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Geology of Diamond Occurrences at Southern Knee Lake, Oxford Lake

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Geology of Diamond Occurrences at Southern Knee Lake, Oxford Lake 80 a Diamond inclusion KL-2 and intergrowth field Summary and guide to figures 70 KL-3 In 2016, bedrock mapping and sampling by the Manitoba Geological Survey resulted Indicator minerals in the conglomerate (MGS sample LX/KL-2) consist of chromite 60 Argyle lo Geology of diamond occurrences at southern Knee Lake, Oxford Lake–Knee Lake lamproite in the discovery of microdiamonds in shoreline outcrop at southern Knee Lake – a and Cr-spinel, whereas the lapilli tuff (MGS sample LX/KL-3) contains chromite, Cr- eo gic g a 50 a l discovery since confirmed and extended by Altius Minerals Corp. (press release, diopside, Cr-spinel and diamond-inclusion Cr-spinel, indicative of a mantle-derived b s 2 3 u o Cr O (wt. %) September 25, 2017). The microdiamonds are hosted by polymictic volcanic magmatic precursor sourced from within the diamond stability field (>140 km 40 t r i v conglomerate and volcanic sandstone belonging to the Oxford Lake group (ca. 2.72 depth). Indicator mineral compositions (Figures 18 & 19), coupled with the absence n e a MGS HP y greenstone belt, Manitoba (NTS 53L14, 15) Kimberlite Ga) of the Oxford Lake–Knee Lake greenstone belt in the northwestern Superior of garnet and ilmenite, suggest that the magmatic precursor was not kimberlitic. 30 province (Figures 1–5). Well-preserved primary features (Figures 6–8) indicate m 20 deposition as debris and turbidity flows in an alluvial or shallow-marine fan setting, Microdiamonds (n = 144; Figures 20 & 21) obtained from a 15.8 kg sample of the 1928 S.D. Anderson (Manitoba Geological Survey) 0 5 10 15 20 whereas interlayers of lapilli tuff (Figure 9) record influxes of pyroclastic material. volcanic conglomerate (sample LX/KL-2) are variable in colour, morphology and MgO (wt. %) 1.0 10 degree of resorption, with the largest stone weighing 0.2071 mg (0.00104 ct); a b c Whole-rock geochemical data (Figures 10 & 11) indicate primitive bulk compositions sample of the lapilli tuff did not yield diamonds, whereas lamprophyre dikes at Knee LIP Komatiite (16.7–18.7 wt. % MgO; >500 ppm Cr, Ni) and alkaline affinities (1.2–3.3 wt. % K₂O) for Lake have yet to be assessed for diamonds or indicator minerals. 0.8 spinel OIB Y/15 the volcanic sedimentary rocks and lapilli tuff, collectively referred to as the 10 1 MORB Station: 52-97-1052 ARC 'ultramafic facies association', and are comparable to primitive alkaline lamprophyre Unconventional diamond occurrences hosted by lamprophyre dikes, polymict a b a b a b 0.6 NAD83 UTM 15N: 379904E, 6079883N dikes in the Knee Lake area (Figure 12), suggesting an association with lamprophyric volcaniclastic breccia and conglomerate in the Michipicoten greenstone belt (south- * Back arc NMORB OIB 2 Channel basin Major zoning TiO (wt. %) volcanism. central Superior province) near Wawa, Ontario, provide a useful analog to the cut VAT Cr/(Cr+Al) 0.4 1 trends of 0.1 occurrence at southern Knee Lake, with implications for exploration. 11413 14 * kimberlite (101) spinel Diverse clast types in the conglomerate include pyroxenite, gabbro and basalt, 0.2 EMORB Kimberlite MORB presumably derived from the ca. 2.83 Ga Hayes River group (Figure 13), representing As is the case in the Wawa area, the types and compositions of indicator minerals 11412 12 Th/Yb groundmass EMORB peridotite (88) spinel SSZ 0.01 peridotite the local basement. Distinctive cored clasts (Figure 14) in the diamondiferous hosted by the lamprophyric rocks at Knee Lake are distinctly different from those 0.1 11411 0.0 0 10 20 30 40 50 conglomerate are interpreted to represent pelletal lapilli that formed during intensive found in surficial sediments. The former are compatible with 'typical' mantle-derived (120) 10 0.0 0.2 0.4 0.6 0.8 1.0 NMORB 2+ 2+ Fe /(Fe +Mg) Al2 O 3 (wt. %) degassing within a diatreme vent and were subsequently reworked by sedimentary rocks such as komatiite or primitive arc basalt, which are major constituents of 11410 Alkaline (99) intercontinental processes. greenstone belts in the Superior province, whereas the latter are indicative of Calcalkaline rifts Figure 18: Bivariate plots for chromite and Cr-spinel grains recovered from MGS samples KL-2 8 Continental 11409 LX/KL-2 0.01 diverse sources, possibly including kimberlite. Consequently, any exploration (112) and KL-3: a) MgO vs. Cr₂O₃; fields for diamond inclusions, Argyle lamproite and HP lamprophyre (83) La/10 Nb/8 0.1 1 10 100 Multiphase ductile deformation fabrics (Figures 15 & 16), greenschist-facies strategy that employs drift prospecting will require careful consideration of not only Nb/Yb are from Fipke et al. (1995); field for kimberlite spinel is from Nowicki et al. (2007); b) 11408 Metres metamorphic assemblages, and interstratification with dacitic volcanic rocks dated at the complex surficial geology and ice-transport history in the Knee Lake region, but c d (75) 6 c d Figure 13: Trace element discrimination diagrams for clasts in conglomerate of Fe²⁺/(Fe²⁺+Mg) vs. Cr/(Cr+Al); compositional field for komatiite spinel is from Barnes and Roeder 2722 ± 3 Ma (Figures 5 & 17) confirm the Archean emplacement age, placing this also the potentially diverse and extensive sources of indicator minerals. 11407 (2001); compositional field and principal zoning trends for kimberlite groundmass spinel are from (95) the UFA (purple diamonds), shown in comparison to mafic volcanic rocks of the diamond occurrence amongst the oldest known on Earth. 4 HRG at Oxford Lake and Knee Lake, representing the local basement during Roeder and Schulze (2008); c) Al₂O₃ vs. TiO₂ (after Kamenetsky et al., 2001). Abbreviations: ARC, 11406 (67) alkaline volcanism (green triangles; unpublished data from Syme et al., 1997, island-arc; LIP, large igneous province; OIB, ocean-island basalt; MORB, mid-ocean–ridge basalt; 2 1998; Anderson et al., 2012, 2013): a) La/10 vs. Y/15 vs. Nb/8 (Cabanis and SSZ, supra-subduction zone. Spinel compositions are consistent with typical arc volcanic rocks 6090000N 390000E Lecolle, 1989); b) Log Nb/Yb vs. Log Th/Yb (Pearce and Peate, 1995). (including lamprophyre) or komatiite. 96° 94° Paleozoic Intrusive rocks Abbreviations: EMORB, enriched mid-ocean–ridge basalt; NMORB, normal Paleoproterozoic WLP CLAIC 0 NKF Cinder Lake alkaline intrusive complex: Opischikona 5 HBT CLAIC mid-ocean–ridge basalt; OIB, ocean-island basalt; VAT, volcanic-arc tholeiite. syenite, monzogranite, pegmatite Narrows MS CS PC CC Figure 19: Bivariate plot Cinder Garnet Oxford Lake–Knee Lake Lake WLP Whitemud Lake pluton: peridotite (Al₂O₃ vs. Cr₂O₃) for Cr- greenstone belt granodiorite, granite Figure 7: Simplified stratigraphic column for bedded polymictic 4 diopside grains from MGS 55° Knee Bayly Lake pluton: Lake granite / granodiorite conglomerate at the discovery outcrop in the eastern bay. Cross Oxford Serpentinite sample KL-3; compositional Lake Lake OSD Normally-graded beds are capped by pebbly sandstone, 3 Inclusion Spinel peridotite field for diamond-inclusion SWF Omusinapis Gods Point Figure 6: Outcrop photographs of diamondiferous polymictic conglomerate of the UFA: a) consistent with sedimentation from high-density turbidity Figure 12: Outcrop photographs of lamprophyre dikes at central Knee Lake: a) primitive (11 wt. % and and off-craton garnet Lake a b intergrowth peridotite and diamond-intergrowth Cr- Lake Pain Killer GF LISZ Bay AF boulder–cobble conglomerate, western bay; b) massive cobble–pebble conglomerate showing currents. Channel samples are from Altius Minerals Corp. (press MgO) lamprophyre dike; b) flow-banded dike of more evolved lamprophyre (7.7 wt. % MgO); c) 2 3 diopside is from Nimis Sharpe Trout 2 Molson Lake TISZ polymictic clast population and rounded clasts, eastern bay; c) cobble–pebble conglomerate beds release, September 25, 2017), following up on the original MGS primitive (11.5 wt. % MgO) lamprophyre dike with thick chilled margins; d) coarse phenocrysts of Cr O (wt.%) Lake (2002); other compositional K2 at the discovery outcrop (MGS sample LX/KL-2 locality) showing size grading and deeply scoured sample (LX/KL-2). Numbers in parentheses indicate diamond phlogopite in primitive lamprophyre dike (same locality as previous photo). 54° Long Trout Opawakow Island Eclogite, fields are from Ramsay and NCT Falls Narrows basal contacts, eastern bay; d) detail of graded bed of polymictic conglomerate, discovery megacryst 0 5 counts in the +0.106 mm size fractions (note higher counts in 1 Tompkins (1994). Celtic and cognate Taskipochikay Knee Kilometres outcrop, eastern bay. sandy beds). Abbreviations: CC, cobble conglomerate; CS, coarse- Hayes Opapuskitew Island Island Bay GF Lake LISZ Lake River grained sandstone; MS, medium-grained sandstone; PC, pebble AF Diamond occurrence 0 F2 Lake Gold showing conglomerate. 0 2 4 6 8 10 12 14 Winnipeg F 2 Fold axial trace a Al2 O 3 (wt.%) 50 Manitoba b kilometres Ontario Shear zone, fault F1 F S0 1 Younging direction Michikanes a b Lake TISZ Figure 1: Regional geological setting of the Oxford Geological contact 363000E 6075000N RU a Lake–Knee Lake greenstone belt in the northwestern Oxford Lake group Hayes River group Superior province (terminology after Stott et al., 2010). Synorogenic sedimentary rocks Volcanic subgroup Intermediate–felsic intrusive rocks; c d feldspar-quartz porphyry Significant gold deposits are indicated by red circles. Greywacke, quartz greywacke Volcanic sedimentary rocks Volcanic conglomerate, greywacke Ultramafic–mafic sills S Abbreviations: HBT, Hudson Bay terrane; NCT, North 0 Volcanic, volcaniclastic and volcanic Volcanic conglomerate, greywacke S sedimentary rocks 1 Caribou terrane; NKF, North Kenyon fault; OSD, Sedimentary subgroup Felsic volcanic and volcaniclastic rocks J LS Oxford–Stull domain; SWF, Stull-Wunnummin fault. Arkosic quartz-greywacke, mudstone Andesitic–dacitic facies association Mafic volcanic rocks (flows): Polymictic conglomerate, greywacke, Basaltic andesite facies association aphyric / amphibolitic Location of Figure 2 is outlined.
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