Southeastern Manitoba Research Articles

<i>Frullania stylifera</i> (Frullaniaceae), a new addition to the liverwort flora of Canada

Frullania stylifera (R.M. Schust.) R.M. Schust. (Frullaniaceae) is newly reported for Canada. The species grows closelyappressed to the bark of trees and shrubs and was recently documented from two areas of mature forest at Buffalo Point First Nation in extreme southeastern Manitoba, Canada. These localities represent a northern range extension for the species on the continent and include two new phorophyte hosts for North America. Features used to distinguish F. stylifera from other known species of Frullania in Manitoba are provided.

Treaty through a Planting Lens: A Study of Manoomin Harvesting Rights in Anishinaabe-Aki, 1873–Present

Abstract: In 1873, the Crown and Anishinaabeg entered Treaty No. 3. This agreement would shape Canadian settlement in what is currently known as northwestern Ontario and southeastern Manitoba. For generations, Anishinaabeg have maintained that manoomin harvesting rights are protected by Treaty No. 3. Colonial officials, by contrast, have narrowly interpreted the archival record and associated references to "gardens" with vegetable cultivation for household use and "farming" with the large-scale cultivation of introduced crops like wheat, barley, and oats thus limiting the ability of First Nations to protect ancestral fields from provincial encroachment. In this article, Anishinaabe ecological knowledge around plant life cycles and needs (i.e., botanical gikendaasowin) is applied to the archival record, revealing how Anishinaabe leaders protected ancestral fields while negotiating access to introduced seeds to diversify food production. Federal activity in the immediate aftermath of 1873 further suggests that the Crown recognized the treaty right to harvest manoomin. Canada's historic attempts to curtail the manoomin harvest after 1876, as well as Ontario's attempts to manage ancestral fields after 1960, constitute violations of Treaty No. 3 when viewed through an Anishinaabe planting lens.

Mercury Contamination of an Introduced Generalist Fish of Intermediate Trophic Level.

Mercury contamination is a global issue because mercury concentrations in aquatic systems are influenced by both natural and anthropogenic pathways. Here, liver and muscle total mercury (THg) concentrations in black crappie Pomoxis nigromaculatus from three boreal lakes in southeastern Manitoba, Canada, were related to age, morphology and physiological traits to better understand the dynamics of mercury accumulation in an introduced generalist fish species. These THg concentrations were then compared to black crappie mercury concentrations in other Canadian water bodies and to mercury concentrations in other freshwater fishes in southeastern Manitoba. Age and size had strong positive correlations (P < 0.001, r ≥ 0.60) with muscle mercury concentrations. No evidence of acute point source contamination or physiological impairment in black crappie was found in the study area. Analysis of liver THg revealed the possible impacts of seasonal and ontogenetic differences in diet on exposure. Furthermore, THg analysis of liver and muscle tissue showed how generalist foraging may curb the progressively greater mercury exposure and resultant physiological consequences expected from ontogenetic diet shifts in black crappie. Although there appeared to be temporally varied levels of mercury exposure (i.e., liver THg) by sex, there was no sex effect observed in long-term mercury accumulation in the muscle. Black crappie bioaccumulated less mercury at age than primary piscivore species in the region. These results will help foster a better understanding of mercury biomagnification within a region impacted by legacy mercury.

Effect of biofuel waste, urea, deer browsing, and vegetation control on Pinus banksiana seedling growth on a dry upland site in central Canada

Ash from biofuels and nitrogen fertilizer are increasingly being used as soil amendments. While this can increase tree growth, it can also increase mammalian grazing and competition with vegetation. We applied moderate amounts of ash (1.5 t·ha−1·year−1) and urea (74 kg N·ha−1·year−1) in each of 2 years to a well-drained site in southeastern Manitoba, planted with Pinus banksiana Lamb. Subplots received deer browsing and (or) vegetation control. The ash resulted in an increase in pH in the upper 15 cm of mineral soil from ∼5.7 to 6.6, and the urea created short-term spikes in soil inorganic nitrogen (NH4 and NO3) levels. Urea combined with ash significantly increased seedling relative growth rates in the first 2 years, with seedlings being largest with urea, with or without ash. However, by the fourth year, seedling growth and size did not differ between the amendments. Urea application increased browsing damage to 91%, but only when vegetation was mowed. Browsing guards resulted in seedlings having 1.6 times greater shoot mass by the end of the fourth growing season. These results suggest that on sandy soils in the dry region of central Canada, P. banksiana may get little benefit from ash applications.

Deep Geothermal Heating Potential for the Communities of the Western Canadian Sedimentary Basin

We summarize the feasibility of using geothermal energy from the Western Canada Sedimentary Basin (WCSB) to support communities with populations &gt;3000 people, including those in northeastern British Columbia, southwestern part of Northwest Territories (NWT), southern Saskatchewan, and southeastern Manitoba, along with previously studied communities in Alberta. The geothermal energy potential of the WCSB is largely determined by the basin’s geometry; the sediments start at 0 m thickness adjacent to the Canadian shield in the east and thicken to &gt;6 km to the west, and over 3 km in the Williston sub-basin to the south. Direct heat use is most promising in the western and southern parts of the WCSB where sediment thickness exceeds 2–3 km. Geothermal potential is also dependent on the local geothermal gradient. Aquifers suitable for heating systems occur in western-northwestern Alberta, northeastern British Columbia, and southwestern Saskatchewan. Electrical power production is limited to the deepest parts of the WCSB, where aquifers &gt;120 °C and fluid production rates &gt;80 kg/s occur (southwestern Northwest Territories, northwestern Alberta, northeastern British Columbia, and southeastern Saskatchewan. For the western regions with the thickest sediments, the foreland basin east of the Rocky Mountains, estimates indicate that geothermal power up to 2 MWel. (electrical), and up to 10 times higher for heating in MWth. (thermal), are possible.

Bryophytes at the Western Limits of Canada's Great Lakes Forest: Floristic Patterns and Conservation Implications

The Great Lakes–St. Lawrence Forest Region in Canada occupies a narrow band of mixed forest from the Gaspe Peninsula to southeastern Manitoba. The westernmost extent of this forest region in Manitoba is expected to support at least some of the bryophyte species that occur further east within the climatic and vegetation boundaries of the region. However, very few bryophytes have been reported from this area in Manitoba either in the scientific literature or herbaria, necessitating further investigation. This study provides the first assessment of bryophytes from mixed forests and treed swamps within the Great Lakes forest in Manitoba and relates them to physiography and habitat, including the first documented bryophytes for Buffalo Point First Nation on Lake of the Woods. Reported here are 153 bryophyte taxa, including 40 liverworts from 29 genera in 23 families, and 113 mosses from 77 genera in 31 families. Among these, 27 bryophytes (9 liverworts and 18 mosses) are new to Manitoba, and an additional 7 bryophytes (1 liverwort and 6 mosses) that previously had incomplete information or were equivocal are now confirmed for Manitoba. These findings expand substantially the provincial bryophyte flora. A further 35 bryophytes are re-discovered for Manitoba after more than 40 years since they were last reported in the literature. The surveyed flora is characterized as predominantly boreal (61.7% of taxa) in terms of world phytogeographic distributions. However, more than one quarter of taxa (27.9%) are temperate and frequently associated with habitats containing Fraxinus nigra (Black Ash) or Fraxinus pennsylvanica (Green Ash) trees. Many of the temperate taxa reported have widespread distributions in eastern North America but approach their western and/or northern distribution limits in the study area. Herein I identify bryophytes of high conservation value for Great Lakes forest in Manitoba using nested subsets of taxa. Additional documentation of bryophytes in the area is needed to help inform the conservation and management of lands that support a high diversity of Great Lakes-associated bryophytes.

S-type granites in the western Superior Province: a marker of Archean collision zones

Detailed field observations indicate that Neoarchean S-type granites were emplaced along and (or) proximal to some terrane (tectonic) boundary zones in the western Superior Province, southeastern Manitoba. These S-type granites are characterized by the presence of at least one diagnostic indicator mineral, such as sillimanite, cordierite, muscovite, garnet, and tourmaline. They are medium- to high-K calc-alkaline, moderately to strongly peraluminous (ANKC &gt;1.1), and contain &gt;1% CIPW normative corundum. Compared with more voluminous, older I-type granitoids in tonalite–trondhjemite–granodiorite suites in the region, the S-type granites occur as relatively small intrusions and have high (SiO2 &gt;72 wt.%) contents with a small silica range (SiO2 = 72.2–81.2 wt.%), but a large range of Zr/Hf (17.1–43.8) and Nb/Ta (0.3–16.0) ratios. These geochemical characteristics suggest that the S-type granites were derived from partial melting of heterogeneous sedimentary rocks deposited as synorogenic flysch that underwent burial and crustal thickening during terrane collision. Although the S-type granites display geochemical variations in individual and between different plutons, their low Sr (&lt;400 ppm) and Yb (&lt;2 ppm) contents and low Sr/Y (&lt;40) and La/Yb (&lt;20) ratios are consistent with a partial melting process that left a granulite-facies residue consisting of plagioclase, pyroxene, and ± garnet. The S-type granites display low zircon saturation temperatures (mostly &lt;800 °C) and low emplacement pressures (&lt;300 MPa), similar to strongly peraluminous leucogranites formed in the Himalayas. Therefore, we propose that the Neoarchean S-type granites in the western Superior Province, whose source rocks were deposited between colliding tectonic blocks between 2720 and 2680 Ma, may serve as a geological marker of some Archean terrane boundary zones.

Natalie Alvarez, Immersions in Cultural Difference: Tourism, War, Performance

This book documents cultural encounters facilitated in immersive role-play scenarios of military and counterinsurgency training, dark tourism, and pedagogy based on history, topography, and the politics of human rights violations. Among the four case studies, two concern training for North American combatants engaged in declared and covert wars in the Middle East; two feature Indigenous cultures claiming control over their narratives, designed to proactively alter a people’s future; and whereas one relies on a built simulation three others utilize the natural landscape to shape encounters. Like studies by Scott Magelssen and Coco Fusco, among others, Natalie Alvarez’s work is invested in formal characteristics of the cases, such as live immersive situations, for paradigm-building. Yet, more importantly, it is cultural ethnography that first destabilizes then reorients the ethnographer’s understanding of kinds of knowledge and leads to sensitization to local cultures and ethical experiences of learning. It matters less whether the cases cohere in terms of formal elements of performance than that, through field work and ethnographic reflection, performance is indispensable in promoting personal encounters imbued with empathy. Over the course of many years, Alvarez’s field work—at CFB Camp Wainwright’s Afghan Village training site, the private company Aeneas Group International’s course on Countering Insurgency in Complex Environments held in the Utah mountains, a nighttime walk led by Indigenous Hñahñu in Hildalgo northeast of Mexico City simulating perils encountered by border-crossing migrants, and a tour of Shoal Lake 40 Reserve conducted by a community leader and settler-ally through this Anishinaabe reserve in southeastern Manitoba—accumulates into a profound meditation on experiences of encounter.

Regionalisation of nitrate leaching on pasture land in Southern Manitoba

Nitrogen is a key agricultural input which is crucial for crop growth, development, and yield. However, an excess application of nitrogen may result in the nitrate contamination of groundwater. This research focused on the estimation of nitrate leaching fluxes upon the application of pig slurry on pasture land in Southern Manitoba using physical based modeling and to further spatially disperse the point estimates of nitrate leaching fluxes at the field scale. The field site was located in Southeastern Manitoba. Grazing livestock tended to concentrate near areas such as mineral feeders, water troughs and shelters causing higher deposition density of urine and feces, resulting in six bare earth areas in the study site. HYDRUS-1D was used to determine continuous recharge and nutrient leaching estimates. The simulated leaching estimates were spatially dispersed using Cokriging. The difference in simulated and observed nitrate concentrations in groundwater was expressed in terms of RMSE between 0.023 and 5.12 mg NO3−-N L-1, NSE between 0.66 and 0.96 and the ME between −1.03 mg NO3−-N L-1 and 1.05 mg NO3−-N L-1. Simulation results suggested that two years of pig slurry application to the hay pasture on a coarse sandy soil did not cause significant accumulation of nitrate in the shallow groundwater since the maximum nitrate leaching flux were below 11 kg ha-1 resulting in NO3− concentration in groundwater below 1.2 mg L-1. In contrast, nitrate concentrations five times above the drinking water threshold were simulated in bare earth areas. Leaching of large amounts of nitrate occurred in these areas. Overall, HYDRUS-1D can be considered a useful tool in quantifying nitrate leaching estimates for pasture in a cold climate such as southeastern Manitoba, and Cokriging can be considered a reliable method for a study site where cross-correlations between variables are important to consider for carrying out interpolation.

Subsurface fracture distribution and its correlation with the shape and thickness of the Lac du Bonnet batholith

The Lac du Bonnet batholith (LDBB) is an Archean pluton in the Superior Province in southeastern Manitoba and formerly the site of the Underground Research Laboratory operated by Atomic Energy of Canada Limited. Batholith-scale investigations were conducted there between the late 1970s and 2005 into the Canadian concept of used nuclear fuel disposal in crystalline rocks of the Canadian Shield. A review of legacy data from this period has revealed that the distribution of the fracture network can be related directly to the thickness of the intrusion, as inferred from the model of the base of the batholith. Fractures have preferentially developed in the shallow and thinner sill-like western remnant of the batholith and, in comparison, they are suppressed within and over its deep root. Fracture orientations, frequency and continuity as seen at the surface are not characteristic of the fractures within the batholith interior, even at relatively shallow depths. A substantial volume of unfractured granite has been preserved despite the LDBB’s great age and its exposure to several cycles of loading and unloading, as revealed by the regional stratigraphic record and a unique apatite fission track dating profile. The LDBB is not a one-of-a-kind anomaly, and similar fracture/host rock relationships can be found in the literature. Based on the data reported here, it can be recommended that site models for any continuance of underground nuclear waste storage define the volume distribution of the host rock and its internal structure as these may serve as guides for subsurface exploration and the development and incremental testing of a conceptual fracture model.

Relationship between lichen species composition, secondary metabolites and soil pH, organic matter, and grain characteristics in Manitoba

Many lichen secondary metabolites have functions related to the environmental conditions of lichen habitats but few studies have compared soil characteristics with lichen species composition or their secondary products. The goal of this study was to investigate the relationship between soil characteristics with lichen species composition and secondary metabolites. Five locations were chosen in Manitoba, each with five sites (transects), and each transect with five quadrats (1 m × 1 m). All species were collected from each of the quadrats, presence of secondary metabolites was determined by thin layer chromatography, and soil characteristics were examined. The results revealed that rounded sand grains were significantly higher in southeastern Manitoba than in other locations, corresponding to a distinct species composition. Angular grains were significantly higher in northern locations, corresponding to a different group of species. Some of the significant relationships between soil characteristics and secondary metabolites include correlations of atranorin with pH, organic matter, and sand content; fumarprotocetraric acid with organic matter and sand content; and usnic acid with pH and organic matter. A better understanding of the role of lichens with respect to soil characteristics will be important for improving soil stabilization in land reclamation.

&lt;i&gt;Fabronia ciliaris&lt;/i&gt;, a Moss New to Canada from Southeastern Manitoba

Fabronia ciliaris (Fabroniaceae, Bryophyta) was recently discovered in the Great Lakes–St. Lawrence forest region in southeastern Manitoba. This collection represents the first record of the species in Canada and the northernmost extent of the species in North America.

Personality predicts ectoparasite abundance in an asocial sciurid

AbstractParasitism is a consequence of complex interactions between host, parasite, and their shared environment, and host behavior can influence parasite risk. Animal personality (i.e., consistent behavioral differences that are repeatable across time and context) can influence parasitism with more explorative individuals typically hosting greater parasite loads. Host “sociality” is known to impact parasite risk with more social individuals typically at higher risk of acquiring or transmitting parasites, but other behaviors could also be important. We quantified personality in least chipmunks (Tamias minimus), including repeatability of behavioral traits, and determined whether these personality traits affected ectoparasite prevalence and abundance. We measured personality using standardized hole‐board tests and quantified ectoparasitism of 39 least chipmunks over 2 years at a site in southeastern Manitoba, Canada. We found that activity and exploration were repeatable within the context of the hole‐board test for least chipmunks, which suggests that these traits reflect personality. More exploratory individuals hosted a greater abundance of ectoparasites compared to less exploratory individuals. Our results are consistent with past studies implicating personality as a factor in host–parasite dynamics and suggest that exploration may be an important behavioral correlate of parasite acquisition.

Range Extension of the Giant Water BugBelostoma flumineumSay 1832 (Hemiptera: Belostomatidae) to Saskatchewan, Canada

Abstract. We collected a single specimen of the giant water bug Belostoma flumineum Say 1832 (Hemiptera: Belostomatidae) during routine macroinvertebrate biomonitoring in the Souris River, Saskatchewan. This is the first record of B. flumineum in the province of Saskatchewan and represents a more northern record than the previously documented locations in Montana, North Dakota, and southeastern Manitoba.

Evaluating the Relative Importance of Shallow Subsurface Flow in a Prairie Landscape

Core Ideas Contrary to established theories, vadose water contributes considerably to Canadian Prairie stormflow. Travel velocities of subsurface flow may exceed surface flow in some cases. Traqnsmissivity feedback and macropore flow promote “old” water deliveryto Prairie streams. Hydrologic literature on the Northern Great Plains, including the Canadian Prairies, has mainly focused on Hortonian overland flow as the dominant runoff generation mechanism. This study focused on subsurface water movement and its contribution to streamflow in high‐latitude, subhumid, prairie landscapes with flat topography. Three “riparian‐to‐stream” sites were monitored in southeastern Manitoba, Canada, and four hydrologic events (snowmelt triggered and rainfall triggered) were selected for analysis. A dual water sourcing strategy, including conservative and non‐conservative tracers, was applied to distinguish the relative importance of event (“new”) and pre‐event (“old”) runoff sources. Surrogate measures of antecedent wetness conditions (AWCs: based on rainfall, snow water equivalent, and soil saturation) data and streamflow and water table fluctuation data were used to identify factors influencing the contributions of different runoff sources. Results showed significant old water contributions during snowmelt‐ and rainfall‐triggered events, especially during the summer season. At an engineered grassland site, decreases in snow cover extent were accompanied by “flashy,” high‐magnitude stream hydrographs. Larger peak discharge was observed coincident to elevated AWCs at all sites. Travel velocities of surface‐applied dye were faster than those of subsurface‐applied dye, except for one rainfall‐triggered event. Rapid delivery of old water to streams was observed and hypothesized to be the result of macropore flow, piston flow, or transmissivity feedback depending on the season and water table position. Further investigations are recommended to increase our understanding of subsurface runoff generation mechanisms that promote the rapid movement of old water from riparian areas to streams in Canadian Prairie landscapes.

Discreditation of diomignite and its petrologic implications

Diomignite (Li 2 B 4 O 7 ) is discredited as a mineral species, and this discreditation has been approved by the International Mineralogical Association, Commission on New Minerals, Nomenclature and Classification. Diomignite was originally reported to occur in virtually every crystal-rich inclusion in spodumene from the Tanco pegmatite in southeastern Manitoba, Canada. However, detailed study of 30 randomly selected crystal-rich inclusions in the purported type material deposited at the U.S. National Museum of Natural History, 30 inclusions in the purported type material from the American Museum of Natural History, and several hundred inclusions in self-collected samples reveals that diomignite is absent in every inclusion examined. Because no holotype specimen exists and no neotype sample was provided by the surviving authors of the original description, the presence of diomignite could not be validated. The evidence provided in the original description to the IMA in 1984 is shown to be insufficient to support the existence of diomignite as a mineral species. The previously reported boron-rich (12 mass% B 2 O 3 ) composition of the melt represented as crystal-rich inclusions in spodumene and petalite from the Tanco pegmatite was predicated on the assumption that diomignite is a common daughter mineral that occurs in most inclusions and that the inclusions are primary melt inclusions. The nonexistence of diomignite, and the absence of other borate daughter minerals, in these crystal-rich inclusions indicates that the boron content was greatly overestimated and so comparisons to experimentally generated boron-rich (>10 mass% B 2 O 3 ) boundary-layer melts are unwarranted. Furthermore, the discreditation of diomignite negates the inferred role of a Li 2 B 4 O 7 -flux-rich melt in the generation of primary pegmatite textures and rare element oxide mineralization in the Tanco pegmatite.

Complex population structure of Borrelia burgdorferi in southeastern and south central Canada as revealed by phylogeographic analysis.

Lyme disease, caused by the bacterium Borrelia burgdorferi sensu stricto, is an emerging zoonotic disease in Canada and is vectored by the blacklegged tick, Ixodes scapularis. Here we used Bayesian analyses of sequence types (STs), determined by multilocus sequence typing (MLST), to investigate the phylogeography of B. burgdorferi populations in southern Canada and the United States by analyzing MLST data from 564 B. burgdorferi-positive samples collected during surveillance. A total of 107 Canadian samples from field sites were characterized as part of this study, and these data were combined with existing MLST data for samples from the United States and Canada. Only 17% of STs were common between both countries, while 49% occurred only in the United States, and 34% occurred only in Canada. However, STs in southeastern Ontario and southwestern Quebec were typically identical to those in the northeastern United States, suggesting a recent introduction into this region from the United States. In contrast, STs in other locations in Canada (the Maritimes; Long Point, Ontario; and southeastern Manitoba) were frequently unique to those locations but were putative descendants of STs previously found in the United States. The picture in Canada is consistent with relatively recent introductions from multiple refugial populations in the United States. These data thus point to a geographic pattern of populations of B. burgdorferi in North America that may be more complex than simply comprising northeastern, midwestern, and Californian groups. We speculate that this reflects the complex ecology and spatial distribution of key reservoir hosts.

Reduced fecundity in non‐parasitic lampreys may not be due to heterochronic shift in ovarian differentiation

Heterochrony is an important mechanism for the evolution of differences between and within species. In lampreys, heterochrony has been suggested as a mechanism contributing to fecundity differences in non-parasitic versus parasitic species. Non-parasitic lampreys, which do not feed at all after metamorphosis, have much smaller body sizes at maturation and therefore much lower fecundity than parasitic lampreys. Previous studies have suggested that this fecundity difference is established in the larval stage through ovarian differentiation at a younger age (and therefore smaller body size) in non-parasitic females, leading to production of fewer oocytes. The current study examined whether this pattern is applicable in two additional lamprey species. The timing of histological ovarian differentiation was determined in larval parasitic chestnut lamprey Ichthyomyzon castaneus and non-parasitic northern brook lamprey I. fossor in southeastern Manitoba, Canada, and potential fecundity was compared through oocyte counts in differentiated females of both species. Ovarian differentiation occurred in age classes 1 and 2 in both chestnut and northern brook lampreys, and there were no significant differences in the timing of ovarian differentiation between species; this indicates that the timing of ovarian differentiation is not a reliable marker of life history type. Factors such as growth and body condition may determine whether an individual undergoes ovarian differentiation in age class 1 or 2. Chestnut and northern brook lampreys had similar minimum oocyte counts; however, the average and maximum oocyte counts were higher in chestnut lamprey in each age class. As the timing of ovarian differentiation is similar in chestnut and northern brook lampreys, the higher potential fecundity of chestnut lamprey must originate through mechanisms other than heterochrony.

Woody debris volumes and carbon accumulation differ across a chronosequence of boreal red pine and jack pine stands

Boreal forests are thought to store more than 30% of the world’s terrestrial carbon (C), much of it in the form of dead wood. Harvesting, stand transformation, and climate change the storage capacity of this carbon pool and improved quantification of C storage is needed to improve the accuracy and coverage of C accounting in Canadian forests. In this study, we compared wood volumes and C storage in coarse woody debris (CWD), fine woody debris (FWD), and standing dead wood (snags) in a 94-year chronosequence of jack pine (Pinus banksiana Lamb.) and red pine (Pinus resinosa Ait.) stands in the Sandilands Provincial Forest, southeastern Manitoba. In our data set of 20 jack pine and 17 red pine stands, jack pine stands supported higher volumes of CWD, snags, and sparsely distributed FWD than red pine stands. Mean CWD volume and C mass were, respectively, 18.6 m3·ha−1 and 2.6 tonnes (t)·ha−1 for jack pine and 11.3 m3·ha−1 and 1.1 t·ha−1 for red pine. Snag volumes and C mass were, respectively, 1.8 m3·ha−1 and 0.25 t·ha−1 for jack pine and 0.26 m3·ha−1 and 0.04 t·ha−1 for red pine. CWD loads in jack pine stands followed a U-shaped distribution with stand age, and snag loads in jack pine increased linearly with time. No such significant trends for CWD or snags were observed in red pine. Our results confirm that stand conversion from fire-origin jack pine to red pine plantations has the potential to significantly reduce and alter temporal patterns of dead wood accumulation across the landscape.

AGE OF THE BIRD RIVER SILL, SOUTHEASTERN MANITOBA, CANADA, WITH IMPLICATIONS FOR THE SECULAR VARIATION OF LAYERED INTRUSION-HOSTED STRATIFORM CHROMITE MINERALIZATION

The Archean Bird River sill, a 20-km-long and up to 800-m-thick mafic-ultramafic layered intrusion in the Bird River greenstone belt of southeastern Manitoba, Canada, contains significant resources of chromium and nickel-copper, and locally anomalous concentrations of platinum group elements. Stratiform chromitite, displaying both remarkable lateral continuity and local irregularities due to synmagmatic disruption, occurs in up to six main intervals over a thickness of ~60 m in the lower ultramafic part of the intrusion. Ni-Cu sulfide mineralization is hosted in ultramafic rocks at the base of, or just below, the sill (e.g., past-producing Maskwa-Dumbarton mines). In this study, we report a U-Pb age (chemical abrasion-thermal ionization mass spectrometry) for a leucogabbro from the Chrome property of 2743.0 ± 0.5 Ma (n = 8 single grains of zircon), which is interpreted as the age of crystallization of the sill and defines the timing of the associated Cr-Ni-Cu ± platinum group element (PGE) mineralization. Other Neoarchean mafic-ultramafic intrusions containing economic concentrations of chromite and/or Ni-Cu-(PGEs) occur in the northwestern part of the Superior province in the Canadian Shield (e.g., McFaulds Lake, Big Trout Lake, Puddy Lake, Shebandowan) and share broadly similar geologic relationships. The age of intrusion-hosted stratiform chromite mineralization worldwide, excluding ophiolite-hosted chromitites, ranges from Archean to Paleoproterozoic, which is comparable to that for komatiite-related Ni deposits. This temporal restriction is consistent with both deposit types requiring the involvement of high-MgO, Cr-rich parent magmas produced during large degrees of mantle melting early in Earth history.