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Hematite and goethite from 100 samples collected from various uranium deposits and prospects, associated alteration zones, and overlying Thelon sandstones in the Kiggavik–Andrew Lake structural trend (KALST; Nunavut, Canada) were investigated by EPMA (electron probe micro-analyzer) and LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometer) to establish discriminant geochemical features in order to constrain indicator mineral exploration for uranium. Three groups of Fe oxides and hydroxides were identified: (1) pre-mineralization—formed during lateritic weathering mainly by replacement of previous metamorphic minerals. This group is variably enriched in P, Pb, Mo, Nb, Cu, Cr, Ni, and Co. (2) Syn-mineralization hydrothermal hematite and goethite intergrown with illite, quartz, chlorite, and calcite ± siderite constituting the typical alteration assemblage associated with the KALST U mineralization. This group can be discriminated because of its enrichment in U, Ca, Mg, Al, Si, Mn, Y, ∑REE, Zr, K, S, and Sr and depletion in P, Fe, Cr, W, Sn and Ta. The flat and un-fractionated REE patterns in this group are comparable with those reported from Kiggavik U mineralized host rocks and are most likely reflecting the signature of parental uraniferous fluids. (3) Post-mineralization hydrothermal specularite and goethite, infilling the KALST host rocks fractures and dissolution pits, are mostly depleted in Mn, Co, Y, Sr, U, and ∑REE but relatively enriched in Cr, Sn, Ta, Ge, and W. Partial least squares-discriminant analysis (PLS-DA) of the geochemical data not only differentiates among different groups of Fe oxides and hydroxides in the KALST samples but also demonstrates the evolution of trace element composition of Fe oxides and hydroxides from the basement host rocks to the mineralization. The results suggest a basement source for the U mineralization in the KALST area.

This thematic issue is in memory of Thomas Kurtis Kyser who died tragically in 2017. Kurt’s contribution to our understanding of the geochemistry of uranium deposits and the application of stable and radiogenic isotope studies to uranium ore systems is incalculable. His research on uranium deposits included the world-class unconformity-related uranium deposits from Canada, Australia, and Russia, and several other major types of uranium deposits worldwide such as Na-metasomatic deposit of Kurupung (Guyana) and Valhalla (Australia), volcanic-hosted deposits of Peru (Macusani), and vein-type uranium deposits from Beaverlodge district (Canada). He was the first to develop methods to use secondary dispersion of pathfinder elements to vector towards deeply buried uranium ore systems. These new geochemical and isotopic approaches are still successfully applied today. Kurt has been a pioneer in applying stable isotopic systems to characterize the paleofluid events associated with ore systems and to distinguish between barren and mineralized fluid pathways. Kurt was a prolific author with over 400 publications and over a hundred articles on uranium deposits. Kurt wrote three short courses for the Mineralogical Association of Canada dedicated to uranium deposits: Fluids and Basin Evolution (2000), Recent and not-so-Recent Developments in Uranium Deposits and Implications for Exploration (2008, in partnership with SGA), and Geology and Geochemistry of Uranium and Thorium Deposits (2015). However, his true legacy is the large number of undergraduate and graduate students, post-docs, and researchers that he taught, supervised, and mentored; most of whom are now working in industry, academia, and government institutions across the globe. He will be missed.

The Thelon Basin, Nunavut, Canada, is similar in age, size, and geology to the U producing Athabasca Basin in Saskatchewan. The Kiggavik project area, located immediately south of the Aberdeen sub-basin of the Thelon Basin, contains U deposits and showings along a ~ 30-km-long NE–SW structural trend. The basement-hosted Kiggavik deposit, comprising three separate mineralized zones (Main, Centre, and East), is located at the northern end of this trend. Multiple styles of U mineralization and alteration were identified at the Kiggavik deposit. Disseminated euhedral uraninite (U1; 1284 ± 53 Ma) has elevated Pb contents up to ~ 14 wt% PbO and is associated with illite and hematite alteration. Vein-style uraninite (U2; 284 ± 19 Ma) forms along fractures, contains lower Pb contents (≤ 3.9 wt% PbO), and is partly altered to Ca–Si-rich (~ 4.5 wt% CaO and ~ 3.3 wt% SiO2) uraninite. Highly altered U2 is associated with galena. Late U minerals (U3) have biogenic textures and are Pb-free, indicating that they formed recently (< 1 Ma). Based on SIMS in situ U–Pb geochronology, the minimum age of uraninite at the Kiggavik deposit is 1284 ± 53 Ma; however, this is likely a reset age associated with the Mackenzie dyke event (~ 1267 Ma). Oxygen and hydrogen stable isotope compositions of illite and muscovite suggest that the Kiggavik deposit has been overprinted by high-latitude surficial fluids derived from snowmelt.

Mineralized shear zones in the Archean Missanabie-Renabie gold district (~ 1.1 Moz Au; Wawa, Ontario, Canada) locally define composite orebodies that record three hydrothermal events: (1) a pre-orogenic Au1 event (pre-D1 and pre-prograde-metamorphic); (2) a syn-orogenic, post-peak-metamorphic Retrograde event (syn-D3); and (3) a late syn-to post-orogenic Au2 event (late syn- to post-D4). Genetic considerations indicate the orebodies are hybrids with early intrusion-related (Au1) and later orogenic (Retrograde + Au2) events. Pearson product-moment correlation coefficients (log10) of whole-rock and LA-ICP-MS pyrite trace metal datasets distinguish Au1 from Au2 mineralization by Au-Ag, Au-Bi, and Au-Te correlations > 0.7 (p < 0.05) in the former, irrespective of sample medium and analytical method. An Au-Mo correlation in whole rock data (0.58–0.76; p < 0.05) further distinguishes Au1 from Au2 and supports an independently inferred intrusion-related origin for Au1. Sulfur isotope data is similar for both Au1 and Au2 pyrite with average δ34S values of − 5.5‰ ± 0.2‰ (1σ) and − 3.5‰ ± 0.3‰ (1σ) and average Δ33S values of 0.4‰ ± 0.1‰ (1σ) and − 0.3‰ ± 0.2‰ (1σ), respectively. SIMS δ18Oquartz values for the Au1, Retrograde, and Au2 events largely overlap and, like δ18Ocarbonate values of previous studies, tend to be lower than values typical of Archean gold deposits. The results of this study suggest that correlation coefficients in trace metal datasets are useful in discriminating and characterizing different gold events. Caution is emphasized with the use of S- and O-isotope datasets for these purposes. The presence of low δ18O values in vein quartz and carbonate is best explained by an 18O-depleted fluid formed during the Retrograde hydrothermal event. The latter is inferred at 2580 ± 21 Ma based on U-Pb geochronlogy of hydrothermal titanite, and relates to deformation and metamorphism in the nearby, amphibolite- to granulite-grade Kapuskasing metamorphic belt. Geochronological and geochemical evidence suggest that the 18O-depleted fluid may have formed via the devolatilization of biotite-bearing granitoids during deep-crustal metamorphism.

Micron-scale pseudomorphs of calcite after ikaite were discovered in microbialites from Manito Lake, a large hypersaline lake in the Great Plains of western Canada. Although environmental conditions in the lakes of this region (seasonally cold temperatures, high salinities and elevated productivity) suggest that ikaite should be common, this is the first documentation of lacustrine ikaite in Canada and the Great Plains region of North America. The calcite pseudomorphs form a porous dendritic fabric, comprise the interiors of massive shoreline microbialite mounds and pinnacles, and are encased in centimeter-scale, laminated dolomite-aragonite rinds. Because of the limited thermodynamic stability of ikaite at or near freezing temperatures, the psuedomorphs of this mineral provide evidence of an extended cold episode during the first millennium AD. The Manito deposits also show high δ¹³C values, indicating elevated productivity and δ¹⁸O signatures are consistent with precipitation in a cold, somewhat fresher lake than present.

Niobium and tantalum, rare metals and high field strength elements (HFSEs) that are essential to modern technologies, are concentrated among others in lithium-cesium-tantalum (LCT) pegmatites and rare metal granites. The most important hosts for Nb-Ta in these types of deposits are the columbite group minerals (columbite-tantalite), but at some ore deposits significant Ta is also contained in wodginite, microlite, and tapiolite. Previous solubility experiments of HFSE minerals have been limited to high temperatures because of the slow diffusivities of HFSEs in granitic melts. An experiment protocol is described herein that allows HFSE mineral solubilities to be determined at lower temperatures, more in line with the estimated solidus temperatures of LCT pegmatites and rare metal granites. This is achieved through the interaction of a melt that is enriched in high field strength elements (e.g., P and Nb or Ta) with a fluid enriched in a fluid-mobile element (FME, e. g., Mn). A starting glass enriched in a slow diffusing HFSE was synthesized, and HFSE mineral saturation is obtained via the diffusion of a FME into the melt via interaction with a fluid. This interaction can occur at much lower temperatures in reasonable experimental durations than for experiments that require diffusion of niobium and tantalum. The solubility product of columbite-(Mn) from the fluid-melt interaction experiment in a highly fluxed granitic melt at 700 °C is the same as those from dissolution and crystallization (reversal) experiments at the same conditions. Thus, both methods produce reliable measurements of mineral solubility, and the differences in the metal concentrations in the quenched melts indicates that the solubility of columbite-(Mn) follows Henry’s Law. Results show that columbite-(Mn) saturation can be reached at geologically reasonable concentrations of niobium in melts and manganese in hydrothermal fluids. This experimental protocol also allows the investigation of HFSE mineral crystallization by fluid-melt interactions in rare-metal pegmatites. Magmatic origins for columbite group minerals are well constrained, but hydrothermal Nb-Ta mineralization has also been proposed for pegmatite-hosted deposits such as Tanco, Greenbushes, and granite-hosted deposits such as Cínovec/Zinnwald, Dajishan, and Yichun. This study shows that columbite-(Mn), lithiophilite, and a Ca-Ta oxide mineral (that is likely microlite) crystallized from experiments in fluid-melt systems at temperatures as low as 650 °C at 200 MPa. It is important to note that HFSE minerals that crystallize from fluid-melt interactions texturally occur as euhedral crystals as phenocrysts in glass, i.e., are purely magmatic textures. Therefore, crystallization of HFSE minerals from fluid-melt interactions in rare metal granites and pegmatite deposits may be more widespread than previously recognized. This is significant because the formation of these deposits may require magmatic-hydrothermal interaction to explain the textures present in deposits worldwide, rather than always being the result of a single melt or fluid phase.

The End deposit is one of several uranium deposits in the Kiggavik area near the Proterozoic Thelon Basin, which is geologically similar to the Athabasca Basin known for its unconformity-related uranium deposits. The mineralization occurs as uraninite and coffinite in quartz veins and wall rocks (psammopelitic gneisses) in the sub-Thelon basement and is associated with clay- and hematite-altered fault zones. Fluid inclusions were studied in quartz cementing unmineralized breccias formed before mineralization (Q2), quartz veins that were formed before mineralization but spatially associated with uranite (Q4), and calcite veins that were formed after mineralization. Four types of fluid inclusions were recognized, namely liquid-dominated biphase (liquid + vapor), vapor-dominated biphase (vapor + liquid), monophase (vapor-only), and triphase (liquid + vapor + halite) inclusions. The first three types were found in Q2, whereas all four types were found in Q4 and calcite. The coexistence of these different types of inclusions within individual fluid inclusion assemblages is interpreted to indicate fluid immiscibility and heterogeneous trapping. Based on microthermometry, the fluids associated with Q2 are characterized by low salinities (0.4 to 6.6 wt%) and moderate temperatures from 148 to 261 °C, and the fluids associated with calcite show high salinities (26.8 to 29.3 wt%) and relatively low temperatures from 146 to 205 °C, whereas the fluids associated with Q4 have a wide range of salinities from 0.7 to 38.8 wt% and temperatures from 80 to 332 °C. Microthermometric and cryogenic Raman spectroscopic studies indicate that the high-salinity fluids in Q4 and calcite belong to the H 2 O-NaCl-CaCl 2 ± MgCl 2 system, with some dominated by NaCl and others by CaCl 2 . The fluid inclusions in Q2 are interpreted to be unrelated to mineralization, whereas those in Q4 and calcite reflect the mineralizing fluids. The fluid inclusion data are consistent with a genetic link of mineralization with basinal brines derived from the Thelon Basin. However, unlike the conventional deep-burial (>5 km) diagenetic-hydrothermal model proposed for the unconformity-related uranium deposits, the uranium mineralization in the End deposit is inferred to have formed in a shallow environment (probably <2 km), based on fluid immiscibility and low fluid pressures obtained in this study. The U-Pb age of uraninite (1295 ± 12 Ma) is interpreted to reflect isotopic resetting after the primary mineralization.

Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth’s Hadean liquid iron core.

The Devonian Sn-mineralized Heemskirk and barren Pieman Heads granites of western Tasmania (southeast Australia) contain abundant tourmaline-rich features, including orbicules, patches, cavities, and veins within their roof zones. The δ11B and δ18O compositions of tourmaline from these texturally different features range from − 21.7 to + 4.1‰ (average − 4.7 ± 4.0‰, n = 127), and from + 6.5 to + 14.9‰ (average + 10.7 ± 1.8‰, n = 38), respectively. These data suggest that the tourmaline-rich assemblages precipitated mostly from magmatic-hydrothermal fluids derived from their host plutons, mixed with minor external components sourced from metasedimentary, meta-ultramafic rocks, and/or meteoric water. The B-isotopic values increase sequentially from tourmaline patches to orbicules and/or cavities in both granites, probably caused by progressive volatile exsolution and systematic fluxing of aqueous boron-rich fluids from the S-type magmas during emplacement into the shallow crust. High degrees of fractional crystallization of the melt may have caused the exsolution of boron-rich hypersaline fluids from which the tourmaline orbicules and cavities formed. Bimodal δ11B populations (− 21.7 to − 12.7‰ and − 3.9 to + 4.0‰) imply that the granitic melts were mostly sourced from 10B-rich metapelitic rocks. The granites underwent fluid exsolution during the late magmatic stage that contributed to the widespread development of tourmaline-rich features and also to the formation of tin deposits associated with the Heemskirk Granite.

Hydrothermal fluids in ultramafic‐hosted hydrothermal systems associated with oceanic detachment faults can be more oxidizing compared to mafic‐hosted vent sites. These fluids form a mineral assemblage of pyrite, magnetite and hematite. At 13°30′N on the Mid‐Atlantic Ridge, chlorite‐quartz breccias recovered from an exposed fault scarp contain pyrite, with abundant magnetite and hematite, indicating that the redox of the fluids was variable. In primary micron‐scale zonations in pyrite, Ni, Co, and Se have a decoupled relationship, recording fluctuations in the chemical composition and temperature of hydrothermal fluid as the grains grew. Secondary zonations that erase and overprint primary zonations are limited to the grain margin and permeable regions within the grain core. Secondary zonations formed via two processes: (a) grain dissolution followed by overgrowth, and (b) remobilization of metals during oxidizing fluid flow events. In both instances, Ni and Co have been mobilized and concentrated, and are not lost to the hydrothermal fluid. Superimposed on these features is evidence of grain scale deformation related to periods of fault movement along the detachment surface. Sulfur isotope ratios (δ34S) in pyrite systematically decrease from the grain margin to the grain core, indicating that increased amounts of sulfur were derived from thermochemical sulfate reduction of seawater. Thus, pyrite records the evolution of fluid flow and deformation events during exhumation along the detachment surface from ∼1 to 2 km below the seafloor at the base of the lava pile, with temporal fluctuations in fluid redox identified as an important process in controlling Ni and Co enrichment in pyrite. Plain Language Summary Detachment faults are long lived faults that can expose ultramafic rocks at the seafloor. We aim to investigate the links between hydrothermal activity and detachment fault formation. To do this we use pyrite as a tape recorder for past fluid flow events. Across individual mineral grains, distinct zonations in metal content and sulfur isotope ratios show that the incursion of seawater occurred periodically during pyrite growth, increasing during fault movement events that lead to changes in the temperature and pH of the fluids in the fault zone. These changes concentrated metals toward the center of individual mineral grains. Zonations were then overprinted by later deformation‐related events, providing evidence that the samples formed at deeper crustal levels below the seafloor and were progressively exhumed at the seafloor over time. Key Points Microtextural, geochemical, and isotopic variations in subseafloor pyrite record the history of sample exhumation along a detachment fault Nickel and Co are remobilized and concentrated in pyrite across individual mineral grains in response to fluctuating fluid redox conditions Evidence of pyrite deformation and alteration mineralogy of samples indicates sample exhumation from a depth of 1–2 km