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Different types of sediment-hosted whole-rock Pb isotope (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) compositions were determined from phyllites, carbonaceous phyllites (>1% TOC), and meta-litharenites belonging to the Serra do Garrote Formation, which is part of the Proterozoic Vazante Group, Brazil. Results were integrated with lithogeochemistry in order to identify the Pb isotopic signature of Zn enrichment (up to 0.24 wt.% Zn) associated with meta-siliciclastic-hosted sulfide mineralization that formed prior to the Brasiliano Orogeny (850 to 550 Ma) in order to (1) understand the nature of siliciclastic sediment sources, (2) identify possible metal sources in pre-orogenic meta-siliciclastic-hosted Zn mineralization, and (3) evaluate the genetic links between the Zn enrichment in the relatively reduced phyllite package, and different styles of syn-orogenic Zn ± Pb mineralization (hypogene Zn-silicate and Zn-Pb sulfide) in overlying dolomitic carbonates throughout the Vazante-Paracatu Zn District, Brazil. The whole-rock 206Pb/204Pb and 207Pb/204Pb isotope ratios of meta-siliciclastic rocks plot as positively sloping, sub-parallel arrays with radiogenic, upper continental crust compositions, which could represent a detrital contribution from at least two upper continental crust sources. However, the 206Pb/204Pb versus207Pb/204Pb isotope system does not distinguish between Zn-enriched samples and un-mineralized samples. In the whole-rock 206Pb/204Pb–208Pb/204Pb plot, Zn-enriched samples form a flat trend of lower 208Pb/204Pb values (38.3 to 39.5) compared to the Zn-poor ones that follow common upper crustal trends. Zinc-enriched samples have low whole-rock Th/U values (

The Jacurici Complex hosts the largest chromite deposit in Brazil in an up to 8-m-thick chromitite layer within a tectonically segmented 300-m-thick intrusion. The ore has been interpreted as the result of crustal contamination-driven crystallization in a magma conduit. This study addresses the stratigraphy, mineralogical and textural relationships, and mineral chemistry of the Monte Alegre Sul segment focusing on chromite-hosted inclusions from the Main Chromitite Layer to understand the role of volatiles in the genesis of the massive chromitite. Silicate inclusions (enstatite, phlogopite, magnesiohornblende, diopside and olivine) are commonly monomineralic and sub- to euhedral, and crystallized prior to, or coeval with, the chromite crystallization. Carbonate inclusions (dolomite and magnesite) are irregular or have negative crystal shapes, suggesting entrapment as melt droplets. Sulfides (pentlandite, millerite, heazlewoodite, polydymite, pyrite, and chalcopyrite) are often polymineralic, irregular, or hexagonal-shaped, indicating entrapment as sulfide melt and as monosulfide solid solution. The inclusions indicate an H2O- and S-saturated resident magma with immiscible droplets of carbonate melt during chromite crystallization. Inclusion-rich and inclusion-free chromites that occur together have similar compositions and are considered to have formed from the same magma in response to variations in the degree of Cr saturation. Hot primitive magma might have heated and mobilized CO2 and probably water from devolatized and assimilated carbonate-rich wall rocks, increasing fO2 and triggering chromite crystallization. We propose that the formation of the chromitite layer started as in situ crystallization with additional material added by slumping of locally remobilized chromite slurries, facilitated by the presence of volatiles.

Applications of the spectral induced polarization (SIP) method to mineral exploration are limited by our knowledge of the relationships among rock texture, mineral composition, and electrical properties. Laboratory SIP responses were measured on rock samples from the Canadian Malartic gold deposit. Field SIP responses were also measured at the outcrop scale, along a profile that intersects a well-studied mineralized zone. The mineralogy and the texture of sedimentary rocks from this deposit were quantitatively determined with mineral liberation analysis. A systematic decrease (Pearson [Formula: see text]) in total chargeability with increasing fraction of the sulfide mineral interfaces associated with feldspar minerals (namely, K-feldspar and albite) was observed. On the other hand, total chargeability increased with the fraction of sulfide mineral interfaces associated with carbonates and micas (Pearson [Formula: see text]). At Canadian Malartic, proximal alteration in the mineralized zones is marked by rocks that lack a foliation plane and that were subjected to pervasive K-feldspar, albite, and pyrite alteration. In contrast, distal alteration in sedimentary rocks is marked by biotite, albite, carbonate, and pyrite that are oriented along the regional [Formula: see text] foliation. In the least-altered (LA) sedimentary rocks, quartz and biotite are associated with pyrrhotite and ilmenite as the main sulfide and oxide mineral phases, respectively. SIP measurements conducted at district and outcrop scales and along a drill core indicated that proximally altered sedimentary rocks were characterized by low total chargeability values ([Formula: see text] to [Formula: see text] in the laboratory and [Formula: see text] in the field). In contrast, the LA sedimentary rocks were characterized by total chargeability values up to [Formula: see text] in the laboratory and [Formula: see text] in the field. We conclude that mineralized zones associated with this type of ore deposit are characterized by low chargeability anomalies.

The Pontiac Subprovince is located in the Superior Province, south of the Abitibi Subprovince. The metasedimentary rocks of the Pontiac Group are characterized by a Barrovian metamorphic gradient increasing from north to south from biotite- through garnet- to staurolite-zone conditions. The Pontiac Subprovince has been interpreted as an accretionary wedge or alternatively as an exotic terrane that was tectonically docked to the Abitibi Subprovince during Archean subduction. However, few studies have attempted to characterize its pressure-temperature-time path to test this hypothesis and little is known about the timing relationship between regional metamorphism and the gold mineralization. An E-W lepidoblastic ductile micaceous foliation defines the regional fabric related to the second episode of deformation. Garnet and staurolite porphyroblasts are interpreted to be late- to post-kinematic to the regional fabric. Locally, cordierite or andalusite porphyroblasts fully pseudomorphed by a polymineralic assemblage of muscovite, feldspar and quartz are wrapped by the main foliation indicating that a potential low pressure - high temperature metamorphism (M1) preceded the main episode of deformation and Barrovian metamorphism (M2). Pressure and temperature (P-T) forward thermodynamic modelling on a water-saturated pseudosection yielded peak conditions of 550–600 °C and 5–6 kbar during a prograde, clockwise P-T path for M2. Lu-Hf dating of garnet from three different locations within the staurolite zone yielded a weighted average age of 2657.5 ± 4.4 Ma (95% confidence level) that is inferred to be representative of these conditions. The age of garnet growth calls into question the previous tectonic interpretation of the Abitibi and Pontiac Subprovinces and is coherent with the new rafted ribbon continent model of Bedard (2018) . Because the Canadian Malartic gold mineralization is older or synchronous with the garnet growth episode in the staurolite zone south of the ore body, this garnet age also provides an estimate of the minimum age of the mineralization event, previously dated at 2664 ± 11 Ma with Re-Os on molybdenite.

The metal-rich Vazante-Paracatu Mineral Belt, in central Brazil, hosts the Zn-Pb sulfide Morro Agudo District in the Mesoproterozoic (1.3–1.1 Ga) upper carbonate sequence of the Vazante Group. The Morro Agudo district is comprised of the Morro Agudo deposit and the Bento Carmelo, Sucuri, and Morro do Capão occurrences. This carbonate sequence also hosts the Fagundes, Ambrósia and Bonsucesso Zn-Pb sulfide deposits (northern part) and the zinc silicate Vazante and North Extension deposits (southern part). The structurally controlled, stratabound and stratiform styles of mineralization in the Morro Agudo orebodies have been variably classified as sedimentary exhalative, Irish-type and Mississippi Valley-type. In this study, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) spot analyses of sphalerite and galena from the Morro Agudo district revealed that red sphalerite (interpreted as the last stage) has higher Fe and Mn and lower Bi, Co, Cu, Ge, Hg, Tl compared to the other types of sphalerite, whereas the first generation of galena (Gn-I) is enriched in Ag, Cd, and Se and depleted in Cu and Mn relative to later galena (Gn-II). Mineral paragenesis and principal component analysis (PCA) of ore mineral composition suggest that the Morro Agudo, Sucuri and Morro do Capão mineralized zones formed by similar processes involving Zn-Pb mineralizing fluids with various compositions, comprising two main elemental associations: (1) Fe, As, In, Mn, Sb, Ag; and (2) Cd, Bi, Co, Ga, and Se. Bento Carmelo is distinguished in PCA by its unique dolomite-hosted sphalerite composition with elevated concentrations of Cu, Ge, Hg and likely formed from distinct fluids or processes. Temperatures of the mineralizing fluids for the Morro Agudo district range from 82 to 320 °C, calculated based on the trace element composition of sphalerite. The styles of mineralization and ore compositions are consistent with MVT deposits; however, fluid temperatures are hotter than typical MVT mineralizing fluids and may reflect a higher geothermal gradient or active advective fluid flow during the Brasiliano orogeny.

International audience; The world-class, oxidized intrusion-related Canadian Malartic gold deposit, with reserves estimated at 5.56 Moz Au grading 1.10 g/t Au, and a total geological endowment of 16.3 Moz Au, is one of the largest gold deposits in the Archean Superior Province of Canada. The gold mineralization is hosted predominantly by Pontiac Group metasedimentary rocks, Piché Group metavolcanic rocks, and quartz monzodiorite to granodiorite porphyritic intrusions. The ore takes the form of a low-grade envelope of disseminated pyrite (0.35 to 1 g/t Au) grading inwards into higher grade (>1 g/t Au) stockwork and breccia zones. Hydrothermal alteration in the metase-dimentary rocks is zonally distributed around the fluid pathways. Proximal alteration is characterized by a microcline±albite-quartz replacement-type assemblage, with lesser phlogopite, calcite±Fe-dolomite, pyrite and rutile. The distal alteration assemblage comprises biotite, microcline±albite, phengite, quartz, calcite, pyrite and rutile. In this study, we assess the magnitude and distribution of fluid-rock interaction in the metasedimentary rocks of the Malartic district. The metaturbidites are separated into four lithotypes based on grain-size to reduce the effects of primary depositional processes on mass change calculations. Despite the variability in protolith composition, the metasedimentary rocks define a geochemically consistent, cogenetic sequence. The results of the mass transfer calculations indicate progressive gains in CO 2 , S, K 2 O and LOI, as well as Au, Te, W, Ag, As, Be, Sb, Bi, Mo and Pb, from background, to distal and proximal alteration zones (relative to the least-altered samples). Molar element ratio analysis (alkali/aluminum) indicates an increase in alkali metasomatism (K and Na) adjacent to the main hydrothermal fluid pathways, which is manifested by the progressive stabilization of microcline and albite at the expense of oligoclase, biotite and white mica. Ore-associated pathfinder elements delineate broad enrichment patterns around the deposit, and are used to understand hydrothermal fluid circulation in the Malartic district. A statistical approach based on a comparison of the mass change results with the background composition provides robust constraints on the magnitude and extent of the lithogeochemical haloes. Generally, the alteration forms envelopes that extend along the S 2 fabric, with the largest lithogeochemical anomalies (e.g., Au, W, Te and Ag) reaching up to 10 km in length, and 2 km in width. The results of this study demonstrate that whole-rock lithogeochemistry can provide a valuable tool with which to define vectors toward gold mineralization in a regional exploration context.

The Malartic gold camp is located in the southern part of the Archean Superior Province and straddles the Larder Lake-Cadillac fault zone that is between the Abitibi and Pontiac subprovinces. It comprises the world-class Canadian Malartic deposit (25.91 Moz, including past production, reserves, and resources), and smaller gold deposits located along faults and shear zones in volcanic and metasedimentary rocks of the Abitibi subprovince. North of the Larder Lake-Cadillac fault zone, the Malartic camp includes 2714 to 2697 Ma volcanic rocks and ≤2687 Ma turbiditic sedimentary rocks overlain by ≤2679 to 2669 Ma polymictic conglomerate and sandstone of the Timiskaming Group. South of the fault, the Pontiac subprovince comprises ≤2685 Ma turbiditic graywacke and mudstone, and minor ultramafic to mafic volcanic rocks and iron formations of the Pontiac Group. These supracrustal rocks were metamorphosed at peak greenschist to lower amphibolite facies conditions at ~2660 to 2658 Ma, during D2 compressive deformation, and are cut by a variety of postvolcanic intrusions ranging from ~2695 to 2640 Ma. The Canadian Malartic deposit encompasses several past underground operations and is currently mined as a low-grade, open-pit operation that accounts for about 80% of the past production and reserves in the camp. It dominantly consists of disseminated-stockwork replacement-style mineralization in greenschist facies sedimentary rocks of the Pontiac Group. The mineralized zones are spatially associated with the Sladen fault and ~2678 Ma subalkaline to alkaline porphyritic quartz monzodiorite and granodiorite. Field relationships and isotopic age data for ore-related vein minerals indicate that gold mineralization in the Canadian Malartic deposit occurred at ~2665 to 2660 Ma and was contemporaneous with syn- to late-D2 peak metamorphism. The smaller deposits in the camp include auriferous disseminated-stockwork zones of the Camflo deposit (1.9 Moz) and quartz ± carbonate-pyrite veins and breccias (0.6 Moz) along faults in chemically and mechanically favorable rocks. The age of these deposits is poorly constrained, but ~2692 Ma postmineral dikes, and ~2625 Ma hydrothermal titanite and rutile from the Camflo deposit highlight a long and complex hydrothermal history. Crosscutting relationships and regional geochronological constraints suggest that an early episode of pre-Timiskaming mineralization occurred at >2692 Ma, shortly after the end of volcanism in the Malartic camp, and postmetamorphic fluid circulation may have contributed to concentration or remobilization of gold until ~2625 Ma. However, the bulk of the gold was concentrated in the Canadian Malartic deposit during the main phase of compressive deformation and peak regional metamorphism.

The Proterozoic Vazante-Paracatu mineral belt hosts distinct styles of mineralization in carbonate rocks, including world-class hypogene silicate-zinc deposits in the south and sulfide Zn-Pb deposits in the north. This study focuses on the Morro Agudo District (Morro Agudo deposit and Bento Carmelo, Sucuri, and Morro do Capao occurrences). The genesis of the Morro Agudo sulfide deposit is controversial (i.e., syn-diagenetic or syn-orogenic), and little was known about the other occurrences. Ore is stratigraphically controlled within distinct dolomitic units of the Morro do Calcario and Lapa formations and structurally controlled at the Morro Agudo deposit along a NW normal fault system. Mineralization is characterized by multiple generations of sphalerite and galena, coeval with hydrothermal Fe-rich dolomite, pyrite, and quartz, which overprint micritic and sparitic dolomite and diagenetic pyrite. The mineralized zones comprises Cd and Hg within the high-grade Zn-Pb ore and Ag, As, Cu, Fe, In, Se, Sb, and Tl extending into the hydrothermal halos surrounding the orebodies. The sphalerite compositions encompass variable proportions of Zn (58.75-66.92 wt. %), Fe (0.15-5.03 wt. %), and Cd (0.18-0.94 wt. %) grouped in five populations based on Cd-Fe ratios and principal component analysis. Sphalerite from various generations and locations in the orebodies and occurrences do not show any specific signature, suggesting episodic contribution of multiple fluids with distinct Fe and Cd contents. The whole rock lithogeochemistry revealed similar trace element signatures for the orebodies of Morro Agudo deposit and surrounding occurrences indicating, despite the textural differences, that these orebodies formed from similar mineralizing fluids and processes, except for the Bento Carmelo occurrence. This study shows clear evidence that the mineralization at the Morro Agudo deposit, Sucuri and Morro do Capao occurrences is epigenetic, forming after the diagenesis of the host carbonate succession, and may have involved progressive mixing of multiple fluids with distinct compositions that interacted with sulfur reservoirs in the dolomitic rocks, similar to other MVT districts worldwide. The Morro Agudo district is one of the rare examples of Proterozoic MVT districts, and this study brings new insights to the exploration of this type of mineralization in similar basins.

Ore-forming hydrothermal fluids react differently with different country rocks, but few studies have applied this knowledge with the specific goal of expanding the size of hydrothermal footprints in mineral exploration. To develop this concept, 122 metamorphosed mafic dykes from the world-class Canadian Malartic gold district (18.6 Moz Au) were sampled and analyzed for mineralogy, physical properties, and lithogeochemistry (partial and total digestion). The mafic dykes intrude mainly metasedimentary rocks, post-date D1 deformation, and cross-cut early-D2 quartz monzodiorite intrusions, but they were deformed and altered during D2 deformation, mineralization, and metamorphism. They can be subdivided into three groups: Group 1 dykes are least-altered, characterized by amphibole-rich regional metamorphic assemblages and distributed throughout the Pontiac Subprovince. Group 2 and 3 dykes underwent ore-related hydrothermal biotite–calcite–pyrite alteration and are associated with density- and volume-adjusted concentration gains of over 100% in Au–W–Te–C–S–Ag–Cs–Mo–Cu–K–Rb–Se–U–Pb–Ba–F–Bi–Sn. They define a metasomatic footprint up to 6 km away from the deposit toward the SE and up to 2 km away from the deposit toward the SW. Fifty-eight variables that define halos around the Canadian Malartic deposit have been identified and integrated using principal component analysis. PC1 explains 30% of the variance, separates least-mobile elements from ore-related elements, and is interpreted to reflect the ore-forming alteration process. PC2 and PC3 represent igneous processes. PC4 highlights the calcite–pyrite and biotite alteration. Spatial variations of the modal abundances of amphibole, biotite, calcite, and pyrite are the simplest expression of the metasomatic footprint, and they can easily be documented during exploration at the camp scale.

The Canadian Malartic deposit is a world class intrusion-related Archean gold deposit hosted in the Pontiac Subprovince, Superior Province, in Quebec, Canada. Laboratory petrophysical properties measurements were performed on 824 rock samples collected from the various rock types observed within the ore body and peripheral host rocks. The various rock types present in the Malartic District, mainly meta-sedimentary rocks, felsic-intermediate intrusive rocks and mafic dykes have contrasting grain densities and magnetic susceptibilities. Using support vector machines, it is shown that these two physical properties can be used to predict the rock type of a sample with an average precision and recall rate of 89%. Within the meta-sedimentary rocks class, variations in magnetic susceptibility are due to the changes in mineralogy associated with hydrothermal alteration. These are caused by the destruction of iron-bearing silicate minerals and magnetite in unaltered rocks (10−4 to 10−3 SI) to form pyrite, carbonates, K-feldspar and Fe-depleted hydrothermal biotite in altered rocks (10−5 to 10−4 SI). Within the felsic-intermediate intrusive rocks, grain densities below 2.7 g/cm3 and magnetic susceptibilities in the 10−6 to 10−4 SI range yield the highest probabilities that a rock has been submitted to carbonate and pyrite alteration. However, magnetic susceptibility and grain density of these intrusive rocks are also dependent on their Fe2O3/Al2O3 and TiO2/Al2O3 ratios, which are not related to the hydrothermal alteration footprint, but are rather due to distinct protolith compositions. Within the mafic dykes, grain density is the best indicator of hydrothermal alteration. Unaltered mafic dykes (2.95 to 3.10 g/cm3) are mostly composed of amphibole, whereas altered mafic dykes (2.70 to 2.95 g/cm3) have reduced amphibole contents and higher abundances of carbonates, pyrite, quartz, and biotite alteration. The support vector machine classifier is extended to predict if meta-sedimentary rocks, felsic-intermediate intrusive rocks and mafic dykes have undergone hydrothermal alteration with average F1 scores of 73%, 69% and 93%, respectively. In altered meta-sedimentary rocks, the integration of grain density and magnetic susceptibility allows the identification of altered but unmineralized samples. The classifier is further extended to predict if the gold content of meta-sedimentary rocks is above or below threshold values of 0.01, 0.1 and 1 ppm with average F1 scores of 83%, 80%, and 76%, respectively. Using conceptual models of the rock physical properties at the Malartic District scale, it is shown that ground magnetic surveys are the most promising geophysical tool for early-stage greenfield exploration of this type of deposit. However, depending on the scale at which the surveys are conducted, magnetic susceptibility contrasts between the various investigated rock types can overshadow the specific signatures of hydrothermally altered rocks. This in part explains why past airborne geophysical exploration campaigns for this type of deposit in the Malartic District were inconclusive. Finally, the machine learning process used in this case study can be applied in advanced exploration stages, during which drilling and either subsequent laboratory petrophysical analyses of core samples or downhole geophysical surveys produce large amounts of data that can be used to train prediction models.

The Proterozoic Vazante Group carbonate rocks were submitted to multiple stages of fluid circulation from diagenesis to orogenesis as documented by detailed mineralogical, fluid inclusion and isotopic studies of the Upper Morro do Pinheiro and Lower Pamplona members from the Serra do Poco Verde Formation in the southern part of the paleo-basin. These units are the main hosts for the hypogene, structurally-controlled zinc silicate deposits in the Vazante Zinc District, including the Vazante mine, which is considered to be the largest willemitic (Zn 2 SiO 4 ) deposit in the world, with estimated total resources of 40–60 Mt at 20% Zn. Five hydrothermal alteration types were identified in the Southern Extension of the Vazante Group. (I) Early stage alteration comprises dolomite substitution bands and nodules, associated with moderate salinity H 2 O-NaCl-CaCl 2 fluids, with temperatures around 90 °C, interpreted as late-diagenetic. The calculated C and O isotopic compositions of the fluids suggest meteoric and/or connate origin and interaction with organic carbon. (II) Pre-ore stage alteration is evidenced by dog-tooth dolomite and quartz with minor Fe-oxi/hydroxides which fills dissolution voids formed by H 2 O-NaCl-CaCl 2 ± MgCl 2 fluids at temperatures around 100° to 150 °C. The C isotopic data also indicate interaction with organic carbon. The two first stages are poor in ore-related elements. (III) Ore stage encompasses four phases of mineral precipitation. The first is distal and characterized by red stained dolostones due to disseminated hematite and red dolomite. The second is the main phase of the ore stage, composed of massive red dolomite, massive hematite and willemite with enrichment mainly in Fe 2 O 3 , SiO 2 , Ag, As, Cd, Cu, Ni, Pb, Se and Zn. The third phase comprises white dolomite, hematite and traces of willemite with enrichment in MnO, Cd, Ni and Pb. The calculated C and O isotopic compositions of the fluids (at 180 °C) associated with pervasive alteration yielded values heavier than the diagenetic stage and lower than the host rock. The fourth phase corresponds to Zn-chlorite and quartz which are associated with H 2 O-NaCl fluids of variable salinities and distinct temperatures (90–140 °C and 170–190 °C). (IV) Pyrite-bearing vein characterized by pyrite, sphalerite, white dolomite, fine hematite and late quartz, with C and O isotopic and fluid inclusion compositions that are similar to the three phases of dolomite of the ore stage. These data suggest a co-genetic relationship of these two alteration styles. (V) The late sulfide stage characterized by rare galena-bearing stringers with sphalerite, chalcocite, greenockite, covellite and white dolomite that cross-cut the main ore stage phases. The ore-related fluid compositions associated with the silicate zinc mineralization in the southern part of the Vazante Group are also similar to the fluids reported in previous work for the sulfide zinc-lead deposits in the northern part, indicating favorable conditions for metal transport during the Brasiliano Orogeny. Sulfide ore would have precipitated in zones where sulfur was available and silicate zinc in structures where mineralizing fluids interacted with evolved meteoric water. This finding implies that carbonate sequences in other districts with sulfide lead-zinc deposits may also host hypogene silicate zinc deposits.

The Northern Ultramafic Centre (NUC) of the Lac des Iles Complex, Northwest Ontario hosts several platinum group element (PGE) occurrences, including the Sutcliffe Zone, which consists of four subparallel, stratiform PGE-enriched intervals exposed within the cyclically layered eastern flank of the NUC. Field relationships, mineral paragenesis and lithogeochemistry allowed for the identification of 14 cyclic cumulate sequences of two distinct types – Cyclic unit type A (CUA) and Cyclic unit type B (CUB). CUA-type and CUB-type units are interpreted to have formed from a Si-enriched and Si-poor parent magmas, respectively. PGE-enriched intervals occur in four of the CUA-type cyclic units (CUA-5, -6, -8 and -11). PGE enriched intervals are commonly associated with websterite, olivine websterite and gabbronorite containing primary disseminated sulfide (0.2–2 vol%) which are dominated by pyrrhotite, chalcopyrite, and pentlandite with minor cubanite, and troilite. In hydrothermally altered rocks enriched in PGE, primary sulfides are locally partially replaced by secondary chalcopyrite, sphalerite, heazlewoodite, and chalcocite. Palladium occurs either in solid solution with primary pentlandite or is associated with platinum group minerals (PGM) such as Pd-plumbide, Pd-telluride, and Pt-bismuthotelluride. PGMs commonly occur within primary sulfides, at contacts between primary sulfide–silicate minerals, or in association with secondary serpentine and actinolite. Gold and silver typically occur as electrum that exhibits similar textural characteristics and mineralogical associations as the PGMs. Two different chemostratigraphic patterns of PGE, Cu and S enrichment can be recognized among the mineralized CUA cycles: The first (top-loaded) occurs near the top of CUA cycles (CUA-6, -8 and -11) in websterite and/or gabbronorite, just below the levels at which CUB magmas were emplaced. The second (middle-loaded), occurs midway through the lower cycle (CUA-5) in the olivine websterite, which is overlain by CUA-6. Within the four mineralized intervals, PGE tenors average 643 ppm Pd + Pt (in 100% sulfide), Pd/Pt and Pd/Ir ratios range from 0.9 to 3.5 and 35 to 537, respectively, and S/Se ratios range between 500 and 6000. The highest PGE tenors (4377 ppm Pd + Pt) are found in the lowermost interval in serpentinized olivine websterite and have an average Pd/Pt ratio of 3.5 and a S/Se ratio of approximately 2000. It is proposed that orthomagmatic processes of fractional crystallization and dynamic magma recharge were the dominant mineralization processes triggering sulfide-saturation and PGE concentration at the Sutcliffe Zone. Textural relationships between PGM, sulfide minerals, and primary and secondary hydrous silicates suggest that late magmatic to postcumulus hydrothermal fluid infiltration occurred locally during and after sulfide mineralization of the PGE-enriched intervals. However, these fluids had a minimal effect on the distribution of PGE in the Sutcliffe Zone. The Sutcliffe Zone shares many similarities with classic stratiform PGE deposits in terms of Pd/Pt ratio, high PGE tenors, low abundance of sulfide, and PGM assemblages. However, it is distinguished from most stratiform PGE deposits by its tectonic environment and lithostratigraphic position and by the intimate spatial association of the two parental magmas that are interpreted to have been responsible for the observed chemostratigraphy and PGE enrichment.

Spectral induced polarization (SIP) measurements are now widely used to infer mineralogical or hydrogeological properties from the low-frequency electrical properties of the subsurface in both mineral exploration and environmental sciences. We present an open-source program that performs fast multi-model inversion of laboratory complex resistivity measurements using Markov-chain Monte Carlo simulation. Using this stochastic method, SIP parameters and their uncertainties may be obtained from the Cole-Cole and Dias models, or from the Debye and Warburg decomposition approaches. The program is tested on synthetic and laboratory data to show that the posterior distribution of a multiple Cole-Cole model is multimodal in particular cases. The Warburg and Debye decomposition approaches yield unique solutions in all cases. It is shown that an adaptive Metropolis algorithm performs faster and is less dependent on the initial parameter values than the Metropolis-Hastings step method when inverting SIP data through the decomposition schemes. There are no advantages in using an adaptive step method for well-defined Cole-Cole inversion. Finally, the influence of measurement noise on the recovered relaxation time distribution is explored. We provide the geophysics community with a open-source platform that can serve as a base for further developments in stochastic SIP data inversion and that may be used to perform parameter analysis with various SIP models.

The structural setting of the Pontiac Subprovince in the vicinity of the world-class Canadian Malartic gold deposit has been revisited by combining and reinterpreting airborne geophysical surveys together with a century of structural observations. Felsic-intermediate intrusive bodies are a key component of this deposit. Defining the regional and local favorable structural setting for intrusive rock emplacement within the clastic meta-sedimentary rocks of the Pontiac Group, south of the Cadillac Larder Lake Deformation Zone, may further assist gold exploration in similar tectonic environment. Three structural domains are interpreted in the area based on the geometry of the bedding, folds and structural fabrics related to the three major phases of deformation. During these events four phases of magmatism and one metamorphic episode occurred. The North domain, which hosts the Canadian Malartic deposit, is characterized by highly variable bedding orientations produced by the interference patterns of isoclinal F1 folds overprinted by open to tight, steeply dipping, F2 folds. The bedding in this domain is cut by a penetrative S2 biotite foliation, which possibly built on rheological changes enhanced by metasomatism in the footprint of the Canadian Malartic deposit. By contrast, the Central and South domains display homogeneous bedding orientations cross-cut by a discrete S2 biotite foliation and syn- to late- D2 metamorphic porphyroblasts. In all domains the L2 stretching lineation consistently plunges at about 60 degrees toward the east. D3 is a minor deformation event in the Pontiac Subprovince, which possibly correlates with late dextral transcurrent movement along the Cadillac Larder Lake Deformation Zone. Intrusive bodies were emplaced throughout the first and second deformation events. Phase 1 monzonite, quartz-monzodiorite and granodiorite plutons (ca. 2683–2680 Ma) intruded into consolidated Pontiac sedimentary rocks during D1. Phase 2 quartz-monzodiorite bodies (ca. 2679–2676 Ma) predominantly formed in the North Domain along F1 fold axial surfaces prior to or at the onset of D2. Phase 3 basic dykes (ca. 2675–2673 Ma) cross-cut earlier felsic-intermediate intrusions across all domains and subsequently developed an S2 metamorphic hornblende foliation. Phase 4 magmatism (ca. 2672–2662 Ma) is related to the Decelles Batholith S-type granite and pegmatite, which is interpreted to be contemporaneous with the peak of regional metamorphism. The Decelles Batholith may project at depth underneath the Canadian Malartic deposit and could be associated with magmatic-hydrothermal mineralizing fluids in the Canadian Malartic deposit. Gold mineralization at Canadian Malartic is spatially located on the contact of Phase 2 quartz-monzodiorite bodies. The proximity to the Cadillac Larder Lake Deformation Zone combined with the rheological contrast between steeply dipping clastic meta-sedimentary rocks and quartz-monzodiorite intrusions favored the protracted failure of the contacts between these two rock masses, thus forming favorable conduits for hydrothermal fluids (e.g., the Sladen Fault Zone). Such specific rheological behavior is demonstrated by domains of structural complexity, emphasized by the variance of the bedding dip. These structurally complex zones systematically host gold mineralization in the Canadian Malartic district of the Pontiac Subprovince proximal to the Cadillac Larder Lake Deformation Zone.

Lithologic, mineralogical, and geochemical attributes of cyclic units are presented for the Northern Ultramafic Center (NUC) of the Lac des Iles Complex, Northwest Ontario. Detailed mapping and core logging show that the NUC is comprised of the Eastern Marginal Zone and the Layered Series. Fourteen cyclic units with two distinct paragenetic sequences of cumulus minerals are identified in the Layered Series. Cyclic Unit type A (CUA) has a cumulus paragenesis of olivine + chrome-spinel → olivine + orthopyroxene + clinopyroxene ± chrome-spinel → clinopyroxene + orthopyroxene → plagioclase + orthopyroxene ± clinopyroxene. Cyclic Unit type B (CUB) has a simpler paragenesis of olivine + chrome-spinel → olivine + clinopyroxene. Boundaries are sharp between similar cyclic units and where CUA overlies CUB. In contrast, where CUB overlies CUA, the cyclic unit contact is gradational and is marked by the development of a geochemical and mineralogical hybrid unit. Within individual cyclic units there are cryptic variations in mg# of pyroxenes (En) and olivine (Fo). Notably, olivines in CUB rocks have lower Ni concentrations despite higher Fo contents than in CUA rocks. Also, incompatible trace element ratios ( e.g ., Zr/Y and Ce/Yb) are distinct between the two types of cyclic units. These mineralogical and geochemical attributes are interpreted to indicate that the CUA and CUB units were generated by fractional crystallization of two compositionally distinct parental magmas. The CUA parental magma was more siliceous, had higher concentrations of Ni and incompatible trace elements, and had a lower Ce/Yb ratio relative to the CUB parental magma. In seeking out possible parent magma analogues for the CUA and CUB within the NUC, it was found that gabbroic rocks forming the Eastern Marginal Zone and hornblende gabbro dikes crosscutting parts of the Layered Series, respectively, have comparable geochemical attributes. The non-systematic sequencing of CUA and CUB units comprising the Layered Series is interpreted to have resulted from the random recharge of CUA and CUB parental magmas feeding the NUC. When similar magma types successively recharged the NUC chamber, or when a CUA magma recharged the chamber previously filled by a CUB, the recharging magma evidently had a higher density than the resident magma and flowed along the cumulus floor to produce sharp cyclic unit boundaries. When more siliceous CUA magma recharged the chamber previously filled by CUB magma, its lower density resulted in its pluming into the resident magma and producing a complex hybrid cyclic unit boundary.

The Vazante deposit, which is the world's largest zinc silicate deposit, occurs in brecciated dolomite and comprises mainly willemite with various proportions of hematite, Fe-carbonate, minor franklinite and magnetite. Exploration for this type of deposit is more challenging than zinc sulfide deposits, as they do not exhibit similar geophysical anomalies. To improve the application of geophysical surveys to the exploration of hypogene silicate zinc deposits, data from 475 samples were investigated from drill holes representative of the various types of ore and host rocks as well as barren zones of known geophysical anomalies in the Vazante District. Lithogeochemical and mineralogical (optical, SEM and MLA) data were integrated with physical rock properties (density, magnetic susceptibility and K U Th gamma-ray spectrometry) to assist in exploring for this type of deposit. The most distinct physical property of the ore is density, compared with the host rocks due to high proportion of denser minerals (hematite and willemite). However, barren hematite breccias also have high densities. The zinc ore and hematite breccias yielded higher magnetic susceptibilities than the surrounding host rocks, with the highest values associated with greater proportions of franklinite and magnetite. The density and magnetic susceptibility contrasts are a result of hydrothermal fluids interacting with and altering the carbonate host rocks. Zinc ore also yielded elevated U concentrations relative to the various host rocks, yielding higher gamma-ray spectrometric values. The results of this investigation indicate that an integration of magnetic, gravimetric and radiometric surveys would be required to identify zinc silicate ore zones.

The Vazante Group comprises a Proterozoic sequence of carbonate and siliciclastic rocks located in Minas Gerais, Brazil. It is the host of the Vazante-Paracatu Zinc District, including world-class hypogene zinc silicate deposits, in the southern part, and several Pb – Zn sulfide deposits in the northern part, all hosted in dolomitic rocks. A recent study revealed the occurrence of base metal sulfide mineralization that formed prior to the Brasiliano orogenic event in the siliciclastic rocks (Serra do Garrote Formation) that underlie these dolomite-hosted silicates and sulfide deposits. These siliciclastic rocks were considered as potential sources of elements for the hydrothermal fluids that formed the dolomite-hosted deposits, however there was little evidence of depletion of the source rocks during the orogenic event. In this paper, Random Forest is used in unsupervised mode along with t-distributed Stochastic Neighbor Embedding (t-SNE) and principal component analysis (PCA) over a lithogeochemical dataset of samples of the Serra do Garrote siliciclastic rocks collected throughout the basin to provide insights about processes related to the mineralizing system at the Vazante-Paracatu District. Multivariate analysis reveals that the Serra do Garrote Formation geochemical signature typical of pre-orogenic mineralization. This is characterized by PC1+ with association of Zn, Cd, Cu, Hg, In, V ± Sb, Se, Mo, Re, which corresponds to nearly 60% of the variance in the data. This suggest that these elements are related to a widespread and/or, long-living hydrothermal activity without an efficient mechanism to focus the metal-bearing fluids, causing basin-scale sub-economical Zn and related metal enrichment. Furthermore, PC2 and PC6 distinguish the zones with signatures interpreted to be related to depletion. PC2+ separates Cd-poor sphalerite, which is typical of the generation of sphalerite that occur in zones with textural evidence of remobilization. PC6+ with association of As, organic carbon and S, identified the zones where sphalerite was leached from the pyrite-rich layers. Areas with multivariate signatures of both pre-orogenic enrichment and syn-orogenic depletion in the source siliciclastic rocks include the Vazante-North Extension and Varginha zinc silicate deposits/occurrence, Ambrosia silicate zinc deposit and the Engenho Velho prospect. Preliminary exploration in this prospect revealed hydrothermal alteration typical of the zinc silicate deposits. This study is pioneering in applying these numerical models in possible source rocks to assist in identifying targets for exploration in basins.

The Proterozoic Vazante zinc silicate district in Minas Gerais, Brazil, hosts world-class hypogene willemite deposits in dolomitic rocks interbedded with siliciclastic rocks deposited in subtidal to supratidal environments. Willemite ore bodies are structurally controlled along regional NE-trending structures which are interpreted as being active during the Neoproterozoic Brasiliano orogeny. The hydrothermal alteration is characterized by an early stage of Fe-dolomite, which replaced the host dolomitic rocks, followed by precipitation of minor sphalerite and then hematite and willemite. Elements commonly enriched in the zinc ore include As, Ba, Be, Bi, Cd, Co, Fe, Ge, In, Mo, Ni, Sb, Se, U, V and W. Mineralogical, fluid inclusion and isotopic data indicates that mixing of S-poor metalliferous saline fluids with meteoric water favored the formation of willemite ore. Carbonaceous phyllites from the underlying thick siliciclastic sequence show evidence of early enrichment in zinc (and ore-related metals) and remobilization, respectively, prior to and during the Brasiliano orogenic event. This unit is interpreted as a possible source of ore-related elements. It is proposed that during the Brasiliano orogeny, hot (T > 170 °C) saline fluids (>15 wt % eq. NaCl) leached metals from siliciclastic source rocks and precipitated willemite ore in the overlying dolomitic sequence along structures that favored mixing with oxidizing meteoric water.

Studies of provenance and tectonic settings of Proterozoic sedimentary basins on the margins of the Sao Francisco Craton (SFC) can enhance understanding of its evolution. The Mesoproterozoic Upper Vazante Sequence is made up of siliciclastic and carbonate rocks (mainly dolomites) that were deposited on the southwestern margin of the SFC between 1300 and 1100 Ma and then thrust eastward on top of the SFC during the Neoproterozoic Brasiliano Orogeny (650–550 Ma). The Serra do Garrote Formation, stratigraphically below the main carbonate sequence, is made up of phyllites, carbonaceous phyllites (>1% TOC), as well as minor meta-arenitic rocks. Serra do Poco Verde Formation meta-siliciclastic rocks, which occur interbedded with dolomitic carbonates, are made up of phyllites, carbonaceous phyllites, dolomitic phyllites, carbonaceous dolomitic phyllites, minor meta-arenitic rocks and silty dolomites. Preserved sedimentary features in the meta-siliciclastic rocks include cross-bedding, ripples and mud drapes that are consistent with a shallow, tidally-influenced marine depositional setting. Three subunits (SG1, SG2 and SG3) were identified within the Serra do Garrote Formation based on molar Al/Ti ratios. Samples from the Serra do Poco Verde Formation have Al/Ti ratios similar to Serra do Garrote samples, indicating they shared similar sediment sources. Geochemical indicators of source rock composition and tectonic discrimination plots (Zr/Sc versus Th/Sc, Th – Sc – Zr/10, Hf versus La/Th, and rare earth element) revealed that andesitic to felsic continental arc rocks were the major source of detritus. The majority of concordant detrital zircons are Paleoproterozoic in age, with 207Pb/206Pb dates between 2350 and 1850 Ma, which overlap with the early and late stages of the Trans-Amazonian orogeny (≈2.3–1.9 Ga) and are coeval with construction of the Paleo-Mesoproterozoic Columbia Supercontinent. Detrital zircon provenance patterns suggest a rift-related tectonic setting during deposition. The youngest concordant U – Pb zircon 207Pb/206Pb dates (≈1340 Ma) are coeval with the formation of regional bimodal volcano – sedimentary sequences and layered that are interpreted to have formed during continental rifting. These events are synchronous with increased tectonic and magmatic activity (Mackenzie dyke swarm, North America; mafic dyke swarms, North China Craton) related to the break-up of the Columbia Supercontinent.

The Mesoproterozoic Upper Vazante Sequence is a mixed carbonate – siliciclastic marine succession, deposited between 1300 and 1100 Ma, which contains a variety of carbonate-hosted Zn silicate and Zn – Pb sulfide deposits in central Brazil. Carbonate-hosted Zn ± Pb mineralization is interpreted to have formed in two stages: one prior to the major deformation event and a second stage during the Brasiliano Orogeny (850 to 550 Ma). All known carbonate-hosted Zn-deposits and occurrences in the 250 km – long Vazante – Paracatu District are underlain by the Serra do Garrote Formation, a 500 + m-thick package made up mostly of phyllites and carbonaceous phyllites (>1% Total organic carbon) with minor interlayered meta-arenitic rocks. Our study revealed that siliciclastic rocks from the Serra do Garrote Formation contain horizons enriched in Zn (up to 0.46%). Siliciclastic-hosted zinc mineralization in the Serra do Garrote Formation is associated with pre-orogenic hydrothermal sulfide – quartz – chlorite layers and veinlets, which preferentially formed in carbonaceous and pyritic phyllites belonging to two subunits. The siliciclastic-hosted mineralization in these layers and veinlets is folded and crenulated (pre-orogenic). The geochemical signature of zinc-enrichment includes Cd, Cu, Hg, In, V ± Sb, Se, Mo, Re. Based on textural relationships and chemical U – Th – Total Pb dates of pre-orogenic and syn-orogenic monazite, sulfide mineralization in the Serra do Garrote Formation formed between 1200 and 650 Ma. Textural features indicate that base metals were locally remobilized within the Serra do Garrote Formation during the Brasiliano Orogeny, precipitating new generations of syn-orogenic sphalerite, chalcopyrite and pyrite. Mineralized zones in the Serra do Garrote Formation are distributed throughout the basin and have a similar geochemical signature as the overlying syn-orogenic carbonate-hosted base metal mineralization. However, there is no compelling evidence of depletion in the Serra do Garrote Formation, at the scale of this study, to suggest the ore-related metals have been leached from these siliciclastic rocks to be concentrated in the carbonate-hosted base metal deposits during the Brasiliano Orogeny. The findings of this study suggest that the Serra do Garrote Formation can be a potential target for siliciclastic-hosted base metal mineralization, similar to other Proterozoic sediment-hosted massive sulfide deposits.