Holocene insect fossils from two sites at Ennadai Lake, Keewatin, Northwest Territories, Canada

Ennadai Lake, in the forest-tundra ecotonal region of Keewatin, Northwest Territories, Canada, has been the subject of several paleoecological investigations (palynology, plant macrofossils, fossil soils). This study concerns Holocene insect fossils at Ennadai, a new approach in a region shown to be sensitive to climatic change. The Ennadai I site yielded 53 taxa, representing 13 families of Coleoptera and 7 families of other insects and arachnids, including abundant ants. These fossils range in age from about 6300 to 630 yr B.P. The Ennadai II site produced fossils of 58 taxa, including 13 beetle families and 15 families of other arthropods, ranging in age from 4700 to 870 yr B.P. The insect evidence suggests the presence of trees in the Ennadai region from 6000 to 2200 yr B.P. A conifer pollen decline from 4800 to 4500 yr B.P. at Ennadai has previously been interpreted as an opening up or retreat of forest in response to climatic cooling, but the insect fossils reveal the continued presence of trees during this interval. Both insect assemblages suggest trends of forest retreat and tundra expansion between about 2200 and 1500 yr B.P., presumably due to climatic cooling, with a return of woodland by about 1000 yr B.P.     

Chemical mineralogy of some cordierite-bearing rocks near Yellowknife,Northwest Territories,Canada

Meta-graywacke and meta-argillite of Archean age near Yellowknife contain biotite, cordierite, gedrite and sillimanite isograds towards the Sparrow Lake granite pluton. The chemistry of biotite, cordierite, gedrite and garnet in rocks that up-grade from the cordierite isograd indicate a small range of chemical composition, particularly with reference to Mg, Fe and Mn. The analyses show further that among the coexisting ferromagnesian minerals Fe/Fe+ Mg ratio decreases in the sequence: garnet, gedrite, biotite, cordierite while Mn/Fe+Mg+Mn ratio decreases in the sequence garnet, gedrite, cordierite, biotite. The same order is also observed in the distribution diagrams. The regular distribution of Mg, Fe and Mn among the coexisting phases demonstrate that chemical equilibrium was attained and preserved in these Archean rocks. Mg-Fe distribution between cordierite and biotite appears to be dependent on the temperature of crystallization or metamorphic grade.     

Late Pleistocene paleoecology of arctic ground squirrel (Urocitellus parryii) caches and nests from Interior Alaska's mammoth steppe ecosystem, USA

Botanical analyses of fossil and modern arctic ground squirrel (Urocitellus parryii) caches and nests have been used to reconstruct the past vegetation from some parts of Beringia, but such archives are understudied in Alaska. Five modern and four fossil samples from arctic ground squirrel caches and nests provide information on late Pleistocene vegetation in Eastern Beringia. Modern arctic ground squirrel caches from Alaska's arctic tundra were dominated by willow and grass leaves and grass seeds and bearberries, which were widespread in the local vegetation as confirmed by vegetation surveys. Late Pleistocene caches from Interior Alaska were primarily composed of steppe and dry tundra graminoid and herb seeds. Graminoid cuticle analysis of fossil leaves identified Calamagrostis canadensis, Koeleria sp. and Carex albonigra as being common in the fossil samples. Stable carbon isotopes analysis of these graminoid specimens indicated that plants using the C₃ photosynthetic pathways were present and functioning with medium to high water-use efficiency. Fossil plant taxa and environments from ground squirrel caches in Alaska are similar to other macrofossil assemblages from the Yukon Territory, which supports the existence of a widespread mammoth steppe ecosystem type in Eastern Beringia that persisted throughout much of the late Pleistocene.     

The geology of kimberlite pipes of the Ekati property, Northwest Territories, Canada

This paper reviews key characteristics of kimberlites on the Ekati property, NWT, Canada. To date 150 kimberlites have been discovered on the property, five of which are mined for diamonds. The kimberlites intrude Archean basement of the central Slave craton. Numerous Proterozoic diabase dykes intrude the area. The Precambrian rocks are overlain by Quaternary glacial sediments. No Phanerozoic rocks are present. However, mudstone xenoliths and disaggregated sediment within the kimberlites indicate that late-Cretaceous and Tertiary cover (likely <200 m) was present at the time of emplacement. The Ekati kimberlites range in age from 45 to 75 Ma. They are mostly small pipe-like bodies (surface area mostly <3 ha but up to 20 ha) that typically extend to projected depths of 400–600 m below current surface. Pipe morphologies are strongly controlled by joints and faults. The kimberlites consist primarily of variably bedded volcaniclastic kimberlite (VK). This is dominated by juvenile constituents (olivine and lesser kimberlitic ash) and variable amounts of exotic sediment (primarily mud), with minor amounts of xenolithic wall-rock material (generally <5%). Kimberlite types include: mud-rich resedimented VK (mRVK); olivine-rich VK (oVK); sedimentary kimberlite; primary VK (PVK); tuffisitic kimberlite (TK) and magmatic kimberlite (MK). The presence and arrangement of these rock types varies widely. The majority of bodies are dominated by oVK and mRVK, but PVK is prominent in the lower portions of certain kimberlites. TK is rare. MK occurs primarily as precursor dykes but, in a few cases, forms pipe-filling intrusions. The internal geology of the kimberlites ranges from simple single-phase pipes (RVK or MK), to complex bodies with multiple, distinct units of VK. The latter include pipes infilled with steep, irregular VK blocks/wedges and at least one case in which the pipe is occupied by well-defined sub-horizontal VK phases, including a unique, 100-m-thick graded sequence. The whole-rock compositions of VK samples suggest significant loss of kimberlitic fines during eruption followed by variable dilution by surface sediment and concurrent incorporation of kimberlitic ash. Diamond distribution within the kimberlites reflects the amount and nature of mantle material sampled by individual kimberlite phases, but is modified considerably by eruption and depositional processes. The characteristics of the Ekati kimberlites are consistent with a two-stage emplacement process: (1) explosive eruption/s causing vent clearing followed by formation of a significant tephra rim/cone of highly fragmented, olivine-enriched juvenile material with varying amounts of kimberlitic ash and surface sediments (predominantly mud); and (2) infilling of the vent by direct deposition from the eruption column and/or resedimentation of crater rim materials. The presence of less fragmented, juvenile-rich PVK in the lower portions of certain pipes and the intrusion of large volumes of MK to shallow levels in some bodies suggest emplacement of relatively volatile-depleted, less explosive kimberlite in the later stages of pipe formation and/or filling. Explosive devolatilisation of CO₂-rich kimberlite magma is interpreted to have been the dominant eruption mechanism, but phreatomagmatism is thought to have played a role and, in certain cases, may have been dominant.     

Temporal, geomagnetic and related attributes of kimberlite magmatism at Ekati, Northwest Territories, Canada

This paper outlines the development of a multi-disciplinary strategy to focus exploration for economic kimberlites on the Ekati property. High-resolution aeromagnetic data provide an over-arching spatial and magnetostratigraphic framework for exploration and kimberlite discovery at Ekati, and hence also for this investigation. The temporal, geomagnetic, spatial and related attributes of kimberlites with variable diamond content have been constrained by judiciously augmenting the information gathered during routine exploration with detailed, laboratory-based or field-based investigations. The natural remanent magnetisation of 36 Ekati kimberlites has been correlated with their age as determined by isotopic dating techniques, and placed in the context of a well-constrained geomagnetic polarity timescale. Kimberlite magmatism occurred over the period 75 to 45 Ma, in at least five temporally discrete intrusive episodes. Based on current evidence, the older kimberlites (75 to 59 Ma) have low diamond contents and are distributed throughout the property. Younger kimberlites (56 to 45 Ma) have moderate to high diamond contents and occur in three distinct intrusive corridors with NNE to NE orientations. Economic kimberlite pipes erupted at 55.4±0.4 Ma along the A154-Lynx intrusive corridor, which is 7 km wide and oriented at 015°, and at 53.2±0.3 Ma along the Panda intrusive corridor, which is 1 km wide and oriented at 038°. The intrusion ages straddle a paleopole reversal at Chron C24n, consistent with the observation that the older economic kimberlites present as aeromagnetic “low” anomalies while the younger economic pipes are characterised as aeromagnetic “highs”. The aeromagnetic responses for these kimberlites are generally muted because they contain volcaniclastic rock types with low magnetic susceptibility. Kimberlites throughout the Ekati property carry a primary natural magnetic remanence (NRM) vector in Ti-bearing groundmass magnetite, and it dominates over vectors related to induced magnetisation. Magnetostratigraphic correlation of Ekati kimberlites may therefore present a powerful adjunct to existing exploration techniques, mainly because the diamond content of Ekati kimberlites apparently is related more to the age of eruption than to any other parameter investigated in this work.     

A late Early Pleistocene pollen record from Fontana Ranuccio (central Italy)

A pollen record from a sediment core drilled underneath the Acheulian site of Fontana Ranuccio provides new data on the vegetation characters in central Italy during the late Early Pleistocene, in a forest phase possibly corresponding to a substage of Marine Isotope Stage 21. A number of tree taxa nowadays extinct in Europe (Cathaya, Tsuga, Taxodium type, Carya and Pterocarya) are present at Fontana Ranuccio in a temperate phase characterised by dense forest conditions, dominated by Alnus and Pterocarya. The comparison of the Fontana Ranuccio record with other dated sites distributed along the Italian peninsula indicates that the floristic and vegetational characters of the Mid Pleistocene revolution present a rather distinct succession of biostratigraphic events. Compared to other Early Pleistocene pollen records from southern Europe, the Italian Peninsula shows greater floristic similarities with the Eastern Mediterranean regions than with the Iberian Peninsula. Copyright © 2011 John Wiley & Sons, Ltd.     

Sedimentology and ecology of the horn plateau formation: A middle devonian coral reef,Northwest Territories,Canada

The Horn Plateau Formation is a Middle Devonian limestone reef approximately 0.4 miles in diameter and 400 feet thick which crops out on the Interior Plains about 110 miles west of Yellowknife, Northwest Territories. The study is based upon examination of material collected from the outcrop and 5 core-holes drilled on and near the reef.Corals are the most important framebuilders present and crinoid ossicles contribute most to the sediment volume. Three macrofacies of this coral patch reef are recognized: 1) organic reef, an area where framebuilders grew profusely, 2) reef flat, an area characterized by sand sized sediments, and 3) reef flank, an area of predominantly gravel sized sediments. Strongly agitated water conditions prevailed over the organic reef and reef flank macrofacies whereas moderately agitated water covered the reef flat. Subdivision of the macrofacies has produced 11 microfacies of distinctive texture-composition combination.

---

Zusammenfassung Die Horn-Plateau-Formation ist ein mitteldevonisches Kalkriff, etwa 0,6 km breit und 120 m mächtig, das in den Interior Plains 180 km westlich von Yellowknife, Northwest Territories, aufgeschlossen ist. Die vorliegende Arbeit beruht auf einer Untersuchung von Proben aus Aufschlüssen und aus 5 Bohrungen im Riff und seiner Umgebung.Korallen sind die wichtigsten der Gerüstbildner, und Krinoiden-Reste die Hauptbestandteile der Sedimente. Drei Makrofazies werden in diesem Krustenriff erkannt: 1. der Riffkern, ein Bereich, worin riffbildende Korallen besonders häufig waren; 2. eine Riffebene charakterisiert durch Sedimente von Sand-Korngröße; und 3. eine Riffflanke mit Sedimenten von Kies-Korngröße. Über dem Kern und der Flanke war das Wasser stark bewegt, während ruhigeres Wasser die Riffebene bedeckte. Eine Unterteilung der Makrofazies ergab 11 Mikrofazies unterschiedlicher Struktur und Zusammensetzung.

---

Résumé Le Horn Plateau est un récif calcaire du Dévonien moyen d'un diamètre de 0.4 mille et d'une épaisseur de 400 pieds environ. Il affleure dans les Plaines environ à 110 milles à l'Ouest de Yellowknife (Territoires du Nord-Ouest). L'étude est basée sur l'examen d'échantillons d'affleurements et de cinq forages effectués dans et près du récif.Les coraux sont les plus importants organismes constructeurs et les entroques forment la majeure partie du volume sédimentaire. Trois macrofaciès ont été reconnus dans le récif de plateforme: 1) récif organique, domaine riche en organismes constructeurs, 2) replat récifal, domaine caractérisé par des sables, et 3) flanc du récif, domaine à débris grossiers. Le récif organique et le flanc du récif se trouvaient dans des eaux très agitées tandis que des eaux modérément agitées couvraient le replat récifal. Les macrofaciès ont été subdivisés en 11 microfaciès présentant des combinaisons texture-composition distinctes.

---

Interior Plains, 180 Yellowknife, Horn, 0,6 120 . , 5- . Crinoiden, — . 3 : 1) , . . , ; 2) , , 3) . , , , , . 11 .

     

The muscovite-melt bathograd and low-P isograd suites in north-central Wopmay Orogen, Northwest Territories, Canada

In north-central Wopmay Orogen, syntectonic low-P(Buchan-type) suites of mineral isograds outline regional metamorphic temperature culminations that are associated, at the higher structural levels, with emplacement of early Proterozoic plutons in the west part of a deformed and eastward transported continental margin prism. The mapped isograds mark the first occurrence of biotite, staurolite, andalusite, sillimanite, sillimanite-K feldspar and K feldspar-plagioclase-quartz ± muscovite (granitic) pods in metapelites, with increasing proximity to the plutons.
Microprobe analyses and field observations have resulted in the formulation of reactions for the 'ideal'pelitic system K₂O-Na₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-Al₂O₃-SiO₂-H₂O, to account for the various mineral assemblages of each metamorphic zone. A P-T petrogenetic grid showing erosion surface P-T curves for the northern Wopmay Orogen pelites, compiled on the basis of the mapped isograds and the inferred reaction(s) for each metamorphic zone, documents a variation in exposed metamorphic pressure ranging between 2 and 4 kbar.
The configuration of a new bathograd, based on the invariant model reaction sillimanite + K feldspar + plagioclase + biotite + quartz + vapor ± muscovite + liquid and interpolated across three metamorphic suites, is consistent with a major regional structure culmination and with independently determined pressures obtained from anorthite-grossular-quartz-Al₂SiO₅ geobarometry. The positive correlation between the configuration of the bathograd and the structural and pressure culmination points to the pressure-dependence of anatectic-granitic-pod mineral associations.     

Uranium-thorium enrichment in alkali olivine basalt magma — Simpson Islands dyke,Northwest Territories Canada

Uranium, thorium and potassium show parallel trends of geochemical enrichment in a differentiated dyke exposed on the Simpson Islands, in Great Slave Lake, Northwest Territories, Canada. Uranium ranges from 0.46 to 4.5 ppm, thorium from 2.2 to 22.6 ppm and K₂O from 0.81 to 8.00%. The correlation coefficient between U and K₂O is +0.985.The dyke is composite, with an early phase of olivine gabbro and the main phase ranging from gabbro to syenite. The inferred parental magma is a potassic olivine basalt. The proposed petrogenetic scheme includes partial melting (5% or less) of upper mantle, gravitational differentiation of the melt within the upper mantle and progressive intrusions of mafic to felsic fractions into a dilational fracture related to the rifting of the Athapuscow aulacogen 2200 m.y. years ago. The uppermost (eastern) levels of intrusion have been removed by erosion. Magmatic concentration of uranium and thorium beyond the levels reported may have occurred in the latest differentiates.     

Proterozoic aeolian quartz arenites from the Hornby Bay Group, Northwest Territories, Canada: Implications for Precambrian aeolian processes

The Hornby Bay Group is a Middle Proterozoic 2.5 km-thick succession of terrestrial siliciclastics overlain by marine siliciclastics and carbonates. A sequence of conglomeratic and arenaceous rocks at the base of the group contains more than 500 m of mature hematitic quartz arenite interpreted to have been deposited by migrating aeolian bedforms. Bedforms and facies patterns of modern aeolian deposits provided a basis for recognizing two sequences of aeolian arenite. Both sequences interfinger with alluvial—wadi fan conglomerates and arenites deposited by braided streams. Depositional processes, facies patterns and paleotopographic position of the arenites are consistent with modern sand sea dynamics.Distal aeolian facies in both sequences are composed of trough crossbed megasets deposited by climbing, sinuous-crested, transverse dunes. Megasets comprise a gradational assemblage of tabular to wedge-planar cosets formed by deflation/reactivation of dune lee slopes and migration of smaller superposed aeolian bedforms (small dunes and wind ripples). Megasets in the proximal facies are thinner, display composite internal stratification and have a tabular-planar geometry which suggests that they were formed by smaller, straight-crested transverse dunes. Most stratification within the crossbeds is inferred to have formed by the downwind climbing of aeolian ripples across the lee slopes of dunes.Remarkably few Precambrian aeolian deposits have been reported previously. This seems anomalous, because most Precambrian fluvial sediments appear to have been deposited by low sinuosity (braided) streams, the emergent parts of which are prime areas for aeolian deflation. Frequent floods and rapid lateral migration of Precambrian humid climate fluvial systems probably restricted aeolianite deposition to arid paleoclimates. Thus the apparent anomaly may reflect non-recognition and/or non-preservation of aeolianites and/or variations in some aspect of sand sea formation and migration unique to the Precambrian. Reconstruction of the Hornby Bay Group aeolianites using recently developed criteria for their recognition suggests that the latter reason did not exert a strong influence.     

A Diamondiferous Lamprophyre Dike,Gibson Lake Area,Northwest Territories

The Akluilak dike is a narrow lamprophyre intrusion of ultrapotassic composition that has yielded a large content of diamonds; it is the first multidiamond occurrence noted from the central Churchill Province. Xenocrysts of apatite containing monazite have been dated at 1832 ± 28 Ma, thus confirming the correlation with Christopher Island Formation volcanic and intrusive rocks. Only 1 or 2 diamonds per 1000 are greater than 0.5 mm, and the majority are less than 0.075 mm. They occur primarily as octahedra and tetrahexahedroids, but cubes, macles, aggregates, and fragments also are present. Their occurrence points to the presence of a “mantleroot-friendly” lithosphere, and confirms the “highly diamond prospective” nature of the area suggested by previous studies.     

Synsedimentary dolospar cementation: a possible Devonian example in the Camsell Formation, Northwest Territories, Canada1

A unique association of beds containing fenestral pore-filling medium crystalline dolomite with beds of dolospar and quartz sandstone occurs in the Lower Devonian Camsell Formation. Many of these sandstones consist almost entirely of dolospar sand. Sedimentological, petrographic, cathodoluminescent and chemical data indicate that the dolospar sands consist of grains that have been eroded and reworked during Camsell deposition from fenestral dolostone beds that directly underlie these sandstones. Erosional truncation of individual dolomite crystals and of intracrystalline cathodoluminescent zonations along the edges of sand grains indicates that either dolomitization, or dolomite cementation of the pore-filling carbonate cements within fenestrae of the fenestral fabrics, occurred before erosion and deposition of dolospar sand. The fine details of the cathodoluminescent zonations suggest that the pore-filling fenestral dolomite originated as primary cements that underwent some annealing recrystallization under the influence of high subsurface temperatures. This unique occurrence may document synsedimentary Devonian cementation by medium to coarsely crystalline dolomite.     

The Campo Felice late Pleistocene Glaciation (Apennines,central Italy)

Study of the glacial deposits and lacustrine sediments of Campo Felice (Apennines, central Italy) has enabled the glacial phases of the last 40 ka to be dated more precisely, and has demonstrated that the maximum glacial advance did not occur in correspondence with the last global glacial maximum and with the coldest and most arid phase suggested by the pollen, but in a period dated between about 33 and 27 ka, characterized by a less extreme climate. Furthermore, a glacial expansion took place also in the period prior to 35 ka. Correlation with the Alpine glacial variations has shown that the Apennine last glacial maximum occurred before that of the southern slope of the Alps. Copyright © 2012 John Wiley & Sons, Ltd.     

Geochemistry and origin of the Archean Prince Albert Group volcanics,western Melville Peninsula,Northwest Territories,Canada

Major and trace element data on the Archean metavolcanic rocks of the Prince Albert Group (PAG), Northwest Territories. Canada, are reported. The following major groups were found, based on combined field and geochemical evidence: ultramafic flows; basaltic rocks, predominantly tholeiites; andesites; heavy REE depleted dacites; and rhyolites.The ultramafic and basaltic rocks are relatively normal Archean volcanics except for the downward bowed REE patterns of the tholeiitic basalts. The andesites, dacites and rhyolites, however, are not typical of Archean terrains. Comparisons between the andesites of the PAG and other Archean and more recent ones show that those of the PAG are most similar chemically to modern high-K andesites. REE patterns in these rocks suggest that partial melting of assemblages with significant garnet are an unlikely source but it is not possible to ascribe their origin to any simple process. Partial melting of a garnet-poor mafic granulite is an acceptable source for the heavy REE depleted dacites. The geochemical characteristics of the rhyolites cannot be explained by partial melting of a mafic source or by fractional crystallization from the daeites. It is suggested that these rocks originated by partial melting of pre-existing sialic crust.     

A late Pleistocene long pollen record from Lake Urmia, NW Iran

A palynological study based on two 100-m long cores from Lake Urmia in northwestern Iran provides a vegetation record spanning 200 ka, the longest pollen record for the continental interior of the Near East. During both penultimate and last glaciations, a steppe of Artemisia and Poaceae dominated the upland vegetation with a high proportion of Chenopodiaceae in both upland and lowland saline ecosystems. While Juniperus and deciduous Quercus trees were extremely rare and restricted to some refugia, Hippophaë rhamnoides constituted an important phanerophyte, particularly during the late last glacial period. A pronounced expansion in Ephedra shrub-steppe occurred at the end of the penultimate late-glacial period but was followed by extreme aridity that favoured an Artemisia steppe. Very high lake levels, registered by both pollen and sedimentary markers, occurred during the middle of the last glaciation and late part of the penultimate glaciation. The late-glacial to early Holocene transition is represented by a succession of Hippophaë, Ephedra, Betula, Pistacia and finally Juniperus and Quercus. The last interglacial period (Eemian), slightly warmer and moister than the Holocene, was followed by two interstadial phases similar in pattern to those recorded in the marine isotope record and southern European pollen sequences.     

Morphological Chronoclines among Late Pleistocene Muskrats (Ondatra zibethicus: Muridae, Rodentia) from Northern Florida

The muskrat (Ondatra zibethicus) is presumed to have undergone a rapid phyletic size decrease near the end of the Pleistocene. Evolutionary changes in the size of middle to late Wisconsinan (ca. 32,000–12,300 ¹⁴C yr B.P.) muskrats from the Aucilla River, Jefferson County, Florida, were reconstructed by examining length and width of the lower first molar (m1). Body mass, estimated from m1 length, was relatively stable from 32,000 to 16,000 ¹⁴C yr B.P. and decreased only slightly by 12,300 ¹⁴C yr B.P. If the size trend found in the Aucilla River material is characteristic of the southeastern United States, a body size decrease after 12,300 ¹⁴C yr B.P. is needed to explain the smaller size of modern populations. It was previously thought that the length/width (l/w) ratio of the muskrat m1 was a paleoenvironmental indicator based on its presumed correlation with latitude in modern populations. We examined the length and width of modern muskrats from several geographic regions and found only a very weak trend in the size of the m1 between northern and southern populations; however, highly significant differences were found between regions of similar latitude. Our data indicate that chronoclines in the m1 of the Aucilla muskrat material and other such documented trends among fossil muskrats have paleoenvironmental significance, but it is not yet clear which environmental variables can best be predicted from them.     

Geothermometry and geobarometry applied to early proterozoic “S-type” granitoid plutons,Wopmay Orogen,Northwest Territories,Canada

Many of the S-type granitoid plutons that comprise Hepburn and Wentzel Batholiths of the Early Proterozoic Wopmay Orogen contain garnet, biotite and rarely cordierite and sillimanite. The garnet, cordierite and sillimanite are interpreted to be relict crystals brought up from the depth of origin of the magmas.Two methods of geothermobarometry were applied to ten samples from the two batholiths: the garnet-biotite Fe-Mg exchange equilibrium (Ferry and Spear 1978) and the garnet-plagioclase-sillimanite-quartz equilibrium (Ghent 1976). The intersection of the two displaced equilibrium curves on a P-T plot provides an estimate of theP-T conditions of equilibration of the minerals. P-T estimates in eight of ten samples ranged between 800 and 1,100°C, and 5 and l0 kbars. These values are in distinct contrast to the typical 650 °C — 3 kb results obtained from the immediately adjacent metapelites, to which the same geothermobarometers were applied.Numerous theoretical and practical problems are encountered both in the application of the methods and in the interpretation of the results. Two of the important theoretical problems are (1) how the anatexis of certain minerals in the equilibria influences the elemental ratios of other minerals that remain as solids, and (2) how far the elemental ratios of the minerals re-equilibrated below the maximalP-T conditions. The most serious practical problem was chloritization of biotite, which generated spuriously high temperatures on the garnet-biotite geobarometer.The significant difference in results between the batholith rocks and the metamorphic rocks indicates that the minerals did not re-equilibrate to sub-magmatic conditions. Using theP-T results as a guide to the conditions of origin of the plutons, it appears that several of the plutons were generated at depths between 21 and 29 km, and rose between 8 and 18 km to the level of emplacement.The core-to-rim zonation of the garnets produces aP-T trend that is towards lower pressure, temperature or both. Such a pattern would be expected in a pluton that rose to the less extreme conditions at higher levels in the crust.The geothermobarometers successfully distinguish between granitoid rocks that were generated at depth and those that were formed by anatexis of country rocks near the level of emplacement.     

Hydrothermal environments during the genesis of silver deposits in the Northwest Territories of Canada: Evidence from fluid inclusions

The Ag-Ni-Co-Bi-As-U veins in the Northwest Territories of Canada are hosted by volcano-sedimentary and intrusive rocks of the Great Bear Batholithic Complex. Fluid inclusion data from the gangue minerals of the veins suggest a wide range in salinity and temperature for the hydrothermal fluids. The salinities of the fluids range from about 15 to 35 wt.% NaCl equivalent. The homogenization temperatures range from 150° to 250°C for stage I fluids; 220° to 480°C for stage II fluids; 250° to 350°C for stage III fluids; 150° to 250°C for stage IV fluids and 90° to 250°C for stage V fluids. The coexistence of liquid-rich and vapour-rich inclusions in stages II and III minerals suggest that the hydrothermal fluids were boiling or effervescing. The coexistence of saturated and unsaturated inclusions in stages II and III minerals may be due to effervescence of a less saline fluid, whilst for stages IV and V it may be due to mixing of a highly saline fluid with a less saline fluid, or due to alternation of hydrostatic and lithostatic pressures at the time of trapping of the inclusions.