Kurt Kyser

Relationships among sedimentology, stratigraphy, and diagenesis in the Proterozoic Thelon Basin, Nunavut, Canada: implications for paleoaquifers and sedimentary-hosted mineral deposits

Abstract The Thelon Basin, Nunavut, Canada, is host to unconformity-type uranium mineralisation and has the potential to host other economic deposits. The Thelon Formation (ca. 1750 Ma) is composed of thick (meters to tens of meters), poorly sorted, trough cross-bedded conglomerate and coarse-grained lithic arenite beds, and to a lesser extent, well-sorted, medium- to coarse-grained quartz arenite beds. Relatively rare, 1–12 cm thick, clay-rich siltstones to fine-grained sandstone layers punctuate the coarser lithofacies. Based on regional analysis of drill cores and outcrops, multiple unconformity-bounded sequences are defined in this fluvial-dominated sedimentary succession. Stratigraphic correlations are based on detailed lithofacies analysis, distinct changes in fining-upward cycle thickness, and intraformational surfaces (unconformities, transgressive surfaces, and paleosols). Diagenetic and paragenetic relationships vary systematically with sedimentology and stratigraphy of the Thelon and provide a framework for understanding the evolution of fluid-flow systems in the context of basin hydrostratigraphy. Stratigraphic units with well-sorted textures, which lacked clay and unstable framework grains, originally were aquifers (depositional aquifers) during early basin evolution. However, pervasive, early quartz cementation radically reduced the porosity and permeability of these units, occluding pore throats and transforming them into aquitards. Proximal fluvial and alluvial fan lithofacies that contained detrital, mechanically infiltrated, and diagenetic clay minerals and/or unstable detrital grains originally had low permeabilities and only experienced minor quartz cementation. In the deep burial setting (2–7 km), these units retained sufficient permeability to allow diagenetic fluid flow (diagenetic aquifers) as suggested by feldspar dissolution, quartz dissolution, and formation and recrystallization of illite and other diagenetic reactions. Tracing potential diagenetic aquifer and aquitard units across the study area allowed development of a hydrostratigraphic model. In this model, diagenetic aquifers onlap onto, and focused basinal fluids into basement rocks to the east in the Thelon Basin (in the vicinity of the Kiggavik uranium deposit).

Direct determination of trace levels of Os, Ir, Ru, Pt and Re in kimberlite and other geological materials using HR-ICP-MS

A fast and simple method for the direct determination of low concentrations of Os, Ir, Ru, Pt and Re from the same sample aliquot has been developed. It exploits the superior mass resolution capabilities of a double focussing sector field mass spectrometer (Finnigan MAT Element), in combination with a highly sensitive, high TDS tolerant teflon® Microflow nebulizer. Samples are spiked with an Os isotopic tracer, and digested in inverse aqua regia in sealed Carius tubes for 24 hours at 240 °C. Osmium is subsequently separated from the matrix (which contains the other PGEs) in a previously developed solvent extraction step, and measured in low resolution mode. The remaining sample solution is dried, redissolved in 2% HCl and analyzed directly for Ir, Ru, Pt and Re using the method of standard addition, in high resolution mode. Procedural blanks are <0.009 ppb for all PGEs except Pt (∼0.046 ppb) and Rh (∼0.0143 ppb). Instrumental detection limits based on the 3σ criterion for background are 0.0009 ppb for Re, 0.0048 ppb for Os, 0.002 ppb for Ru, and 0.003 and 0.01 ppb for Ir and Pt, respectively. The accuracy of the analytical method has been verified by comparing the results obtained in this study for three PGE reference materials (komatiite WITS1, komatiite KAL1, and peridotite GP13) with those reported in the literature.

Stable isotope composition of impact glasses from the Nordlinger Ries impact crater, Germany

Abstract The Ries impact, which today is represented by a 24-km-diameter complex crater, occurred at 15 Ma. It is estimated that a projectile of about 1 km diameter formed a transient crater of 6 to 7 km radius and 2.8 km depth. Impact melt glasses investigated are found within the suevite, the breccia that forms the uppermost layers of the ejecta blanket around the crater. The glasses are considered to represent quenched melts produced as a result of the impact. The oxygen isotope compositions of several glasses sampled from widely spaced localities are very homogeneous with δ 18 O values in the range of 6.7 to 7.4‰ and δ 17 O of 3.3 to 3.7‰. With increasing devitrification and alteration the δ 18 O values increase up to 15.8‰, δ 17 O up to 8.1‰. Water content also increases with devitrification from fresh glasses with about 1.3 wt.% to 3.8 wt.% in devitrified glass. δD values decrease with increasing water content from about −87 to −127‰. Whole-rock oxygen isotope compositions of the sedimentary cover sequence range from 17 to 27‰, and crystalline basement rocks range from 8.8 to 13.5‰ for granites and gneisses, whereas amphibolites have δ 18 O values of 5.2 and 6.1‰. Models suggesting that the glass in the suevite represents a mixed melt derived from all the rocks present in the suevite in proportion of their occurrence (Engelhardt, 1997) do not agree with the oxygen isotope composition of the glass. The simplest explanation for the homogeneous chemical and oxygen isotope composition is that the glass represents quenched melts from a few closely spaced lithologies only. High Fe and Ni contents in the glass, Cr/Ni and Cr/Co ratios, and relatively low δ 18 O values indicate involvement of amphibolites during melting. The balance of the melt was made up of spatially associated granites or gneisses as indicated by the major element and rare-earth element geochemistry. The best agreement between major element and oxygen isotope composition of the glass and model melts is obtained by mixtures of amphibolite and granite in proportions similar to their average occurrence in the fallout suevite.

Fluids in sedimentary basins: an introduction

Understanding paleohydrologic systems in terms of basin evolution requires the integration of information derived from the sedimentology, stratigraphy, diagenesis and geology of basin-filling successions. Combination of these is prerequisite for realistic basin analysis and to guide any hydrologic or geochemical modeling. Ancient basins, in particular, represent systems that can record protracted burial histories, thereby constraining the composition of specific fluid events that normally affected vast areas. The papers in this volume are concerned with tracing the fluid history of several sedimentary basins. These papers, which were presented in a special session at the Geological Association of Canada and Mineralogical Association of Canada meeting in Calgary, Alberta, Canada in May 2000, illustrate some of the methods, techniques and approaches required to document significant fluid events in basins and how this information can be used in some cases to evaluate the economic potential of basins. The focus of these studies deals with the interaction between basinal fluids and both chemical and clastic sediments. Both types of sediments can act as principal aquifers or aquitards for fluids in basins because of their changing reactivity and permeability as basins evolve. D 2003 Elsevier B.V. All rights reserved.

Trophodynamics and distribution of silver in a Patagonia mountain lake.

Silver (Ag) ions are among the most toxic metallic ions to aquatic biota. In southern Argentina, fish from Patagonian lakes have liver Ag concentrations [Ag] among the highest ever reported globally. Silver concentration in phytoplankton from Lake Moreno (1.82±3.00μgg(-1) dry weight, DW) was found to be significantly higher than [Ag] in zooplankton (0.25±0.13μgg(-1)). Values in snails and decapods (0.60±0.28μgg(-1) and 0.47±0.03μgg(-1) respectively), were higher than in insect larvae (0.28±0.39μgg(-1) for Trichoptera). We examined trophic transfer of Ag in the biota using stable nitrogen and carbon isotopes ratios (δ(15)N and δ(13)C respectively). Silver concentrations in the biota of Lake Moreno were not associated with any particular C source, as assessed by δ(13)C. Hepatic [Ag] significantly increased with trophic position, as measured by δ(15)N, within the brook trout sample set. Biodilution of Ag was observed between primary producers and small forage fish when whole body [Ag] was analyzed. Nevertheless, when considering whole food web biomagnification and hepatic [Ag] of top predator fish, a significant positive regression was found between [Ag] and trophic position, as measured by δ(15)N. The importance of species-specific and tissue-specific considerations to obtain more information on Ag trophodynamics than that usually presented in the literature is shown. To the best of our knowledge, this is the first study in assessing Ag trophodynamics and tissue-specific biomagnification in a whole freshwater food web.

Geochemical and Nd isotopic compositions of the metasedimentary rocks in the La Ronge Domain, Trans-Hudson Orogen, Canada: implications for evolution of the domain

Abstract Metasedimentary rocks of greenschist to amphibolite facies occur in various belts in the Paleoproterozoic La Ronge Domain of the Trans-Hudson Orogen. Metasedimentary rocks in the Central Metavolcanic greenstone belt are low to intermediate in SiO2 contents, have ϵNd values of +2.3 to +4.5, with two rare earth element (REE) patterns of (La/Yb)n 8.5, respectively. The predominant component, meta-arkoses, of the overlying McLennan Lake belt, have distinctly high K2O and SiO2 contents, high (La/Yb)n ratios (11–46), large negative Eu anomalies (Eu/Eu*=0.1–0.58) and uniformly low ϵNd values (−0.8 to +0.2). The MacLean Lake belt is comprised of a variety of metasedimentary gneisses. The volumetrically minor amphibole- and diopside-rich gneisses have high ferromagnesian element (Sc and Co) concentrations, high ϵNd values of +4.1 to +5.7, and flat REE patterns with no Eu anomalies. The predominant lithological unit in the MacLean Lake belt, the psammitic gneisses, is geochemically more variable. The mafic psammitic gneiss samples in the MacLean Lake belt and the metasedimentary rocks in the Central Metavolcanic belt are similar in trace and major element geochemistry and Nd isotope composition, whereas those more felsic psammitic gneisses are similar to the McLennan Lake meta-arkoses. Geochemical data, combined with lithological and geochronological evidence, indicate that sediments in the Central Metavolcanic belt were derived mainly from contiguous volcanic debris, with minor addition of felsic components from exotic sources. In contrast, the McLennan Lake meta-arkoses were derived from a distinct felsic source, whereas the psammitic gneisses in the MacLean Lake belt are mixtures of volcanic materials in the Central Metavolcanic belt and meta-arkoses-like felsic components. Nd isotopic data and ages of detrital zircon suggest that Proterozoic juvenile materials were dominant sources for metasedimentary rocks in the La Ronge Domain. Comparison of geochemical signatures of the metasedimentary rocks with those of the felsic plutonic rocks in the Central Metavolcanic belt suggests that the felsic materials in the McLennan Lake and MacLean Lake belts could not have been derived predominantly from the plutonic rocks, implying more distal sources. Topographic changes during collision may have resulted in a shift from mixing of both local and distal sources for the rocks in the MacLean Lake belt, to the dominance of a more distal source for the McLennan Lake meta-arkoses.

Application of sulphur isotopes to discriminate Cu–Zn VHMS mineralization from barren Fe sulphide mineralization in the greenschist to granulite facies Flin Flon–Snow Lake–Hargrave River region, Manitoba, Canada

The Flin Flon Belt in northern Canada is one of the largest Palaeoproterozoic volcanic-hosted massive sulphide (VHMS) districts in the world, but up to 20 000 km2 of prospective Palaeoproterozoic basement south of this belt is buried beneath 10 to 100 m of Phanerozoic calcareous cover. The recent acquisition of airborne SPECTREM geophysics data south of the Flin Flon Belt has resulted in the discovery of Cu-Zn sulphide prospects comprising pyrrhotite, pyrite, chalcopyrite, sphalerite and galena, but numerous barren Fe sulphide occurrences comprising only pyrite and pyrrhotite have also been intersected. The problem for explorers is trying to determine whether a barren Fe sulphide intersection that has just been cored is part of a larger Cu-Zn mineralized system, or nothing more than a pyrite–pyrrhotite occurrence. A sulphur isotope study of sulphides from the Flin Flon–Snow Lake–Hargrave River–Talbot area shows that sulphides from the Cu-Zn VHMS deposits have δ34S values that range between −1.4 and 6.4‰, with a mean δ34S value of 1.6 ± 1.7‰ (2σ error). More than 95% of these samples have δ34S values of <3.3‰. In contrast, pyrite and pyrrhotite separates from barren Fe sulphide deposits have δ34S values between 1.8 and 10.0‰, with a mean δ34S value of 4.3 ± 1.8‰ (2σ error). In this case, >84% of these samples have δ34S values of >3.3‰. The results imply that the barren Fe sulphide deposits can be statistically distinguished from Cu-Zn VHMS mineralization based on S isotopic composition, which should make future exploration drilling decisions easier.

Formation of the enigmatic Matoush uranium deposit in the Paleoprotozoic Otish Basin, Quebec, Canada

The Matoush uranium deposit is situated in the Paleoproterozoic Otish Basin, northern Quebec, Canada, and is hosted by the Indicator Formation sandstones. Its sheet-like ore bodies are closely associated with the steeply dipping Matoush Fracture, which hosts mafic dykes and minor quartz–feldspar–tourmaline pegmatites. Regional diagenesis, involving oxidizing basinal fluids (δ2H ∼−15‰, δ18O ∼8‰), produced mostly illite and possibly leached U from accessory phases in the Indicator Formation sandstones. The bimodal Matoush dyke intruded the Indicator Formation along the Matoush Fracture, and the related metasomatism produced Cr-rich dravite and muscovite in both the dyke and the proximal sandstones. Uraninite formed when U6+ in the basinal brine was reduced to U4+ in contact with the mafic dyke and by Fe2+ in Cr–dravite and Cr–muscovite, and precipitated together with eskolaite and hematite. Because of its unique characteristics, the Matoush deposit cannot be easily classified within the generally accepted classification of uranium deposits. Two of its main characteristics (unusual reduction mechanism, structural control) do not correspond to the sandstone-hosted group of deposits (unconformity type, tabular, roll front), in spite of uranium being derived from the Otish Group sandstones.

Using groundwater chemical and isotopic composition in the search for base metal deposits: hydrogeochemical investigations in the Hinta and Kayar Pb-Zn districts, India

ABSTRACT As part of mineral exploration campaigns carried out by Anglo American plc in India, groundwater samples were collected from over 200 wells in the Hinta and Kayar sedimentary exhalative base metal districts to evaluate the usefulness of hydrogeochemistry in mineral exploration. Mineralization occurs here mostly as Pb and Zn sulphides in metamorphic silicic–felsic and carbonate basement rocks. Groundwater chemical and isotopic data from 27 wells at Hinta and 181 wells at Kayar were interpreted using major element concentrations, their ratios to a conservative element (e.g. Cl−), isotope systematics and thermodynamic modelling. The results indicate that the following processes take place: evaporation, evapotranspiration, water–regolith–rock interaction (e.g. weathering of carbonate, silicate, and sulphate or sulphide minerals) and water mixing. Certain gradients, rather than absolute values, may reflect proximity to mineralization; these include: (1) concentrations of ore and related elements normalized to a conservative element (e.g. Zn/Cl− molar ratios from 1 × 10−7 up to 2 × 10−2; SO42 − /Cl− molar ratios from 1 × 10−5 up to 3.3); (2) isotopic ratios (e.g. δ34S from +23 down to +5‰ V-CDT, with two samples at −5‰); (3) saturation indices for ore, alteration or secondary minerals (e.g. SIjarosite from −15 up to zero). Hydrogeochemistry may be a useful tool to help vector toward ore deposits in deep or covered settings.

Mesoproterozoic rift sedimentation, fluid events and uranium prospectivity in the Cariewerloo Basin, Gawler Craton, South Australia

ABSTRACT The Cariewerloo Basin formed in the Mesoproterozoic following assembly of the Gawler Craton, South Australia, and was filled by arenaceous redbeds of the Pandurra Formation. While previous regional-scale work reveals a basin with similar size and sedimentary fill to the Proterozoic Athabasca and Kombolgie basins that host unconformity-related uranium deposits, few details of the Cariewerloo Basin are known. In this study, stratigraphy, petrography, lithogeochemistry, stable isotope geochemistry and 40Ar/39Ar geochronology are integrated to clarify the depositional history of the Pandurra Formation, and to assess fluid events in the basin that could be linked to the formation of uranium deposits. In the study area, the Pandurra Formation was deposited in two eastward-thickening packages that terminate at faulted basement uplifts, interpreted as half-grabens that formed in a continental rift system as the eastern Gawler Craton underwent extension. Deposition occurred between 1575 Ma (latest Hiltaba Suite age) and ca 1490 Ma, the 40Ar/39Ar age of diagenetic illite in the basal Pandurra. Diagenesis involving fluids having δ18O and δ2H values between –2.1 and 3.6‰, and between –66 and –8‰, respectively, occurred at around 150°C. Protracted diagenesis preferentially occurred in the upper Pandurra Formation based on petrography and Pearce Element Ratios that show complete replacement of detrital lithic and feldspathic grains by diagenetic phyllosilicates, and younger 40Ar/39Ar ages between ca 1330 and 1200 Ma that record fluid events later into basin history. Conversely, the basal Pandurra Formation shows better preservation of detrital grains, and older 40Ar/39Ar ages around 1450 Ma that suggest these strata became closed to fluid flow earlier in basin history. Although, based on O-isotope ratios, fluid–rock interaction did not occur in the Cariewerloo Basin to the same extent as that in the Athabasca or Kombolgie basins, it is possible that a uranium deposit formed where the upper Pandurra Formation was in contact with metasedimentary basement units outside the present basin margins.