of regional folds give ages of ca. 1.77 Ga and are interpreted to record a later episode of hematite upgrading. The ca. 1.8 Ga and ca. 1.77 Ga phosphate ages coincide with the timing of regional tectonic events, suggesting hematite mineralization was coeval with the Penokean orogeny and Yavapai-interval accretion. Our results support the proposal that the formation of high-grade hematite ore bodies is a multistage process linked to discrete orogenic events.

U–Pb geochronology of baddeleyite from the Belleview metadiabase: Age and geotectonic implications for the Negaunee Iron Formation, Michigan

The Belleview metadiabase sill crosscuts the Siamo Slate and the Negaunee Iron Formation of the Marquette Range Supergroup, Michigan. It is one of a series of metamorphosed diabase sills that intrudes the Menominee Group and possibly the Baraga Group. Chloritic sills within the Marquette Range Supergroup have yet to be officially grouped as one or more series or swarms and therefore stratigraphic correlations and temporal relationships between these sills remain speculative. The recovery of the first sensitive high-resolution ion mass spectrometry (ID-TIMS) U–Pb date of 6 baddeleyite grains from the Belleview sill yielding an age of 1891±3Ma is a positive step to classifying these chloritic metadiabases. This new date has three important implications: (1) it gives a definitive minimum age of Negaunee Iron Formation deposition; (2) it confirms recent stratigraphic correlations that suggest that the Negaunee Iron Formation is older than the Mesabi, Gunflint and Gogebic Iron Ranges and not contemporaneous; and (3) it does not support recent correlations of the Menominee Group chloritic sills with the 1874Ma Hemlock Formation within the Baraga Group of the Marquette Range Supergroup.This new date does not support a more recently proposed synorogenic foredeep depositional environment for the deposition of the Negaunee Iron Formation. Instead, this age date supports an earlier interpretation that the Negaunee Iron Formation is part of continental shelf deposits laid down prior to onset of 1875–1835Ma collision. The age of the Belleview sill suggests that it is the result of extension-related magmatism that is prior to the currently defined Penokean orogenic event.

Sedimentary and metamorphic lithofacies of the Lower Negaunee Iron Formation, Marquette District, Michigan, USA

The base of the Proterozoic Negaunee Iron Formation is exposed in the open pit at Tilden Mine, Marquette, Michigan. Juxtaposed against the Archean-aged Palmer Gneiss, it is bounded by the regional-scale Southern Shear Zone and cut by two sets of dykes: an older chloritic and schistose set and a younger 1.1 Ga Keweenawan set. Tilden Mine is dominated by a 100 m scale plunging northwest-anticline and is cut by a growth fault locally termed the Tower Hill Fault that intersects the Southern Shear Zone. The base of the exposed iron formation is composed of three lithofacies, including lower clastics that grade into the overlying banded iron formation that in turn grades upward into granular iron formation. This succession is capped by chloritic metadiabases locally termed the Summit Hill Sill and Pillar Intrusive. Petrographic and mineral chemical investigations document primary or early diagenetic hematite, siderite and possibly ferri-hydrite, metamorphic and related hydrothermal magnetite, chlorite, late martite overgrowing earlier magnetite and growth of specularite. All three lithofacies are cut by brittle fractures and late quartz veins. Brittle fractures are coated with chlorite, carbonate minerals, fluor-apatite, and sparse Cu-sulphides. These lithofacies document initial clastic sedimentation of strained detrital quartz into a subsiding fault trough. Over time, as subsidence slowed or sea level fluctuated, clastic deposition competed with quiescent chemical sedimentation, leading to deposition of the banded iron formation facies. As a stable shelf platform emerged, the granular iron formation facies was deposited via wave reworking of hardgrounds. Subsequent diagenesis initiated dissolution of carbonate and chert and promoted diagenetic replacement of primary iron minerals and chert. Regional metamorphism during Penokean orogeny at 1875–1835 Ma produced a suite of secondary metamorphic and related hydrothermal minerals. Metamorphism and hydrothermal flux related to the 1750 Ma development of the Republic Metamorphic Node overprinted the iron formation at Tilden to greenschist facies and infilled brittle fractures with a unique mineral assemblage. This unique mineral assemblage exhibits some striking similarities to Mn, Au, and Cu-sulphides documented at Champion Mine, west of Tilden, and proximal to the core of the Republic Node.

Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region

A rhyolite in the Hemlock Formation, a mostly bimodal submarine volcanic deposit that is laterally correlative with the Negaunee Iron-formation, yields a sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon age of 1874 ± 9 Ma, but also contains inherited Archean zircons as old as 3.8 Ga. This precise age determination for the classic Paleoproterozoic stratigraphic sequence of northern Michigan, the Marquette Range Supergroup (MRS), necessitates modification of previous depositional and tectonic models. Our new data indicate that the Menominee Group, previously ascribed to continental rifting during early, pre-collision phases of the Penokean orogenic cycle, is coeval with arc-related volcanic rocks now preserved as accreted terranes immediately to the south and is more aptly interpreted as a foredeep deposit. We interpret these to be second-order basins created by oblique subduction of the continental margin rather than basins formed on a rifting margin. Along with a recently reported age for the Gunflint Formation in Ontario of 1878 ± 2 Ma, our data suggest that an extensive foredeep in the western Lake Superior region was the locus of iron-formation deposition during arc accretion from the south. Further, we interpret the lower MRS (Chocolay Group), a glaciogenic and shallow-marine succession that lies atop Archean basement, to be equivalent to the upper part of the Huronian Supergroup of Ontario and to represent the original continental rifting and passive-margin phase of the Penokean cycle. The upper MRS (Baraga Group) represents deeper marine basins, dominated by turbidites and lesser volcanic rocks, resulting from increased subsidence and continued collision. A stitching pluton, which cuts correlatives of the Hemlock Formation in a thrust sheet, yielded a U–Pb zircon age of 1833 ± 6 Ma, consistent with other post-tectonic plutons in Wisconsin and northern Michigan, indicating that Penokean convergence lasted no longer than ~40 million years.

Neoproterozoic fossils from the Franklin Mountains, northwestern Canada: stratigraphic and palaeobiological implications

Shales from the Lone Land Formation of the Cap Mountain inlier, southern Franklin Mountains, and ‘pre-Palaeozoic’ horizons of a nearby drillhole (Shell Blackwater Lake G-52) have yielded two distinct fossil assemblages. The presence of Trachyhystrichosphaera aimika (in G-52) and Chuaria circularis (in the Lone Land) identifies both sections as Neoproterozoic; previous lithostratigraphic correlations with Mesoproterozoic strata on the Coppermine Homocline are not supported. Marked differences in thermal alteration of the fossils suggest that the two successions are not related despite their present geographic proximity; whereas the G-52 fossils are medium orange in colour, those in the Lone Land are brown. We argue that the Cap Mountain succession is allochthonous and originated probably to the west of its present position. By contrast, the G-52 assemblage shows marked similarity to a fossil in the Wynniatt Formation (Shaler Supergroup) of Victoria Island. Compound and/or coiled filaments are abundant in the G-52 assemblage and appear to be associated generally with exceptional fossil biotas in the Neoproterozoic; insofar as these are likely to document particular environmental conditions, they may help to identify the locus of early eukaryotic diversification. The tendency of some prokaryotes to aggregate and form compound structures also has implications for interpreting various problematic macrofossils, including putative seaweeds from the Palaeoproterozoic Negaunee Iron Formation. We erect a new genus and species, Gemmuloides doncookii n. gen., n. sp. from the Lone Land Formation.

Algae and Oxygen in Earth's Ancient Atmosphere

T.-M Han and B. Runnegar report finding eukaryotic microfossils, tentatively identified as Grypania, in the 2.1-billion-year-old Negaunee iron formation in northern Michigan. They suggest that free oxygen (O[sub x]) was a significant component of the atmosphere between 2.0 and 2.5 billion years ago on the basis of the metabolic requirements of these organisms. This argument does not account for the possibility that ocean surface waters might have been locally enriched in photosynthetically derived O[sub 2], while the atmosphere itself remained free of O[sub 2]. Present rates of marine photosynthesis and of diffusive loss of O[sub 2] through the ocean-atmosphere interface suggest that dissolved O[sub 2] concentrations could have been up to eight times higher than the 0.01 present atmospheric level that is needed to sustain Grypania. The existence of an atmosphere free of O[sub 2] before about 2.0 billion years ago would be consistent with several other types of geologic evidence. 3 refs.

Megascopic eukaryotic algae from the 2.1-billion-year-old negaunee iron-formation, Michigan.

Hundreds of specimens of spirally coiled, megascopic, carbonaceous fossils resembling Grypania spiralis (Walcott), have been found in the 2.1-billion-year-old Negaunee Iron-Formation at the Empire Mine, near Marquette, Michigan. This occurrence of Grypania is 700 million to 1000 million years older than fossils from previously known sites in Montana, China, and India. As Grypania appears to have been a photosynthetic alga, this discovery places the origin of organelle-bearing eukaryotic cells prior to 2.1 billion years ago.

Early proterozoic geology of the great lakes region

The Belleview metadiabase sill crosscuts the Siamo Slate and the Negaunee Iron Formation of the Marquette Range Supergroup, Michigan. It is one of a series of metamorphosed diabase sills that intrudes the Menominee Group and possibly the Baraga Group. Chloritic sills within the Marquette Range Supergroup have yet to be officially grouped as one or more series or swarms and therefore stratigraphic correlations and temporal relationships between these sills remain speculative. The recovery of the first sensitive high-resolution ion mass spectrometry (ID-TIMS) U–Pb date of 6 baddeleyite grains from the Belleview sill yielding an age of 1891 ± 3 Ma is a positive step to classifying these chloritic metadiabases. This new date has three important implications: (1) it gives a definitive minimum age of Negaunee Iron Formation deposition; (2) it confirms recent stratigraphic correlations that suggest that the Negaunee Iron Formation is older than the Mesabi, Gunflint and Gogebic Iron Ranges and not contemporaneous; and (3) it does not support recent correlations of the Menominee Group chloritic sills with the 1874 Ma Hemlock Formation within the Baraga Group of the Marquette Range Supergroup.This new date does not support a more recently proposed synorogenic foredeep depositional environment for the deposition of the Negaunee Iron Formation. Instead, this age date supports an earlier interpretation that the Negaunee Iron Formation is part of continental shelf deposits laid down prior to onset of 1875–1835 Ma collision. The age of the Belleview sill suggests that it is the result of extension-related magmatism that is prior to the currently defined Penokean orogenic event.

Metamorphic petrology of the Negaunee Iron Formation, Marquette District, northern Michigan; mineralogy, metamorphic reactions, and phase equilibria

Progressive regional metamorphism of the Negaunee Iron Formation in the Marquette district from chlorite to sillimanite grade has resulted in the development of three metamorphic zones and two subzones. The diagnostic zone assemblages are, in order of increasing grade: 1, siderite + quartz + or - ankerite + or - minnesotaite + or - stilpnomelane + or - magnetite; 2, grunerite + or - quartz + or - ankerite + or - magnetite and grunerite + or - siderite + or - ankerite + or - magnetite; 3a, grunerite + or - garnet + or - quartz + or - magnetite and grunerite + or - Ca amphibole + or - quartz + or - calcite + or - magnetite; 3b, grunerite + or - olivine + or - clinopyroxene + or - quartz + or - calcite + or - magnetite and grunerite + or - Ca amphibole + or - olivine + or - magnetite. The grunerite, garnet, and Ca amphibole, and olivine and clinopyroxene isograds define the boundaries between the respective zones and subzones. Temperature estimates are approximately 300 degrees C for zone 1 and approximately 600 degrees C for subzone 3b. Pressure is estimated at between 200 to 300 MPa. Microprobe analyses of coexisting minerals indicate general attainment of exchange equilibrium and a strong localized influence of bulk composition on mineral chemistry.Theoretical analysis of phase relations on isobaric T-X (sub CO 2 ) diagrams leads to the following conclusions:1. Throughout zone 1, minnesotaitc-bearing assemblages are diagnostic of low X (sub CO 2 ) values, stilpnomelane-bearing assemblages represent low to medium values, and chlorite-bearing assemblages represent medium to high X (sub CO 2 ) values. Juxtaposition of these three assemblages at one locality indicates that fluid phase compositional gradients most likely existed.2. Throughout zone 3, actinolite-, hornblende-, and garnet-bearing assemblages reflect similar low to high X (sub CO 2 ) values. The continued juxtaposition of these various assemblages suggests the continued presence of fluid-phase compositional gradients into the highest grade metamorphic zones.3. Metamorphic reactions caused substantial, localized CO 2 enrichment in the coexisting fluid phase. A diversity of T-X (sub CO 2 ) paths was followed, typically differing on the scale of individual mesobands.

Hard iron ore of the Marquette Range, Michigan

Hard iron ore on the Marquette Range consists of nearly pure masses of specularite, magnetite, or martite and occurs near the unconformable contact of the Negaunee Iron-Formation with the overlying Goodrich Quartzite. Most ore is in an upper jaspilite unit of the Negaunee, but some orebodies are entirely within the basal part of the Good-rich Quartzite or span the contact of the two formations.In the past, hard ore has been considered a single ore class having a single mode of origin. It is argued here, however, that it is three distinct types of ore, each having a different mode of origin.Specularite ore varies from virtually pure specularite to ore containing as much as about 10 percent magnetite. It was formed by the leaching of silica from the Negaunee Iron-Formation and the precipitation of iron oxide in its place during the period of erosion immediately after deposition of the iron-formation, and it was later deformed and metamorphosed to its present hard specular form.Magnetite ore is nearly pure magnetite, containing in places as much as 10 percent specularite. It was formed during regional metamorphism and localized in the upper jaspilite unit of the Negaunee because of oxidation by the jaspilite of iron-bearing metamorphic fluids generated in lower parts of the iron-formation. Flow of the fluids was concentrated along structurally or stratigraphically controlled channelways, and magnetite was precipitated as iron solubility was lowered by oxidation of the fluid.Clastic ore contains specularite and lesser amounts of magnetite. It is a placer concentrate of iron oxides in the basal Goodrich Quartzite. The iron oxides in the ore were derived from the Negaunee Iron-Formation and were concentrated during the transgression marked by the onset of Goodrich deposition.Hard ore accounted for about 20 percent of high-grade ore production from the Marquette Range; about 67 million tons were produced. It is no longer mined, except incidentally at the Republic open pit where hard-ore bodies are within the taconite presently being mined as concentrating-grade ore.

Stratigraphic Zones in the Negaunee Iron Formation of Marquette County, Michigan

The Negaunee iron formation, heretofore treated as a lithologic unit, may be divided into four stratigraphic zones in the area studied. Beginning with the oldest these are: the Makasin Hill zone, characterized by straight bedding throughout and abundant martite in the lower one-third; the North Lake zone, characterized by lenticular groups of strata having concretionary structure, interbedded in a straight bedded formation; the Corning Creek zone, characterized by large, lenticular, wavy-bedded bodies, interbedded in a straight bedded formation; and the Jasper Knob zone, characterized by straight bedding throughout and abundant red jasper, specularite, and magnetite or martite at least in the upper portion, and in most places extending all the way to its base. At the base of this zone there is a thick uralitic dolerite sill of which the possible genetic relation to the distinctive mineralogy of the zone is considered. Structural relations brought out by the subdivision of the Negaunee formation are also d...