NWT Geoscience Office

Yellowknife, Canada

NWT Geoscience Office

Yellowknife, Canada
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Acosta-Gongora P.,University of Alberta | Acosta-Gongora P.,Geological Survey of Canada | Gleeson S.A.,University of Alberta | Samson I.M.,University of Windsor | And 5 more authors.
Precambrian Research | Year: 2015

The NICO deposit is located in the southern portion of the Paleoproterozoic Great Bear magmatic zone, Northwest Territories, Canada. The majority of the mineralization lies within the Bowl Zone, hosted by the Treasure Lake Group (TLG), where heavily altered precursor rocks are interpreted to be carbonate-rich wackes and siltstones. These rocks are crosscut by a set of pre-ore quartz ± calcite + amphibole. + K-feldspar veins (S1). The Co-Au-Bi (±W-Cu-Mo) mineralization at NICO is contained in stratabound ore lenses within the Bowl Zone, and comprises a prograde assemblage of Co-rich arsenopyrite (arsenopyrite I) and loellingite, cobaltite, pyrite, actinolite, ferrohornblende, biotite and rare scheelite (±molybdenite) along with minor magnetite and amphibole. Retrograde assemblages resulted from re-crystallization of the Co-bearing phases to form arsenopyrite II and III, along with precipitation of marcasite, pyrite, hastingsite, native Bi-Au (±Te) and minor bismuthinite and magnetite. The latest stage of retrograde mineralization comprises chalcopyrite, hastingsite, chlorite, and hematite (±emplectite). Two sets of barren quartz ± dolomite + amphibole + K-feldspar + chalcopyrite veins (S2 and S3) post-date the mineralization. The Southern Breccia zone hosts minor U-Cu-Mo mineralization and is interpreted to be the deepest portion of the NICO system. Two molybdenite samples from the Bowl Zone and the Southern Breccia yielded Re-Os ages of 1865 ± 9 and 1877 ± 8 Ma, respectively, consistent with the interpreted ca 1870 Ma age of the NICO deposit. This age is also synchronous with the onset of magmatism in the Great Bear magmatic arc (ca. 1875-1850. Ma).Ore mineral δ34S values (3.3-6.7‰, sulfides) indicate that crustal sulfur was assimilated by upwelling felsic magmatic melts. The δ18O values of the fluids precipitating magnetite and Co-rich arsenopyrite (6 and 8‰), and from pre- and syn-mineralization magnetite (-0.8 to 1.5‰) support a magmatic-hydrothermal origin of the fluids. Two out of three calcite samples from pre-(S1) and post-ore (S3) veins have also δ13C values consistent with a magmatic origin (-5.5 to -3.6‰). However, one calcite sample from the S3 veins has a value that indicates a reduced sulfur source (-15.6‰). This value is similar to those of the much younger (<1843Ma) giant quartz veins cutting the GBMZ rocks. The quartz δ18O values suggest that S1 (12.7‰) formed at higher temperatures than S2 (13.2-19.14‰) and S3 (9.4-17.1‰), or that in the latter two generations of veins, 18O was enriched during fluid/rock equilibration.Secondary trails of native Bi in S1 vein quartz are associated with liquid-vapor (LV) and liquid-vapor-halite (LVS) inclusions, which indicates that Bi, and possibly Au, were transported in saline to hyper-saline brines (LV-Bi, 2-16wt.% NaCl equiv., 8-22wt.% CaCl2 equiv.; LVS-Bi, >37wt.% NaCl equiv.), with homogenization temperatures of 137-216°C and 192 to >350°C for LV-Bi (ThL+V→L) and LVS-Bi (ThL+S→L), respectively. The presence of calcium-rich fluids might indicate extensive equilibration of those solutions with the host TLG rocks. If a pressure correction is applied to the LV inclusions using a minimum entrapment temperature of 271.4°C (the Bi melting point), a minimum crystallization depth of between approximately 5 and 8km is indicated.Trace element analyses carried out in this study and compiled from Acosta-Góngora et al. (2014) show that the least altered metasedimentary TLG rocks contain up to six times more As (Carbonate unit, 30.5. ppm) than the average upper continental crust. Conversely, concentrations of Au (<2. ppb), Co (10. ppm) and Cu (12. ppm) are lower than the crustal values. As such, it is possible that the TLG was a source of As, but is a less likely source of Au, Co and Cu for the NICO deposit; this further supports a magmatic-hydrothermal origin for the metals. Nonetheless, the plurikilometer alteration halo of the NICO system indicates that large amounts of elements could have been leached from the TLG, and potentially some were incorporated to the system. However, detailed studies on structural geology, geochemical modeling, and mass balance calculations need to be carried out to consider if such a scenario is feasible. © 2015.

Slack J.F.,U.S. Geological Survey | Falck H.,NWT Geoscience Office | Kelley K.D.,U.S. Geological Survey | Xue G.G.,Selwyn Chihong Mining Ltd.
Mineralium Deposita | Year: 2016

Detailed lithogeochemical data are reported here on early Paleozoic sedimentary rocks that host the large Howards Pass stratiform Zn-Pb deposits in Yukon-Northwest Territories. Redox-sensitive trace elements (Mo, Re, V, U) and Ce anomalies in members of the Duo Lake Formation record significant environmental changes. During the deposition of lower footwall units (Pyritic siliceous and Calcareous mudstone members), bottom waters were anoxic and sulphidic, respectively; these members formed in a marginal basin that may have become increasingly restricted with time. Relative to lower members, a major environmental change is proposed for deposition of the overlying Lower cherty mudstone member, which contains phosphorite beds up to ∼0.8 m thick in the upper part, near the base of the Zn-Pb deposits. The presence of these beds, together with models for modern phosphorite formation, suggests P input from an upwelling system and phosphorite deposition in an upper slope or outer shelf setting. The overlying Active mudstone member contains stratabound to stratiform Zn-Pb deposits within black mudstone and gray calcareous mudstone. Data for unmineralized black mudstone in this member indicate deposition under diverse redox conditions from suboxic to sulphidic. Especially distinctive in this member are uniformly low ratios of light to heavy rare earth elements that are unique within the Duo Lake Formation, attributed here to the dissolution of sedimentary apatite by downward-percolating acidic metalliferous brines. Strata that overlie the Active member (Upper siliceous mudstone member) consist mainly of black mudstone with thin (0.5–1.5 cm) laminae of fine-grained apatite, recording continued deposition on an upper slope or outer shelf under predominantly suboxic bottom waters. Results of this study suggest that exploration for similar stratiform sediment-hosted Zn-Pb deposits should include the outer parts of ancient continental margins, especially at and near stratigraphic transitions from marginal basin facies to overlying slope or shelf facies. © 2016 Springer-Verlag Berlin Heidelberg(outside the USA)

Thienpont J.R.,Brock University | Steele C.,Carleton University | Vermaire J.C.,Carleton University | Pisaric M.F.J.,Brock University | And 2 more authors.
Journal of Paleolimnology | Year: 2014

Low-lying Arctic coastal environments are threatened by marine storm surges, which are predicted to increase in frequency and intensity as a result of decreasing sea ice, rising sea levels and altered intensity and frequency of storm activity. The Mackenzie Delta of Canada’s Northwest Territories, a vast, low-lying wetland ecosystem, is particularly susceptible to such storm surges, because much of the outer alluvial plain is below 2-m elevation. A large storm-surge event in September 1999 flooded >13,000 ha of alluvial terrain and impacted the terrestrial and freshwater ecosystems of the region. Previous research on the limnological impacts of the storm surge recorded a shift from freshwater to brackish diatom taxa, and a change in cladoceran assemblages to more saline-tolerant species. We examined the remains of Chironomidae (Insecta, Diptera) in sediment cores from two lakes impacted by the 1999 saltwater inundation to determine whether the storm surge also affected benthic macroinvertebrate communities, which are particularly important to lake ecosystem function in Arctic regions. We observed an increase in the relative abundance of saline-tolerant taxa in the two impacted lakes, including Paratanytarsus and Cricotopus/Orthocladius, and decreases in saline-intolerant Sergentia and Corynocera oliveri-type, coincident with the 1999 storm. We observed no major assemblage changes after 1999 in a control lake located beyond the zone of inundation. The number of head capsules recovered from sediments of the impacted lakes increased after the 1999 storm, suggesting no negative impact on overall chironomid abundance as a result of the shift to brackish conditions. There has, however, been no recovery of the chironomid community to the pre-1999 composition. Earlier assemblage changes in both impacted lakes likely tracked regional climate warming in the region, known to have begun in the late nineteenth century. © 2014, Springer Science+Business Media Dordrecht.

Kelley K.D.,U.S. Geological Survey | Selby D.,Durham University | Falck H.,NWT Geoscience Office | Slack J.F.,U.S. Geological Survey
Mineralium Deposita | Year: 2016

Stratiform Zn-Pb deposits hosted in unmetamorphosed carbonaceous and siliceous mudstones of the Ordovician to Silurian Duo Lake Formation define the Howards Pass district in Yukon Territory and Northwest Territories, western Canada. Collectively, the deposits are amongst the largest in the world, containing drill-indicated and inferred resources of 423 Mt at 4.84 % Zn and 1.59 % Pb. Sulphide textures include (a) fine-scale laminations of sphalerite, galena, and pyrite from <0.05 mm to 1 cm thick, interbedded with carbonaceous sedimentary rock; (b) layers of coarse sulphide that are structurally controlled by microfolds; and (c) veins that cut bedding and sulphide laminations. The finely interlaminated nature of sulphides with mudstone has been used as evidence for syngenetic mineralizing processes, whereas paleomagnetic data determined on coarse layered sulphides suggest a Middle Jurassic age of mineralization. Here, we present new rhenium-osmium (Re-Os) isotopic data for 12 pyrite separates obtained from 4 laminated sulphide-rich samples from the XY Central (XYC) and Don (DON) deposits and for 1 unmineralized organic-rich mudstone ∼20 m stratigraphically below the sulphide-bearing zone. Pyrite separates that lack mudstone inclusions (“pure”) from the XYC deposit contain 2.2 to 4.0 ppb Re and 93.4 to 123.4 ppt Os; pure pyrite from the DON deposit is significantly more enriched in Re and Os (34–37 ppb Re; 636.8–694.9 ppt Os). The 187Re/188Os values of pure pyrite separates from the XYC and DON deposits range from 137.6 to 197 and 182.1 to 201.4, respectively. Regression of all pure pyrite Re-Os data from both deposits yields an isochron age of 442 ± 14 Ma (MSWD = 7.4) and an initial 187Os/188Os (Osi) value of 0.71 ± 0.07. The Re-Os age indicates that the early phase of pyrite precipitation (and by inference, sphalerite and galena) occurred during the early Silurian, consistent with biostratigraphic ages of the host rocks. The Osi value of ∼0.8 for earliest Silurian seawater recorded from organic-rich shale in the basal Silurian Global Stratotype Section and Point (GSSP) at Dobs Linn, Scotland is very similar to that provided by the Howards Pass pyrite regression and hence suggests a hydrogenous (seawater) source of Os for the pyrite. Therefore, two possible sources of Os are (1) the Zn- and Pb-bearing hydrothermal fluid that leached Os from footwall sedimentary rocks, which were deposited in seawater, or (2) directly from seawater during precipitation of the pyrite, which suggests that the Os content of the hydrothermal fluid was minor relative to that of seawater. © 2016 Springer-Verlag Berlin Heidelberg (outside the USA)

Jackson V.A.,NWT Geoscience Office | van Breemen O.,Geological Survey of Canada | Ootes L.,NWT Geoscience Office | Bleeker W.,Geological Survey of Canada | And 4 more authors.
Canadian Journal of Earth Sciences | Year: 2013

New field and U-Pb zircon data from the south-central Wopmay Orogen (south of 65°N) establish crystallization ages of basement and plutonic phases and bring to the forefront questions on the architecture of the orogen. The complex and extensive >3300-2575 Ma basement domain was derived from the adjacent Slave craton. Paleoproterozoic supracrustal rocks, dominated by an extensive pelitic-psammitic sequence with basal arenite, conglomerate, and carbonate, unconformably overlie this basement. Pre- to post-kinematic Paleoproterozoic plutons intrude both basement and overlying strata and crystallized at 1877, 1867-1862, and 1858-1850 Ma. The first pulse of plutonism constrains an early generation of metamorphism and deformation to younger than 1877 Ma and is corroborated by metamorphic zircon in Archean basement. The interval from 1867 to 1862 Ma brackets ductile deformation in granite and granodiorite intrusions; although corresponding structure in the metasedimentary strata is equivocal. Post-kinematic ca. 1858-1850 Ma plutons were coeval with localized high-grade metamorphism and concomitant recrystallization of Archean basement. The absence of older (>1880 Ma) phases of the Hepburn intrusive suite in south-central Wopmay Orogen demonstrates a previously unrecognized north-south plutonic dichotomy and that the historical assignment of Calderian metamorphism to ca. 1885 Ma may be too old. The designation of part of the area to a klippe of Hottah terrane is not compatible with field and zircon age data which shows that basement and the overlying strata can be tied directly to the Slave craton. The study raises questions regarding the tectonic relationship between the Hottah terrane and Slave craton.

Malone L.,University of Ottawa | Lacelle D.,University of Ottawa | Kokelj S.,NWT Geoscience Office | Clark I.D.,University of Ottawa
Chemical Geology | Year: 2013

Retrogressive thaw slumps are one of the most dramatic thermokarst landforms in periglacial regions. This study investigates the impacts of one stable and two active thaw slumps on the geochemistry of streams in the Stony Creek watershed (Peel Plateau, NWT, Canada). The objective of this study is to elucidate the geochemical processes associated with ground ice ablation in retrogressive thaw slumps and the geochemical evolution of slump runoff to streams. This is accomplished by describing the geochemical composition of runoff across active mega-slumps, impacted and pristine tundra streams, as well as that of the ice-rich permafrost exposed in the slump headwalls. In the Stony Creek watershed, runoff derived from active and stable thaw slumps is characterized by a Ca(Mg)-SO4 geochemical facies with conductivity and solute concentrations approximately one order of magnitude higher than in pristine streams. The elevated solute concentrations in the slump runoff are directly related to thawing of highly weatherable Late Pleistocene age ice-rich and solute-rich permafrost exposed in the headwalls of slumps, which has solute concentrations nearly 100 times higher than those measured in the uppermost 1-2m (i.e., above the early Holocene thaw unconformity). An examination of ionic relations revealed a strong relation between Ca2+ and SO4 2- and (Ca2++Mg2+)-SO4 2-, suggestive that sulfate dissolution is the main process responsible for the geochemical composition of slump impacted streams. Thaw slumps significantly impact the geochemistry of streams, by increasing their solute load well above that of pristine streams along any reach of impacted streams. Unlike shallow active layer disturbances, the thaw slumps can degrade permafrost to depths of 10m or more and the impacts of abundant slump activity on stream geochemistry can be detected at the 10^2km2 watershed-scale. © 2013.

Houben A.J.,University of Ottawa | D'Onofrio R.,University of Ottawa | Kokelj S.V.,NWT Geoscience Office | Blais J.M.,University of Ottawa
PLoS ONE | Year: 2016

Gold mines in the Yellowknife, NT, region-in particular, the Giant Mine-operated from 1949-99, releasing 237,000 tonnes of waste arsenic trioxide (As2O3) dust, among other compounds, from gold ore extraction and roasting processes. For the first time, we show the geospatial distribution of roaster-derived emissions of several chemical species beyond the mine property on otherwise undisturbed taiga shield lakes within a 25 km radius of the mine, 11 years after its closing. Additionally, we demonstrate that underlying bedrock is not a significant source for the elevated concentrations in overlying surface waters. Aquatic arsenic (As) concentrations are well above guidelines for drinking water (10 μg/L) and protection for aquatic life (5 μg/L), ranging up to 136 μg/L in lakes within 4 km from the mine, to 2.0 μg/L in lakes 24 km away. High conversion ratios of methyl mercury were shown in lakes near the roaster stack as well, with MeHg concentrations reaching 44% of total mercury. The risk of elevated exposures by these metals is significant, as many lakes used for recreation and fishing near the City of Yellowknife are within this radius of elevated As and methyl Hg concentrations. © 2016 Houben et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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