Eric E. Hiatt

Mobile Pb-isotopes in Proterozoic sedimentary basins as guides for exploration of uranium deposits

Lead isotope ratios and associated trace element concentrations (U, Th and Pb) extracted by partial-leaching with 2% nitric acid from Proterozoic sandstones and basement rocks reveal much about the fluid evolution of sedimentary basins hosting unconformity-type uranium deposits. In addition, these techniques have great potential as a guide for exploration of uranium and other types of deposits in basins of any age. Isotope ratios of Pb in Proterozoic sandstones from basins known to contain high-grade uranium deposits are radiogenic at key geological localities and settings distal to known mineralization and particularly in altered zones proximal to mineralization. Sandstones completely cemented by quartz overgrowths typically have non-radiogenic Pb isotope ratios, indicating early closure of porosity and isolation of these rocks from later fluid events. Alternatively, the unconformity served as both a source of uranium and radiogenic Pb as well as an avenue for late-stage (<250–900 Ma) fluid flow. The mafic volcanic units, which are relatively reducing lithologies and therefore have removed uranium from basinal brines, have uranium-supported radiogenic Pb isotope ratios. Comparison of 238U/206Pb and 206Pb/204Pb ratios is useful in determining the timing and nature of U and Pb migration before, during and after mineralization in these basins. This comparison can be used to delineate the presence of radiogenic Pb isotope ratios that are not internally supported by uranium and thorium in rocks, eventually providing the explorationist with geochemical vectors that point toward sites of high potential for economic uranium mineralization.

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).

Mineralogical Stabilization of High-magnesium Calcite: Geochemical Evidence for Intracrystal Recrystallization Within Holocene Porcellaneous Foraminifera

Mineralogical stabilization of porcellaneous foraminifera is known to consist of chemical change without textural alteration at any scale. However, the nature of the alteration process has not been fully reconciled. Porcellaneous foraminifera from the freshwater diagenetic zone of two small islands in the Schooner Cays, Bahamas, are in the midst of Mg loss and delta 18 O change with no textural alteration at any scale. These data indicate that the mineralogical stabilization process, or recrystallization, is a repetitive intracrystal incongruent dissolution-precipitation reaction. Each recrystallization produces a calcite with a slightly lower Mg content than its predecessor. The rate of stabilization is dependent on time and hydrologic flux; older phreatic-zone material is the most altered and younger vadose-zone material is the least altered. Numerical modeling of the chemical diagenesis indicates that the molar water: rock ratio of a single recrystallization is Mg ) between 0.0001 and 0.0003. These values are two orders of magnitude less than that generally assigned to calcite precipitation from a large fluid reservoir, and they suggest that D Mg is dependent on the type and scale of reaction, though the nature of that dependency is unknown.

Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan

An ejecta layer produced by the Sudbury impact event ca. 1850 Ma occurs within the Baraga Group of northern Michigan and provides an excellent record of impact-related depositional processes. This newly discovered, ∼2–4-m-thick horizon accumulated in a peritidal environment during a minor sea-level lowstand that punctuated a period of marine transgression. Common ejecta clasts include shock-metamorphosed quartz grains, splash-form melt spherules and tektites, accretionary lapilli, and glassy shards, suggesting sedimentation near the terminus of the continuous ejecta blanket. Sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact-generated tsunami wave train. Observed platinum group element anomalies (Ir, Rh, and Ru) within the Sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite. Documenting and interpreting the detailed characteristics of the Sudbury ejecta horizon in Michigan have yielded a fingerprint to identify this chronostratigraphic marker in other Paleoproterozoic basins. For the first time a foundation exists to assess the consequences of the Sudbury impact on Precambrian ocean chemistry and early life.

Sedimentary phosphate formation in warm shallow waters: new insights into the palaeoceanography of the Permian Phosphoria Sea from analysis of phosphate oxygen isotopes

Abstract Oxygen isotopic analyses of the structural phosphate ( δ 18 O P ) in sedimentary phosphorites of the Upper Permian Phosphoria Formation reveal that phosphogenesis occurred across a broad range of palaeoenvironments. The δ 18 O P values from well-preserved hand-picked phosphate-grain separates range from 20.2 to 13.7 (±0.5)‰ SMOW, with the lowest values in the most landward shallow-water deposits and the highest values in the most basinward deeper water deposits. The relationship between the maximum burial depth and δ 18 O P is opposite to that predicted if burial diagenesis was responsible for the isotopic signature of these phosphate grains. This suggests that the regional trend in oxygen isotopic data is not due to burial diagenesis. The range in oxygen isotopic values does correspond to the potential range of palaeotemperatures in the shallow and broad epicontinental Phosphoria Sea. The range is 14–42°C (±3.5°C) assuming a seawater δ 18 O of −2.5 ‰ SMOW. Palaeotemperatures along an offshore to onshore transect indicate that upwelling involved temperate (14–26°C; mean of 21°C) but not cold waters, and that warming across the palaeoshelf resulted in mean shallow-water temperatures of 34–37°C. The latter values approximate those predicted by climate and ocean circulation models. These warm palaeotemperatures provide further evidence that phosphogenesis in the epicontinental Phosphoria Sea occurred under palaeoceanographic conditions that were different from modern phosphorite depositional systems. This suggests that a purely actualistic approach to the interpretation of the Phosphoria phosphorites is not valid.

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.

Extreme paleoceanographic conditions in a Paleozoic oceanic upwelling system; organic productivity and widespread phosphogenesis in the Permian Phosphoria Sea

High-resolution geochemical data from phosphorites and associated lithofacies of the Permian Phosphoria Rock Complex suggest that organic matter deposition and phosphogenesis occurred in fundamentally different oceanographic conditions than those of modern oceanic upwelling systems. Unlike modern phosphorites, those of the Phosphoria accumulated in a shallow marginal sea within a semi-restricted epicontinental embayment that extended landward into proximal environments bordered by evaporative lagoons. The Phosphoria Rock Complex phosphorites formed in outer ramp (<200 m water depth), organically productive mid-ramp, and very shallow and restricted inner-ramp environments. Chemostratigraphic data (total organic carbon, sulfur, phosphate, δ 13 CPO4‐CO3 , Ni, Cr, and Cd) indicate that this wide range of paleoenvironments was largely dysoxic or anoxic; euxinic conditions developed sporadically. High cadmium and nickel concentrations suggest maximum paleoproductivity (preserved total organic carbon up to 15 wt%) was associated with anoxic and euxinic conditions. Water column oxygen and trophic levels are interpreted to have been the primary controls over macrofaunal distribution in the Phosphoria, not coldwater temperatures as has been previously inferred. These findings, augmented by recent Permian paleoclimate and ocean circulation models, suggest that an oxygen-poor, nutrient-rich intermediate water mass flowed into the Phosphoria embayment and impinged on the mid-ramp area. Seasonal coastal upwelling brought this water to the surface, where it mixed with warm waters flowing seaward from the restricted shallow lagoons in west-central Wyoming, resulting in high paleoproductivity and organic matter accumulation and oxygen depletion in the water column. Warming of the waters on the broad, shallow ramp, coupled with seasonal attenuation of the coastal upwelling system, is predicted to have led to a positive feedback between productivity and phosphogenesis through a wide range of environments. This new model and our findings illustrate that paleoceanographic setting and paleoenvironment must be taken into account to fully understand the geochemical variation seen in ancient phosphorites.

Advances in understanding the Kombolgie Subgroup and unconformity-related uranium deposits in the Alligator Rivers Uranium Field and how to explore for them using lithogeochemical principles

The Alligator Rivers Uranium Field (ARUF) includes the mined and unmined Jabiluka, Ranger, Koongarra and Nabarlek unconformity-related uranium deposits and several small prospects including the newly discovered King River prospect. Uranium mineralisation is hosted by a variety of metamorphosed Nimbuwah Domain lithologies that are unconformably overlain by the Kombolgie Subgroup, a basin package of unmetamorphosed arenites and mafic volcanics. All of the uranium deposits and prospects preserve an identical alteration assemblage that is subdivided into a distal and proximal alteration zone. The distal alteration zone comprises an assemblage of sericite and chlorite that replace albite and amphibole. In some cases, this alteration can be traced >1000 m from the proximal alteration zone that is dominated by uraninite, hematite, chlorite and sericite. Uranium precipitated in the basement as uraninite at 1680 Ma at around 200°C from a fluid having δ18Ofluid values of 3.0±2.8‰ and δDfluid values of −28±13‰ VSMOW reflecting an evolved marine source. These geochemical properties are indistinguishable from those recorded by diagenetic illite and chlorite that were collected from the Kombolgie Subgroup sandstones across the ARUF. The illite and chlorite formed in diagenetic aquifers, and where these aquifers intersected favourable basement rocks, such as those containing graphite or other reductants, U was precipitated as uraninite. Therefore, it is proposed that the Kombolgie Subgroup is the source for fluids that formed the deposits. A post-ore alteration assemblage dominated by chlorite, but also comprising quartz±dolomite±sulfide veins cut the uranium mineralisation at all deposits and has historically been recorded as part of the syn-ore mineralisation event. However, these minerals formed anywhere between 1500 to 630 Ma from fluids that have distinctly lower δ18Ofluid values around 1.5‰ and lower δDfluid values around −45‰ reflecting a meteoric water origin. Despite unconformity-related uranium deposits having a large alteration halo, they remain difficult to find. The subtle alteration of albite to sericite several hundred metres from mineralisation occurs in isolation of any increase in trace elements such as U and radiogenic Pb and can be difficult or impossible to identify in hand specimen. Whole rock geochemical data indicate that Pearce Element Ratio (PER) analysis and General Element Ratio (GER) analysis may vector into this subtle alteration because it does not rely on an increase in trace elements to identify proximity to ore. PER and GER plots, Al/Ti vs (2Ca + Na + K)/Ti, Na/Al vs (Na + K)/Al, K/Al vs (Na + K)/Al and (Fe + Mg)/Al vs (Na + K)/Al provide a visual guide that readily distinguish unaltered from altered samples. A plot of (Na + K)/Al and (Fe + Mg)/Al on the x-axis against the concentration of trace elements on the y-axis reveals that U, Pb, Mo, Cu, B, Br, Ce, Y, Li, Ni, V and Nd are associated with the most intensely altered samples. The lithogeochemical vectors should aid explorers searching for uranium mineralisation in a prospective basin environment, but exploration must first focus on the characteristics of the basin to assess its mineralisation potential. A holistic model that describes the evolution of the Kombolgie Subgroup from deposition through diagenesis to formation of the uranium deposits in the underlying basement rocks is presented and has application to other basins that are considered prospective for unconformity-related uranium deposits. The model outlines that explorers will need to consider the thickness of the sedimentary pile, its lithological composition relative to depositional setting, the depth to which the sediments were buried during diagenesis and the degree of diagenesis achieved, which may be time dependant, before deciding on the prospectivity of the basin.

Riverine mixing and fluvial iron formation: A new type of Precambrian biochemical sediment

Precambrian iron formations are biochemical sediments that record ocean chemistry and circulation on the early Earth. The appearance of large, economically important continental margin iron formation reflects the creation of extensive continental shelves and oxygenation of the ocean-atmosphere system near the end of the Archean. Exhalative iron formation contains a record of hydrothermal vent chemistry through time. We introduce here fluvial iron formation, a new type of Fe-rich microbial-biochemical sediment that formed by mixing river discharge and seawater in coastal environments. The Paleoproterozoic Chiall Formation (ca. 1.8 Ga), Earaheedy Basin, Western Australia, contains laminated and granular hematitic iron formation in delta channel deposits. Where mixing occurred in adjacent peritidal settings, laminated iron formation and hematitic oncoids formed. Because fluvial iron formations precipitated at the interface between terrestrial and marine realms, the locus of known Fe precipitation processes is shifted landward into paleoestuarine settings and reflects Fe derived from both terrestrial weathering and coastal upwelling, providing a new window into ocean-atmosphere evolution.

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.