Nonesuch Formation Research Articles

End-Mesoproterozoic (ca. 1.08 Ga) epeiric seaway of the Nonesuch Formation, Wisconsin and Michigan, USA

Abstract The Nonesuch Formation and related sedimentary units of the Oronto Group, southern Lake Superior region, midwestern United States, are commonly held to have been deposited in a lacustrine rift basin within interior continental Laurentia. Here, we present new sedimentologic and stratigraphic evidence that shows a marine influence on deposition. Tidally influenced shallow-marine sandstone and evaporitic, sandy and muddy tidal flat facies pass upward into fine-grained estuarine and sandy turbidite deposits, which are sharply overlain by mixed sandy and muddy tidal flat and floodplain deposits. These observations are evidence that the lower Oronto Group was deposited in an epeiric seaway, one of several such seaways that developed during the final amalgamation of Rodinia at a time of globally high sea level. Retrogradational-aggradational-progradational-degradational stratal architecture records changes in the relative balance between generation of accommodation space and sedimentation rates, which we interpret to reflect the combined influence of Grenvillian Ottawan phase tectonic subsidence and thermal subsidence from earlier Midcontinent Rift magmatism. We use this revised stratigraphic framework to show that the geochemical proxies of the Nonesuch Formation are tied closely to sedimentary facies and reflect intrabasinal redox heterogeneity rather than global anoxia at the end-Mesoproterozoic. Further, our sedimentology shows that the microfossils recovered from the Nonesuch rocks are primarily associated with tidal flat facies. The combined influence of marine and local nonmarine conditions must be considered when invoking the Nonesuch Formation, or similar marine-influenced interior basin deposits, as global analogues.

Reconstructing the paleoenvironment of an oxygenated Mesoproterozoic shoreline and its record of life

The Nonesuch Formation microbiota provide a window into ca. 1075 Ma life within the interior of ancient North America. The Nonesuch water body formed following the cessation of widespread volcanism within the Midcontinent Rift as the basin continued to subside. In northern Michigan and Wisconsin, USA, the Copper Harbor Conglomerate records terrestrial alluvial fan and fluvial plain environments that transitioned into subaqueous lacustrine deposition of the Nonesuch Formation. These units thin toward a paleotopographic high associated with the Brownstone Falls angular unconformity. Due to these “Brownstone Highlands,” we were able to explore the paleoenvironment laterally at different depths in contemporaneous deposits. Rock magnetic data constrain that when the lake was shallow, it was oxygenated as evidenced by an oxidized mineral assemblage. Oxygen levels were lower at greater depth—in the deepest portions of the water body, anoxic conditions are recorded. An intermediate facies in depth and redox between these endmembers preserves detrital magnetite and hematite, which can be present in high abundance due to the proximal volcanic highlands. This magnetic facies enabled the development of a paleomagnetic pole based on both detrital magnetite and hematite that constrains the paleolatitude of the lake to 7.1 ± 2.8°N. Sediments of the intermediate facies preserve exquisite organic-walled microfossils, with microfossils being less diverse to absent in the anoxic facies where amorphous organic matter is more likely to be preserved. The assemblage of cyanobacteria and eukaryotes (both photoautotrophs and heterotrophs) lived within the oxygenated waters of this tropical Mesoproterozoic water body.

Fluid Flow, Alteration, and Timing of Cu-Ag Mineralization at the White Pine Sediment-Hosted Copper Deposit, Michigan, USA

Abstract White Pine, located in Michigan’s Upper Peninsula, is an archetypal sediment-hosted stratiform copper deposit. The Midcontinent rift system is one of only seven basins globally that host a giant sediment-hosted stratiform copper deposit. Despite many similarities with other deposits of this type, White Pine displays some important differences, including the late Mesoproterozoic age, a thick basalt sequence, an apparent lack of evaporites, and a lacustrine depositional setting. This study analyzes paleofluid flow related to the formation of White Pine and places a particular emphasis on structural and diagenetic fluid pathways. Most of the ore is located in a 30-m-wide zone spanning the Copper Harbor Formation red beds and the overlying Nonesuch Formation shales. Sedimentation of these units was accompanied by subtle synsedimentary faulting. Premineralization phases include calcite concretions and nodules, illite and hematite grain coatings, isopachous chlorite rims, emplacement of liquid petroleum (now pyrobitumen), and bleaching. Mineralization introduced native copper into the footwall sandstones and a zoned suite of native copper and sulfur-poor copper sulfide minerals across a migrating redox front in the overlying shales where copper minerals nucleated on authigenic and detrital chlorite grains. Postmineralization phases include quartz cement, calcite cement, and calcite veins that partially overlapped inversion of synsedimentary faults. Contrary to previous studies, we identified evidence for only one phase of mineralization. An Re-Os chalcocite age of 1067 ± 11 Ma places mineralization 11 to 17 m.y. after host-rock deposition. Sulfide δ34S values of –14.0 to 29.9‰ suggest an important contribution from sour gas and thermochemical sulfate reduction of seawater. Carbon (δ13C) and oxygen (δ18O) isotope compositions of five calcite generations range from –15.1 to –1.3‰ and 10.4 to 41.3‰, respectively, and record early meteoric pore water displaced by later seawater. White Pine is both a sediment-hosted stratiform copper deposit and a paleo-oil field. Synsedimentary faults controlled the sedimentological character of the upper Copper Harbor Formation, and together these imparted a strong control on fluid flow and later diagenetic processes. Early oxidized meteoric fluids were displaced by liquid petroleum and sour gas, which were in turn succeeded by metal-rich but sulfate-poor oxidized seawater. Burial compaction during deposition of the overlying Freda Formation drove fluids through White Pine due to its situation on a paleotopographic high near the basin margin. Mineralization occurred at ~125°C at depths of ~2.0 km and spanned incipient basin inversion related to the distal effects of Grenvillian orogenesis. The hightenor copper mineral assemblage is the product of an abundant supply of metal from basaltic volcanic detritus in the Copper Harbor Formation and low seawater sulfate concentrations in late Mesoproterozoic oceans. This demonstrates that viable sediment-hosted stratiform copper systems can form when a readily leachable metal source rock is present, even if hypersaline and sulfate-rich brines are not.

Controls on the Stratiform Copper Mineralization in the Western Syncline, Upper Peninsula, Michigan

The Western Syncline hosts reduced-facies, or Kupferschiefer-type, sedimentary rock-hosted stratiform Cu deposits (SSC) in the lowermost meters of the Nonesuch Formation, which is part of a thick section of clastic sedimentary rocks that comprise the upper fill of the Mesoproterozoic Midcontinent Rift of North America. Located in the Porcupine Mountains Cu district in Upper Peninsula, Michigan, these blind deposits were discovered in 1956, but are not yet developed, although recent renewed interest may result in near-term production. The deposits are distinguished by their relatively undeformed nature and lack of superposed hydrothermal events. Prior to lithification, chalcocite mineralization replaced diagenetic pyrite within two discrete tabular, albeit discontinuous, potential orebodies referred to as the lower Cu-bearing sequence (LCBS) and the upper Cu-bearing sequence (UCBS). The Top Cu Zone transgresses lithologic boundaries, suggesting that a limited volume of Cu-bearing fluids moved vertically upwards through the unlithified stratigraphy, since reductant pyritic rocks above this zone are essentially barren of Cu. The total Cu inventory that has a reasonable expectation of economic extraction is 3678 M lbs. of Cu with 15.3 M oz. of byproduct Ag. When a cutoff grade of 0.9% Cu over a minimum thickness of 2 m is applied to justify an underground room-and-pillar mine, the LCBS and UCBS are not continuous over the Western Syncline. Sedimentology is the first-order control of potential ore and its continuity; dark-gray shales and siltstones deposited under low-energy, anoxic conditions are preferred host rocks, whose thickness must be >2 m to be potential ore since host-rock thickness determines economic viability of extraction. Furthermore, stratigraphy influences the time constraints on mineralization as the lithification process impedes vertical permeability and thus the flow of Cu-bearing fluids upward through the unlithified section. Syn-sedimentary tectonic movements, likely along pre-existing buried faults, are a third-order control as the thickness of host rocks is enhanced under such conditions. Therefore, an understanding of the depositional and tectonic history throughout the Western Syncline is fundamental to understanding the limits of possible economic exploitation and to optimizing ore extraction.

The Nonesuch Formation Lagerstätte: a rare window into freshwater life one billion years ago

The Nonesuch Formation in the clastic sedimentary Oronto Group on the Keweenaw Peninsula of the Upper Peninsula, Michigan, USA most likely represents an ancient lake that formed between 1083 and 1070 Ma. Exceptional preservation, seen in palynological preparations, provides a snapshot of cell morphology, biological complexity and ecology at an early stage in the evolution of the eukaryotes. A wide range of unicellular organization is documented in both vegetative and encysted cell morphologies, but the extent to which multicellularity is developed seems very limited at this time. Overall, the Nonesuch microbiota, when viewed as a Lagerstätte, opens up a window onto the early evolution of unicellular eukaryotes, presenting an essential baseline of both eukaryotic diversity and cell structure well in advance of eukaryotic diversification documented in marine deposits from the later Neoproterozoic. Supplementary material: Materials and methods with a detailed specimen list are available at: https://doi.org/10.6084/m9.figshare.c.5183652

Geochemical fingerprints of seawater in the Late Mesoproterozoic Midcontinent Rift, North America: Life at the marine-land divide

The 1.1 Ga Midcontinent Rift (MCR) is a thick volcanic-sedimentary succession that forms a curvilinear belt through central North America and crops out along its northern apex around Lake Superior. Sedimentary units of the MCR have been long interpreted as fluvial-lacustrine and invited a number of studies on the early evolution of life in non-marine habitats. One of the key units is the siliciclastic Nonesuch Formation, thought to record deposition in a large lake. However, recent sedimentological observations indicate the presence of marine incursions. To further test this interpretation, we analysed trace element abundances in a broad suite of samples from multiple drill cores through the Nonesuch Formation. We aimed to differentiate geochemical influences of sediment provenance from post-depositional hydrothermal overprint and thereby identify authigenic enrichments in fluid-mobile elements that are indicators of primary environmental conditions. Our results reveal discrete enrichments in Mo and U in organic- and sulphide-rich horizons, which are most parsimoniously interpreted as marine signatures. This conclusion is supported by Sr/Ba ratios, which suggest mixing between freshwater and saltwater, and by rare cm-thick gypsum in the upper Copper Harbor Formation immediately below the Nonesuch rocks. The gypsum displays δ34S values of +25.9 ± 0.6‰, consistent with input of marine sulphate at least during parts of the basin's history. Collectively, our geochemical data support the sedimentological interpretation that this portion of the MCR archives a marine-influenced estuarine system. Although this conclusion rules out that microbial life documented from the MCR was living in exclusively freshwater habitats, the Nonesuch Fm and associated rocks still hold important clues about organisms that were capable of withstanding salinity gradients and bridging the gap between the marine and non-marine environments of the mid-Proterozoic.

A marine origin for the late Mesoproterozoic Copper Harbor and Nonesuch Formations of the Midcontinent Rift of Laurentia

The c. 1.1 Ga Copper Harbor and Nonesuch Formations of the Keweenawan Supergroup exposed along the Canadian-United States shorelines of Lake Superior are part of the surface exposures of the Laurentian Midcontinent Rift. These units have long been considered non-marine in origin and have figured prominently in ideas regarding the evolution of microbial life and the redox conditions of Earth’s ocean-atmosphere system at the close of Mesoproterozoic time. However, these rocks also host hydrothermal metal deposits, the emplacement of which may have compromised primary geochemical signals that are used to underpin those ideas. Here we highlight new sedimentological observations to provide an independent framework for assessing the depositional setting and geochemistry of those strata. We show that the totality of sedimentological features leads to the conclusion that parts of the upper Copper Harbor Formation and the entirety of the Nonesuch Formation were deposited along tide- and wave-influenced shorelines and in shallow-marine settings under evaporitic conditions. Evidence for this interpretation includes the abundance of flaser, wavy, linsen and pinstripe bedding, ubiquity of reactivation surfaces and mud drapes associated with all ripple forms, superposed sets of ripple cross-lamination showing bimodal (herring-bone) sediment transport directions, desiccation cracks and metre-scale hummocky cross-stratification. Further, evaporite fabrics and pseudomorphs after gypsum in the upper 200 m of the Copper Harbor Formation and in numerous stratigraphic positions within the Nonesuch Formation indicate that the water body was saline, not fresh. The emerging palaeogeographic image is one of a large, shallow-marine embayment with fringing sabkha-like shorelines. Ideas about late Mesoproterozoic biospheric evolution and Earth’s surface redox and oxygenation that rely on the Nonesuch Formation and Copper Harbor stromatolites having been deposited within a lacustrine setting require reassessment.

Redox Chemistry and Molybdenum Burial in a Mesoproterozoic Lake

AbstractWhile marine sediments have been used to constrain a history of redox chemistry throughout the Precambrian, far fewer data have been generated from lakes. With major biological innovations thought to have occurred in Proterozoic lakes, understanding their chemistry is critical for understanding the evolution of eukaryotic life. We use sediment geochemistry to characterize the redox conditions of the Nonesuch Formation (~1.1 Ga) and a modern analogue for the Proterozoic: the Middle Island Sinkhole in Lake Huron (USA). Iron speciation, Mo contents, and Mo‐U covariation demonstrate oxic and anoxic—not euxinic—environments, with no clear indicators of enhanced biological productivity in the Nonesuch Formation. Moderate Mo enrichments observed in the Nonesuch Formation are not attributed to euxinia, but instead to an authigenic particulate shuttle. We suggest that the Fe and Mo sediment geochemistry of these lacustrine systems reflect only local water column and sediment burial conditions and not atmospheric oxygenation.

Oxygenated Mesoproterozoic lake revealed through magnetic mineralogy

Terrestrial environments have been suggested as an oxic haven for eukaryotic life and diversification during portions of the Proterozoic Eon when the ocean was dominantly anoxic. However, iron speciation and Fe/Al data from the ca. 1.1-billion-year-old Nonesuch Formation, deposited in a large lake and bearing a diverse assemblage of early eukaryotes, are interpreted to indicate persistently anoxic conditions. To shed light on these distinct hypotheses, we analyzed two drill cores spanning the transgression into the lake and its subsequent shallowing. While the proportion of highly reactive to total iron (FeHR/FeT) is consistent through the sediments and typically in the range taken to be equivocal between anoxic and oxic conditions, magnetic experiments and petrographic data reveal that iron exists in three distinct mineral assemblages resulting from an oxycline. In the deepest waters, reductive dissolution of iron oxides records an anoxic environment. However, the remainder of the sedimentary succession has iron oxide assemblages indicative of an oxygenated environment. At intermediate water depths, a mixed-phase facies with hematite and magnetite indicates low oxygen conditions. In the shallowest waters of the lake, nearly every iron oxide has been oxidized to its most oxidized form, hematite. Combining magnetics and textural analyses results in a more nuanced understanding of ambiguous geochemical signals and indicates that for much of its temporal duration, and throughout much of its water column, there was oxygen in the waters of Paleolake Nonesuch.

Sedimentology, sequence-stratigraphy, and geochemical variations in the Mesoproterozoic Nonesuch Formation, northern Wisconsin, USA

We use core descriptions and portable X-ray fluorescence analyses to identify lithofacies and stratigraphic surfaces for the Mesoproterozoic Nonesuch Formation within the Ashland syncline, Wisconsin. We group lithofacies into facies associations and construct a sequence stratigraphic framework based on lithofacies stacking and stratigraphic surfaces. The fluvial-alluvial facies association (upper Copper Harbor Conglomerate) is overlain across a transgressive surface by the fluctuating-profundal facies association (lower Nonesuch Formation). The fluctuating-profundal facies association comprises a retrogradational sequence set overlain across a maximum flooding surface by an aggradational-progradational sequence set comprising fluctuating-profundal, fluvial-lacustrine, and fluvial-alluvial facies associations (middle Nonesuch through lower Freda Formations). Lithogeochemistry supports sedimentologic and stratigraphic interpretations. Fe/S molar ratios reflect the oxidation state of the lithofacies; values are most depleted above the maximum flooding surface where lithofacies are chemically reduced and are greatest in the chemically oxidized lithofacies. Si/Al and Zr/Al molar ratios reflect the relative abundance of detrital heavy minerals vs. clay minerals; greater values correlate with larger grain size. Vertical facies association stacking records depositional environments that evolved from fluvial and alluvial, to balanced-fill lake, to overfilled lake, and returning to fluvial and alluvial. Elsewhere in the basin, where accommodation was greatest, some volume of fluvial-lacustrine facies is likely present below the transgressive stratigraphic surface. This succession of continental and lake-basin types indicates a predominant tectonic driver of basin evolution. Lithofacies distribution and geochemistry indicate deposition within an asymmetric half-graben bounded on the east by a west-dipping growth fault. While facies assemblages are lacustrine and continental, periodic marine incursions are probable, especially across maximum transgressive surfaces.We demonstrate a sequence-stratigraphic approach may be applied to fine-grained Precambrian sediments using traditional rock description and supporting lithogeochemistry. Identification of a characteristic lithofacies succession in Mesoproterozoic sediments demonstrates fundamental controls commonly interpreted for Phanerozoic lake systems may be extended into the Precambrian. These controls result in a predictable association of lithofacies, with distinct physical, biological, and geochemical properties. This has regional significance for carbon sequestration and the distribution of mineral and hydrocarbon resources and broader significance for addressing Mesoproterozoic paleogeographic reconstructions and questions related to the evolution of terrestrial life.

Palaeoecology of a billion-year-old non-marine cyanobacterium from the Torridon Group and Nonesuch Formation.

A new chroococcalean cyanobacterium is described from approximately 1‐billion‐year‐old non‐marine deposits of the Torridonian Group of Scotland and the Nonesuch Formation of Michigan, USA. Individual cells of the new microfossil, Eohalothece lacustrina gen. et sp. nov., are associated with benthic microbial biofilms, but the majority of samples are recovered in palynological preparations in the form of large, apparently planktonic colonies, similar to extant species of Microcystis. In the Torridonian, Eohalothece is associated with phosphatic nodules, and we have developed a novel hypothesis linking Eohalothece to phosphate deposition in ancient freshwater settings. Extant cyanobacteria can be prolific producers of extracellular microcystins, which are non‐ribosomal polypeptide phosphatase inhibitors. Microcystins may have promoted the retention and concentration of sedimentary organic phosphate prior to mineralization of francolite and nodule formation. This has a further implication that the Torridonian lakes were nitrogen limited as the release of microcystins is enhanced under such conditions today. The abundance and wide distribution of Eohalothece lacustrina attests to the importance of cyanobacteria as oxygen‐producing photoautotrophs in lacustrine ecosystems at the time of the Mesoproterozoic–Neoproterozoic transition.

LATENT THERMAL EFFECTS FROM PORCUPINE VOLCANICS CALDERAS UNDERLYING THE WHITE PINE-PRESQUE ISLE STRATIFORM COPPER MINERALIZATION, NORTHERN MICHIGAN

Economic stratiform copper mineralization in the fine-grained, carbonaceous basal Nonesuch Formation of the ~50-km-wide White Pine-Presque Isle mining district, northern Michigan, has long been explained by the essentially vertical infiltration of cupriferous brine across the full width of basal Nonesuch strata from the underlying Copper Harbor Conglomerate aquifer, followed by copper precipitation mainly by reactions with in situ iron sulfide. Based on that model, why do lesser amounts of total copper, measured in vertical sections, occur in coarser grained (presumably more permeable) basal Nonesuch strata situated laterally to the east and west of the mining district? The answer may lie in increased infiltration rates for Copper Harbor-hosted ore-forming brine warmed locally by latent volcanic heat from a resurgent caldera (or calderas) located within the large Porcupine Volcanics dome underlying the mine district. Not only would caldera heat have warmed the brine and lowered its density, but more importantly it would have produced a remarkable decrease in the brine viscosity. An increase in temperature from 15° to 100°C could have increased the brine infiltration rate by more than four times in very early diagenetic time when the sediments were poorly compacted and highly porous. Because progressive compaction would have opposed the temperature effect, a very early diagenetic timing is suggested for main-stage copper mineralization over the Porcupine Volcanics caldera. Textural evidence has also long supported an early mineralization event. At the rift-basin scale, the exceptionally rapid upward flux of warmed brine into fine-grained Nonesuch over the caldera would have required equivalent volumes of replacement brine, which would in turn have required a lateral convergence of brine within the footwall Copper Harbor Conglomerate aquifer toward the White Pine-Presque Isle district. This lateral focus of brine toward the warm caldera area would have created a more efficient basin-scale ore-forming system than that resulting from a linear hydrogeologic flow. Most concepts described here deserve further evaluation through quantitative hydrogeologic modeling. Thermal effects on ore brine viscosities and densities, developed here for the sediment-hosted stratiform copper mineralization of the White Pine-Presque Isle district, could be considered in genetic models for other deposits and deposit types. The effects are especially strong for solutions warmed from low temperatures (e.g., 20°C or lower) to 100°C, and could also be important to ore fluid flow for ores involving much higher hydrothermal temperatures if solutions with initially low temperatures played a role in their genesis.

The Mesoproterozoic Copperwood Sedimentary Rock-Hosted Stratiform Copper Deposit, Upper Peninsula, Michigan

Copperwood is located in Gogebic County, Upper Peninsula, Michigan, within the Porcupine Mountains copper district. It was discovered in 1956 and, in 2008, Orvana Minerals Corp began exploration and development work with production projected in the near future. Stratiform copper mineralization is hosted by gray to black shales and siltstones of the basal Nonesuch Formation. Copper mineralization is overlain by laminated gray siltstones of the Nonesuch Formation and is underlain by red-bed sandstones, conglomerates, and minor siltstones of the Copper Harbor Formation; these rocks are part of a thick section of clastic sedimentary rocks that comprise the upper fill of the Mesoproterozoic (1.1–1.0 Ga) Midcontinent rift. The copper deposit is a tabular body that averages 2.5 m thick, lacks significant basal undulations, and dips 8° to 12° northward on the southwest limb of the Western syncline, an open fold with a shallow plunge to the northwest. Chalcocite is the only Cu-bearing mineral in the orebody and occurs as concentrations along stratigraphic laminations and as disseminated grains. Within the orebody, there is no mineral or metal zoning; Cu, Ag, and S are strongly enriched, Zn is very slightly enriched and Pb and Co lack enrichment. Gangue minerals in the orebody are quartz, clinochore, muscovite/illite, plagioclase, K-feldspar, calcite, and hematite, which are consistent with temperatures of less than 130°C. Only one significant fault, a shallow NW-dipping reverse fault with displacement of up to 7 m, has disturbed the section. The mineralized zone at Copperwood is stratigraphically equivalent to that at the White Pine mine, but Copperwood lacks the structural complexity and the hydrothermal overprint manifested by the native Cu mineralization at White Pine. Reported Canadian National Instrument 43-101-compliant measured, indicated, and inferred resources are 33 million metric tonnes (Mt) of 1.60% Cu and 4.1 ppm Ag for total contained Cu of about 1.165 billion lb and 4.32 Moz of Ag. The characteristics of Copperwood are consistent with those of reduced-facies, or Kupferschiefer-type, sedimentary rock-hosted stratiform copper deposits. The proposed genetic model is chalcocite replacement of pyrite in unlithified sediments during diagenesis as a result of emanating upward-focused, compaction-driven, Cu-bearing saline basinal waters with Cu leached from the underlying red-bed paleoquifer. The simplicity of the mineralizing event and the undeformed nature of the deposit distinguish Copperwood from most other examples of reduced-facies, or Kupferschiefer-type, sedimentary rock-hosted stratiform copper deposits throughout the world.

Anoxia in the terrestrial environment during the late Mesoproterozoic

A significant body of evidence suggests that the marine environment remained largely anoxic throughout most of the Precambrian. In contrast, the oxygenation history of terrestrial aquatic environments has received little attention, despite the significance of such settings for early eukaryote evolution. To address this, we provide here a geochemical and isotopic assessment of sediments from the late Mesoproterozoic Nonesuch Formation of central North America. We utilize rhenium-osmium (Re-Os) geochronology to yield a depositional age of 1078 ± 24 Ma, while Os isotope compositions support existing evidence for a lacustrine setting. Fe-S-C systematics suggest that the Nonesuch Formation was deposited from an anoxic Fe-rich (ferruginous) water column. Thus, similar to the marine realm, anoxia persisted in terrestrial aquatic environments in the Middle to Late Proterozoic, but sulfidic water column conditions were not ubiquitous. Our data suggest that oxygenation of the terrestrial realm was not pervasive at this time and may not have preceded oxygenation of the marine environment, signifying a major requirement for further investigation of links between the oxygenation state of terrestrial aquatic environments and eukaryote evolution.

Age and genesis of the White Pine stratiform copper mineralization, northern Michigan, USA, from paleomagnetism

AbstractAt White Pine, the stratiform sedimentary copper (SSC) mineralization occurs mostly as chalcocite in gray‐black shales in the basal approximately 6 m of the 1081 ± 9 Ma Nonesuch Formation of the Keweenawan Supergroup. Paleomagnetic analysis of 176 specimens (19 sites) isolated collinear characteristic remanent magnetization (ChRM) directions in both magnetite and hematite in both the oxidized SSC zones and overlying pyritic and upper zones of the Nonesuch Formation. A paleomagnetic fold test shows that the normal‐polarity ChRM is prefolding in origin. Combining the ChRM directions from this study (18 sites) and from (Canadian Journal of Earth Sciences, 14, 1977, 1128; 11 sites), the Nonesuch mineralized and nonmineralized sites give a paleopole at 7.3°N, 174.7°E (N = 29, A95 = 3.0°), yielding an age of 1063 ±8 Ma on the Keweenawan apparent polar wander path. The observed 1063 ± 8 Ma age is interpreted to date both the average age of oxidation and mineralization of the SSC zone. Thus, ore genesis occurred after burial by the overlying Freda Formation and after conversion by oxidation of the underlying original clastic sediments of the Copper Harbor Formation to red beds, most likely by gravity‐driven recharge of subsurface brines by meteoric water from adjacent highlands on the southeast side of the Keweenawan Rift.

GENESIS OF NATIVE COPPER LODES IN THE KEWEENAW DISTRICT, NORTHERN MICHIGAN: A HYBRID EVOLVED METEORIC AND METAMOPHOGENIC MODEL

The Keweenaw Peninsula of northern Michigan is well known for early diagenetic sulfide-dominant sediment-hosted stratiform copper mineralization hosted mainly by basal siltstone-shale units of the Nonesuch Formation in the White Pine district, and later native copper lodes hosted mainly by flow-top breccias and amygdaloidal portions of flood basalts and interbedded conglomerates of the stratigraphically lower Portage Lake Volcanics. The native copper mineralization is generally attributed to downdip burial metamorphism of the host strata. An alternative model is proposed here, involving a complementary influx of evolved meteoric water (elevated in salinity and progressively equilibrating with its aquifers), similar to the fluid which formed the White Pine mineralization. This model has several advantages over the metamorphogenic model alone. It would provide the salinity (by dissolution of evaporites) and moderately oxidizing conditions (as a result of hematitization, i.e., reddening, of coarse continental rift sediments) necessary for copper transport, and the evolved meteoric water would have been an important cupriferous brine owing to the leaching of trace amounts of copper from the voluminous aquifers. Further reduction of the evolved fluid would also explain the updip deposition of native copper in association with dehematitized pumpellyite- and chlorite-dominant (i.e., reduced) hydrothermal mineral assemblages. At the same time, this influx of meteoric water from highlands adjacent to the Keweenaw rift basin would have provided a mechanism to move the deep-seated hot, dense ore brine upward along the aquifers of the Portage Lake Volcanics. On its ascent from deep portions of the rift basin, the gravity-driven evolved meteoric brine would have mixed with the fluid generated by burial metamorphism to form a hybrid evolved meteoric-metamorphogenic oreforming hydrothermal brine.

A hydrogeologic model of stratiform copper mineralization in the Midcontinent Rift System, Northern Michigan, USA

AbstractThis paper presents a suite of two‐dimensional mathematical models of basin‐scale groundwater flow and heat transfer for the middle Proterozoic Midcontinent Rift System. The models were used to assess the hydrodynamic driving mechanisms responsible for main‐stage stratiform copper mineralization of the basal Nonesuch Formation during the post‐volcanic/pre‐compressional phase of basin evolution. Results suggest that compaction of the basal aquifer (Copper Harbor Formation), in response to mechanical loading during deposition of the overlying Freda Sandstone, generated a pulse of marginward‐directed, compaction‐driven discharge of cupriferous brines from within the basal aquifer. The timing of this pulse is consistent with the radiometric dates for the timing of mineralization. Thinning of the basal aquifer near White Pine, Michigan, enhanced stratiform copper mineralization. Focused upward leakage of copper‐laden brines into the lowermost facies of the pyrite‐rich Nonesuch Formation resulted in copper sulfide mineralization in response to a change in oxidation state. Economic‐grade mineralization within the White Pine ore district is a consequence of intense focusing of compaction‐driven discharge, and corresponding amplification of leakage into the basal Nonesuch Formation, where the basal aquifer thins dramatically atop the Porcupine Mountains volcanic structure. Equilibrium geochemical modeling and mass‐balance calculations support this conclusion. We also assessed whether topography and density‐driven flow systems could have caused ore genesis at White Pine. Topography‐driven flow associated with the Ottawan orogeny was discounted because it post‐dates main‐stage ore genesis and because recent seismic interpretations of basin inversion indicates that basin geometry would not be conductive to ore genesis. Density‐driven flow systems did not produce focused discharge in the vicinity of the White Pine ore district.

The 1.1-Ga Midcontinent Rift System, central North America: sedimentology of two deep boreholes, Lake Superior region

Abstract The Midcontinent Rift System (MRS) of central North America is a 1.1-Ga, 2500-km long structural feature that has been interpreted as a triple-junction rift developed over a mantle plume. As much as 20 km of subaerial lava flows, mainly flood basalts, are overlain by as much as 10 km of sedimentary rocks that are mostly continental fluvial red beds. This rock sequence, known as the Keweenawan Supergroup, has been penetrated by a few deep boreholes in the search for petroleum. In this paper, two deep boreholes in the Upper Peninsula of Michigan are described in detail for the first time. Both the Amoco Production #1-29R test, herein referred to as the St. Amour well, and the nearby Hickey Creek well drilled by Cleveland Cliffs Mining Services, were 100% cored. The former is 7238 ft (2410 m) deep and the latter is 5345 ft (1780 m) deep. The entirety of the stratigraphic succession of the Hickey Creek core correlates very well with the upper portion of the St. Amour core, as determined by core description and point-counting of 43 thin sections selected out of 100 studied thin sections. Two Lower Paleozoic units and two Keweenawan red bed units—the Jacobsville Sandstone and the underlying Freda Sandstone—are described. The Jacobsville is largely a feldspatholithic sandstone and the Freda is largely a lithofeldspathic sandstone. Below the Freda, the remaining footage of the St. Amour core consists of a thick quartzose sandstone unit that overlies a heterogenous unit of intercalated red bed units of conglomerate, sandstone, siltstone, and shale; black shale; individual basalt flows; and a basal ignimbritic rhyolite. This lower portion of the St. Amour core presents an enigma, as it correlates very poorly with other key boreholes located to the west and southwest. While a black shale sequence is similar to the petroleum-bearing Nonesuch Formation farther west, there is no conglomerate unit to correlate with the Copper Harbor Conglomerate. Other key boreholes are distributed over a 1300-km distance along the better known southwest arm of the triple-junction MRS, and can be correlated rather well with the units that are exposed in the Lake Superior region. However, a definitive explanation of the anomalous, deeper St. Amour stratigraphy is elusive and any explanation is tenuous. A possible explanation for this anomalous stratigraphy may be the geographic proximity of the St. Amour borehole to the Keweenawan Hot Spot (mantle plume), the suggested thermal force behind the development of the MRS. Similarly, a drastic change in structural architecture may be explained by this geographic relationship. Thus, within the locale of this rifting center, complexities of expansion tectonics may well be responsible for igneous and sedimentary sequences that differ considerably from those found farther west along the rift arm.

The role of basin-scale transgression and sediment compaction in stratiform copper mineralization: implications from White Pine, Michigan, USA

We present a paleohydrologic model of stratiform copper mineralization in the Keweenawan Midcontinent Rift System. We attribute regional-scale mineralization in the basal facies of the transgressive Nonesuch Formation to compaction of the subjacent fluvial Copper Harbor Formation in response to deposition of the superjacent fluvial Freda Formation. The transgressive sequence focused compaction-driven discharge of cupriferous brines in the Copper Harbor Formation to the basin margins, where leakage into the overlying Nonesuch Formation drove redox-controlled copper precipitation. Intense fluid focusing atop a basement high formed the White Pine ore body. The timing of this simulated event is consistent with the inferred timing of stratiform mineralization. Our model satisfies mass-balance constraints at White Pine for a geologically plausible range of permeabilities and copper concentrations.

Relationship between organic matter and copper mineralization in the Proterozoic Nonesuch Formation, northern Michigan

Comparison of organic and inorganic geochemical data of stratigraphically equivalent, mineralized and unmineralized samples from strata hosting the White Pine stratiform copper deposit in northern Michigan provides insight into the ore forming process. Whole rock and organic extract analyses were performed on mine and core samples from five gray beds (“stripey” shale, “UZV” siltstone, “thinly” shale, “DGM” siltstone, “domino” shale) of the Proterozoic Nonesuch Formation and from the “lower” sandstone of the underlying Copper Harbor Conglomerate. Bitumen is degraded in both mineralized and unmineralized samples of the “lower” Nonesuch Formation. Presumably, this degradation is not a direct result of the mineralizing process, but represents either regional-scale water-washing or biodegradation. In contrast, the pristane/n-C 17 and phytane/n-C 18 ratios are lower in mineralized samples than in unmineralized samples, suggesting thermal maturation of the “lower” Nonesuch Formation during mineralization. Copper mineralization most likely proceeded via three mechanisms: (1) deposition of native copper; (2) conversion of precursor pyrite to chalcocite; and (3) deposition of chalcocite with copper and sulfur derived entirely from mineralizing fluids. A positive correlation between organic carbon and copper contents in mineralized strata implies that organic matter served as a reducing agent during copper mineralization. However, redox reactions must have conserved total organic carbon since the total organic carbon contents of mineralized and unmineralized samples are similar.