Proterozoic low-sulfidation epithermal Au-Ag mineralization in the Mallery Lake area, Nunavut, Canada

The Mallery Lake area contains pristine examples of ancient precious metal-bearing low-sulfidation epithermal deposits. The deposits are hosted by rhyolitic flows of the Early Proterozoic Pitz Formation, but are themselves apparently of Middle Proterozoic age. Gold mineralization occurs in stockwork quartz veins that cut the rhyolites, and highest gold grades (up to 24 g/t over 30 cm) occur in the Chalcedonic Stockwork Zone. Quartz veining occurs in two main types: barren A veins, characterized by fine- to coarse-grained comb quartz, with fluorite, calcite, and/or adularia; and mineralized B veins, characterized by banded chalcedonic silica and fine-grained quartz, locally intergrown with fine-grained gold or electrum. A third type of quartz vein (C), which crosscuts B veins at one locality, is characterized by microcrystalline quartz intergrown with fine-grained hematite and rare electrum. Fluid inclusions in the veins occur in two distinct assemblages. Assemblage 1 inclusions represent a moderate temperature (Th=150 to 220 °C), low salinity (~1 eq. wt% NaCl, with trace CO₂), locally boiling fluid; this fluid type is found in both A and B veins and is thought to have been responsible for Au-Ag transport and deposition. Assemblage 2 inclusions represent a lower temperature (Th=90 to 150 °C), high salinity calcic brine (23 to 31 wt% CaCl₂-NaCl), which occurs as primary inclusions only in the barren A veins. Assemblage 1 and 2 inclusions occur in alternating quartz growth bands in the A-type veins, where they appear to represent alternating fluxes of dilute fluid and local saline groundwater. No workable primary fluid inclusions were observed in the C veins. The A-vein quartz yields '¹⁸O values from 8.3 to 14.5‰ (average=10.9ǃ.7‰ [1C], n=30), whereas '¹⁸O values for B-vein quartz range from 11.2 to 14.0‰ (average=13.0ǂ.9‰, n=12). Calculated '¹⁸O_H2O values for the dilute mineralizing fluid from B veins range from -2.6 to 0.2‰ (average=-0.8ǂ.9‰, n=12) and are consistent with a dominantly meteoric origin. No values could be calculated for the brine, however, because all A-vein quartz samples contain mixed fluid inclusion populations. However, the fact that A-vein quartz samples extend to lower '¹⁸O values than the B veins suggests that the brine had a lighter isotopic signature relative to the dilute fluid. Hydrogen isotopic ratios of fluid inclusion waters extracted from eleven quartz samples of both vein types range from 'D_FI=-56 to -134‰, but show no particular correlation with vein type. In most respects, the mineralogical and fluid characteristics of the Mallery Lake system are comparable to those of Phanerozoic low-sulfidation deposits, and although the presence of high salinity brines is unusual in such deposits, it is not unknown (e.g., Creede, Colorado). In addition, one of the few other examples of well-preserved, Precambrian, low-sulfidation epithermal deposits, from the Central Pilbara tectonic zone, Australia, contains a similarly bimodal fluid assemblage. The significance of these saline brines is not clear, but from this study we infer that they were not directly involved with Au-Ag transport or deposition.     

红十字湖地区锂云母-透锂长石伟晶岩脉中锡锰钽矿的发现

锡锰钽矿(wodginite)理想式为MnSnTa2O8,Z=4,其结构为锡铁钽矿(ixiolite)结构的有序化形式。它是钽的主要工业矿物,是高度分异演化伟晶岩的标志矿物之一。在加拿大红十字湖地区锂云母-透锂长石伟晶岩脉中发现了这一钽矿物。矿物为黑褐色,呈半自形,颗粒达0.2mm,分布于由锂云母钠长石构成的条带中,与之共生的是一套典型的、高演化的花岗伟晶岩矿物组合。锡锰钽矿具有Mn/Fe、Sn/Ti、Ta/Nb比值高,杂质少的特点。矿物平均分子式为:Mn1.00(Sn0.64Ta0.24Ti0.04Mn0.04Zn0.02Fe0.01)0.99(Ta1.69Nb0.31)2.00O8;晶胞参数a=0.9534nm,b=1.1482nm,c=0.5123nm,β=91°07',V=0.5607nm3。依据矿物β>91°、V>0.560nm3、对偶面网221与221具强分裂衍射双峰、I021/I220为0.19和Mn/(Fe+Mn)≈1等特征,在锡锰钽矿与锡铁钽矿所构成的同质多像、有序-部分有序-无序这样一个连续系列中,本区所产锡锰钽矿属有序端员矿物,其阳离子在八面体中分布的有序度约为95%。在钽资源的寻找和开发利用中,应重视含锡锰钽矿的锂云母-透锂长石伟晶岩脉。     

Seismic stratigraphy of Waterton Lake, a sediment-starved glaciated basin in the Rocky Mountains of Alberta, Canada and Montana, USA

Upper and Middle Waterton lakes fill a glacially scoured bedrock basin in a large (614 km²) watershed in the eastern Front Ranges of the Rocky Mountains of southern Alberta, Canada and northern Montana, U.S.A. The stratigraphic infill of the lake has been imaged with 123 km of single-channel FM sonar (‘chirp') reflection profiles. Offshore sonar data are combined with more than 2.5 km of multi-channel, land-based seismic reflection profiles collected from a large fan-delta. Three seismic stratigraphic successions (SSS I to III) are identified in Waterton Lake resting on a prominent basal reflector (bedrock) that reaches a maximum depth of about 250 m below lake level. High-standing rock steps (reigels) divide the lake into sub-basins that can be mapped using lake floor reflection coefficients. A lowermost transparent to poorly stratified seismic succession (SSS I, up to 30 m thick) is present locally between bedrock highs and has high seismic velocities (1750–2100 m/s) typical of compact till or outwash. A second stratigraphic succession (SSS II, up to 50 m thick), occurs throughout the lake basin and is characterised by continuous, closely spaced reflectors typical of repetitively bedded and rhythmically laminated silts and clays most likely deposited by underflows from fan-deltas; paleo-depositional surfaces identify likely source areas during deglaciation. Intervals of acoustically transparent seismic facies, up to 5 m thick, are present within SSS II. At the northern end of Upper Waterton Lake, SSS II has a hummocky surface underlain by collapse structures and chaotic facies recording the melt of buried ice. Sediment collapse may have triggered downslope mass flows and may account for massive facies in SSS II. A thin Holocene succession (SSS III, <5 m) shows very closely spaced reflectors identified as rhythmically laminated fine pelagic sediment deposited from interflows and overflows. SSS III contains Mt. Mazama tephra dated at 6850 yr BP.     

Seismic activity changes progressing simultaneously with slow-slip in the Tokai area

Anomalous movements were detected simultaneously in both the seismic and the GPS observations in the Tokai area, the central part of the Japanese islands from the late 1990s to 2000. The anomalies are of great concern since the pending risk of a large megathrust earthquake in this area has been predicted for more than 20 years. The GPS data revealed that a slow-slip on the plate interface had commenced beneath Lake Hamana, the center of which is positioned around the edge of the assumed focal zone. On the other hand, the seismic data indicated that a delicate but clear quiescence appeared over a wide area that spreads into the main focal zone. Analyses of the seismicity changes in space and time confirmed that the contrast in the seismicity rate is distinct inside the focal zone. While the integrated seismicity indicated quiescence, some locations were distinguished as activated zones, possibly indicating the appearance of asperities. The rise of the seismicity rate in a quasi-stationary manner suggests an increase in the stress rate at that location. The following hypothesis is proposed based on the simultaneously detected evidences. The slow-slip progressing beneath Lake Hamana will induce a stress shift that invades the interior of the main locked zone, which will increase the contrast of the seismicity rate, possibly reflecting inhomogeneity in the locking strength. Even in this stage, the activated zones still maintain a locked state to prevent overall breakage. Investigations of the b-value changes and of tidal dependence in seismicity that reveal the stress-concentrated state also support the hypothesis. If this is the case, the observed change in seismicity would indicate the process of stress redistribution in the locking state, which represents the preparatory process toward final breakage. Tracking such seismicity changes would yield valid information for predictions of the next Tokai earthquake.     

Sapphirine bearing granulites from the Sipiwesk Lake area of the late Archean Pikwitonei granulite terrain,Manitoba, Canada

Sapphirine occurs in the orthopyroxene-cordierite and feldspar-sillimanite granulites in the Sipiwesk Lake area of the Pikwitonei granulite terrain, Manitoba (97°40W, 55°05N). The orthopyroxene-cordierite granulites have extremely high Al₂O₃ (24.5 wt%) and MgO (24.6 wt%) contents and contain sapphirine (up to 69.2 wt% Al₂O₃), aluminous orthopyroxene (up to 8.93 wt% Al₂O₃), cordierite, spinel, phlogopite, and corundum. Sapphirine forms coronas mantling spinel and corundum. Corona sapphirine is zoned and its composition varies through the substitution (Mg, Fe, Mn) Si=2 Al as a function of the phases with which it is in contact. Textural and chemical relationships of sapphirine with coexisting phases indicate that spinel + cordierite reacted to form orthopyroxene + sapphirine under conditions of increasing pressure. Moreover, decreasing core to rim variation of Al₂O₃ in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation. On the basis of experimentally determined P-T stability of the assemblage enstatite + sapphirine + cordierite, and the Al content of hypothetical Fe²⁺-free orthopyroxene associated with sapphirine and cordierite, metamorphic temperatures and pressures are estimated to be 860–890° C and 3.0–11.2 kbar.In the feldspar-sillimanite granulites, sapphirine occurs as a relict phase mantled by sillimanite and/or by successive coronas of sillimanite and garnet. These textural relations suggest the reaction sapphirine + garnet + quartz = orthopyroxene + sillimanite with decreasing temperature. Compositions of minerals in the assemblage garnet-orthopyroxene-sillimanite-plagioclase-quartz, indicate metamorphic P-T conditions of 780–880° C and 9±1 kb.The metamorphic conditions estimated in this study suggest that the sapphirine bearing granulites in the Sipiwesk Lake area represent Archean lower crustal rocks. Their formation might be related to the crustal thickening processes in this area as suggested by Hubregtse (1980) and Weber (1983).     

Glacial Lake development and marine inundation,Deer Lake area,Newfoundland, Canada: Topographically controlled deglaciation of an interior Basin

A large interior basin in west-central Newfoundland (covering the area of modern Deer Lake, Grand Lake, Sandy Lake and Birchy Lake) is connected to the sea by a narrow breach of a coastal mountain range. During retreat of Late Wisconsinan glaciers, this basin was occupied by a short-lived glacial lake impounded by remnant ice in coastal fjords, and drained by a spillway at the western end of the lake. Evidence for this lake is fragmentary, and consists of strandline features that fall on a plane of elevation consistent with the post-glacial isostatic tilt. Following collapse of the ice dams and subsequent lake drainage at some time prior to 12 220 yr BP, the Deer Lake basin was inundated by the sea to an elevation of about 45 m a.s.l. Deltas were formed at the basin edges, and thick successions of fine-grained rhythmites blanketed the basin floor. Isostatic rebound resulted in falling relative sea levels, and, following a stillstand marked by a period of deltaic deposition at 33 m a.s.l., isolation of the Deer Lake basin from marine influence in the early Holocene. This style of deglaciation differs from previously accepted models for this part of eastern Canada, which showed progressive retreat from the coast to remnant centres on topographic highs. In the model proposed here, a large, low-elevation basin was deglaciated early, at a similar time to ice retreat to coastal positions from offshore. This pattern of deglaciation may be found in other areas with similar topographic settings.     

Holocene carbonate sedimentation in Lake Manitoba, Canada

The carbonate mineral suite of the modern offshore bottom sediment of the South Basin of Lake Manitoba consists mainly of high magnesian calcite and dolomite with minor amounts of low-Mg calcite and aragonite. The high-Mg calcite is derived from inorganic precipitation within the water column in response to supersaturation brought about by high levels of organic productivity in the basin. Both dolomite and pure calcite are detrital in origin, derived from erosion of the surrounding carbonate-rich glacial deposits. Aragonite, present only in trace amounts in the offshore sediments, is bioclastic in origin. The upward increase in the amount of magnesian calcite in the post-glacial sediment record is attributed to increasing photosynthetic utilization of CO₂ in the lake. Stratigraphic variation in the amount of magnesium incorporated into the calcite lattice is interpreted as reflecting a variable magnesium input to the lake from ground water and surface runoff, and possibly variable calcium removal in the precipitating lake water. The effects of long-term chemical weathering at the source and size segregation explain the changes in dolomite content throughout the section.     

Subglacial Lake McGregor, south-central Alberta, Canada

It is proposed that a lake, here named “Subglacial Lake McGregor”, existed beneath the Laurentide Ice Sheet at, or near, the last glacial maximum. The lake resided in the ancient buried McGregor and Tee Pee preglacial valleys, which are now mostly filled with glacigenic deposits. The greatest thickness of sediment in the valleys is in the form of chaotically deposited lake beds that were laid down in a subaqueous environment by a number of process: gravity flow, water transport, and suspension settling. Topographic, sedimentary, and stratigraphic evidence point to a subglacial, not a proglacial, origin for the beds. During the early stages of lake existence, ice movement was significant as there are numerous sets of shear planes in the sedimentary beds. This indicates that the lake filled (lake sedimentation) and drained (shearing of the beds by overlying ice when ice contacted the bed) often. Thus, early in its history, the lake(s) was/were ephemeral. During the later stages of lake existence, the lake was relatively stable with no rapid draining or influx of sediment. Gradual drainage of the lake resulted in lowering of the ice onto the lake beds resulting in subglacial till deposition. Drainage was not a single continuous event. Rather it was characterized by multiple phases of near total drainage (till deposition), followed by water accumulation (lake sedimentation). Water accumulation events became successively less significant reflected by thinning of lake beds and thickening of till beds higher in the stratigraphic sequence. Since subglacial lake sedimentation appears to be restricted to the subglacial valleys, it is suggested that the valleys acted as a large-scale interconnected cavity system that both stored and transported water. It is also suggested that these acted as the main routes of water flow beneath the Laurentide Ice Sheet.     

Seismic reflection imaging of thin, kimberlite dykes and sills: exploration and deposit characterization of the Snap Lake dyke, Canada

Seismic reflection techniques are, for the first time, used to image a thin, diamondiferous, kimberlite dyke from subcrop to depths greater than 1300 m. Exploration for vertical kimberlite pipes generally utilizes potential field techniques that often fail to reveal subhorizontal or shallow-dipping intrusions. In contrast, seismic reflection methods are well suited for imaging targets with this geometry. Therefore, in order to evaluate seismic reflection as a tool for subhorizontal kimberlite dyke/sill exploration and mine planning, a feasibility study and subsequent seismic survey were undertaken on the diamondiferous Snap Lake dyke (Northwest Territories, Canada). A substantial drilling program has mapped the dyke as a gently dipping sheet that averages 2–3 m in thickness. The detailed structural and composition data available at Snap Lake provides a unique opportunity to test reflection techniques on a well-sampled deposit. The feasibility study involved measuring P-velocities and densities of cores drilled from the kimberlite and host rocks. These data were used to model reflection amplitudes, evaluate resolution limitations, and determine the acquisition parameters for the reflection survey. Two 2-D lines were acquired that provide comparative datasets for different sources (explosive and vibroseis) and ground types (land and lake ice). In addition, the exploration-scale survey incorporated high fold (40–260 nominal) and long offsets (3260 m). The explosive-source profile recorded on land yielded a superb image of the dyke from depths of 60 m to more than 1300 m over a lateral distance of 5700 m. The seismic image correlates well with adjacent drill hole data and adds considerable detail to the topography of the kimberlite sheet determined by drilling. The vibroseis source also imaged the dyke, but only when sources and geophones were on land; the dyke was not imaged beneath the ice due to reverberation and attenuation effects. The frequency response and unusually strong reflection amplitudes from the dyke indicate the importance of tuning effects and multiples for this type of target and acquisition environment. Apparent correlations between reflection amplitudes and dyke structure (e.g., thickness, feathering, 3-D geometry) suggest that seismic reflection data may be valuable for guiding drilling programs. The results demonstrate that, in the appropriate situation, seismic methods have great potential for use in kimberlite exploration, subsurface mapping, and detailed imaging for mine development purposes.     

Cupriferous bogs in the Sackville area, New Brunswick, Canada

Two cupriferous bogs in the Sackville area of New Brunswick, Canada, are described. The organic matter (muck) in these bogs contains from 2 to 6% Cu, derived from cupriferous springs that carry copper in amounts ranging from 0.005 to 1 ppm. The source of the copper in the springs is unknown, but the metal probably originates from the leaching of cupriferous (chalcocitic) deposits in grits and conglomerates at the base of the Boss Point Formation of Carboniferous (Pennsylvanian) age.The copper occurs in the muck as copper humate(s), the precise chemical characterization of which is unknown.Examples of copper bogs related to copper mineralization in many parts of the world are reviewed, and it is pointed out that such bogs are good geochemical indicators of cupriferous deposits.The moss, Pohlia nutans, grows in the springs in the copper bogs and in other wet sites in the area where large concentrations of copper occur. This moss is tolerant to large amounts of available copper and markedly accumulates the metal. When slightly chlorotic Pohlia nutans may indicate the presence of higher than normal amounts of copper.     

Seismic convolutions in the Quaternary deposits of Lake Sevan,Armenia

The aim of the study is to detect deformations in the soft sediments in a tectonically active area (the Armenian Highland) and to examine the significance of the deformations as paleoseismicity indicators. Deformations in the form of pillows, pockets, sharp waves, and ovoids are exposed in the Sevan basin, within interbedded shallow lacustrine, beach, and fluvial sediments. Also, broken beds and low-amplitude thrusts are observed. Eight field criteria for assigning soft-sediment deformations to paleoseismic triggering provide strong evidence for the seismic origin of the deformations. According to the local relative stratigraphic scale, the Sevan seismites are of Pleistocene–Holocene age.     

Petrology of the Hara Lake paragneisses,northeastern Saskatchewan,Canada

Precambrian aluminous paragneisses in the Hara Lake area of northeastern Saskatchewan are characterized by the stable association of cordierite and garnet with either sillimanite or biotite; alkali feldspar is an important mineral of this association. The aluminous gneisses are interlayered with pyroxene-plagioclase gneisses that contain orthopyroxene, clinopyroxene, hornblende, and biotite. The gneisses in the study area and in the adjacent Charcoal Lake area were metamorphosed under conditions of the granulite facies. The petrologic evidence is consistent with a first metamorphic event during which the association sillimanite-biotite was stable, followed by a second event during which cordierite-garnet was stable. Consideration of mineral assemblages and mineral chemistries of the aluminous gneisses in terms of experimentally established or thermodynamically derived reactions suggests that recrystallization from the sillimanite-biotite association to cordierite-garnet took place at 680 °–750 °C and 3.2–4.4 kb.     

U−Ti phases from Precambrian quartz-pebble conglomerates of the Elliot Lake area,Canada, and the Pongola basin,South Africa

Summary In the present study inhomogeneous, partly uraniferous leucoxene/rutile aggregates and brannerite grains from the Elliot Lake area, Canada, and the Pongola basin, South Africa, were identified by ore microscopy. This observation accords with the fact that U–Ti phases are second only to uraninite as the most important uranium minerals in Precambrian conglomerates. The observed U–Ti phases, however, are somewhat indefinitive. They often merge into each other and represent authigenic constituents pseudomorphous after detrital titanium minerals. Microprobe work has confirmed the existence of a continuous mineral series recognized optically which ranges from uranium-free leucoxene/rutile to uranium-enriched brannerite. However, the presence of metamict brannerite can only be verified by X-ray diffraction after prolonged heating, a process which may synthesize the latter mineral. The actual occurrence of brannerite in Precambrian conglomerates must, therefore, be regarded with reservation and it is suggested that redistribution and subsequent adsorption of uranium on Ti phases during diagenesis and/or metamorphism of the conglomerates did not lead to the formation of authigenic brannerite, but rather resulted in microcrystalline leucoxene/rutile admixtures containing uranium in varying amounts.

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U–Ti-Phasen aus präkambrischen Quarz-Geröll-Konglomeraten des Elliot Lake-Gebietes, Canada, und des Pongola-Beckens, Südafrika

Zusammenfassung Im Untersuchungsmaterial, das aus dem Elliot Lake-Gebiet, Canada, und dem Pongola-Becken, Südafrika, stammt, konnten Leukoxen/Rutil-Aggregate, die teilweise Uran-führend sind, und Brannerit mikroskopisch bestimmt werden. Dies stimmt mit vielfach mitgeteilten Beobachtungen überein, daß in präkambrischen Konglomeraten U–Ti-Phasen, nach Uraninit, die wichtigsten Uranminerale darstellen. Die festgesteilten U–Ti-Phasen sind recht wenig definiert und gehen oft graduell ineinander über. Es sind authigene Konglomeratkomponenten, die im allgemeinen pseudomorph nach detritischen Titanmineralen sind. Ein kontinuierlicher Übergang von Uran-freiem Leukoxen/Rutil zu Uran-reichem Brannerit konnte mittels der Mikrosonde ebenfalls mineralgeochemisch bestätigt werden. Wegen des metamikten Charakters des Probenmaterials kann jedoch die Anwesenheit von Brannerit, mittels Röntgendiffraktometrie, erst nach längerem Glühen der Proben verifiziert werden, was möglicherweise zu einer Synthetisierung von Brannerit führt. Sein Auftreten in präkambrischen Konglomeraten ist deshalb nicht gesichert, und es wird vorgeschlagen, daß diagenetische oder/und metamorphe Umlagerungen von Uran eher zur Bildung uranhaltiger mikrokristalliner Leukoxen/Rutil-Aggregate als von authigenem Brannerit führen.

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With 10 Figures     

Seismic faults in the Aegean area

Reliable fault plane solutions of shallow earthquakes and information on surface fault traces in combination with other seismic, geomorphological and geological information have been used to determine the orientation and other properties of the seismic faults in the Aegean and surrounding area.Thrust faults having an about NW-SE strike occur in the outer seismic zone along western Albania-westernmost part of mainland of Greece-Ionian Sea-south of Crete-south of Rhodes.The inner part of the area is dominated by strike-slip and normal faulting. Strike-slip with an about NE-SW slip direction occurs in the inner part of the Hellenic arc along the line Peloponnesus-Cyclades-Dodecanese-southwest Turkey as well as along a zone which is associated with the northern Aegean trough and the northwesternmost part of Anatolia. All other regions in the inner part of the area are characterized by normal faulting. The slip direction of the normal faults has an about SW-NE direction in Crete (N38°E) and an about E-W direction (N81°E) in a zone which trends N-S in eastern Albania and its extension to western mainland of Greece. In all other regions (central Greece-southern Yugoslavia and Bulgaria, western Turkey) the slip of the normal faults has an about N-S direction.     

Late quaternary history of Lake Manitoba,Canada

The postglacial history of Lake Manitoba has been deduced from a study of the changes in physical, mineralogical, and chemical variables in sediment cores collected from the lake. Six lithostratigraphic units are recognized in the South Basin of the lake. Weakly developed pedogenic zones, reflecting dry or extremely low water conditions in the basin, separate five of these six units. The initial phase of lacustrine sedimentation in the Lake Manitoba basin began shortly after 12,000 yr B.P. as water was impounded in front of the receding glacier to form Lake Agassiz. By 11,000 yr ago, continued retreat of the ice sheet opened lower outlets to the east and much of Lake Agassiz drained, including the Lake Manitoba basin. Water levels again rose at 9900 yr B.P., but by about 9200 yr B.P. the South Basin was again dry. For the next 4700 yr there was an alternation of wet and dry conditions in the basin in response to the interaction of a warmer and drier climate and differential crustal rebound of the basin. About 4500 yr ago a new phase of Lake Manitoba sedimentation was initiated when the Assiniboine River began to discharge into the South Basin. The Assiniboine River was diverted out of the Lake Manitoba watershed about 2200 yr ago. Erosion and redistribution of the sandy deltaic sediments deposited by the Assiniboine River has created the barrier beach that now separates the extensive marsh to the south of the lake from the main lake.     

Rb-Sr dating of the Lackner Lake Complex,northern Ontario,Canada

The Lackner Lake Complex of northern Ontario consists of nepheline syenite and ijolite with associated bodies of apatite—magnetite rock and carbonatite. A whole-rock Rb-Sr isochron indicates that these rocks crystallized 1078 ± 7 Ma ago ($\text{λ}{}^{87}\text{Rb = 1.39·10}{}^{\mathrm{?11}}\text{a}{}^{\mathrm{?1}}$) and had an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.070282 ± 0.00011. This date is slightly lower than a K-Ar date of 1090 Ma for biotite reported previously, but is in agreement with K-Ar dates of several nearby syenite-carbonatite complexes.     

Indicator mineralogy of kimberlite boulders from eskers in the Kirkland Lake and Lake Timiskaming areas, Ontario, Canada

Sixteen kimberlite boulders were collected from three sites on the Munro and Misema River Eskers in the Kirkland Lake kimberlite field and one site on the Sharp Lake esker in the Lake Timiskaming kimberlite field. The boulders were processed for heavy-mineral concentrates from which grains of Mg-ilmenite, chromite, garnet, clinopyroxene and olivine were picked, counted and analyzed by electron microprobe. Based on relative abundances and composition of these mineral phases, the boulders could be assigned to six mineralogically different groups, five for the Kirkland Lake area and one for the Lake Timiskaming area. Their indicator mineral composition and abundances are compared to existing data for known kimberlites in both the Kirkland Lake and Lake Timiskaming areas. Six boulders from the Munro Esker form a compositionally homogeneous group (I) in which the Mg-ilmenite population is very similar to that of the A1 kimberlite, located 7–12 km N (up-ice), directly adjacent to the Munro esker in the Kirkland Lake kimberlite field. U–Pb perovskite ages of three of the group I boulders overlap with that of the A1 kimberlite. Three other boulders recovered from the same localities in the Munro Esker also show some broad similarities in Mg-ilmenite composition and age to the A1 kimberlite. However, they are sufficiently different in mineral abundances and composition from each other and from the A1 kimberlite to assign them to different groups (II–IV). Their sources could be different phases of the same kimberlite or—more likely—three different, hitherto unknown kimberlites up-ice of the sample localities along the Munro Esker in the Kirkland Lake kimberlite field. A single boulder from the Misema River esker, Kirkland Lake, has mineral compositions that do not match any of the known kimberlites from the Kirkland Lake field. This suggests another unknown kimberlite exists in the area up-ice of the Larder Lake pit along the Misema River esker. Six boulders from the Sharp Lake esker, within the Lake Timiskaming field, form a homogeneous group with distinct mineral compositions unmatched by any of the known kimberlites in the Lake Timiskaming field. U–Pb perovskite age determinations on two of these boulders support this notion. These boulders are likely derived from an unknown kimberlite source up-ice from the Seed kimberlite, 4 km NW of the Sharp Lake pit, since indicator minerals with identical compositions to those of the Sharp Lake boulders have been found in till samples collected down-ice from Seed. Based on abundance and composition of indicator minerals, most importantly Mg-ilmenite, and supported by U–Pb age dating of perovskite, we conclude that the sources of 10 of the 16 boulders must be several hitherto unknown kimberlite bodies in the Kirkland Lake and Lake Timiskaming kimberlite fields.     

Late Holocene history of Waldsea Lake,Saskatchewan, Canada

The post-Hypsithermal history of Waldsea Lake, a saline meromictic lake located in south-central Saskatchewan, has been deduced from a study of the changes in physical, mineralogical, and paleobiological parameters in sediment cores from the basin. Six lithostratigraphic units and three palynological zones are identified in the most recent sediment. These units and zones indicate that a shallow hypersaline lake with extensive mudflats existed about 4000 yr B.P. In response to the subsequent trend toward a cooler and wetter climate, deeper water conditions ensued, and by about 3000 yr ago a relatively deep stratified lake occupied the Waldsea Basin. A short climatic reversal about 2500 yr B.P. again caused low-water and mudflat conditions, but by 2000 yr ago the lake had regained its higher levels. The past 2000 yr of Waldsea's history have been relatively uneventful, except for a minor lowering of the lake about 700 yr B.P.