Northwest Ontario Research Articles

The utility of freshwater dinoflagellate cyst assemblages as a paleoecological proxy: An assessment from boreal lakes (northwest Ontario, Canada)

Aquatic palynomorphs have the potential to provide valuable insights into past environments but little has been done to assess the utility of freshwater dinoflagellate cysts as palaeoecological proxies of lacustrine environments. This study is the first to compare the distribution of freshwater dinoflagellate cysts found in surface sediments from 32 boreal lakes with varying physical and chemical characteristics. Cysts of Fusiperidinium wisconsinense, Parvodinium umbonatum, and Peridinium willei were nearly ubiquitous, but of the remaining cyst taxa, only Parvodinium inconspicuum, Peridinium limbatum, and Peridinium volzii were abundant but present in a smaller number of lakes. Assemblage composition broadly clustered into three groups: 1) an assemblage (primarily Peridinium spp.) associated with relatively shallow, mesotrophic lakes; 2) an assemblage dominated by F. wisconsinense in mesotrophic lakes with intermediate depth and relatively high pH and alkalinity; and 3) an assemblage dominated by P. umbonatum in deeper oligotrophic lakes. Moreover, changes in dinoflagellate cyst assemblage were assessed in a sediment core spanning the Holocene from nearby Gall Lake and interpreted alongside other previously published paleoecological proxies from this core. High abundances of many dinoflagellate cysts including P. limbatum, P. willei, F. wisconsinense, and P. inconspicuum were observed when lake production was high at a time when lake-levels were low and climate was more arid than today. This study suggests that dinoflagellate cysts in lacustrine sediments are, when abundant, promising both in modern and core samples, and the relationships between assemblages today and in the past can contribution additional information in understanding past ecological conditions.

Erratum: Rendezvous procedure, a simultaneous endoscopic retrograde cholangiopancreatography and laparoscopic cholecystectomy for choledocholithiasis, in a rural surgical program in Northwest Ontario.

Erratum: Rendezvous procedure, a simultaneous endoscopic retrograde cholangiopancreatography and laparoscopic cholecystectomy for choledocholithiasis, in a rural surgical program in Northwest Ontario.

Examining Access to Vascular Surgery Care for Indigenous Patients in North West Ontario

Examining Access to Vascular Surgery Care for Indigenous Patients in North West Ontario

Progression From Gestational Diabetes Mellitus to Type 2 Diabetes Mellitus Among First Nations Women in Northwest Ontario: A Retrospective Cohort Study

ObjectiveOur aim in this study was to identify the incidence of type 2 diabetes mellitus among First Nations women in northwest Ontario with a history of gestational diabetes mellitus (GDM). MethodsThis work was a retrospective cohort study of women diagnosed with GDM using a 50-gram oral glucose challenge test or a 75-gram oral glucose tolerance test from January 1, 2010, to December 31, 2017, at the Sioux Lookout Meno Ya Win Health Centre. Outcomes were assessed based on glycated hemoglobin (A1C) measurements performed between January 1, 2010, and December 31, 2019. ResultsThe cumulative incidence of T2DM among women with a history of GDM was 18% (42 of 237) at 2 years and 39% (76 of 194) at 6 years. Women with GDM who developed T2DM were of similar age and parity and had equivalent C-section rates (26%) compared to those who did not develop T2DM. They had higher birth weights (3,866 grams vs 3,600 grams, p=0.006) and rates of treatment with insulin (24% vs 5%, p<0.001) and metformin (16% vs 5%, p=0.005). ConclusionsGDM confers a significant risk for the development of T2DM in First Nations women. Broad community-based resources, food security, and social programming are required.

Sedimentology and geochemistry of basinal lithofacies in the Mesoarchean (2.93 Ga) Red Lake carbonate platform, northwest Ontario, Canada

The > 200 m thick Red Lake carbonate platform of the 2.940-2.925 Ga Ball Assemblage (Red Lake Greenstone Belt, Superior Province, Canada) is the oldest known carbonate platform preserved on Earth. This study examined surface outcrops and multiple industry cored drill holes comprising a diverse assemblage of chemical and siliciclastic sedimentary rocks of the deeper water lithofacies associated with the shallow water carbonates. The offshore lithologies, underlying, interlayered with, and overlying the mainly stromatolitic carbonates, consist of alternating packages of sandstone, siltstone, carbonaceous slate, pyritic carbonaceous slate, chert, and oxide facies iron formation (OFIF). These interlayer with carbonate in transitional slope environments, which consist in places of carbonate with magnetite laminae above slumped carbonate debris in iron formation and/or carbonaceous slate. Chert and OFIF were deposited further offshore in a suboxic environment, with lithology dependent on concentrations of seawater Fe(II) and silicic acid. The iron sulfides were deposited in a reducing, anoxic, and organic-rich bottom water and muddy pore water environment not conducive to Fe hydroxide or chert precipitation. Carbonaceous mud was the background sediment, accumulating during intervals when the precipitation of chert, Fe hydroxides, or massive layers of Fe sulfides was inhibited. The interlayering of differing assemblages of chemical and siliciclastic sedimentary rocks indicates temporal and spatial fluctuations in the chemistry of local paleo-seawater and variable sedimentation rates on the margins of the Red Lake carbonate platform. Post-Archean Australian Shale (PAAS)- normalized rare earth element (REE) systematics of carbonate and OFIF show positive La and Gd anomalies, super-chondritic Y/Ho ratios, and depleted light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), similar to modern seawater, while the positive Eu anomalies in the carbonate and OFIF samples reflect hydrothermally influenced Archean seawater. The chemical sedimentary rocks exhibit enrichment of redox-sensitive elements (i.e., Cr, Mo, V, and U) compared to their siliciclastic associates. This, combined with concentrations of MnO up to 6.3 wt% and positive Ce anomalies in the iron formation directly below the carbonate platform, all indicate the likely presence of some O2 ca 2.93 Ga.

Rendezvous procedure, a simultaneous endoscopic retrograde cholangiopancreatography and laparoscopic cholecystectomy for choledocholithiasis, in a rural surgical program in Northwest Ontario.

Northwest Ontario has a high prevalence of cholelithiasis, at 1.6 times the provincial norm. There is a concomitant 14% rate of choledocholithiasis. Accessing surgical services in the region often requires extensive travel by air. Choledocholithiasis management is typically with a 2-staged approach, an endoscopic retrograde cholangiopancreatography (ERCP) followed several days or weeks later by laparoscopic cholecystectomy (LC). Regional surgeons were concerned about the patient burden of travel and the loss to follow-up inherent in scheduling two independent procedures at separate hospital admissions. They adopted a 1-stage management, called the rendezvous procedure, which describes the simultaneous performance of an ERCP and LC. We accessed Sioux Lookout Meno Ya Win Health Centre hospital data for all patients receiving an ERCP and LC between 1 June 2019 and 1 December 2022. We documented patient demographics, operative outcomes, length of stay and transfer to other facilities. There were 29 rendezvous procedures performed, with successful cannulation of the ampulla of Vater in 27 (93%) cases and stone removal in 23 (79%), with a complication rate of 7%. The operating time averaged 136 min, and two patients required transfer to a tertiary care centre and four were stented locally and required a return trip to Sioux Lookout for repeat ERCP and successful stone removal. The average length of stay was 2.1 ± 1.3 days. Patients who could not access a rendezvous procedure averaged 46.1 ± 78.1 days between procedures. Managing choledocholithiasis with a 1-stage approach was safe and effective and reduced patient travel, time to definitive care and hospital admissions.

A collaborative approach to hepatitis C testing in two First Nations communities of northwest Ontario.

Two remote First Nations communities each collaborated with an urban-based liver clinic to organize wide-spread testing, followed by linkage to care for hepatitis C virus (HCV). Involvement of community members was central to planning and conduct of the programs. Samples were obtained using dry blood spot cards (DBS). A week-long pilot study in Community 1 investigated the effectiveness of the program, using DBS. Community 2, being larger, more remote, and known to be endemic for HCV was more challenging. Three-week-long testing drives plus a stand-alone testing day were used to collect samples over 5 months. Public Health Agency (PHAC)'s National Laboratory for HIV Reference Services (NLHRS) received and tested the DBS samples for HCV and other blood-borne infections. Outcomes were measured by number of people tested, the quality of the tests, and community members' satisfaction with the program and retained knowledge about HCV, based on interviews. In Community 1, 226 people were tested for HCV over 4 days. 85% agreed to human immunodeficiency virus (HIV) testing as well. In Community 2, 484 people, one-half of the adult population, were tested. Surveys of participants showed food was the most significant draw, and Facebook the most effective way to inform people of the events. Interviews with staff and participants showed a high level of satisfaction. The results suggest this is an effective approach to testing for HCV in unusually challenging settings. Lessons from the program include the power of community involvement; and the effectiveness of a highly targeted health initiative when developed through collaboration.

Demographics of mental healthcare presentations in a northwest Ontario emergency department.

Providing emergency mental health services for populations in remote rural areas of Canada is challenging. Program needs are distinct. We describe the emergency mental health workload and service needs at the Sioux Lookout Meno Ya Win Health Centre (SLMHC) in northwest Ontario. Emergency department (ED) data were collected for mental health, addiction and self-harm diagnoses (MHA) in 2018/2019. Comparisons were made to similar sized provincial hospitals and EDs. Mental health admissions data from Oct 1, 2018 to Dec 31, 2019 were manually collected from hospital medical charts for demographics, suicide attempts/ideation and frequency of applications for Form 1 psychiatric assessment. The volume of MHA ED visits as a percentage of total ED visits was 4 times higher at SLMHC when compared to both the 67 other Ontario level C hospitals (< 100 beds) and the 15 level C hospital with a similar volume of ED visits (15,000-20,000), (15% vs 4%). Self-harm presentations were 308 at SLMHC versus an average of 42 ± 37 at the 15 level C hospitals with a similar ED volume. From Oct 1, 2019 to Dec 31, 2019, there were 49 patients requiring a Form 1, with an average wait time of 55h before transfer to a schedule 1 facility. There is an increased level of mental health, addiction and self-harm presentations in this northern ED. Lack of alternative resources indicate the need for the development of an integrated model of mental health care service. Reliance on the ED for crisis management indicates the need for the development of more regionally relevant models of care.

Changes in the prairie–forest ecotone in northwest Ontario (Canada) across the Holocene

AbstractPollen and diatom assemblages from well-dated sediment cores from three lakes forming a west-to-east transect across the boreal forest in northwest Ontario (Canada) were used to evaluate the timing and nature of the movement of the prairie–forest ecotone (PFE) across the Holocene. Changes in vegetation, temperature, and effective moisture were inferred from pollen and pollen-based transfer functions. Analyses indicated site-specific vegetational and climate changes across short spatial distances, with prolonged prairie-like conditions during the middle Holocene at the westernmost site. Increased reconstructed temperatures at this westernmost site occurred from ~9000 to 3000 cal yr BP, alongside increases in diatom-inferred lake levels beginning at ~6000 cal yr BP. The abundance of Quercus peaked concurrently with rising lake levels before declining to trace levels by ~3000 cal yr BP. Increases in the abundance of non-arboreal pollen between ~8500 and ~4500 cal yr BP at the more eastern lakes suggest relatively delayed and truncated PFE influence, before the reestablishment of primarily boreal taxa by ~4500 cal yr BP, coincident with diatom-inferred increases in lake levels. This study shows that the PFE moved both farther east and north than previously determined, but generally agrees with established patterns in vegetation from other studied regions along the PFE.

Cloud-to-Ground Lightning in Canada: 20 Years of CLDN Data

ABSTRACT This study presents the spatial and temporal features of more than 45 million cloud-to-ground (CG) lightning flashes recorded by the Canadian Lightning Detection Network for the years 1999–2018. Although sensor upgrades have improved the detection efficiency and location accuracy of CG lightning, the large-scale spatial patterns remain about the same as found in a previous study covering the years 1999–2008. Analyses, using equal-area squares with 10 km sides, describe the regional and seasonal characteristics of negative and positive flashes, the percentage and flash density of positive lightning, and the first-stroke peak currents of both polarities. Lightning activity over the provinces and territories is greatest in the summer, varying from 95.9% to 76.8% of the annual activity in the Northwest Territories and Ontario, respectively. Winter lightning is rare, usually occurring in extreme southern Ontario and the Atlantic Provinces, as well as over offshore regions west of Vancouver Island and the coastal waters off Nova Scotia. Preliminary analysis suggests that, compared with the 1999–2008 period, the majority of western and northern Canada has experienced more lightning days during the 2009–2018 period, whereas much of eastern Canada has experienced fewer lightning days. A statistical analysis performed on 154 stations across Canada found that the decadal increases (decreases) at 5 (31) stations were significant at the 90% confidence level or higher, and 4 (16) of these were significant at the 95% confidence level.

Atmospheric trace metal deposition to remote Northwest Ontario, Canada: Anthropogenic fluxes and inventories from 1860 to 2010

National and global inventories of anthropogenic trace element emissions to air is a comparatively recent phenomenon (post-1993 in Canada) as is the monitoring of atmospheric metal deposition, the latter being also very spatially limited. Paleo-reconstructive methods offer a contiguous record of environmental contamination providing a needed framework to establish locally relevant “pre-industrial” (~natural) conditions which can be compare with relative and quantitative deviations away from reference conditions. In this study, we reconstruct the history of the long-range, anthropogenic sourced atmospheric trace element deposition to the remote region of Northwestern Ontario Canada (Experimental Lakes Area (ELA)) using dated sediment records from five lakes. Several elements are shown to be highly enriched in lake sediments relative to pre-1860 sediments (Antimony, Lead, Tellurium, Tin, Arsenic, Bismuth, Cadmium and Mercury) and moderately (Zinc, Tungsten, Thallium, Copper, Silver, Selenium, Nickel and Vanadium). Mean decadal anthropogenic atmospheric fluxes (mg m-2 yr-1) are reconstructed for 1860–2010 and compare well with available local (ELA), regional (NW Ontario Canada, N Michigan USA) monitoring data, as well as global assessments of anthropogenic contribution to atmospheric trace metal burdens. Quantitative paleo reconstructions of atmospheric contamination history using the collective signal from multiple lakes provide a rigorous methodology to assess trends, uncertainties, evaluation with monitoring data and, provide an opportunity to explore landscape processes of contaminant transport and storage. Further study of the latter is recommended to understand the latency of legacy anthropogenic contamination of the environment.

Outcomes of Pregnancies Affected by Gestational Diabetes and Type 2 Diabetes in a Rural First Nations Obstetrical Program in Northwest Ontario

ObjectivesIn this work, we describe diabetes prevalence and birth outcomes in a primarily First Nations obstetrical population in northwest Ontario. MethodsThis retrospective, observational study of maternal and neonatal characteristics and birth outcomes was performed at the Sioux Lookout Meno Ya Win Health Centre between April 1, 2012 and March 31, 2017. ResultsThe prevalence of pre-existing type 2 diabetes mellitus was 3.7% and gestational diabetes mellitus was 7.9%. Mothers with diabetes, compared to those without diabetes, were, on average, 5 years older and were of greater parity. Average prepregnancy weight was higher, with an increased incidence of hypertension, inductions and caesarean sections. Neonates had increased incidence of macrosomia, hypoglycemia and hyperbilirubinemia. All maternal cohorts had high rates of alcohol, tobacco and illicit opioid use. ConclusionsWe have identified a high prevalence of diabetes in this First Nations obstetrical population, with associated adverse maternal and neonatal outcomes.

Diabetes prevalence and demographics in 25 First Nations communities in northwest Ontario (2014-2017).

First Nations communities are known to have high rates of diabetes. The rural First Nations communities in northwest (NW) Ontario are particularly affected. Regional studies in 1985 and 1994 found a high prevalence of diabetes. More recently, they are estimated to have the highest prevalence in Ontario at 19%, double the provincial norm. The purpose of this study is to examine the epidemiology and prevalence of diabetes in the total population and cardiovascular comorbidities in the adult population of 25 First Nations communities in NW Ontario. This retrospective diabetes prevalence study used primary care electronic medical record data for a 3-year period, 1 August 2014-31 July 2017. Diabetes prevalence was calculated for both the total and the adult (18+) populations and comorbid hypertension and dyslipidaemia were identified in adults. The age-adjusted diabetes prevalence for the total population was 15.1% versus a Canadian prevalence of 8.8%. The age-adjusted adult prevalence was 14.1%, double Canada's average of 7.1%. The average age of adults with diabetes was 52 years (±14.9); 57% were female. Comorbid hypertension (58%) and dyslipidaemia (73%) were common. Metformin was the most commonly used medication (58%), followed by insulin/analogues (23%) and sulphonylureas (13%). The diabetes prevalence in the First Nations population of NW Ontario is double Canada's norm. Addressing it will require addressing relevant social determinants of health, including poverty and food security.

Prevalence of chronic kidney disease and cardiovascular comorbidities in adults in First Nations communities in northwest Ontario: a retrospective observational study.

The prevalence of adult chronic kidney disease and cardiovascular comorbidities in Canadian Indigenous communities is largely unknown. We conducted a study to document the prevalence of chronic kidney disease and concurrent diabetes mellitus, hypertension and dyslipidemia in a First Nations population in northwest Ontario. In this observational study, we used retrospective data collected from regional electronic medical records of 16 170 adults (age ≥ 18 yr) from 26 First Nations communities in northwest Ontario from May 2014 to May 2017. Demographic and laboratory data included age, gender, prescribed medications, estimated glomerular filtration rate, urine albumin:creatinine ratio, low-density lipoprotein cholesterol (LDL-C) level and glycated hemoglobin (HbA1c) concentration. We identified patients with diabetes by an HbA1c concentration of 6.5% or higher, or the use of a diabetic medication, those with dyslipidemia by an elevated LDL-C level (≥ 2.0 mmol/L) or use of lipid-lowering medication, and those with hypertension by use of antihypertensive medication. Of the 16 170 adults residing in the communities, 5224 unique patients (32.3%) had renal testing (albumin:creatinine ratio and/or estimated glomerular filtration rate). The age-adjusted prevalence of chronic kidney disease was 14.5%, and the prevalence of stage 3-5 chronic kidney disease (estimated glomerular filtration rate < 60 mL/min) was 7.0%. Most patients with chronic kidney disease (1487 [80.0%]) had at least 1 cardiovascular comorbidity. A total of 1332 patients (71.6%) had diabetes, 1313 (70.6%) had dyslipidemia, and 1098 (59.1%) had hypertension; all 3 comorbidities were present in 716 patients (38.5%). We document a high prevalence of advanced chronic kidney disease in this First Nations population, 7.0%, double the rate in the general population. High rates of cardiovascular comorbidities were also common in these patients with chronic kidney disease, which places them at increased risk for cardiovascular disease.

Increased relative abundance of colonial scaled chrysophytes since pre-industrial times in minimally disturbed lakes from the Experimental Lakes Area, Ontario

A top–bottom paleolimnological analysis of 30 undisturbed lakes in the Experimental Lakes Area (ELA) in northwest Ontario showed marked increases in the relative abundance of colonial scaled chrysophyte taxa in most lakes since pre-industrial times. The increase in abundance of colonial taxa was primarily driven by Synura sphagnicola in small, high-nutrient, and warm lakes. The colonial taxa Synura petersenii and Synura echinulata also increased in the deeper study lakes. Detailed analysis of sediment cores from six lakes revealed that these changes occurred in the early to mid-1900s. Based on the modern distribution of scaled chrysophytes relative to lake physicochemical data, we propose multiple processes that may be contributing to these changes, such as changes in lake thermal properties and historical changes in the flux of important particles and solutes to lakes. Additional mechanisms that may contribute to the observed changes are discussed, including trophic interactions and changes in limnetic pCO2. Because the ELA is relatively undisturbed, our findings provide important information on the drivers responsible for increases in colonial chrysophyte abundance, which have been documented in studies of lakes in central Ontario and northeast United States.

Self-administered versus provider-directed sampling in the Anishinaabek Cervical Cancer Screening Study (ACCSS): a qualitative investigation with Canadian First Nations women

BackgroundWhile (Pap)anicolaou screening has helped to decrease cervical cancer incidence in Canada, First Nations women continue to have a higher burden and mortality relative to mainstream populations. Many First Nations...

Factors Associated with Residential Long-Term Care Wait-List Placement in North West Ontario.

This article is based on a study that investigated factors associated with long-term care wait list placement in Ontario, Canada. We based the study's analysis on Resident Assessment Instrument for Home Care (RAI-HC) data for 2014 in the North West Local Health Integration Network (LHIN). Our analysis quantified the contribution of three factors on the likelihood of wait list placement: (1) care recipient, (2) informal caregiver, and (3) formal system. We find that all three factors are significantly related to wait list placement. The results of this analysis could have implications for policies aimed at reducing the number of wait-listed individuals in the community.

Water depth is a strong driver of intra-lake diatom distributions in a small boreal lake

There has been much debate over the relative importance of environmental selection and spatial variation on community organization in microorganisms. To assess the importance of environmental or spatial variables in diatom species assemblages in Gall Lake, northwest Ontario, 41 surface-sediment samples were collected in a two-dimensional gridded pattern along and across depth contours. A depth-constrained cluster analysis separated the diatom flora into three communities: a shallow-water benthic zone (B1); a deeper-water benthic zone (B2); and a planktonic zone (P). Redundancy analysis (RDA) confirmed that water depth was a major predictor of variation in the flora. Further RDAs and variation partitioning using orthogonal polynomials and Moran’s eigenvector maps showed that spatial location had minimal effect on the diatom assemblages. Principal components analysis grouped the diatom flora not only by assemblage, but also by water depth, regardless of two-dimensional spatial separation, suggesting the importance of the environmental gradients associated with lake depth. These findings indicate that environment is a more important explanatory variable than spatial variables for diatoms within lakes, suggesting dispersal plays a limited role in intra-lake diatom distributions.

Stratiform platinum-group element mineralization in the layered Northern Ultramafic Center of the Lac des Iles Intrusive Complex, Ontario, Canada

The Northern Ultramafic Centre (NUC) of the Lac des Iles Complex, Northwest Ontario hosts several platinum group element (PGE) occurrences, including the Sutcliffe Zone, which consists of four subparallel, stratiform PGE-enriched intervals exposed within the cyclically layered eastern flank of the NUC. Field relationships, mineral paragenesis and lithogeochemistry allowed for the identification of 14 cyclic cumulate sequences of two distinct types – Cyclic unit type A (CUA) and Cyclic unit type B (CUB). CUA-type and CUB-type units are interpreted to have formed from a Si-enriched and Si-poor parent magmas, respectively. PGE-enriched intervals occur in four of the CUA-type cyclic units (CUA-5, -6, -8 and -11). PGE enriched intervals are commonly associated with websterite, olivine websterite and gabbronorite containing primary disseminated sulfide (0.2–2vol%) which are dominated by pyrrhotite, chalcopyrite, and pentlandite with minor cubanite, and troilite. In hydrothermally altered rocks enriched in PGE, primary sulfides are locally partially replaced by secondary chalcopyrite, sphalerite, heazlewoodite, and chalcocite. Palladium occurs either in solid solution with primary pentlandite or is associated with platinum group minerals (PGM) such as Pd-plumbide, Pd-telluride, and Pt-bismuthotelluride. PGMs commonly occur within primary sulfides, at contacts between primary sulfide–silicate minerals, or in association with secondary serpentine and actinolite. Gold and silver typically occur as electrum that exhibits similar textural characteristics and mineralogical associations as the PGMs.Two different chemostratigraphic patterns of PGE, Cu and S enrichment can be recognized among the mineralized CUA cycles: The first (top-loaded) occurs near the top of CUA cycles (CUA-6, -8 and -11) in websterite and/or gabbronorite, just below the levels at which CUB magmas were emplaced. The second (middle-loaded), occurs midway through the lower cycle (CUA-5) in the olivine websterite, which is overlain by CUA-6. Within the four mineralized intervals, PGE tenors average 643ppm Pd+Pt (in 100% sulfide), Pd/Pt and Pd/Ir ratios range from 0.9 to 3.5 and 35 to 537, respectively, and S/Se ratios range between 500 and 6000. The highest PGE tenors (4377ppm Pd+Pt) are found in the lowermost interval in serpentinized olivine websterite and have an average Pd/Pt ratio of 3.5 and a S/Se ratio of approximately 2000.It is proposed that orthomagmatic processes of fractional crystallization and dynamic magma recharge were the dominant mineralization processes triggering sulfide-saturation and PGE concentration at the Sutcliffe Zone. Textural relationships between PGM, sulfide minerals, and primary and secondary hydrous silicates suggest that late magmatic to postcumulus hydrothermal fluid infiltration occurred locally during and after sulfide mineralization of the PGE-enriched intervals. However, these fluids had a minimal effect on the distribution of PGE in the Sutcliffe Zone. The Sutcliffe Zone shares many similarities with classic stratiform PGE deposits in terms of Pd/Pt ratio, high PGE tenors, low abundance of sulfide, and PGM assemblages. However, it is distinguished from most stratiform PGE deposits by its tectonic environment and lithostratigraphic position and by the intimate spatial association of the two parental magmas that are interpreted to have been responsible for the observed chemostratigraphy and PGE enrichment.

Petrogenesis of Cyclic Units In the Northern Ultramafic Center of the Lac Des Iles Complex, Ontario, Canada

Abstract Lithologic, mineralogical, and geochemical attributes of cyclic units are presented for the Northern Ultramafic Center (NUC) of the Lac des Iles Complex, Northwest Ontario. Detailed mapping and core logging show that the NUC is comprised of the Eastern Marginal Zone and the Layered Series. Fourteen cyclic units with two distinct paragenetic sequences of cumulus minerals are identified in the Layered Series. Cyclic Unit type A (CUA) has a cumulus paragenesis of olivine + chrome-spinel → olivine + orthopyroxene + clinopyroxene ± chrome-spinel → clinopyroxene + orthopyroxene → plagioclase + orthopyroxene ± clinopyroxene. Cyclic Unit type B (CUB) has a simpler paragenesis of olivine + chrome-spinel → olivine + clinopyroxene. Boundaries are sharp between similar cyclic units and where CUA overlies CUB. In contrast, where CUB overlies CUA, the cyclic unit contact is gradational and is marked by the development of a geochemical and mineralogical hybrid unit. Within individual cyclic units there are cryptic variations in mg# of pyroxenes (En) and olivine (Fo). Notably, olivines in CUB rocks have lower Ni concentrations despite higher Fo contents than in CUA rocks. Also, incompatible trace element ratios ( e.g ., Zr/Y and Ce/Yb) are distinct between the two types of cyclic units. These mineralogical and geochemical attributes are interpreted to indicate that the CUA and CUB units were generated by fractional crystallization of two compositionally distinct parental magmas. The CUA parental magma was more siliceous, had higher concentrations of Ni and incompatible trace elements, and had a lower Ce/Yb ratio relative to the CUB parental magma. In seeking out possible parent magma analogues for the CUA and CUB within the NUC, it was found that gabbroic rocks forming the Eastern Marginal Zone and hornblende gabbro dikes crosscutting parts of the Layered Series, respectively, have comparable geochemical attributes. The non-systematic sequencing of CUA and CUB units comprising the Layered Series is interpreted to have resulted from the random recharge of CUA and CUB parental magmas feeding the NUC. When similar magma types successively recharged the NUC chamber, or when a CUA magma recharged the chamber previously filled by a CUB, the recharging magma evidently had a higher density than the resident magma and flowed along the cumulus floor to produce sharp cyclic unit boundaries. When more siliceous CUA magma recharged the chamber previously filled by CUB magma, its lower density resulted in its pluming into the resident magma and producing a complex hybrid cyclic unit boundary.