Paul Alexandre - Academia.edu (original) (raw)
Papers by Paul Alexandre
Economic Geology, 2013
The McArthur River unconformity-related uranium deposit, located near the southeastern margin of ... more The McArthur River unconformity-related uranium deposit, located near the southeastern margin of the Proterozoic Athabasca basin in northern Saskatchewan, Canada, contains both basement-hosted and sandstone-hosted orebodies, one of which is the sandstone-hosted Zone 4 orebody and its alteration system. Early pre-ore quartz cementation forming immediately above Zone 4 produced a 200-m-thick silicified zone within the lower Manitou Falls Formation. Late pre-ore oxidizing basinal fluids ( δ 18 O = 1.1–8.9‰, δ 2 H = −70 to −34‰) formed zones of dickite, magnesiofoitite, and sudoite-rich alteration in the Manitou Falls Formation above the deposit. These zones were overprinted by an outer halo of illite and illite-chlorite mixed-layer clay that coincided with illite and clinochlore alteration of the Wollaston Group basement rocks. Circulation of basinal fluids into the fault zone, promoted by fault reactivation, created a reducing fluid ( δ 18 O = 3.9–7.8‰, δ 2 H= −62 to −25‰) through basement interaction, which formed a narrow sudoite alteration halo near the faulted unconformity. Mixing of these two contrasting fluids initiated uranium mineralization at ca. 1600 Ma. Post-ore alteration caused isotopic resetting of primary uraninite, broadly coincident with the intrusion of the Mackenzie dikes into the basin, followed by alteration of uraninite to coffinite, poorly crystalline uraninite, and gummite during the Neoproterozoic. Cool (ca. 10°C), oxidizing, meteoric waters ( δ 18 O = −15.7 to −13.2‰, δ 2 H = −120 to −93‰) later infiltrated fractures, produced post-ore kaolinite, and caused partial oxidation of uraninite to uranophane. The silicified zone at Zone 4 functioned as a physical barrier to fluid flow, creating favorable conditions for uranium mineralization by focusing basinal fluids into the fault zone, localizing reducing basement-modified basinal fluids, and enhancing ore preservation. However, the silicified zone limited the spatial extent of late pre-ore sudoite alteration and obstructed the post-ore mobilization of components such as radiogenic Pb and other pathfinder elements into the overlying strata.
Economic Geology, 2010
Previous studies on Athabasca basin unconformity-related uranium deposits have focused on major d... more Previous studies on Athabasca basin unconformity-related uranium deposits have focused on major deposits and have not investigated sites with barren alteration systems that could clarify some of the critical factors controlling mineralization processes. A paragenetic study of the Wheeler River area reveals the presence of minerals that formed during the diagenetic, the main hydrothermal, which is subdivided into early, mid-, and late hydrothermal substages, and the late alteration stages. The diagenetic stage consists of early quartz overgrowths, siderite, rutile, hematite, and abundant dickite in the pore spaces of the Manitou Falls Formation. The early hydrothermal alteration substage is characterized by pervasive 1Mc muscovite alteration and minor goyazite clusters, which formed from oxidizing basinal fluids at temperatures around 240°C prior to 1550 Ma, based on Ar-Ar dates. The mid-hydrothermal alteration substage comprises dravite and sudoite in the basal 200 m of the Manitou Falls Formation, which are interpreted to have formed at temperatures around 175°C from fluids chemically distinct but isotopically similar to the basinal fluids involved during the early hydrothermal alteration substage. The late hydrothermal substage was observed only at zone K of the Wheeler River area and is characterized by the precipitation of clinochlore, copper sulfides, and florencite from reducing basement fluids emerging into the Manitou Falls Formation at temperatures around 230°C, creating a ~250-m-high bỹ 250-m-wide reducing halo. Oxidized uranium-bearing basinal fluids interacted with the Manitou Falls Formation during the early hydrothermal substage prior to the arrival of the reducing fluids during the mid-and late hydrothermal substages and this precluded uranium precipitation. The post-hydrothermal alteration stage is characterized by formation of kaolinite after late hydrothermal clinochlore near fractures by meteoric waters. A minimal amount of leachable radiogenic Pb, with a Pb-Pb model age of 1907 Ma that is older age than both the Athabasca basin and the main mineralization event of 1590 Ma, was encountered at zone K, indicating low probability of this area to host uranium mineralization. However, areas of possible unconformity-related uranium deposits were identified outside zone K wherein significant amounts leachable radiogenic Pb were observed.
Economic Geology, 2009
ABSTRACT The Spring Point barren alteration system and the Maurice Bay subeconomic uranium deposi... more ABSTRACT The Spring Point barren alteration system and the Maurice Bay subeconomic uranium deposit, situated oil the northwest rim of the Athabasca Basin, northern Saskatchewan,an, are hosted by the sandstones and conglomerates of the Fair Point Formation of them Athabasca Group. Macroscopic observations indicate two different styles of alteration for the two systems: chlorite-dominated, partly reduced alteration at Spring Point, and illite-dominated, strongly oxidized alteration at Maurice Bay. The paragenetic sequence's developed for each area are similar, but the crystal chemistry of the alteration minerals differs markedly, indicating that the alteration fluids at Spring Point were richer ill Mg and Ca than at Maurice Bay; this is consistent with a predominantly basement origin for the former. Stable isotope analyses of clay alteration minerals indicate that, in particular, the chlorite-related fluids from Spring Point have a basement signature delta O-18 = 3.5 parts per thousand and delta D = -13 parts per thousand), whereas chlorite front Maurice Bay is more consistent with a mixture of basement and basinal fluids (delta O-18 = 5.2 parts per thousand and delta D = -28 parts per thousand). The absence of uranium-transporting basinal fluids at the critical time when reducing basement fluids were producing chlorite alteration is likely the single most important cause for the lack of uranium mineralization at Spring Point.
Economic Geology, 2009
ABSTRACT The Millennium uranium basement-hosted unconformity-related deposit is one of the most r... more ABSTRACT The Millennium uranium basement-hosted unconformity-related deposit is one of the most recent discoveries of basement-hosted deposits, and may hold as much as 46.8 million Pounds (18,000 metric tonnes U) at an average grade of 4.53 percent U3O8. The alteration zone associated with the deposit is atypical, as it consists predominantly of muscovite rather than the chlorite and muscovite commonly found in the other basement-hosted deposits of the Athabasca basin. The preore stage consists of early minor clinochlore alteration at temperatures of about 275 degrees C, interpreted to result from retrograde metamorphism of the basement rocks prior to the deposition of the Athabasca basin. This clinochlore event is followed by all extensive 1Mc muscovite alteration of the basement rocks by Na-K-Fe-rich basinal brines at temperatures of about 250 degrees C, creating tip to 25 percent void space in the basement rocks. Subsequently, a weak chamoisite alteration resulted from Fe-Mg-Ca-rich fluids with temperatures of about 300 degrees C that evolved from reactions between the basinal brines and the basement rocks. Ore stage uraninite has an extrapolated chemical age of 1590 Ma and is contemporaneous with synore 1Mc muscovite and it-oil oxide deposited at temperatures near 250 degrees C from fluids with similar delta O-18 but lower delta D values to preore basinal fluids. The low delta D values indicate postdepositional exchange of H isotopes with Cenozoic fluids. Postore alteration consists of dravite and sudoite precipitated from Mg-Ca rich fluids, from the same evolved basinal brines that precipitated preore chamoisite, at temperatures of about 185 degrees C, filling voids created during the preore stage. Our results indicate that the pre- and synore minerals were formed dominantly by basinal fluids, with no contribution from basement fluids, which differ from some, other basement-host deposits in the Athabasca basin where basement-derived fluid influx was more significant. Several U-Pb and Ar-Ar resetting events recorded oil preore muscovite and on uraninite are coincident with fluid flow induced by distal orogenies, remobilizing radiogenic Pb from the deposit. The radiogenic Pb remobilization events were not widespread as the presence of radiogenic Pb is restricted to basement rocks proximal to the deposit and along the unconformity.
Springer textbooks in earth sciences, geography and environment, 2020
Springer textbooks in earth sciences, geography and environment, 2020
Isotopes and the Natural Environment, 2020
The atmosphere is a complex and dynamic component of our natural environment; it has intricate an... more The atmosphere is a complex and dynamic component of our natural environment; it has intricate and multiple interactions with the other “spheres” of the Earth, including the geosphere, the hydrosphere, and the biosphere. The isotopes of the elements present in the atmosphere fractionate and the record of past climate changes can be found in ancient ice or in sedimentary rocks. In this chapter, we will also discuss mass-independent fractionation.
Isotopes and the Natural Environment, 2020
Geology (the study of the Earth and the rocks and minerals within it) is the one science that mos... more Geology (the study of the Earth and the rocks and minerals within it) is the one science that most heavily relies on and is assisted by the use of isotopes as an investigation tool. The recent period of strong and sustained development in geological understanding—starting in the 1950s—coincides with very significant advances in isotope research (and in geophysics), itself strongly dependent on technological advances such as electronics and modern computers. The reason for this quasi-dependence of geology on isotopes (to the point of colloquially equating geochemistry to isotope geochemistry), is that every and each process that occurred in the past and affected a rock and its components also affected the isotopic composition of many elements within the rock. By studying the isotopic composition of a geological sample, we are able to understand not only what processes affected that rock, but also when and under what physical and chemical conditions these processes occurred. As a consequence, isotope geochemistry is very well understood and widely applied. In this chapter, we will briefly examine some of the many applications of isotopes, both stable and radiogenic, in geology.
The very first thing that we have to examine is the nature of isotopes: what they are, when and h... more The very first thing that we have to examine is the nature of isotopes: what they are, when and how they were formed, and what their characteristics are. The word isotope itself means “same place”, from the Greek words ισοσ (“same”) and τοποσ (“place”), and refers to their position in the periodic table of the elements. Here, we will discuss how isotopes are affected by various physical and chemical processes and the passage of time.
Archaeology is a well-established science, with its own methods and practices. It can and often d... more Archaeology is a well-established science, with its own methods and practices. It can and often does make use of isotopes to unlock secrets of past societies and we will consider a few specific examples.
Springer Textbooks in Earth Sciences, Geography and Environment, 2021
Geological Society, London, Special Publications, 2021
The True North orogenic gold deposit is situated in the 2.99–2.70 Ga Rice Lake greenstone belt ne... more The True North orogenic gold deposit is situated in the 2.99–2.70 Ga Rice Lake greenstone belt near Bissett (Manitoba, Canada). This belt is the western equivalent of northern Ontario's Abitibi gold-producing region. The lithology and alteration, structural control, geochronological framework, and ore geology of the True North orogenic gold deposit have been addressed, but its formation temperature and age are poorly constrained. The deposit's gold-bearing veins are composed mostly of quartz (40 to 80%), ankerite (20 to 30%), and albite (5 to 25%), with minor muscovite, sericite, chlorite, and calcite. Sulfide minerals are dominated by pyrite and chalcopyrite and with minor pyrrhotite; native gold appears as inclusions in sulfides or as free gold. In general, most of the major minerals, the sulfides, and gold all precipitated during the main ore stage. The oxygen isotopic composition of co-existing quartz–albite and quartz–ankerite couples were used to calculate a formation ...
Springer Textbooks in Earth Sciences, Geography and Environment, 2020
This book provides straightforward and practical information on isotopes applied to a variety of ... more This book provides straightforward and practical information on isotopes applied to a variety of natural sciences. It covers the basics of isotopes and includes detailed examples from a range of natural sciences: ecology, biology, human health, environment and climate, geography, and geology, highlighting their applicability in these fields. It is a must-read for all advanced-undergraduate and graduate students working with isotopes, regardless of the area, and is a very useful one-stop resource for scientists starting in isotope research.
Springer Textbooks in Earth Sciences, Geography and Environment, 2021
The purpose of sampling is, in a nutshell, to collect representative samples appropriate for the ... more The purpose of sampling is, in a nutshell, to collect representative samples appropriate for the specific intended purpose of the study. In other words, when planning and conducting sampling we must always keep in mind the purpose and the characteristics of the sampled medium, be it rock or another type of material. There are several very good publications that offer solid and detailed advice on sampling and should be consulted (e.g., [8, 9]); here we will consider some general ideas and pointers.
Isotopes and the Natural Environment, 2020
Economic Geology, 2013
The McArthur River unconformity-related uranium deposit, located near the southeastern margin of ... more The McArthur River unconformity-related uranium deposit, located near the southeastern margin of the Proterozoic Athabasca basin in northern Saskatchewan, Canada, contains both basement-hosted and sandstone-hosted orebodies, one of which is the sandstone-hosted Zone 4 orebody and its alteration system. Early pre-ore quartz cementation forming immediately above Zone 4 produced a 200-m-thick silicified zone within the lower Manitou Falls Formation. Late pre-ore oxidizing basinal fluids ( δ 18 O = 1.1–8.9‰, δ 2 H = −70 to −34‰) formed zones of dickite, magnesiofoitite, and sudoite-rich alteration in the Manitou Falls Formation above the deposit. These zones were overprinted by an outer halo of illite and illite-chlorite mixed-layer clay that coincided with illite and clinochlore alteration of the Wollaston Group basement rocks. Circulation of basinal fluids into the fault zone, promoted by fault reactivation, created a reducing fluid ( δ 18 O = 3.9–7.8‰, δ 2 H= −62 to −25‰) through basement interaction, which formed a narrow sudoite alteration halo near the faulted unconformity. Mixing of these two contrasting fluids initiated uranium mineralization at ca. 1600 Ma. Post-ore alteration caused isotopic resetting of primary uraninite, broadly coincident with the intrusion of the Mackenzie dikes into the basin, followed by alteration of uraninite to coffinite, poorly crystalline uraninite, and gummite during the Neoproterozoic. Cool (ca. 10°C), oxidizing, meteoric waters ( δ 18 O = −15.7 to −13.2‰, δ 2 H = −120 to −93‰) later infiltrated fractures, produced post-ore kaolinite, and caused partial oxidation of uraninite to uranophane. The silicified zone at Zone 4 functioned as a physical barrier to fluid flow, creating favorable conditions for uranium mineralization by focusing basinal fluids into the fault zone, localizing reducing basement-modified basinal fluids, and enhancing ore preservation. However, the silicified zone limited the spatial extent of late pre-ore sudoite alteration and obstructed the post-ore mobilization of components such as radiogenic Pb and other pathfinder elements into the overlying strata.
Economic Geology, 2010
Previous studies on Athabasca basin unconformity-related uranium deposits have focused on major d... more Previous studies on Athabasca basin unconformity-related uranium deposits have focused on major deposits and have not investigated sites with barren alteration systems that could clarify some of the critical factors controlling mineralization processes. A paragenetic study of the Wheeler River area reveals the presence of minerals that formed during the diagenetic, the main hydrothermal, which is subdivided into early, mid-, and late hydrothermal substages, and the late alteration stages. The diagenetic stage consists of early quartz overgrowths, siderite, rutile, hematite, and abundant dickite in the pore spaces of the Manitou Falls Formation. The early hydrothermal alteration substage is characterized by pervasive 1Mc muscovite alteration and minor goyazite clusters, which formed from oxidizing basinal fluids at temperatures around 240°C prior to 1550 Ma, based on Ar-Ar dates. The mid-hydrothermal alteration substage comprises dravite and sudoite in the basal 200 m of the Manitou Falls Formation, which are interpreted to have formed at temperatures around 175°C from fluids chemically distinct but isotopically similar to the basinal fluids involved during the early hydrothermal alteration substage. The late hydrothermal substage was observed only at zone K of the Wheeler River area and is characterized by the precipitation of clinochlore, copper sulfides, and florencite from reducing basement fluids emerging into the Manitou Falls Formation at temperatures around 230°C, creating a ~250-m-high bỹ 250-m-wide reducing halo. Oxidized uranium-bearing basinal fluids interacted with the Manitou Falls Formation during the early hydrothermal substage prior to the arrival of the reducing fluids during the mid-and late hydrothermal substages and this precluded uranium precipitation. The post-hydrothermal alteration stage is characterized by formation of kaolinite after late hydrothermal clinochlore near fractures by meteoric waters. A minimal amount of leachable radiogenic Pb, with a Pb-Pb model age of 1907 Ma that is older age than both the Athabasca basin and the main mineralization event of 1590 Ma, was encountered at zone K, indicating low probability of this area to host uranium mineralization. However, areas of possible unconformity-related uranium deposits were identified outside zone K wherein significant amounts leachable radiogenic Pb were observed.
Economic Geology, 2009
ABSTRACT The Spring Point barren alteration system and the Maurice Bay subeconomic uranium deposi... more ABSTRACT The Spring Point barren alteration system and the Maurice Bay subeconomic uranium deposit, situated oil the northwest rim of the Athabasca Basin, northern Saskatchewan,an, are hosted by the sandstones and conglomerates of the Fair Point Formation of them Athabasca Group. Macroscopic observations indicate two different styles of alteration for the two systems: chlorite-dominated, partly reduced alteration at Spring Point, and illite-dominated, strongly oxidized alteration at Maurice Bay. The paragenetic sequence's developed for each area are similar, but the crystal chemistry of the alteration minerals differs markedly, indicating that the alteration fluids at Spring Point were richer ill Mg and Ca than at Maurice Bay; this is consistent with a predominantly basement origin for the former. Stable isotope analyses of clay alteration minerals indicate that, in particular, the chlorite-related fluids from Spring Point have a basement signature delta O-18 = 3.5 parts per thousand and delta D = -13 parts per thousand), whereas chlorite front Maurice Bay is more consistent with a mixture of basement and basinal fluids (delta O-18 = 5.2 parts per thousand and delta D = -28 parts per thousand). The absence of uranium-transporting basinal fluids at the critical time when reducing basement fluids were producing chlorite alteration is likely the single most important cause for the lack of uranium mineralization at Spring Point.
Economic Geology, 2009
ABSTRACT The Millennium uranium basement-hosted unconformity-related deposit is one of the most r... more ABSTRACT The Millennium uranium basement-hosted unconformity-related deposit is one of the most recent discoveries of basement-hosted deposits, and may hold as much as 46.8 million Pounds (18,000 metric tonnes U) at an average grade of 4.53 percent U3O8. The alteration zone associated with the deposit is atypical, as it consists predominantly of muscovite rather than the chlorite and muscovite commonly found in the other basement-hosted deposits of the Athabasca basin. The preore stage consists of early minor clinochlore alteration at temperatures of about 275 degrees C, interpreted to result from retrograde metamorphism of the basement rocks prior to the deposition of the Athabasca basin. This clinochlore event is followed by all extensive 1Mc muscovite alteration of the basement rocks by Na-K-Fe-rich basinal brines at temperatures of about 250 degrees C, creating tip to 25 percent void space in the basement rocks. Subsequently, a weak chamoisite alteration resulted from Fe-Mg-Ca-rich fluids with temperatures of about 300 degrees C that evolved from reactions between the basinal brines and the basement rocks. Ore stage uraninite has an extrapolated chemical age of 1590 Ma and is contemporaneous with synore 1Mc muscovite and it-oil oxide deposited at temperatures near 250 degrees C from fluids with similar delta O-18 but lower delta D values to preore basinal fluids. The low delta D values indicate postdepositional exchange of H isotopes with Cenozoic fluids. Postore alteration consists of dravite and sudoite precipitated from Mg-Ca rich fluids, from the same evolved basinal brines that precipitated preore chamoisite, at temperatures of about 185 degrees C, filling voids created during the preore stage. Our results indicate that the pre- and synore minerals were formed dominantly by basinal fluids, with no contribution from basement fluids, which differ from some, other basement-host deposits in the Athabasca basin where basement-derived fluid influx was more significant. Several U-Pb and Ar-Ar resetting events recorded oil preore muscovite and on uraninite are coincident with fluid flow induced by distal orogenies, remobilizing radiogenic Pb from the deposit. The radiogenic Pb remobilization events were not widespread as the presence of radiogenic Pb is restricted to basement rocks proximal to the deposit and along the unconformity.
Springer textbooks in earth sciences, geography and environment, 2020
Springer textbooks in earth sciences, geography and environment, 2020
Isotopes and the Natural Environment, 2020
The atmosphere is a complex and dynamic component of our natural environment; it has intricate an... more The atmosphere is a complex and dynamic component of our natural environment; it has intricate and multiple interactions with the other “spheres” of the Earth, including the geosphere, the hydrosphere, and the biosphere. The isotopes of the elements present in the atmosphere fractionate and the record of past climate changes can be found in ancient ice or in sedimentary rocks. In this chapter, we will also discuss mass-independent fractionation.
Isotopes and the Natural Environment, 2020
Geology (the study of the Earth and the rocks and minerals within it) is the one science that mos... more Geology (the study of the Earth and the rocks and minerals within it) is the one science that most heavily relies on and is assisted by the use of isotopes as an investigation tool. The recent period of strong and sustained development in geological understanding—starting in the 1950s—coincides with very significant advances in isotope research (and in geophysics), itself strongly dependent on technological advances such as electronics and modern computers. The reason for this quasi-dependence of geology on isotopes (to the point of colloquially equating geochemistry to isotope geochemistry), is that every and each process that occurred in the past and affected a rock and its components also affected the isotopic composition of many elements within the rock. By studying the isotopic composition of a geological sample, we are able to understand not only what processes affected that rock, but also when and under what physical and chemical conditions these processes occurred. As a consequence, isotope geochemistry is very well understood and widely applied. In this chapter, we will briefly examine some of the many applications of isotopes, both stable and radiogenic, in geology.
The very first thing that we have to examine is the nature of isotopes: what they are, when and h... more The very first thing that we have to examine is the nature of isotopes: what they are, when and how they were formed, and what their characteristics are. The word isotope itself means “same place”, from the Greek words ισοσ (“same”) and τοποσ (“place”), and refers to their position in the periodic table of the elements. Here, we will discuss how isotopes are affected by various physical and chemical processes and the passage of time.
Archaeology is a well-established science, with its own methods and practices. It can and often d... more Archaeology is a well-established science, with its own methods and practices. It can and often does make use of isotopes to unlock secrets of past societies and we will consider a few specific examples.
Springer Textbooks in Earth Sciences, Geography and Environment, 2021
Geological Society, London, Special Publications, 2021
The True North orogenic gold deposit is situated in the 2.99–2.70 Ga Rice Lake greenstone belt ne... more The True North orogenic gold deposit is situated in the 2.99–2.70 Ga Rice Lake greenstone belt near Bissett (Manitoba, Canada). This belt is the western equivalent of northern Ontario's Abitibi gold-producing region. The lithology and alteration, structural control, geochronological framework, and ore geology of the True North orogenic gold deposit have been addressed, but its formation temperature and age are poorly constrained. The deposit's gold-bearing veins are composed mostly of quartz (40 to 80%), ankerite (20 to 30%), and albite (5 to 25%), with minor muscovite, sericite, chlorite, and calcite. Sulfide minerals are dominated by pyrite and chalcopyrite and with minor pyrrhotite; native gold appears as inclusions in sulfides or as free gold. In general, most of the major minerals, the sulfides, and gold all precipitated during the main ore stage. The oxygen isotopic composition of co-existing quartz–albite and quartz–ankerite couples were used to calculate a formation ...
Springer Textbooks in Earth Sciences, Geography and Environment, 2020
This book provides straightforward and practical information on isotopes applied to a variety of ... more This book provides straightforward and practical information on isotopes applied to a variety of natural sciences. It covers the basics of isotopes and includes detailed examples from a range of natural sciences: ecology, biology, human health, environment and climate, geography, and geology, highlighting their applicability in these fields. It is a must-read for all advanced-undergraduate and graduate students working with isotopes, regardless of the area, and is a very useful one-stop resource for scientists starting in isotope research.
Springer Textbooks in Earth Sciences, Geography and Environment, 2021
The purpose of sampling is, in a nutshell, to collect representative samples appropriate for the ... more The purpose of sampling is, in a nutshell, to collect representative samples appropriate for the specific intended purpose of the study. In other words, when planning and conducting sampling we must always keep in mind the purpose and the characteristics of the sampled medium, be it rock or another type of material. There are several very good publications that offer solid and detailed advice on sampling and should be consulted (e.g., [8, 9]); here we will consider some general ideas and pointers.
Isotopes and the Natural Environment, 2020