Midcontinent Rift Research Articles

Architecture of the hidden Penokean terrane suture and Midcontinent rift system overprint in eastern Minnesota and western Wisconsin from magnetotelluric profiling

Architecture of the hidden Penokean terrane suture and Midcontinent rift system overprint in eastern Minnesota and western Wisconsin from magnetotelluric profiling

U-Pb baddeleyite ages of key dyke swarms in the Amazonian Craton (Carajás/Rio Maria and Rio Apa areas): Tectonic implications for events at 1880, 1110 Ma, 535 Ma and 200 Ma

U-Pb baddeleyite ages for key mafic dykes of the Amazonian Craton reveal four significant intraplate episodes that allow connections with global igneous activity through time and supercontinent cycles. The oldest dykes (Carajás-Rio Maria region) are diabases with ages of 1880.2 ± 1.5 Ma and 1884.6 ± 1.6 Ma, respectively, corresponding with the Tucumã swarm which crops out to the west and is age-equivalent. The magmatic activity has a genetic link with the ca. 1.88 Ga Uatumã Silicic Large Igneous Province (SLIP), characterized by felsic plutonic-volcanic rocks. There is an age correlation with LIP events (ca. 1880 Ma) in the Superior, Slave, Indian and other cratons. This magmatism could be derived from significant perturbations of the upper mantle during the partial assembly of Columbia. Gabbronorite of the Rio Perdido Suite (Rio Apa Terrane) crystallized at 1110.7 ± 1.4 Ma, and is identical to that of the Rincón del Tigre-Huanchaca LIP event of the Amazonian Craton. This event was synchronous with the initiation of Keweenawan magmatism of central Laurentia (Midcontinent Rift) and also with coeval units in the Kalahari, Congo and India cratons. The two youngest U-Pb dates (535 and 200 Ma) occur in the Carajás region. Diabase of the Paraupebas swarm yields an age of 535.1 ± 1.1 Ma, which may be correlative with the giant Piranhas swarm located ca. 900 km apart to the west. The Paraupebas swarm is correlative with post-collisional plutonism within the Araguaia marginal belt. Therefore, the Cambrian dykes may reflect reactivation of cooled lithosphere, due to crustal extension/transtension active along the craton’s margin during assembly of West Gondwana. This magmatism is also contemporaneous with the 539–530 Ma Wichita LIP of southern Laurentia. The youngest studied Carajás region dyke was emplaced at ca. 200 Ma, corresponding with 40Ar/39Ar ages for the Periquito dykes west of Carajás and with most K-Ar ages of the giant Cassiporé swarm, located north of the study area. The newly dated ca. 200 Ma dyke fits well into the known, brief span of ages for the CAMP Large Igneous Province event, around the present central and northern Atlantic Ocean.

U–Pb baddeleyite crystallization age for a Corson diabase intrusion: possible Midcontinent Rift magmatism in eastern South Dakota

The largely buried basement of the northern Great Plains includes suture zones and terrane boundaries that represent a significant part of the growth of Laurentia in the Proterozoic. Basement exposures in this region east of the Black Hills are rare. In southeastern South Dakota, southwestern Minnesota, northeastern Nebraska, and northwestern Iowa, small outcrops of the Proterozoic Sioux Quartzite occur. In southeastern South Dakota, Corson diabase sills or dykes have intruded the quartzite. U–Pb ID–TIMS baddeleyite data from a Corson diabase sample yield an upper intercept date of 1149.4 ± 7.3 Ma, suggesting the diabase is related temporally to the Midcontinent Rift (MCR). The similarity in age of this diabase to the Inspiration sill, Pigeon River, Kipling, and Abitibi dykes suggests that early Midcontinent Rift development was not localized within the Nipigon Embayment, but extended along a roughly east–northeast zone from the Abitibi dykes to the Corson diabase. The presence of the Corson intrusions 250 km west of the MCR is hypothesized to represent a failed rift arm within the Superior craton. The greater strength of the Superior craton relative to lithosphere south of the Spirit Lake tectonic zone resulted in a shift of the southwestern rift arm in southern Minnesota along the Belle Plaine fault southeastward to the Iowa border. Alternatively, the apparent northeast trend of known occurrences of the Corson diabase is also consistent with a mantle plume centre explanation for early Midcontinent rifting.

Is the “Grenville Front” in the central United States really the Midcontinent Rift?

Is the “Grenville Front” in the central United States really the Midcontinent Rift?

Copper and sulfur isotope ratios in Paleozoic-hosted Mississippi Valley-type mineralization in Wisconsin, USA

We provide the first Cu-isotope data for chalcopyrite in Paleozoic carbonate-hosted Mississippi Valley-type (MVT) mineralization, along with corresponding sulfur isotopes, for eastern Wisconsin, USA. Chalcopyrite in this region occurs mainly as disseminated crystals and is associated with regionally extensive dolomite-hosted subeconomic MVT mineralization on the western edge of the ancestral Michigan Basin. A narrow range in δ65Cu values (0 ± 0.5‰) for chalcopyrite is observed along a large region (125 km) of the western margin of the basin that is consistent with a homogeneous Cu-source in the crust with a δ65Cu composition near 0‰, along with limited redox modification during regional flow or hydrothermal precipitation processes. Cu and S-isotope values, in conjunction with existing lead, strontium, and sulfur isotope data from other sulfide minerals in the region, are consistent with a Precambrian source similar to parts of the Midcontinent Rift System, but distinct from some other Precambrian deposits in the region.

Emplacement of ultramafic-carbonatite intrusions along reactivated North American mid-continent rift structures

Understanding the various emplacement pathways of ultramafic‑carbonatite intrusive complexes and mapping of their global distribution are in an early stage. Discerning the pattern of their emplacement can reveal information about earth's dynamics and guide exploration for associated economic deposits. Here we apply multiple spatial analysis methods and synthetic aperture radar (SAR) data to assess the correlation between outcrops of ultramafic‑carbonatite intrusives and regional craton-scale fractures and assign targets for further exploration. Kernel density cluster, two-point azimuth, and the Fry method two-dimension spatial analyses are applied to 58 known outcrops of intrusives in the Avon Volcanic District (AVD), southeast Missouri USA. The results of the spatial analysis show linear trends among the outcrops with three primary orientations: northwest, northeast, and east-west and a strong correlation with regional fault zones. SAR data processing and analysis guided by the spatial results reveals rhombic lineament patterns with sides that parallel the northwest-northeast emplacement patterns of the AVD. These rhombic structures are related to possible extensional stepover duplexes, likely formed during Precambrian North American plate rifting along transfer faults. Reactivation of the Precambrian structures during high angle dip slip and wrench faulting that was penecontemporaneous with emplacement of the AVD in the Devonian likely provided pathways for the mantle-derived intrusives. These findings can guide exploration efforts and help us understand the emplacement of a unique intracontinental intrusive complex.

Concentration of Metals Associated with the Native Copper Deposits of Northern Michigan

The Keweenaw Peninsula of northern Michigan is home to the largest accumulation of native copper in the world. Native copper deposits are hosted in the 1.1 billion-year old midcontinent rift which extends from Kansas, up through Lake Superior, around into lower Michigan, but is only exposed in the Keweenaw region. Native copper is found in the vesicular and brecciated flow tops of the volcanism associated with this rift, within the interflow conglomerate that developed in-between individual flows, and also minor native copper deposits are found with the copper sulfide deposits in the Nonesuch Shale. The purpose of this study was to examine the concentrations of zinc, lead, and silver associated with native copper in each of these depositional environments in hopes of gaining an understanding of the geochemistry of the native copper in this area. Eleven copper samples were examined using X-Ray fluorescence spectroscopy. Three geologic areas of copper deposition studied were: the brecciated and amygdaloidal flow tops of the Portage Lake Volcanics (PLV), conglomerate layers in between PLV basalt flows, and fracture fillings of the Nonesuch Shale, as seen in the White Pine Copper Mine. The volcanic flow tops exhibited decreased Zn, Pb, and Ag with increasing Cu. Within the interflow conglomerates, chemical relationships showed an increase in Zn but a decrease in Pb and Ag with increasing Cu. In both of these sets, chemical ratios of Pb/Zn decreased with increasing Cu/Ag in a decay-like curve. The native copper as fracture filling in shale had a different chemical signature in which the Ag increased with Cu. Chemical data from this study may help in our understanding of fluid composition associated with hydrothermal copper mineralization.

A REVIEW OF THE APPLICATION OF MULTIPLE S ISOTOPES TO MAGMATIC Ni-Cu-PGE DEPOSITS AND THE SIGNIFICANCE OF SPATIALLY VARIABLE Δ33S VALUES

The determination of Δ 33 S values in conjunction with δ 34 S values provides an effective means of evaluating the importance of S derivation from Archean country rocks in magmatic Ni-Cu-PGE ore genesis, particularly where δ 34 S values of potential S source rocks are near 0‰ and similar to most S of mantle derivation. However, Δ 33 S values of sulfides in some Archean rocks may also be near 0‰; care should be taken in the evaluation of S derivation from Archean rocks based solely on this parameter. Although anomalous Δ 33 S values are found primarily in Archean rocks, rare occurrences of nonzero Δ 33 S values have been identified in sulfides from younger rocks, and hence knowledge of the multiple S isotope composition of potential contaminants is essential for an accurate assessment of S contamination of mantle-derived magmas. Restricted ranges in Δ 33 S values may be characteristic of particular Archean formations, and when used in conjunction with mixing calculations may fingerprint a distinct S source rock. Spatial variability in Δ 33 S values within different types of mineralized systems may be caused by the emplacement of isotopically distinct magma pulses, by locally variable mixing proportions, and the addition of secondary sulfide to the primary assemblage. In the case of conduit-type deposits in the Midcontinent rift system, three-component mixing calculations show that both high and low Δ 33 S values in some units that are similar to end-member values in the Archean country rocks must be a function of assimilation of primarily Archean S even though Proterozoic carbonaceous sedimentary country rocks contain in excess of 1 wt % S and are particularly viable S source rocks.

Detrital zircon geochronology and provenance of the Middle Cambrian Flathead Sandstone, Park County, Wyoming

We report the results of analyses of detrital zircon from the middle Cambrian Flathead Sandstone from four locations in Park County, WY. The Flathead U-Pb zircon age spectra (n=355) includes one peak at 2702–3345 Ma (95%) and two small age peaks at 1784 Ma (4%) and ∼1830 Ma (1%). Regional paleocurrent data for the Flat-head indicate sediment transport from east to west but the dominant Archean detrital zircons in our sample suite indicates a proximal source in structurally and topographically high Wyoming Province crust. The Archean ages are consistent with the >3.0 Ga fraction being derived from basement rocks present in the northern Beartooth Range and areas further to the west. The 2.8–3.0 Ga grains were derived from the Beartooth-Bighorn magmatic zone, which directly underlies and extends to the south and east of these Flathead sampling localities. The 1.7–1.8 Ga grains were derived from 100s of km to the south and east, in the Cheyenne Belt or more distal Yavapai Province rocks. This 1.7–1.8 Ga crust was uplifted along the Transcontinental Arch during late Precambrian time. The Transcontinental Arch, and perhaps the Midcontinent Rift further to the east as well, prevented easterly-sourced (i.e. Grenville) zircons from reaching the Wyoming Sauk shoreline.

Insights into the extreme PGE enrichment of the W Horizon, Marathon Cu-Pd deposit, Coldwell Alkaline Complex, Canada: Platinum-group mineralogy, compositions and genetic implications

The W Horizon, Marathon Cu-Pd deposit in the Mesoproterozoic Midcontinent rift is one of the highest grade PGE repositories in magmatic ore deposits world-wide. The textural relationships and compositions of diverse platinum-group mineral (PGM) and sulfide assemblages in the extremely enriched ores (>100ppm Pd-Pt-Au over 2m) of the W Horizon have been investigated in mineral concentrates with ∼10,000PGM grains and in situ using scanning electron microprobe and microprobe analyses.Here we show, from ore samples with concentrations up to 23.1 Pd ppm, 8.9 Pt ppm, 1.4 Au ppm and 0.73 Rh ppm, the diversity of minerals (n=52) including several significant unknown minerals and three new mineral species marathonite (Pd25Ge9; McDonald et al., 2016), palladogermanide (Pd2Ge; IMA 2016-086, McDonald et al., 2017), kravtsovite (PdAg2S, IMA No 2016-092, Vymazalová et al., 2017). The PGM are distributed as PG-, sulfides (52vol%), -arsenides (34vol%), -intermetallics of Au-Ag-Pd-Cu and Pd-Ge(10vol%) and -bismuthides and tellurides (4vol%). The discovery of abundant (>330 grains) large unknown sulfide minerals with Rh allows us to present analyses three significant potentially new minerals (WUK-1, WUK-2, WUK-3) that are all clearly enriched in Rh (averaging 4.2, 8.5 and 28.21wt% Rh respectively). Several examples of paragenetic sequences and mineral chemical changes for enrichment of Cu, Pd and Rh with time are revealed in the PGM and base-metal sulfides. We suggest this enhanced metal enrichment formed in response to increasing fO2 causing the oxidation of Fe2+ to Fe3+ and to a lesser extent, S.Phase relations in the Ag-Pd-S, Rh-Ni-Fe-S, Pd-Ge, Au-Pd-Cu-Ag, Pd-Ag-Te systems help constrain the crystallization temperatures of the majority of ore minerals in the W Horizon at ∼500°C or moderate to high subsolidus temperatures (400–600°C). Local transport by aqueous fluids becomes evident as minerals recrystallize down to <300°C. The PGE-enriched W Horizon ores exhibit a complex post-magmatic history dominated by the effects of oxidation during cooling of a Cu-PGE enriched magma source from a deep reservoir.

Constraining the thermal history of the North American Midcontinent Rift System using carbonate clumped isotopes and organic thermal maturity indices

The Midcontinent Rift System (MRS) is a Late Mesoproterozoic (∼1.1Ga) sequence of volcanic and sedimentary rocks exposed in the Lake Superior Region of North America. The MRS continues to be the focus of much research due to its economic mineral deposits as well as its archive of Precambrian life and tectonic processes. In order to constrain the post-depositional thermal history of the MRS, samples were analyzed for carbonate clumped isotope composition and organic thermal maturity. Clumped isotope values from sedimentary/early-diagenetic samples were partially reset during burial to temperatures between 68 and 75°C. Solid-state reordering models indicate that maximum burial temperatures of 125–155°C would reset the clumped isotope values to the observed temperature range prior to the onset of regional cooling and uplift. Clumped isotope results from late-stage veins in the White Pine Mine encompass a greater temperature range (49–116°C), indicative of spatially variable hydrothermal activity and vein emplacement after burial temperatures fell below 100°C during regional cooling and uplift. Clumped isotope and organic thermal maturity data do not indicate significant spatial differences in thermal history along the MRS. Observed variability in bulk organic matter composition and biomarker indices are therefore more likely a result of shifts in primary productivity or early-degradation processes. These results demonstrate that the MRS experienced a spatially consistent, relatively mild thermal history (125–155°C) and is therefore a valuable archive for understanding the Late Mesoproterozoic environment.

A self-consistent top–down model for differentiation in bimodal suites: application to the Sonju Lake Intrusion–Finland granite system (MN)

ABSTRACTThe Sonju Lake Intrusion (SLI) is a 1200-meter thick layered mafic intrusion that directly underlies an equally large silicic pluton, the Finland granophyre (FG) within the Beaver Bay Complex of the Mid-Continent Rift (MN, USA). The SLI, with a simple mineralogical and compositional stratigraphy, provides an excellent case study for examining the changes in iron isotope ratios (δ56Fe). Here new Fe isotope data along with 87Sr/86Sr for a set of stratigraphically controlled samples from the SLI and FG are presented. The Fe isotope data show systematic changes within two differentiation sequences found in the lowermost FG as well as the upper portion of the SLI. Specifically, δ56Fe is observed to start at low values and increase to heavy values going stratigraphically up through each differentiation sequence. Within the middle portion of the SLI, δ56Fe varies between 0 and 0.1. Two samples from the SLI bottom are isotopically lighter than the middle SLI. The origin of the Fe isotope variations is discussed in terms of recently proposed explanations. A quantitative model shows that the observed spatial variation is consistent with the prediction of a temperature gradient model. Using present constraints on equilibrium phase partitioning, the iron isotope variations do not appear consistent with production by fractional crystallization. Based on these observations, a top–down sill emplacement process coupled with in situ differentiation remains a viable alternative model for forming this layered intrusion.

The end of Midcontinent Rift magmatism and the paleogeography of Laurentia

Paleomagnetism of the North American Midcontinent Rift provides a robust paleogeographic record of Laurentia (cratonic North America) from ca. 1110 to 1070 Ma, revealing rapid equatorward motion of the continent throughout rift magmatism. Existing age and paleomagnetic constraints on the youngest rift volcanic and sedimentary rocks have been interpreted to record a slowdown of this motion as rifting waned. We present new paleomagnetic and geochronologic data from the ca. 1090–1083 Ma “late-stage” rift volcanic rocks exposed as the Lake Shore Traps (Michigan), the Schroeder-Lutsen basalts (Minnesota), and the Michipicoten Island Formation (Ontario). The paleomagnetic data allow for the development of paleomagnetic poles for the Schroeder-Lutsen basalts (187.8°E, 27.1°N; A 95 = 3.0°, N = 50) and the Michipicoten Island Formation (174.7°E, 17.0°N; A 95 = 4.4°, N = 23). Temporal constraints on late-stage paleomagnetic poles are provided by high-precision, 206 Pb- 238 U zircon dates from a Lake Shore Traps andesite (1085.57 ± 0.25 Ma; 2s internal errors), a Michipicoten Island Formation tuff (1084.35 ± 0.20 Ma) and rhyolite (1083.52 ± 0.23 Ma), and a Silver Bay aplitic dike from the Beaver Bay Complex (1091.61 ± 0.14 Ma), which is overlain by the Schroeder-Lutsen basalt flows. These Michipicoten Island Formation dates are the youngest yet obtained from Midcontinent Rift volcanic rocks and indicate that rift magmatism was active for at least 25 m.y. The addition of these late-stage paleomagnetic poles to the Laurentian apparent polar wander path suggests that rapid motion of Laurentia continued throughout the entirety of rift volcanism.

Distinct crustal structure of the North American Midcontinent Rift from P wave receiver functions

AbstractEighty‐two broadband seismic stations of the Superior Province Rifting Earthscope Experiment (SPREE) collected 2.5 years of continuous seismic data in the area of the high gravity anomaly associated with the Midcontinent Rift (MCR). Over 100 high‐quality teleseismic earthquakes were used for crustal P wave receiver function analysis. Our analysis reveals that the base of the sedimentary layer is shallow outside the MCR, thickens near the flanks where gravity anomalies are low, and shallows again in the MCR's center where the gravity anomalies peak. This pattern is similar to that found from local geophysical studies and is consistent with reverse faulting having accompanied the cessation of rifting at 1.1 Ga. Intermittent intracrustal boundaries imaged by our analysis might represent the bottom of the MCR's mostly buried dense volcanic layers. Outside the MCR, the Moho is strong, sharp, and relatively flat, both beneath the Archean Superior Province and the Proterozoic terranes to its south. Inside the MCR, two weaker candidate Mohos are found at depths up to 25 km apart in the rift's center. The intermediate layer between these discontinuities tapers toward the edges of the MCR. The presence of this transitional layer is remarkably consistent along the strike of the MCR, including beneath its jog in southern Minnesota, near the Belle Plaine Fault. We interpret these results as evidence for extensive underplating as a defining characteristic of the rift, which remains continuous along the Minnesota jog, where due to its orientation, it is minimally affected by the reverse faulting that characterizes the NNE striking parts of the rift.

New Insights into North America's Midcontinent Rift

The Midcontinent Rift has characteristics of a large igneous province, causing geologists to rethink some long-standing assumptions about how this giant feature formed.

Maximum depositional age of the Neoproterozoic Jacobsville Sandstone, Michigan: Implications for the evolution of the Midcontinent Rift

A crucial constraint on the evolution of the ca. 1100 Ma Midcontinent Rift (MCR) in North America comes from the Jacobsville Sandstone, Bayfield Group, and other equivalent sedimentary rocks (JBE) that overlie the volcanics and sediments deposited in the MCR basin near Lake Superior. The MCR began extending ca. 1120 Ma and failed—ceased extending—at ca. 1096 Ma, although volcanism continued for ∼10 m.y. The JBE’s age is poorly constrained, with proposed ages ranging from ca. 1100 to ca. 542 Ma (i.e., youngest Precambrian). It has been proposed that the JBE were deposited shortly prior to or during the time when the MCR failed due to regional compression occurring ca. 1060 Ma as part of the Grenville orogeny (1300–980 Ma). However, the JBE are not conformable with the youngest rift-filling strata and differ compositionally from them. We present an analysis of 2050 new detrital-zircon ages showing that the JBE are younger than 959 ± 19 Ma. Thus, the JBE and the compression recorded in them that inverted the basin postdate the Grenville orogeny and are unrelated to the rift’s failure. The JBE may be significantly, perhaps ∼200–300 m.y., younger than the maximum age from zircons and may have been deposited shortly after a Snowball Earth event.

Anisotropic zonation in the lithosphere of Central North America: Influence of a strong cratonic lithosphere on the Mid-Continent Rift

We present shear-wave splitting analyses of SKS and SKKS waves recorded at sixteen Superior Province Rifting Earthscope Experiment (SPREE) seismic stations on the north shore of Lake Superior, as well as fifteen selected Earthscope Transportable Array instruments south of the lake. These instruments bracket the Mid-Continent Rift (MCR) and sample the Superior, Penokean, Yavapai and Mazatzal tectonic provinces. The data set can be explained by a single layer of anisotropic fabric, which we interpret to be dominated by a lithospheric contribution. The fast S polarization directions are consistently ENE-WSW, but the split time varies greatly across the study area, showing strong anisotropy (up to 1.48s) in the western Superior, moderate anisotropy in the eastern Superior, and moderate to low anisotropy in the terranes south of Lake Superior. We locate two localized zones of very low split time (<0.6s) adjacent to the MCR: one in the Nipigon Embayment, an MCR-related magmatic feature immediately north of Lake Superior, and the other adjacent to the eastern end of the lake, at the southern end of the Kapuskasing Structural Zone (KSZ). Both low-splitting zones are adjacent to sharp bends in the MCR axis. We interpret these two zones, along with a low-velocity linear feature imaged by a previous tomographic study beneath Minnesota and the Dakotas, as failed lithospheric branches of the MCR. Given that all three of these branches failed to propagate into the Superior Province lithosphere, we propose that the sharp bend of the MCR through Lake Superior is a consequence of the high mechanical strength of the Superior lithosphere ca. 1.1Ga.

Making it and breaking it in the Midwest: Continental assembly and rifting from modeling of EarthScope magnetotelluric data

A three-dimensional lithospheric-scale resistivity model of the North American mid-continent has been estimated based upon EarthScope magnetotelluric data. Details of the resistivity model are discussed in relation to lithospheric sutures, defined primarily from aeromagnetic and geochronologic data, which record the southward growth of the Laurentian margin in the Proterozoic. The resistivity signature of the 1.1Ga Mid-continent Rift System is examined in detail, in particular as relates to rift geometry, extent, and segmentation. An unrecognized expanse of (concealed) Proterozoic deltaic deposits in Kansas is identified and speculated to result from axial drainage along the southwest rift arm akin to the Rio Grande delta which drains multiple rift basins. A prominent conductor traces out Cambrian rifting in Arkansas, Missouri, Tennessee, and Kentucky; this linear conductor has not been imaged before and suggests that the Cambrian rift system may have been more extensive than previously thought. The highest conductivity within the mid-continent is imaged in Minnesota, Michigan, and Wisconsin where it is coincident with Paleoproterozoic metasedimentary rocks. The high conductivity is attributed to metallic sulfides, and in some cases, graphite. The former is a potential source of sulfur for multiple mineral deposits types, occurrences of which are found throughout the region. Finally, the imprint left within the mantle following the 1.1Ga rifting event is examined. Variations in lithospheric mantle conductivity are observed and are interpreted to reflect variations in water content (depleted versus metasomatized mantle) imprinted upon the mantle by the Keweenawan mantle plume.

Chalcophile element (Ni, Cu, PGE, and Au) variations in the Tamarack magmatic sulfide deposit in the Midcontinent Rift System: implications for dynamic ore-forming processes

The Tamarack magmatic sulfide deposit is hosted by the Tamarack Intrusive Complex (1105.6 ± 1.2 Ma) in the Midcontinent Rift System. The most important sulfide mineralization in the Complex occurs in the northern part, which consists of two separate intrusive units: an early funnel-shaped layered peridotite body containing relatively fine-grained olivine (referred to as the FGO Intrusion) at the top, and a late gabbro-troctolite-peridotite dike-like body containing relatively coarse-grained olivine (referred to as the CGO Intrusion) at the bottom. Disseminated, net-textured, and massive sulfides occur in the base of the FGO Intrusion as well as in the upper part of the CGO Intrusion. The widest part of the CGO Intrusion also hosts a large semi-massive (net-textured) sulfide ore body locally surrounded by disseminated sulfide mineralization. Small massive sulfide veins occur in the footwall of the FGO Intrusion and in the wall rocks of the CGO dike. The sulfide mineralization is predominantly composed of pyrrhotite, pentlandite, and chalcopyrite, plus minor magnetite. Pyrrhotite containing the highest Ni and Co contents occurs in the FGO disseminated sulfides and in the CGO semi-massive sulfide ores, respectively. The most important platinum-group minerals associated with the base metal sulfides are sperrylite (PtAs2), sudburyite (PdSb), and michenerite (PdBiTe). Nickel shows a strong positive correlation with S in all types of sulfide mineralization, and Cu shows a strong positive correlation with S in the disseminated sulfide mineralization. At a given S content, the concentrations of Pt, Pd, and Au in the CGO disseminated sulfides are significantly higher than those in the FGO disseminated sulfides. The semi-massive sulfide ores are characterized by significantly higher IPGE (Ir, Os, Ru, and Rh) concentrations than most of the massive sulfide ores. With few exceptions, all of the various textural types of sulfide mineralization collectively show a good positive correlation between Pt and Pd, and between individual IPGE. At a given Pt or Pd content, however, the semi-massive sulfide ores have higher IPGE contents than the disseminated sulfide samples. Modeling results show that the variations in PGE tenors (metals in recalculated 100 % sulfide) in the Tamarack magmatic sulfide deposit are mainly controlled by variable R factors (magma/sulfide-liquid mass ratios) during sulfide-liquid segregation and subsequent monosulfide solid solution (MSS) fractionation during cooling. The initial contents of Ir, Pt, and Pd in the parental magma, estimated from the metal tenors of the disseminated sulfides, are 0.2, 2, and 1.8 ppb, respectively, which are ∼1/5 of the values for the PGE-undepleted primitive basalts of the Midcontinent Rift System. The variations of PGE tenors in the semi-massive and massive sulfide ores can be explained by MSS fractional crystallization from sulfide liquids. Extreme variations in the PGE contents of the massive sulfides may also in part reflect metal mobility during post-crystallization hydrothermal processes. The higher PGE tenors for the disseminated sulfides in the CGO dike relative to those in the FGO Intrusion are consistent with formation in a dynamic conduit where the early sulfide liquids left in the conduit by the FGO magma were subsequently upgraded by the subsequent surge of the CGO magma. The relatively low PGE tenors for the semi-massive and massive sulfides can be explained by lack of such an upgrading process for the sulfide due to their distal locations in a migrating conduit.

Emplacement age and isotopic composition of the Prairie Lake carbonatite complex, Northwestern Ontario, Canada

AbstractAlkaline rock and carbonatite complexes, including the Prairie Lake complex (NW Ontario), are widely distributed in the Canadian region of the Midcontinent Rift in North America. It has been suggested that these complexes were emplaced during the main stage of rifting magmatism and are related to a mantle plume. The Prairie Lake complex is composed of carbonatite, ijolite and potassic nepheline syenite. Two samples of baddeleyite from the carbonatite yield U–Pb ages of 1157.2±2.3 and 1158.2±3.8 Ma, identical to the age of 1163.6±3.6 Ma obtained for baddeleyite from the ijolite. Apatite from the carbonatite yields the same U–Pb age of ~1160 Ma using TIMS, SIMS and laser ablation techniques. These ages indicate that the various rocks within the complex were synchronously emplaced at about 1160 Ma. The carbonatite, ijolite and syenite have identical Sr, Nd and Hf isotopic compositions with a 87Sr/86Sr ratio of ~0.70254, and positive εNd(t)1160 and εHf(t)1160 values of ~+3.5 and ~+4.6, respectively, indicating that the silicate and carbonatitic rocks are co-genetic and related by simple fractional crystallization from a magma derived from a weakly depleted mantle. These age determinations extend the period of magmatism in the Midcontinent Rift in the Lake Superior area to 1160 Ma, but do not indicate whether the magmatism is associated with passive continental rifting or the initial stages of plume-induced rifting.