The Archean Goldex deposit, located in the Val-d'Or gold camp in the southern Abitibi greenstone belt, Canada, comprises two contrasting styles of gold mineralization that result from two distinct ore-forming events that happened >= 25 m.y. apart. The first is an early (pre-2685 Ma), pre-D-3 event forming the volcanic-hosted South Zones vein system in the immediate structural footwall (south side) of the Goldex quartz diorite. The second is a late, ca. 2660 to 2640 Ma syn-D-3 event forming an orogenic vein system within the 2686.3 +/- 1.2 Ma quartz diorite. The South Zones consist of deformed smoky quartz veins and diffuse domains of intense silicification with variable amounts of disseminated pyrrhotite-pyrite +/- sphalerite-chalcopyrite native gold, bordered by a metamorphosed assemblage of pervasive biotite-quartz-pyrrhotite +/- amphibole-epidote alteration. Many characteristics of the South Zones, such as folding, boudinage, and dynamic recrystallization, indicate that they predate the main phase of regional deformation (D-3) shortening and peak metamorphism. In contrast, the bulk of the Goldex deposit consists of a network of steeply and shallowly S-dipping quartz-tourmaline-carbonate-pyrite vein stockworks and breccias with proximal albite-pyrite alteration. The South Zones deposit is part of a group of early structurally controlled gold deposits that are spatially associated with a WNW-trending structural corridor (referred to as the Marbenite-Norbenite corridor) that originated prior to D-3, which plausibly represents a significant D-1 to D-2 fault zone. The juxtaposition of these two temporally and geologically distinct gold deposits (i.e., pre-D-3 vs. syn-D-3) attests to the longevity of the Marbenite-Norbenite structural corridor and highlights the importance of early structures as gold metallotects and multiple mineralizing events in the formation of world-class gold camps in polydeformed terranes.

Sections of Michigan Basin sediments were exposed to hydrothermal fluids during the Ordovician. Secondary calcite and dolomite occur as fracture-filling veins/vugs and replacement minerals in host Ordovician limestones-which occur at depth beneath the Bruce Nuclear site (near Tiverton) in southwestern Ontario. To provide insights on the evolution of diagenetic fluids responsible for secondary mineral growth and the effects of temperature-dependent alteration in the basin, clumped isotope geothermometry was applied to these carbonates. Ordovician calcite in veins/vugs have delta 18OC values of +20.0 to +23.4 %o (VSMOW) and are estimated to have formed at 65-83 degrees C from various waters (delta 18Ow = +0.1 to +4.4 %o) including Ordovician to modified (an evolved 18O-enriched) seawater, and mixed seawater and basinal brines at depth. Calcite veins/vugs in the lower Cambrian units formed at 74-91 degrees C, have lower delta 18OC values (+14.9 to +17.0 %o) and precipitated from 18O-depleted hydrothermal brines (delta 18Ow =-4.6 to-3.1 %o). Calcite in the matrix of host limestones yielded higher apparent temperatures (T(Delta 47) = 49-82 degrees C)) that are inconsistent with precipitation from Ordovician seawater and burial diagenesis of the sediments but instead, reflect a solid-state isotope reordering due to a Late Devonian-Mississippian rift-related basinal heating. Diagenetic dolomitization in the basin occurred during shallow to intermediate burial, where dolomite primarily replaces calcite in the limestone matrix or occur as secondary infill in fractures and vugs. The clumped isotopic signature in dolomite is more resistant to low-temperature heating than calcite, whose Delta 47 composition reflects a 367-322 Ma regional heating event at 125-150 degrees C during burial, in agreement with, but more precise than previous estimates based on fluid inclusion micro-thermometry in secondary quartz and saddle dolomite.

The large range of Rb and Sr mass fractions in natural micas presents a challenge for LA-ICP-MS/MS Rb-Sr geochronology. Mica with disparately high Rb and low common Sr mass fractions are particularly problematic given the limited linear dynamic range (1-2 & times; 10(6) cps) of ICP-MS/MS systems. Here we quantify sources of uncertainty and inaccuracy associated with the in situ Rb-Sr method, including assessing laser-ICP-MS/MS timing issues, evaluating statistical bias at low count rates and quantifying non-linearity of the electron multiplier between pulse and analogue detection. These considerations are tested on natural mica with high Rb-87/Sr-86 ratios (> 1000), including ca. 92 Ma biotite (Tombstone pluton, Yukon) and ca. 2620 Ma muscovite (Tanco pegmatite, Manitoba), with Rb mass fractions of ca. 2300 and 30000 mu g g(-1), respectively. Our results highlight the importance of: (1) optimising analytical parameters (e.g., laser energy, spot size), (2) potential application of non-linearity corrections for high Rb signals and (3) judicious selection of data reduction methods (e.g., signal integration methods, incorporating all sources of uncertainty, error correlation) for the determination of precise and accurate Rb-Sr ages. With these considerations in mind, in situ Rb-Sr measurements of Tombstone and Tanco mica yielded isochron ages within similar to 1% of their accepted Rb-Sr TIMS ages.

Sulphide minerals within xenoliths provide critical records for melting and metasomatism in the lithospheric mantle and crust. During the transport of these mantle and crustal xenoliths to the surface, high-temperature sulphide minerals cool and transform to a low-temperature assemblage that includes pyrrhotite, pentlandite, and chalcopyrite. However, the small size and complex intergrowths between these low-temperature minerals present analytical challenges for determining their trace element concentrations, and for estimating the bulk sulphide composition prior to cooling. Herein we combine geochemical mapping by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) with machine learning for a suite of basalt-hosted peridotite, pyroxenite, gabbro, and anorthosite xenoliths from British Columbia, Canada, to reconstruct the bulk sulphide composition and constrain the petrology of the Cordilleran lithospheric mantle. Sulphide mineral compositions extracted from the segmented geochemical maps for the peridotite and pyroxenite xenoliths yield chondritic to sub-chondritic Os/Ir and Pd/Ir ratios, indicating that these samples represent mantle residues that underwent low to moderate degrees of partial melting. However, reconstructed bulk sulphide compositions also yield suprachondritic Re/Os ratios and high concentrations of relatively incompatible and highly chalcophile trace elements (e.g., Te, Ag, and Se). These enriched geochemical signatures indicate residual sulphide phases must have mixed with a metasomatic and compositionally fractionated sulphide melt. Small volumes of alkaline silicate melts partially preserved with sulphide, feldspar, and other metasomatic minerals in the same micro-textural settings further indicate that sulphide metasomatism occurred in the mantle and prior to volcanism. New two-pyroxene and olivine-spinel geothermometry results demonstrate that these sulphide-bearing lherzolite, websterite, and wehrlite xenoliths equilibrated at a range of temperatures (931-1129 degrees C). Sulphide minerals with similar compositions from peridotite xenolith localities across British Columbia of varying equilibration temperatures and volcanic ages (i.e., Oligocene to Pleistocene) suggests that sulphide metasomatism is a characteristic feature of the Cordilleran lithospheric mantle.

Porphyry deposits are major sources of copper, gold, molybdenum, and silver globally. However, the potential for critical raw materials (CRM) to be mined as by-products (e.g., antimony, bismuth, platinum group elements, and tellurium) at these deposits is poorly understood. Herein we present results from a lithogeochemical survey (n = 331), detailed mineralogy, and trace element mapping to characterize the concentrations of CRM and their deportment within the Golden Triangle, northwest British Columbia, Canada. We demonstrate that the host rocks to porphyry copper-gold (i.e., Galore Creek, Copper Canyon, KSM, Dok, Yeti, and Burgundy) and epithermal gold-silver (i.e., Brucejack) deposits were derived from oxidized and water-rich parental melts that suppressed sulphide crystallization, resulting in the pre-enrichment of CRM relative to other arc rocks globally. The structural juxtaposition, local thickening, and preservation of these prospective source rocks is likely one of the factors contributing to the mineral district's exceptional gold and CRM endowment. Multiple analytical methods (e.g., aqua-regia, four-acid, fusion) for the same samples further demonstrate that the highest Bi (39 ppm), Pd (460 ppb), Sb (375 ppm), and Te (15 ppm) concentrations are associated with sulphide and/or other non-resistate minerals within the most hydrothermally altered samples. Detailed mineralogy and trace element mapping by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) reveals that CRM within the Golden Triangle are hosted by at least 58 different minerals. We apply these results to estimate the contained CRM at the Galore Creek and KSM deposits and to discuss their potential to be recovered as by-products. Lithogeochemical results are also applied to predict the mineral potential of different igneous suites within the Golden Triangle based on their CRM concentrations.

This study explores the application of machine learning techniques for an enhanced interpretation of pyrite laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) maps. The Colosseum Au deposit, in southern California, was considered as a case study. Colosseum is genetically related to a rhyolitic breccia-pipe complex, where Au mineralization is associated with two main pyrite generations—early pyrite and late pyrite. Our machine learning workflow involves the detection of distinct compositional zones in individual maps through unsupervised clustering, and a second clustering step where these zones are grouped by compositional similarity, enabling the direct comparison between different maps and providing a compositional overview of pyrite representative of the various styles of mineralization present in the deposit. Clustering of individual maps correctly differentiated between distinct growth zones in early pyrite, fractures that crosscut early pyrite growth, and zones of late pyrite growth, matching petrographic observation. All the zones detected by this first step, in turn, were classified into two compositionally distinct groups and a third transitional group, enabling the direct comparison between maps while keeping petrographic consistency. For Colosseum, our approach revealed that (1) Au is more abundant in late pyrite than early pyrite, but significant amounts can be found in both generations and in both Colosseum mineralized breccia pipes; (2) the transition from early to late pyrite is represented by a change from a Co-Ni-Te–rich end member to a Cu-Ag-Zn-Sb-Tl–rich end member; and (3) Au is directly correlated with As in both pyrite generations.

ABSTRACT Integrated field mapping, phase equilibria modelling and in situ U–Pb monazite geochronology from the northern margin of the Rae craton on Baffin Island document three metamorphic events during the Neoarchean to the middle Paleoproterozoic. The Qimivvik area comprises Neoarchean tonalitic gneiss structurally juxtaposed over Neoarchean metasedimentary rocks along the Paleoproterozoic Qimivvik thrust and associated shear zone. High‐grade metamorphism at ca. 2.56–2.50 Ga supports a footprint for cryptic late Neoarchean metamorphism over a distance of ∼600 km along the northwestern Rae margin from southern Boothia Peninsula to northern Baffin Island. Thermal peak mineral assemblages in the Qimivvik area equilibrated at ca. 1.9 Ga at conditions of ~710°C–790°C and 4.3–5.5 kbar. The dominant Paleoproterozoic foliation is defined by peak metamorphic phases and is reoriented by folds related to the Qimivvik thrust. Peak metamorphism and associated deformation, including the Qimivvik thrust, are interpreted as a manifestation of the Ellesmere‐Inglefield belt of Ellesmere Island and West Greenland, which links with the ca. 1.9 Ga Thelon orogen of western Canada. Partial melting also occurred at ca. 1.8 Ga, possibly resulting from decompression of the Churchill domain following the collisional‐accretionary events related to the late stages of amalgamation of Laurentia and supercontinent Nuna. Quantitative trace element maps (acquired using LA‐ICP‐MS) of monazite reveal distinct trace element signatures associated with each of three growth stages. Ca. 2.5 Ga monazite exhibits complex intragrain compositional zoning, has elevated Y and heavy rare earth elements (HREEs) relative to ca. 1.9 Ga monazite and has higher Th/U overall than both ca. 1.9 Ga and ca. 1.8 Ga monazite. These signatures suggest that ca. 2.5 Ga monazite growth was concomitant with partial melting and preceded the majority of garnet growth. The ca. 1.9 Ga monazite grains are comparatively less zoned and have lower Y + HREE contents than both ca. 2.5 Ga and 1.8 Ga monazite, consistent with the ca. 1.9 Ga monazite forming after most garnet growth. Elevated Y + HREE in the ca. 1.8 Ga monazite imply that it formed after retrograde resorption of garnet rims. In our samples, Y + HREE generally exhibit stronger correlations with monazite age and/or petrographic context than Eu/Eu* and Th/U. As some compositional overlap exists between monazite of different ages and petrographic contexts, quantitative limits (‘cut‐offs’) based on trace element concentrations or ratios (e.g., Th/U, Eu/Eu*, La CN /Yb CN ) are unreliable for distinguishing between monazite populations. In addition to providing important constraints on the early tectonic evolution of northeastern Laurentia, our study offers new insights into trace element behaviour in a key accessory mineral during three metamorphic events occurring over a ~700 Ma time period.

Mineral exploration in remote mountain belts represents an exceptional challenge due to high operational costs, limited accessibility, and complex geology. New tools are urgently needed to improve discovery rates in these types of highly prospective but challenging mineral exploration frontiers. Herein we apply deep learning to predict deposit types in a rugged and remote part of northwest British Columbia (Canada) based on laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of pyrite to address that knowledge gap. The combined pyrite dataset represents four mineral deposit types at various stages of development, including porphyry copper-gold (i.e., Galore Creek, Copper Canyon, Kerr, Mitchell, Sulphurets, Iron Cap), epithermal goldsilver (i.e., Brucejack), magmatic nickel-copper (i.e., E&L), and volcanogenic massive sulphide copper-lead-zinc (i.e., A6). Trace element mapping, spot analysis, and quantitative mineralogy are applied to characterize the petrogenetic context and composition of each pyrite sample. Geochemical data were first pre-processed with principal component analysis and autoencoders to extract new features from the training data. The pre-processed pyrite data were then combined to train a series of feed-forward artificial neural networks to predict deposit types. The preferred deep learning classification model yields an overall accuracy of 99% for a subset of pyrite analyses that were not included in the training process. We then apply that classifier to show that hydrothermally altered rocks from early-stage mineral exploration projects (i.e., Dok and Yeti) are most likely related to porphyry copper-gold mineralization based on the composition of pyrite. Statistical analysis of the model results further demonstrates that pyrite morphology, texture, grain size, and paragenesis are important predictors of deposit type when combined with trace element concentrations and the new latent variables identified by the preferred autoencoder. We suggest that the pyrite library and modelling methodology can be used to support early-stage mineral targeting in remote exploration frontiers that are prospective for a range of deposit types.

The fate of metals, such as Cu, in stream waters draining porphyry mineralization is commonly controlled by several natural processes such as sorption, microbial processes, and ligand availability. Isotopes of Cu offer a novel approach to understanding these processes and determining metal sources within complicated mineralogical systems. Drainages at the Casino Cu-Au-Mo porphyry deposit, Yukon, Canada exhibit circumneutral (pH > 5) in Casino Creek and natural acid rock drainage (pH < 3.5) in Proctor Gulch with the precipitation of schwertmannite (Fe3+). Isotopic systems delta Cu-65 and delta S-34(sulfate) indicate different metal sources, with signatures of both hypogene and supergene mineralization. Waters from Proctor Gulch contain delta Cu-65 values (< -0.5 parts per thousand) consistent with supergene Cu sources from the leached or oxide portion of the mineralization. Comparatively, drainage in the upper part of Casino Creek contains a delta Cu-65 composition (> 0.5 parts per thousand) characteristic of Cu sourced from hypogene sulfide mineral oxidation. Variation in metal sources is similarly supported by aqueous delta S-34(sulfate) values in the stream waters, which suggest mixing of S derived from a sulfide mineral phase and a much heavier sulfate mineral (e.g., gypsum or anhydrite). Isotopic fractionation of Cu in the dissolved (<0.45 mu m) phase presents two predominant controls on Cu dispersion. The natural acid conditions in Proctor Gulch favor the preferential co-precipitation of Cu-63 with schwertmannite but could be influenced by intracellular assimilation or adsorption by microbes, which also has been shown to preferentially favor Cu-63. Copper isotopic fractionation results in a gradient of increasing delta Cu-65 values in waters downstream. In Casino Creek, higher pH conditions favor the precipitation of Fe(OH)(3) and the preferential adsorption of Cu-65, resulting in decreasing delta Cu-65 values downstream. Copper concentrations in stream waters remain elevated (up to 4.1 mu g/L) above ambient background (1.9 mu g/L) levels up to 11 km downstream of the deposit. Given the abundance of surface water in many parts of northern Canada, hydrogeochemical prospecting using broad scale stream water catchment analysis is clearly a viable greenfield exploration methodology.

The purpose of our study was to test the hypothesis that apatite provides a detailed record of the evolution of carbonatitic magmas. To this end, we investigated the chemistry and textures of apatite in the various rock units (banded carbonatite, biotitised syenite, biotitite, magnetite-biotite rock and massive carbonatite) of the St Honore carbonatite using a combination of micro-analytical and imaging techniques. Subtle changes in the uptake of a variety of trace elements during growth led to corresponding changes in the cathodoluminescence response that are recorded as distinct zones in the apatite. In the banded carbonatite, the first apatite to crystallise was fluid/mineral inclusion-bearing and was followed by apatite displaying oscillatory zoning and, in turn, by apatite that replaced the earlier apatite through dissolution-reprecipitation. In contrast, the earliest apatite in the biotitised syenite displays oscillatory zoning and was variably replaced by later apatite. The subsequent crystallisation stages duplicate those of apatite in the banded carbonatite. Apatite crystallisation in the magnetite-biotite rock duplicated the stages recorded by apatite in the banded carbonatite. Finally, the apatite of the massive carbonatite contains representatives of all the apatite types mentioned above. A model is presented in which inclusion-rich apatite records aqueous-carbonic fluid exsolution from the magma, oscillatory zoned apatite records periods of quiescent growth and replacement apatite records dissolution-reprecipitation induced by the differential stresses that accompanied fluid overpressures. Apatite in the massive carbonatite was incorporated from the other units. Based on the above model, we propose that banded carbonatites at St Honore and other similar complexes formed during an early stage of carbonatitic magma emplacement, when thermal gradients between the magma (hot) and the host rocks (cooler) were steep and the calcite liquidus was reached before significant biotitisation. With the emplacement of additional batches of magma, the thermal gradient was gradually flattened and biotitisation was extensive, producing massive biotitite. This hypothesis explains the occurrence, spatial distribution and genesis of the biotitite (glimmerite) and banded carbonatite observed in many carbonatite complexes.

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.

In the Gaspé Peninsula, karsts associated with subaerial exposure are interpreted to have developed either during the Silurian or the Carboniferous. Determining the timing of karst formation is valuable for economic reasons (including petroleum system characterization) and for the understanding of basin/climate evolution. In this study, a laminated calcite speleothem coating a karst wall in the McInnis cement plant quarry is studied in detail through petrographic observations, stable and clumped isotope analysis, and geochronology. The speleothem is made of laminae of different colours that are geochemically and isotopically distinct. White and red laminae have δ 18 O and δ 13 C values comparable to contemporaneous (within uncertainty) phreatic calcrete hardpans described in southern Gaspé Peninsula and northern New Brunswick, suggesting that both the speleothem and calcrete precipitated from primarily fresh groundwaters that interacted with soils. Changes in laminae colour most likely reflect different kinetics of precipitation and amounts of impurities. LA-ICP-MS in situ dating of calcite from white laminae gives a date of 331.8 ± 9.1 Ma, which is the first direct evidence that the cavity formed during the Visean (Carboniferous). From a regional point of view, subaerial exposure and karst formation during the Visean could have resulted from a glacially induced sea-level lowstand and/or from an uplift episode associated with magmatic underplating/plume-related magmatism in the offshore part of the Maritimes Basin.

The crystal growth history of an Au-rich sedimentary pyrite nodule from the Timmins-Porcupine Au camp, Ontario, Canada, has been investigated using Electron Backscattered Diffraction and Laser Ablation Inductively Coupled Plasma Mass Spectrometry techniques to study the crystallographic processes controlling metal deportment in the pyrite structure. Results show four distinct growth stages characterized by different pyrite microstructures, crystal forms and trace element compositions. A direct link is observed between the growth of octahedral facets in pyrite and the development of primary (non-tectonic) subgrain boundaries. Furthermore, zones with a high abundance of subgrain boundaries have the highest Au, As, Ag and Cu (and other metals) contents - suggesting metal distribution is linked to the development of microstructures. Finer-grained aggregates are characterized by higher grain boundary density than in coarse areas, making higher trace element concentrations inversely proportional to grain size. Our results indicate that the high Au concentrations (similar to 100 ppm) in pyrite represent a primary feature related to nodule growth, instead of secondary enrichment processes, and highlight the possibility that sediment-hosted pyrite nodules could represent a metal-rich geochemical reservoir for the formation of younger orogenic Au deposits.

Canada's northern cratons record a complex poly-metamorphic evolution in response to multiple orogenic events. The Snowbird Tectonic Zone (STZ), the principal suture between the Archean Rae and Hearne cratons, is a highly debated structure; it is argued to have accommodated modern-style accretion in either the Neoarchean (2.6-2.5 Ga) or Palaeoproterozoic (ca. 1.9 Ga). To resolve this controversy, we use Lu-Hf garnet geochronology to date an eclogite collected from the East Athabasca mylonite triangle of the STZ. Garnet shows prograde trace-element zoning, and we obtained an age of 1914.5 +/- 3.4 Ma. The age is interpreted to date the timing of collision related to deep burial of crust along the STZ. This strongly supports Palaeoproterozoic terminal collision between the Rae and Hearne cratons, and that to constrain high-pressure metamorphism during supercontinent assembly, dating eclogite itself is key.

Distal alteration related to porphyry Cu mineralization is typically characterized by an abundance of green minerals, such as epidote, tremolite, and chlorite, within the propylitic and sodic-calcic alteration zones and extends far outside (>1 km) the mineralized zone(s). Glacial erosion and dispersal derived from rocks affected by propylitic and sodic-calcic alteration have resulted in the development of extensive dispersal trains of epidote in till (glacial sediment) that can reach 8 to 330 km(2) as observed at four porphyry Cu study sites in the Quesnel terrane of south-central British Columbia: Highland Valley Copper, Gibraltar, Mount Polley, and Woodjam deposits. At each of these sites, epidote is more abundant in heavy mineral concentrates of till collected directly over and down-ice from mineralization and associated alteration. Epidote grains in till with >0.6 ppm Sb and >8 ppm As (as determined by laser ablation-inductively coupled plasma-mass spectrometry) are attributed to a porphyry alteration provenance. There is a greater abundance of epidote grains with high concentrations of trace elements (>12 ppm Cu, >2,700 ppm Mn, >7 ppm Zn, and >37 ppm Pb) in each porphyry district compared to background regions. This trace element signature recorded in till epidote grains is heterogeneously distributed in these districts and is interpreted to reflect varying degrees of metal enrichment from a porphyry fluid source. Tracing the source of the epidote in the till (i.e., geochemically tying it to porphyry-related propylitic and/or sodic-calcic alteration), coupled with porphyry vectoring tools in bedrock, will aid in the detection of concealed porphyry Cu mineralization in glaciated terrains.

Garnet is an exceptionally useful mineral for reconstructing the evolution of metamorphic rocks that have experienced multiple tectonic or thermal events. Understanding how garnet crystallizes and its mechanical behaviour, is important for establishing a petrological and temporal record of metamorphism and deformation, and to recognize multiple geologic stages within the growth history of an individual crystal. In this study, we integrate fine-scale microstructural (EBSD) and microchemical (LA-ICP-MS mapping) data obtained on a polycyclic garnet-bearing micaschist from the Alpine belt. Results suggest that fragmentation of pre-Alpine garnet porphyroblasts occurred during the late pre-Alpine exhumation and/or the onset of the Alpine burial, such that the older pre-Alpine garnet fragments were transported/redistributed during Alpine deformation and acted as new nucleation sites for Alpine garnet growth. These processes produced a bimodal garnet size distribution (macro mm-sized and micro sub-mm-sized grains). Thermodynamic modelling indicate that Alpine garnet grew during the final stage of burial (from 1.9 GPa 480 °C to 2.0 GPa 520 °C) and early exhumation (down to 1.6 GPa 540 °C) forming continuous idioblastic rims on macro- and micro-grains, and sealing fractures preserved in pre-Alpine garnet porphyroblasts. We propose that fragmentation-overgrowth processes coupled with ductile deformation in polycyclic rocks may produce a bimodal garnet size distribution and form multistage crystals resembling neoblasts. This study highlights the importance of linking microstructural (EBSD) and microchemical (LA-ICP-MS mapping) data by providing valuable information about the dominant deformation mechanisms at a given site by identifying potential links between major/trace element mobility and crystal deformation.

The Bakken Formation is a hydrocarbon-rich, metalliferous shale with subordinate sandstone and limestone that was deposited in the Williston Basin in Canada and the USA. Deposition of the Bakken Formation spanned the Devonian-Carboniferous boundary - a key interval in Earth's history representing profound biotic and geochemical changes in the terrestrial and marine realms, leading to deposition of metal-rich shales in the lower part of the sequence. Most studies of metal enrichment in the Bakken Formation have focused solely on the bulk rock litho-geochemistry, with little attention given to the organic matter, mineralogical, and diagenetic controls on metal sequestration and distribution. The current study combined bulk sulfur isotope and litho-geochemistry, along with reflected light and scanning electron microscopy (SEM) petrography to elucidate organic matter and mineralogical controls on trace element geochemistry. We have also used laser ablation-inductively couple plasma-mass spectrometry (LA-ICP-MS) to quantitatively assess the distribution of trace elements among different mineral phases, shale matrix, and organic matter. We show that sphalerite is the main zinc host, and that sphalerite sequesters Cu, Ga, Ge, and Cd within its crystal structure. Pyrite is the main Fe mineral and has typical abundances for sedimentary pyrite, however, the total bulk rock abundances of Ni, Mo, and Cu cannot be accounted for in pyrite alone, and indeed, are present within early mature organic matter in the shale matrix. Vanadium (V) is associated with organic carbon but is also high in aluminosilicates within the matrix. Collectively, this study demonstrates the need for detailed petrographic analyses even in seemingly monotonous black shales.

The Archean Horne 5 deposit, located in the Rouyn-Noranda district in the southern Abitibi greenstone belt, Canada, contains a total resource of 172.4 t Au (5.6 Moz) from 112.7 Mt of ore grading at 1.53 g/t Au. The deposit is part of the Au-rich Horne volcanogenic massive sulfide (VMS) complex that also includes the past -producing Horne mine (i.e., the Upper and Lower H zones plus small subsidiary lenses) that yielded 325.4 t Au (10.5 Moz Au) from 53.7 Mt of ore grading at 6.06 g/t Au. Combined, the Horne mine and Horne 5 deposit contain similar to 500 t Au (16 Moz), making them the world's single largest accumulation of VMS-related Au. The Horne 5 deposit consists of stacked lenses of massive to semimassive sulfides alternating with extensive zones of disseminated and stringer sulfides. The mineralization is hosted within thick accumulations of steeply dipping dacitic to rhyodacitic volcaniclastic units of transitional to calc-alkaline magmatic affinity. Dacitic-rhyo-dacitic synvolcanic units (lobes, sills, and/or domes) intrude the host succession, which is also crosscut by a series of post-ore mafic and younger intermediate to felsic feldspar +/- quartz porphyry dikes. A broad and diffuse halo of distal sericite-chlorite-epidote alteration extends outboard of intensely sericite-altered zones proximal to the sulfide lenses. Gold is interpreted to be synvolcanic on the basis of Au-rich mas-sive sulfide clasts in the volcaniclastic units, the presence of preserved Au-rich primary pyrite, Au zones limited to the sulfide envelope, crosscutting deformed but unaltered and barren dikes, and the absence of typical syn-deformation, orogenic-style alteration and mineralization despite overprinting high-strain corridors and faults. Gold is spatially associated with pyrite, sphalerite, and chalcopyrite, and its distribution is largely controlled by the higher porosity and permeability of the volcaniclastic host rocks, which are interpreted to have facili-tated hydrothermal fluid circulation in the subseafloor environment. Synvolcanic intrusions and fine-grained tuffs overlying auriferous zones also influenced the distribution of the mineralization by acting as cap rocks to ascending fluids. Evidence suggests that Au enrichment at the Horne 5 deposit is due to efficient transport and precipitation of Au in the subseafloor environment, a favorable geodynamic setting (transitional to calc-alkaline magmatism over thick crust), and possible input of magmatic fluids as suggested by high Te and Cu in the mineralization. Minor and very local remobilization of metals occurred in response to regional deformation and associated greenschist facies metamorphism. The detailed study of the Horne 5 deposit geology and a review of the available information on the Horne mine and recent 3-D modeling indicate that the Horne 5 deposit may have formed higher in the stratigraphy than the Upper and Lower H orebodies of the former Horne mine, which originally formed a single lens. Therefore, the Horne Au-rich VMS complex originally formed as a stacked system in which the Horne 5 deposit was deposited above the Upper and Lower H zones and not in a distal or lateral position as previously proposed, indicating that a robust hydrothermal system was responsible for the formation of the world's largest Au-rich VMS complex.