The Windy Craggy deposit in northwestern British Columbia is the world's largest mafic siliciclastic (Besshi-type) volcanogenic massive sulfide (VMS) deposit, containing similar to 300 Mt grading 1.38 wt.% Cu, 0.069 wt.% Co, and 0.2 g/t Au. To evaluate the sources of sulfur and carbon, and assess magmatic contributions to ore formation, we analysed sulfur, carbon, and oxygen isotopes in massive sulphide, carbonate gangue minerals, and host rocks. Whole-rock sulfur isotope compositions range from delta 34S = -10.1 to +4.8 parts per thousand and are consistent with mixing between sulfur leached from footwall volcanic rocks and magmatic sulfur modified by SO2 disproportionation, with only a minor possible sedimentary contribution. Carbonate delta 13C values range from -11.8 to +29.6 parts per thousand (mostly -5.9 to +7.7 parts per thousand), whereas delta 18O values range from +7.0 to +26.6 parts per thousand. Most samples define a mixing trend between mantle-derived carbon and marine carbonate, indicating variable inputs of magmatic CO2 and seawater-derived carbon. Trace elements commonly enriched in magmatic fluids (As, Ag, Mo, Sb, and Tl) are strongly correlated and increase with decreasing delta 34S. Principal component analysis similarly links these elements with delta 13C and total carbon, providing compelling evidence that magmatic volatiles supplied sulfur, carbon, and metals to the Windy Craggy hydrothermal system.

Sulfide minerals, in a variety of deposit types, are important hosts to numerous critical- and precious-metals. During the original mineralization processes, various factors affect the distributions and abundances of these metals within deposits. In addition, modification of sulfides during deformation and metamorphism may cause some sulfide-hosted metals to be expelled, but others to become concentrated. This study integrates microstructural analysis of sulfides from the Co-bearing Windy Craggy volcanogenic massive sulfide deposit with quantitative trace- and major- element mapping, to investigate the relationship between sulfide modification and the distribution of various sulfide-hosted critical- and precious- metals. Cobalt in pyrite and pyrrhotite is minimally affected by mechanical remobilization involving dislocation creep and dynamic recrystallization. However, additional Co introduced during deformation resulted in the growth of new, Co-rich metamorphic pyrite and the formation of local cobaltite. In contrast, Ag (along with Pb-Bi +/- Sb +/- Te +/- Au) was expelled from pyrite and other sulfides during deformation and metamorphism by pipe- or dislocation-impurity-pair diffusion, and was concentrated along intra-grain boundaries. Gallium was lost from sphalerite by diffusion, and In was expelled from chalcopyrite during dissolution-reprecipitation.

Pyrite and pyrrhotite from the Windy Craggy volcanogenic massive sulfide (VMS) deposit, British Columbia, Canada, were investigated using combined in situ compositional mapping (by laser ablation-inductively coupled plasma-mass spectrometry [LA-ICP-MS]) and microstructural mapping (by electron backscatter diffraction [EBSD]) to elucidate how their compositions were affected by hydrothermal processes and subsequent deformation and metamorphism. Early pyrite was precipitated rapidly from cool (<250°C) hydrothermal fluids with a significant seawater component and incorporated substantial quantities of a wide variety of trace elements, including the low-temperature suite As-Ag-Sb-Au-Tl-Pb. As the hydrothermal system evolved, this early pyrite was overgrown during subseafloor zone refining and replaced by massive pyrrhotite-chalcopyrite-pyrite mineralization containing Co-Ni-Cu-Se-Mo-Bi, under high-temperature (~350°–380°C), reducing conditions. During deformation and metamorphism at greenschist facies conditions (≥370°C), pyrrhotite was mechanically remobilized by dislocation creep and dynamic recrystallization, expelling elements hosted in mineral inclusions (Cu, Mo, Ag, Sb, Pb, and Bi) but largely retaining direct, stoichiometric substitution elements (Co, Ni, and Se). Pyrite is more competent than pyrrhotite, but local dynamic recrystallization did occur and similarly expelled most elements incorporated by coupled substitution or in inclusions (Cu, Zn, Mo, Ag, Sn, Sb, some Te, Au, Tl, Pb, and Bi), while retaining direct substitution elements (Co, As, Se, and some Te). Synmetamorphic phases, like pyrite overgrowths and minor cobaltite, are distinctly Co and As rich. Within the Windy Craggy deposit, significant variations exist in pyrite and pyrrhotite trace element compositions, reflecting both syn- and postdepositional processes. In general, low-temperature elements are present in sulfide mineral inclusions or as coupled substitutions and may be positive indicators of proximity to hydrothermal mineralization, but they are readily remobilized during hydrothermal, metamorphic, or deformational sulfide modifications. Several high-temperature elements are incorporated tightly into the crystal lattice of pyrite and pyrrhotite and are thus better retained through such modifications.

The Eastern Haft-Savaran Zn-Pb-(Ba) deposit, located in the southeastern part of the Arak Mining District of the Malayer-Esfahan Metallogenic Belt, Iran, is hosted in the uppermost part of an early Cretaceous massive limestone unit that is capped by shale. The mineralization has a sheet-like geometry and is associated with intense dolomitization and silicification. The mineralization is mineralogically zoned: chalcopyrite-galena-pyrite-sphalerite-tetrahedrite occurs in the southern part of the deposit; sphalerite-galena-pyrite occurs in the central part of the deposit; and barite-galena-sphalerite-pyrite occurs in the northern part of the deposit. The thickest mineralization and most intense alteration are in the southern part of the deposit, associated with an aphyric rhyodacite flow containing minute euhedral barite crystals and Cu-bearing sulphides in vein/veinlets and disseminations, indicating this was the mineralizing fluid upflow site. Three hydrothermal mineralization stages are recognized. The first is mud lime sedimentation, framboidal pyrite, minute euhedral barite, sphalerite, galena and early dolomite. Minute euhedral barites are cut by microsparite formed by micrite recrystallization during diagenesis. First stage sphalerite and galena occur as inclusions in framboidal pyrite, and these were remobilized into inclusions in euhedral pyrite during recrystallization. The second (main) stage mineralization includes sphalerite, galena, chalcopyrite, tetrahedrite, pyrite and barite that with dolomite and quartz as gangue minerals replaced first stage mineralization and is associated with silicification and dolomitization. The third stage of mineralization comprises sphalerite and galena that is associated with dolomitization and calcitization. The occurrence of mineralization in an extensional back arc setting, massive limestone host rock, intense and pervasive host rock alteration, Fe-dolomite associated with main stage mineralization, the presence of mineralization load casts and minute euhedral barite, the remobilization of first stage of mineralization during diagenetic evolution of framboidal pyrite to euhedral pyrite, and timing of mineralization during early diagenesis of mud lime collectively indicate the Eastern Haft-Savaran deposit can be classified as an Irish-type deposit.

The Early Cretaceous Shams-Abad deposit, with proven reserves of 48 Mt grading 35 wt% Fe, is the largest Fe-(Mn) deposit in the Malayer-Esfahan metallogenic belt (MEMB), Iran. The mineralization is stratabound and restricted to Early Cretaceous dolomitic limestone (Kld) and felsic tuffs. The primary ore consists mainly of siderite-ankerite, and minor pyrite, galena, and chalcopyrite. Secondary Fe-oxide-hydroxides (hematite, goethite) are also present. Iron mineralization in the Shams-Abad deposit was emplaced in two paragenetic stages: stage 1, a large volume of host rocks (Kld) was replaced by fine-grained siderite (Sid1) and ankerite (Ank1); stage 2, coarse-grained siderite (Sid2) and ankerite (Ank2) show vein-veinlets and massive textures and were formed by replacement of stage 1 mineralization. Fluid inclusions in hydrothermal dolomite (stage 2) have homogenization temperatures of 170 to 283 °C, with salinities ranging from 2.50 to 11.70 eq. wt. % NaCl. These temperatures and salinities are similar to the ranges reported for some sideritic Fe exhalative deposits elsewhere. The δ13CPDB and δ18OSMOW values of stages 1 and 2 hydrothermal ankerite and siderite suggest that CO2 (or H2CO3) in the hydrothermal fluid mainly originated from marine carbonate rocks. The textural, mineral, chemical, and isotopic evidence suggests that main-stage (stage 2) ore (primarily siderite) was precipitated by mixing between hydrothermal fluid and seawater below the seafloor in the dolomitic limestone host rocks. Subsequently, siderite and ankerite were converted to secondary iron oxides such as goethite and hematite during meteoric water flow through the inverted normal fault and thrust faults and uplift.

ABSTRACT The Lower Proterozoic, Lake Superior-type Sokoman Iron Formation of the Labrador Trough is one of the world's largest iron formations. It represents a unique, major event in the history of the Trough. Originally a largely irregularly bedded, intraclastic, granular, locally oolitic, conglomeratic iron formation, it is highly variable in its stratigraphy, mineralogy, and textures, which are the consequence of sedimentology, diagenesis, metamorphism, structural deformation, and magmatic overprint. Despite its complexity, the regional characteristics of the iron formation within the 1200 km length of the Labrador Trough indicate three main stratigraphic units, defined by their dominant iron minerals: the lower and upper parts of the formation are characterized by the abundance of iron silicates and carbonates (silicate-carbonate facies), and the middle part is characterized by the dominance of iron oxides (oxide facies). The origin of these lithostratigraphic units of the iron formation is attributed to three main sea-level changes which changed the chemistry (oxidation–reduction potential) and the physical energy (wave and current action) of the sedimentary environment. The vast amount of iron and some of the silica required for deposition of the Sokoman Formation is inferred to be the consequence of intense hydrothermal activity within a major rift created by the eastward extension of the Labrador Trough ca 1.88 Ga. The hydrothermal fluids venting within the rift saturated the deep and likely anoxic sea of the Trough with ferrous iron and some silica which then upwelled onto its oxygenated shallow waters to deposit the iron formation. The end of the processes involved in creating the iron formation ca. 1.82 Ga is attributed to the westward contraction of the Trough induced by the Hudsonian (Trans-Hudson) orogeny, which closed the iron- and silica-generating rift and at the same time ended all magmatic activities and related sedimentation coeval with the deposition of the iron formation.

Radiogenic (e.g., U-Th-Pb, Rb-Sr, Sm-Nd, Lu-Hf), traditional stable (e.g., S, O, H, C, He, B, and Li), and non-traditional metal stable (e.g., Se, Fe, Zn, Cu, Mo, and Tl) isotopes are increasingly being recognized as powerful geochemical tools for understanding metal and sulfur sources, depositional ages, ambient basin redox, area selection, and vectoring in the exploration for volcanogenic massive sulfide (VMS) deposits. Volcanogenic massive sulfide deposits are metal sulfide deposits that are globally economically important sources of base metals (Zn, Cu, Pb), and, in some deposits, precious (Au, Ag) and by-product critical (e.g., As, Bi, Co, Ge, In, Sn, Sb, Ga, Se, and Te) metals. These deposits are closely associated in space and time with submarine volcanism. Seawater is drawn down into the subsurface, magmatically heated by magma chambers and/or subvolcanic intrusions, and chemically modified during hydrothermal circulation; metals and other solutes in the rocks are leached along the flow path and precipitated at or near the seafloor in response to strong physicochemical gradients between the mineralizing fluid and cold, ambient seawater at the depositional site. Isotope data are not routinely used in mineral exploration due to perceived cost and complexity in interpretation; however, advances in analytical technologies and techniques, refinements to VMS deposit genetic and exploration models, and data integration and reduction algorithms have facilitated the practical potential application of isotope data in exploration for many mineral deposits, including VMS. New technologies have resulted in lower cost, facile analysis, and better understanding of the processes that govern isotopic fractionation, in particular for non-traditional (metal) isotope systems. Radiogenic isotopes are primarily used to date the lithologies (e.g., U-Pb, Ar-Ar) that host the VMS mineralization or the mineralization itself (e.g., Re-Os, Rb-Sr), or hydrothermal alteration products (clays, white micas) (e.g., Ar/Ar). Radiogenic isotopes (e.g., Pb, Sr, Nd, Hf) are also used as tracers to understand metal and solute sources (e.g., mantle versus crust) and processes. Stable isotopes are also used to elucidate metal and solute sources such as seawater, igneous, sedimentary (e.g., S, C, O, H, He), mineralizing processes such as boiling/phase separation (e.g., O, H), and to track hydrothermal fluid-rock interaction (e.g., O). Non-traditional metal isotopes (e.g., Fe, Cu, Zn, Se, Mo, Ni, Hg) have only been applied to studies of VMS deposition for the last decade or so. For these isotope systems, there are still only limited data for ancient VMS. However, most VMS deposits have isotopic values that are essentially similar to mantle values, only rarely showing more extreme fractionation. Several isotope systems (e.g., O, H, C, Sr) are commonly used to recognize and quantify water-rock interactions attendant with hydrothermal alteration associated with VMS mineralizing processes. The most well-studied isotope system in this regard is O, with large isotopic exchange occurring as a function of increasing temperature and water/rock interaction. Strontium isotopes are also useful for understanding alteration around VMS deposits, with potentially significant differences in Sr-87/Sr-86 as a function of composition of the magma feeding the hydrothermal system, the isotopic composition of extant seawater, and the age and composition of the footwall lithologies.

Polymetallic (Ni-Mo-Zn-Pt-Pd-Au-Re) hyper-enriched black shales in the northern Canadian Cordillera consist of thin, semi-massive sulfides interbedded with black shale. We studied HEBS deposits at Nick, Peel River, Monster River, and Moss in northern Yukon, and at a single locality underlying the Cardiac Creek Pb-Zn-Ag deposit in northeastern British Columbia. High-grade mineralization contains up to 7.4 weight per cent Ni, 2.7 weight per cent Zn, 0.38 weight per cent Mo, 400 ppb Pt, 250 ppb Pd, 160 ppb Au, and 58.5 ppm Re. Sulfide mineralization formed during syngenesis to later diagenesis. Analyses by LA-ICP-MS indicate that pyrite is the principal host of platinum-group elements, Au, and Re. Mineralization and sedimentation were coeval based on the overlap between Re-Os geochronology of HEBS at Nick and Peel River (390.7 ± 5.1 and 387.3 ± 4.4 Ma, respectively) and conodont biostratigraphic ages of sedimentary host rocks. Bulk S isotope composition of HEBS is uniformly negative, indicating that bacterial reduction of seawater sulfate generated sulfur to precipitate sulfide minerals. The initial Os ratios at Peel River (0.25 ± 0.07) and Nick (0.32 ± 0.20) overlap with Middle Devonian seawater, suggesting that elemental enrichment was derived from seawater.

Achieving net-zero carbon emissions goals will increasingly rely on critical mineral resources while simultaneously decreasing the extraction, processing and use of hydrocarbons as the primary provider of energy. Canada is well-positioned to contribute to this effort through a series of innovative policy and research initiatives, and it is Canada's goal to be a stable supplier of critical minerals into the future. To this end, Natural Resources Canada and the Geological Survey of Canada invest financial resources into critical mineral research initiatives. This research aims to generate precompetitive baseline geological, geochemical and geophysical data for large, underexplored regions within Canada, whereas targeted studies focus on mineral systems science and improved exploration models for the large variety of critical mineral resources distributed throughout Canada. These research approaches can be combined, digitally, to generate mineral potential models. These ongoing efforts by the Geological Survey of Canada enhance the viability of Canada being (or maintaining its status as) a hub for critical mineral resource development and processing well into the future.

The trace element composition of detrital magnetite grains recovered from six local streams around the Casino high-grade porphyry Cu–Au–Mo deposit, west-central Yukon, is compared with igneous and magmatic-hydrothermal magnetite recovered from mineralized and unmineralized host rocks at the deposit. Linear discriminant analysis of 12 elements (Mg, Al, Ti, V, Mn, Co, Cr, Ni, Cu, Zn, Ga and Ge) and plots of Ti v. Ni/Cr are used to discriminate between magmatic-hydrothermal magnetite from the potassic alteration zone and igneous magnetite from granodiorite and quartz monzonite hosting the deposit. Magmatic-hydrothermal magnetite with a trace element composition similar to that from the potassic alteration zone at Casino is identifiable in stream sediments draining the deposit. Copper in magmatic-hydrothermal magnetite, present as minute inclusions of sulfide minerals such as chalcopyrite or substituted within the magnetite crystal lattice, is a strong indicator of Cu mineralization. We show that the chemical compositions of magnetite recovered from stream sediments can be used to explore for porphyry systems. Thematic collection: This article is part of the Applications of Innovations in Geochemical Data Analysis collection available at: https://www.lyellcollection.org/cc/applications-of-innovations-in-geochemical-data-analysis Supplementary material: Laser ablation data for major, minor and trace elements in magnetite from bedrock and stream sediment samples from Casino are available at https://doi.org/10.6084/m9.figshare.c.5896900

Volcanogenic massive-sulfide (VMS) deposits may have had metal contributions from magmatic degassing and leaching of footwall rocks. The Windy Craggy Cu-Co-Zn VMS deposit in northwestern British Columbia may include magmatic contributions, based on laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of fluid inclusions (enriched in Sb, Sn, and Bi) and lithogeochemistry. Sulfide-mineral trace-element abundances in the massive-sulfide orebody, underlying stockwork zone, gold zone, and altered and unaltered mafic rock and argillite were analyzed by LA-ICP-MS. Elevated Au, W, As, Bi, Sb, Se, Te, Tl, Ag, Co, and Mo contents occur within the gold and/or stockwork zones. Increasing 'magmatic metals' with increasing Co/Ni values suggest direct magmatic contribution to the deposit. Covariation of Co with these so-called 'magmatic elements' indicates that it, too, may be of magmatic origin, sourced via fluids exsolved from a crystallizing magma; however, evidence from the composition of rocks and sulfide minerals from Windy Craggy and other VMS deposits suggests that there is probably no meaningful distinction between hydrothermal leaching and direct magmatic contributions and that most - if not all - fluids that form VMS deposits should be termed 'magmatic-hydrothermal'.

Northern Yukon hosts occurrences of Middle Devonian hyper-enriched black shale (HEBS) Ni-Mo-Zn-platinum-group element-Au-Re mineralization, including the Monster River showing in the Ogilvie Mountains. This mineralization has been documented predominantly in the Paleozoic Richardson trough; however, the Monster River showing is atypical, occurring within the Blackstone trough, more than 200 km to the west on the southern margin of the Yukon block. The ambient paleoredox conditions of the marine water column and sediments may be primary controlling factors in HEBS formation. We use major and trace element lithogeochemistry to better understand ambient paleoenvironmental redox conditions through the application of robust redox proxies to HEBS mineralization and host rocks. Uniformly negative Ce anomalies (0.6-0.9) indicate that the water column was predominantly suboxic throughout the deposition interval, even during HEBS mineralization. Although there is a strong terrigenous influence on the rare earth element-yttrium (REE-Y) abundances of the sedimentary rocks, superchondritic Y/Ho ratios (>27) indicate that seawater contributed REE-Y to the host rocks and HEBS. High (>10) authigenic Mo/U ratios indicate that a Fe-Mn particulate shuttle operated in the water column; this is corroborated by negative Ce anomalies and high Y/Ho ratios. The data indicate that metalliferous sedimentary rocks formed by hydrogenous metal enrichment (e.g. Ni, Mo, Pt) caused by ferromanganese oxyhydroxide particulate shuttling as chemical sediments; moreover, the REE- and Mo-based paleoenvironmental indicators suggest a complexly redox-stratified depositional environment with an abundant supply of metals, metalloids, and sulfur.

This volume presents results of research conducted during phase 5 of the Volcanic- and Sedimentary-hosted Base Metals Ore Systems project of the Geological Survey of Canada's Targeted Geoscience Initiative (TGI) program. The papers in this volume include syntheses and primary scientific reports. We present here a synopsis of the findings during this TGI project. Research activities have addressed several mineral deposit types hosted in sedimentary rocks: polymetallic hyper-enriched black shale, sedimentary exhalative Pb-Zn, carbonate-hosted Pb-Zn (Mississippi Valley-type; MVT), and fracture-controlled replacement Zn-Pb. Other carbonate-hosted deposits studied include a magnesite deposit at Mount Brussilof and a rare-earth element-F-Ba deposit at Rock Canyon Creek, both of which lack base metals but are spatially associated with the MVT deposits in the southern Rocky Mountains. Volcanogenic massive-sulfide deposits hosted in volcanic and mixed volcanic-sedimentary host rock settings were also examined. Through field geology, geochemical (lithogeochemistry, stable and radiogenic isotopes, fluid inclusions, and mineral chemistry), and geophysical (rock properties, magnetotelluric, and seismic) tools, the TGI research contributions have advanced genetic and exploration models for volcanic- and sedimentary-hosted base-metal deposits and developed new laboratory, geophysical, and field techniques to support exploration.

Demand for critical raw materials is expected to accelerate over the next few decades due to continued population growth and the shifting consumption patterns of the global economy. Sedimentary basins are important sources for critical raw materials and new discoveries of sediment-hosted Mississippi Valley-type (MVT) and/or clastic-dominated (CD) Zn-Pb deposits are likely required to mitigate future supply chain disruptions for Zn, Pb, Ag, Cd, Ga, Ge, Sb, and In. Herein we integrate public geoscience datasets using a discrete global grid to system to model the mineral potential for MVT and CD deposits across Canada, the United States of America, and Australia. Statistical analysis of the model results demonstrates that surface-wave tomography and derivative products from satellite gravity datasets can be used to map the most favourable paleo-tectonic settings of MVT and CD deposits inboard of orogenic belts and at the rifted edges of cratonic lithosphere, respectively. Basin development at pre-existing crustal boundaries was likely important for maintaining the low geothermal-gradients that are favourable for metal transport and generating the crustal fluid pathways that were reactivated during ore-formation, as suggested by the statistical association of both sediment-hosted mineral deposit types with the edges of upward-continued gravity and long-wavelength magnetic anomalies. Multivariate statistical analysis demonstrates that the most prospective combination of these geophysical datasets varies for each geological region and deposit type. We further demonstrate that maximum and minimum geological ages, coupled with Phanerozoic paleogeographic reconstructions, represent mappable proxies for the availability of oxidized, brine-generating regions that are the most likely source of ore-forming fluids (e.g., low-to mid-latitude carbonate platforms and evaporites). Ore deposition was likely controlled by interaction between oxidized, low-temperature brines and sulfidic and/or carbonaceous rocks, which, in some cases, can be mapped at the exposed surface or identified using the available rock descriptions. Baseline weights-of-evidence models are based on regional geophysics and are the least impacted by missing surface information but yield relatively poor results, as demonstrated by the low area-under-the-curve (AUC) for the spatially independent test set on the success-rate plot (AUC = 0.787 for MVT and AUC = 0.870 for CD). Model performance can be improved by: (1) using advanced methods that were trained and validated during a series of semi-automated machine learning competitions; and/or (2) incorporating geological and geophysical datasets that are proxies for each component of the mineral system. The best-performing gradient boosting machine models yield higher AUC for the test set (AUC = 0.983 for MVT and AUC = 0.991 for CD) and reduce the search space by >94%. The model results highlight the potential benefits of mapping sediment-hosted mineral systems at continental scale to improve mineral exploration targeting for critical raw materials.

Paleozoic strata of the Selwyn Basin host sedimentary exhalative (SEDEX) Pb-Zn deposits, and age-correlative strata of the Richardson trough host polymetallic hyper-enriched black shale (HEBS) deposits. In both deposit types, organic matter is spatially and temporally associated with mineralization. We investigated the characteristics of organic matter in mineralization and unmineralized host rocks in the XY Central SEDEX deposit in the Howard's Pass district, and the Nick and Peel River HEBS deposits in the Richardson trough using Rock-Eval pyrolysis, organic petrography, and solvent extraction and gas chromatography mass spectrometry (GCMS) analysis of the soluble organic matter (SOM). All samples experienced extremely high thermal maturity (Tmax up to 599°C), indicating they contain low SOM. Rock-Eval parameters S1, S2, HI, and OI values are low. Total organic carbon (TOC) values are low for Nick and Peel River and are generally higher for XY Central. Residual carbon values are universally high. Mineral carbon values are low for deposits studied (one outlier). Pyrobitumen reflectance is mostly below 5.80%. Full-scan GCMS analyses of SOM reveal that most, if not all, high molecular weight hydrocarbons, including biomarkers, have been lost due to thermal cracking and many detected peaks are likely due to contaminants introduced during sampling.

Understanding the controls on the behaviour of metalloids (Se, As) and metals (Cu, Zn, Pb) in natural aqueous systems is vital to interpreting hydrogeochemical data in environmental and mineral exploration applications. Geochemical, isotopic and redox measurements of a suite of groundwaters sampled from around the ABM zone of the Kudz Ze Kayah (KZK) volcanogenic massive sulfide (VMS) deposit in the Yukon, Canada are presented and contrasted with other case studies from a variety of mineral deposit types. This deposit has atypically high As (up to 4.3 wt%, average 2457 ppm) and Se (up to 2620 ppm, average 157 ppm) contents in the sulfide mineralization. As a relatively undisturbed deposit (unmined), it is an ideal site to study the mobility and solubility of trace metals in groundwaters. Herein we present field measurements (pH, dissolved oxygen, specific conductance, oxidation–reduction potential and temperature), major ion, trace element, anion (Cl, Br, SO 4 , PO 4 ), and stable isotope ( δ 2 H, δ 13 C DIC , δ 18 O, δ 18 O SO4 , δ 34 S) data. Waters are dominantly low-salinity HCO 3 to HCO 3 –SO 4 -type waters with variable sulfate (4.83 to 601 mg l −1 ), Ca (23–235 mg l −1 ) Mg (3.1–96.8 mg l −1 ), Na (0.30–66.9 mg l −1 ) and K (0.55 to 6.25 mg l −1 ) concentrations. These waters also have variable trace element concentrations that include As (0.01 to 148 µg l –1 ), Se (<0.02 to 1.01 µg l –1 ), Fe (0.01 to 3.84 mg l −1 ), Zn (<0.2 to 1070 µg l –1 ), Pb (<0.01 to 8.4 µg l –1 ), Cu (0.03 and 24.5 µg l –1 ) and Sb (0.01 to 54.4 µg l –1 ). Some waters also have elevated concentrations (compared to most meteoric waters) of Nb (up to 0.3 µg l –1 ), Y (up to 1.42 µg l –1 ), Zr (up to 18 µg l –1 ), and the rare-earth elements (REEs) ( Σ REE up to 2.04 µg l –1 ). The δ 18 O (−22.8 to −20.9 ‰) and δ 2 H (−174 to −158 ‰), together with the δ 13 C DIC (−10.6 to +1.9 ‰), δ 34 S (+10 to +12 ‰) and δ 18 O SO4 (15.5 to −4.75 ‰) all suggest that local meteoric water has interacted with massive sulfide mineralization at the ABM zone. Our results demonstrate the requirement for the use of multiple techniques in hydrogeochemical studies, with dissolved concentrations of major and trace elements coupled with a suite of stable isotopes that help define a larger geochemical footprint for the KZK deposit. Water–mineral interaction between groundwater aquifers and VMS deposits like the ABM zone are distinctly different from dispersion halos described from other deposit types (i.e. Cu porphyry, unconformity U). Thematic collection: This article is part of the Hydrochemistry related to exploration and environmental issues collection available at: https://www.lyellcollection.org/cc/hydrochemistry-related-to-exploration-and-environmental-issues

ABSTRACT The MacMillan Pass District in Yukon, Canada, hosts the Tom and Jason clastic sediment-hosted Zn-Pb-Ag-(Ba) deposits. Bulk geochemical paleoredox proxies (Eu/Eu*, Ce/Ce*, Mo, Re/Mo, and Ni/Co) indicate anoxic–dysoxic water column and sulfidic porewater conditions persisted during the Late Devonian deposition of the Lower Earn Group host rocks. Positive Eu/Eu* anomalies (up to 3.31) in sulfide mineralization at the Tom deposit are consistent with relatively high temperature (probably >250 °C), reducing, acidic hydrothermal fluids that infiltrated laterally through unconsolidated sediments proximal to the hydrothermal upflow zone and/or exhaled at the seafloor as moderate- to high-density brines. Molybdenum and U enrichment factors (relative to upper continental crust) and Mo/organic C values are consistent with a moderately restricted basin; Mo/C values fall between those of the Black Sea (highly restricted) and the Framvaren Inlet (moderately restricted). A Ba-rich shale was identified in rocks that are distal and time-equivalent units to the Pb-Zn mineralization; based on the bulk chemical compositions and on previous S and Sr isotope studies, we interpret the baryte in this unit to be largely hydrothermal in origin and perhaps remobilized and reprecipitated during hydrothermal base-metal mineralization.

ABSTRACT Hyper-enriched black shale (HEBS) deposits in northern Yukon, consist of thin (<10 cm), laterally extensive (tens of thousands of km2) stratiform sulfide mineralization layer(s) that are hyper-enriched in Ni, Mo, Zn, platinum group elements, Re, and Au. The genesis of HEBS deposits and the ambient paleoenvironment in which they formed are the subject of vigorous debate. Non-traditional stable isotopes, particularly molybdenum and thallium, are robust paleoredox indicators and we have employed these isotope systems in this study of Yukon HEBS. Systematic sampling and Mo and Tl isotopic analysis of a continuous 200 m stratigraphic section through the Yukon HEBS mineralization and footwall and hanging-wall strata at the Peel River north and south bank showings (spaced five km apart) give δ98Mo –1.24 to –0.53‰ and –8.1 to –5.2 ε-units for the mineralization and –0.70 to 0.60‰ and –6.5 to –2.0 ε-units for the unmineralized strata. These values preclude a hydrothermal origin and strongly suggest that redox processes were responsible for the Yukon HEBS mineralization. The isotopic compositions, together with rare earth element (REE) systematics (REE profile, Y positive anomalies, Ce negative anomalies, and Y/Ho values) and other bulk geochemical redox indicators (Mo, V, Re/Mo, Ni/Co, U/Th, and V/Cr) indicate that the Peel River HEBS mineralization formed because of metal scavenging from seawater in a quiescent, euxinic basinal paleoenvironment.