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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.

The Mangabeira deposit is the only known Brazilian tin mineralization with indium. It is hosted in the Paleo- to Mesoproterozoic Mangabeira within-plate granitic massif, which has geochemical characteristics of NYF fertile granites. The granitic massif is hosted in Archean to Paleoproterozoic metasedimentary rocks (Ticunzal formation), Paleoproterozoic peraluminous granites (Aurumina suite) and a granite–gneiss complex. The mineralized area comprises evolved Li-siderophyllite granite, topaz–albite granite, Li–F-rich mica greisens and a quartz–topaz rock, similar to topazite. Two types of greisens are recognized in the mineralized area: zinnwaldite greisen and Li-rich muscovite greisen, formed by metasomatism of topaz–albite granite and Li-siderophyllite granite, respectively. Cassiterite occurs in the quartz–topaz rock and in the greisens. Indium minerals, such as roquesite (CuInS2), yanomamite (InAsO4·2H2O) and dzhalindite (In(OH3)), and In-rich cassiterite, sphalerite, stannite group minerals and scorodite are more abundant in the quartz–topaz rock, and are also recognized in albitized biotite granite and in Li-rich muscovite greisen. The host rocks and mineralized zones were subsequently overprinted by the Brasiliano orogenic event. Primary widespread two-phase aqueous and rare coeval aqueous-carbonic fluid inclusions are preserved in quartz from the topaz–albite granite, in quartz and topaz from the quartz–topaz rock and in cassiterite from the Li-rich muscovite greisen. Eutectic temperatures are − 25 °C to − 23 °C, allowing modeling of the aqueous fluids in the system H2O–NaCl(–KCl). Rare three-phase H2O–NaCl fluid inclusions (45–50 wt.% NaCl equiv.) are restricted to the topaz–albite granite. Salinities and homogenization temperatures of the aqueous and aqueous-carbonic fluid inclusions decrease from the topaz–albite granite (15–20 wt.% NaCl equiv.; 400 °C–450 °C), to the quartz–topaz rock (10–15 wt.% NaCl equiv.; 250 °C–350 °C) and to the greisen (0–5 wt.% NaCl equiv.; 200 °C–250 °C). Secondary fluid inclusions have the same range of salinities as the primary fluid inclusions, and homogenize between 150 and 210 °C. The estimated equilibrium temperatures based on δ18O of quartz–mica pairs are 610–680 °C for the topaz–albite granite and 285–370 °C for the Li-rich muscovite greisens. These data are coherent with measured fluid inclusion homogenization temperatures. Temperatures estimated using arsenopyrite geothermometry yield crystallization temperatures of 490–530 °C for the quartz–topaz rock and 415–505 °C for the zinnwaldite greisens. The fluids in equilibrium with the topaz–albite granite have calculated δ18O and δD values of 5.6–7.5‰ and − 67 to − 58‰, respectively. Estimated δ18O and δD values are mainly 4.8–7.9‰ and − 60 to − 30‰, respectively, for the fluids in equilibrium with the quartz–topaz rock and zinnwaldite greisen; and 3.4–3.9‰ and − 25 to − 17‰, respectively, for the Li-rich muscovite greisen fluid. δ34S data on arsenopyrite from the quartz–topaz rock vary from − 1.74 to − 0.74‰, consistent with a magmatic origin for the sulfur. The integration of fluid inclusion with oxygen isotopic data allows for estimation of the minimum crystallization pressure at ca. 770 bar for the host topaz–albite granite, which is consistent with its evolved signature. Based on petrological, fluid inclusion and isotope data it is proposed that the greisens and related Mangabeira Sn–In mineralization had a similar hydrothermal genesis, which involved exsolution of F-rich, Sn–In-bearing magmatic fluids from the topaz–albite granite, early formation of the quartz–topaz rock and zinnwaldite greisen, progressive cooling and Li-rich muscovite greisen formation due to interaction with meteoric water. The quartz–topaz rock is considered to have formed in the magmatic-hydrothermal transition. The mineralizing saline and CO2-bearing fluids are interpreted to be of magmatic origin, based on the isotopic data and paragenesis, which has been documented as characteristic of the tin mineralization genetically related to Proterozoic within-plate granitic magmatism in the Goias Tin Province, Central Brazil.

Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2012-02-29 16:45:36.41

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.

The Flin Flon greenstone belt is best known for its world-class volcanogenic massive sulfide deposits. However, the recent discovery of the McBratney platinum group element (PGE)-Au occurrence and other PGE occurrences has drawn attention to the potential for Ni-Cu-PGE (Au) deposits in this belt. The McBratney occurrence contains some of the highest PGE and Au grades found in this type of deposit in North America (up to 207 g/t Pd, 34 g/t Pt, 2.6 g/t Rh, and 75 g/t Au). It is hosted in chlorite-actinolite and chlorite schists of the Bear Lake magmatic unit in the Bear Lake block of the Flin Flon greenstone belt. The metamorphosed host rocks are komatiitic and tholeiitic in composition. The mineralization is hydrothermal and postdates the regional, retrograde metamorphic event. It occurs as sulfide-rich veins and surrounding disseminated zones controlled by faults, as well as isolated disseminated zones within chlorite-actinolite and chlorite schists. Platinum group minerals (PGM) and gold are spatially and temporally associated with pyrrhotite, chalcopyrite, carbonate, second generation chlorite, chamosite and, locally, stilpnomelane. In the disseminated zones, these phases replace the metamorphic actinolite-chlorite assemblage in both types of host rocks. Locally the veins are layered with pyrrhotite-rich zones and chalcopyrite-rich zones. The PGM include, in order of abundance, Te-rich sudburyite, borovskite, sperrylite, sudburyite, an unknown Pd-Te-Sb mineral, temagamite, and merenskyite, which occur mainly as inclusions in the sulfides. Minor amounts of PGE also occur within chlorite, carbonate, and stilpnomelane, and Au-Ag alloy is observed locally included in Fe sulfides in association with sphalerite. The mantle-normalized metal distribution shows that the McBratney mineralization is enriched in Bi, As, Au, Pd, Cu, Pt, Rh, Os, Ru, Zn, Re, and Ag; the mineralization is both enriched and depleted in Ir and Cr, and it is depleted in Ni in all analyzed samples when compared with typical komatiitic magmatic ore. The mineral assemblage suggests that the hydrothermal fluids were reduced (pyrrhotite-pyrite stable), neutral to alkaline, and CO 2 bearing (carbonate-chlorite stable). Chlorite geothemometry indicates that the hydrothermal assemblage formed at temperatures ranging from 250° to 350°C. Under these conditions, PGE may have been transported mainly as bisulfide complexes and their precipitation likely occurred due to reactions of the hydrothermal fluids with the Fe-rich host rocks, which led to the formation of Fe sulfides, reduction of the activity of the bisulfide, and formation of PGM. Palladium precipitated mainly as tellurides and antimonides; however, Pt formed an arsenide. The abundant As, Te, and Sb may have been crucial in forming high-grade PGE ore. Sulfur isotope data indicate that magmatic rocks or fluids were likely the sources of sulfur, which may include the metamorphosed basic-ultrabasic sequence, hidden magmatic Ni-Cu sulfide mineralization, or fluids derived directly from postmetamorphic magmatic rocks.

The Camaguey district, Cuba, is known for its epithermal precious metal deposits in a Cretaceous volcanic arc setting. Recently, the La Union prospect was discovered in the southern part of the district, containing gold and minor copper mineralization interpreted as porphyry type. Mineralization is hosted in a 73.0 ± 1.5 Ma calc–alkaline I-type oxidized porphyry quartz diorite intrusive within volcanic and volcaniclastic rocks of the early Cretaceous Guaimaro Formation. The porphyry is affected by propylitic alteration and crosscut by a network of quartz and carbonate veinlets and veins. Chlorite, epidote, sericite, quartz, and pyrite are the main minerals in the early veins which are cut by late carbonate and zeolite veins. Late barite pseudomorphously replaces pyrite. Gold is associated with pyrite as disseminations in the altered quartz diorite and in the veins, occurring as inclusions or filling fractures in pyrite with 4 g/t Au in bulk samples, and up to 900 ppm Au in in pyrite. Fluid inclusion and oxygen isotope data are consistent with a H2O–NaCl–(KCl) mineralizing fluid, derived from the quartz diorite magma, and trapped at least at 425°C and 1.2 kbar. This primary fluid unmixed into two fluid phases, a hypersaline aqueous fluid and a low-salinity vapor-rich fluid. Boiling during cooling may have played an important role in metal precipitation. Pyrite δ34S values for the La Union prospect range between 0.71‰ and 1.31‰, consistent with a homogeneous magmatic sulfur source. The fluids in equilibrium with the mineralized rocks have estimated δ18O values from 8‰ to 11.8‰, calculated for a temperature range of 480–505°C. The tectonic environment of the La Union prospect, its high gold and low copper contents, the physical–chemical characteristics of the mineralizing fluids and the isotopic signature of the alteration minerals and fluids indicate that the La Union gold mineralization is similar to the porphyry gold type, even though the ore-related epidote–chlorite alteration can be classified as propylitic and not the classic potassic and/or phyllic alteration. The low copper contents in the prospect could be due to a mineralizing fluid previously saturated in copper, which is indicated by trapped chalcopyrite crystals in high-temperature fluid inclusions. The low-temperature paragenesis, represented by carbonate, zeolite and barite, indicates epithermal overprint. The study shows the potential for other gold porphyry-type deposits in the Cretaceous volcanoplutonic arc of Cuba.

Sulfide minerals in the Otago and Alpine schists, New Zealand, a metasedimentary belt exposed from unmetamorphosed greywackes up to amphibolite facies, underwent systematic changes in abundance, composition and texture during prograde metamorphism. In unmetamorphosed rocks, the most common sulfide mineral is framboidal pyrite, which contains abundant As (up to 14000 ppm), Co (up to 4000 ppm), Cu (up to 14000 ppm), Ni (up to 1100 ppm) and, locally, Ag (up to 270 ppm), Au (up to 90 ppm), and Sb (up to 240 ppm). Chalcopyrite, sphalerite, and galena also occur as isolated grains. Chalcopyrite and sphalerite contain few trace elements, whereas galena contains significant Se (up to 1600 ppm) and locally abundant Hg (up to 600 ppm). The distribution of these trace and minor elements is extremely heterogeneous. In subgreenschist-facies rocks, pyrrhotite replaces pyrite, and there is a clear textural change from framboidal pyrite to composite grains of pyrrhotite, sphalerite, chalcopyrite, galena and cobaltite. Pyrrhotite contains Co (average values 1100 ± 490 ppm), Cu (up to 17000 ppm), and Ni (up to 11000 ppm). Antimony and Hg are above detection in rare individual grains, but none of the other trace and minor elements sought are detectable. Sphalerite, galena and cobaltite increase in proportion in subgreenschist-facies rocks, and also contain higher concentrations of Ag (up to 1480 ppm in galena), Au (up to 230 ppm in galena and 110 ppm in cobaltite), As (38 ± 6 wt. % in cobaltite), Co (26 ± 4 wt. % in cobaltite), Hg (up to 4500 ppm in galena and 1100 ppm in sphalerite), and Sb (up to 1280 ppm in cobaltite and 770 ppm in galena). Pyrite, sphalerite, galena, and cobaltite become less abundant from subgreenschist- to amphibolite-facies rocks. In amphibolite-facies rocks, only pyrrhotite, chalcopyrite and trace amounts of galena and molybdenite occur, and none of these minerals contain detectable levels of Ag, Au, As, or Hg. Mass-balance calculations between sulfide minerals and whole rocks show that sulfides are important host minerals for S, Cu, and As, but host a minor proportion of Fe, Zn, and Pb. As pyrite, sphalerite, galena, and cobaltite become less abundant at higher metamorphic grade, Pb, Zn and Co are retained in the rock incorporated in other minerals, whereas As, Sb, Hg, Au, and Ag are removed from the rock, most likely by metamorphic devolatilization.

The Athabasca Basin hosts many world-class unconformity-related uranium deposits. Recently, uranium reserves for the Eagle Point basement-hosted deposit have increased with the discovery of new mineralized zones within secondary structures. A paragenetic study of Eagle Point reveals the presence of three temporally distinct alteration stages: a pre-Athabasca alteration, a main alteration and mineralization comprised of three substages, and a post-main alteration and mineralization stage that culminated in remobilization of uraninite from primary to secondary structures. The pre-Athabasca alteration stage consists of minor amounts of clinochlore, followed by dolomite and calcite alteration in the hanging wall of major fault zones and kaolinitization of plagioclase and K-feldspar caused by surface weathering. The main alteration and uranium mineralization stage is related to three temporally distinct substages, all of which were produced by isotopically similar fluids. A major early alteration substage characterized by muscovite alteration and by precipitation Ca–Sr–LREE-rich aluminum phosphate-sulfate minerals, both from basinal fluids at temperatures around 240°C prior to 1,600 Ma. The mineralization substage involved uraninite and hematite precipitated in primary structures. The late alteration substage consists of dravite, uranophane-beta veins, calcite veins, and sudoite alteration from Mg–Ca-rich chemically modified basinal fluids with temperatures around 180°C. The post-main alteration and mineralization stage is characterized by remobilization of main stage uraninite from primary to secondary structures at a minimum age of ca. 535 Ma. U–Pb resetting events recorded on primary and remobilized uraninites are coincident with fluid flow induced by distal orogenies, remobilizing radiogenic Pb to a distance of at least 225 m above the mineralized zones.

This study was undertaken to characterize the mineral paragenesis and metal zoning at the property scale, evaluate the potential sources of ore-related metals, quantify the relationship between intensity of alteration and gold grade, and propose a comprehensive genetic model for the Carlin-type Au deposits at the southern part of the Goldstrike property, Nevada. Mineralogy, textural relationships, whole-rock composition, and spatial distribution of the studied samples revealed two types of gold ore: Ore I and II. The former, which is hosted by the Roberts Mountains and Rodeo Creek Formations, and the Wispy, Planar, and Upper Mud units of the Popovich Formation, is the most abundant and widespread in the property. Ore I is characterized by intense hydrothermal alteration (e.g., carbonate dissolution, silicification, and precipitation of pyrite) and high amounts of trace elements (e.g., Ag, As, Au, Ba, Cd, Cu, Hg, Mo, Ni, S, Sb, Se, Te, Tl, and Zn). On the other hand, Ore II, which is hosted in the Wispy, Planar, and Soft Sediment Deformation units of the Popovich Formation, is mainly confined to the central-north-northwest portion of the Screamer deposit and is weakly altered with low concentration of trace elements. Both Ores I and II contain similar average concentrations of Au in whole rock (14 and 19 g/t Au, respectively) and in pyrite (290 and 540 ppm, respectively); however, auriferous pyrite from Ore I has higher trace element (As, Ag, Cu, Hg, Ni, Sb, Se, and Tl)/Au ratios than Ore II. The sedimentary units are interpreted to be the major local source of Cd, Mo, Ni, U, V, and Zn and minor As, Cu, Hg, and Se as denoted by the composition of least altered samples and diagenetic pyrite and sphalerite. this study reveals that Al2O3 and TiO2 are the most immobile compounds, and their distribution indicates a homogeneous source for the detrital components in the sedimentary rocks. Among the ore-related trace elements, Tl best correlates with Au grade ( R 2 = 0.69) and shows some relationship with the calculated amount of pyrite ( R 2 = 0.49), indicating that Tl would be the best element to vector toward zones of high-grade Carlin-type Au mineralization. Gold grades do not correlate with the amount of pyrite, degree of alteration, or organic C. Our results integrated with available thermodynamic data for AU, ore-related elements, and SiO2 lead us to suggest that the formation of Ore I occurred more proximal to the major mineralizing conduits as the hot, more acid, SiO2- and trace element-rich auriferous fluids interacted with Fe-bearing impure carbonate host rocks, intensely dissolving the carbonate rocks and precipitating quartz and auriferous pyrite in the Betze-Post and Rodeo deposits. As the fluids moved laterally throughout the favorable host rocks, the pH increased, leading to a decreasing in the rate of carbonate dissolution and in the solubility of silica, favoring the formation of more distal Ore II in the central-northern part of the Screamer deposit. Significantly, the gold concentrations in whole rock and in pyrite are, in some way, very similar in both ore types, being slightly higher in Ore II, suggesting that less acidic conditions were still favorable for the incorporation of gold in the structure of pyrite, even at lower concentrations of other trace elements.

The Fortaleza de Minas Ni–Cu–PGE sulfide deposit is hosted by Archean komatiitic rocks of the Morro do Ferro greenstone belt, near the southwestern margin of the Sao Francisco craton, Minas Gerais state, Brazil. The deposit contains 6 million tonnes of ore with an average grade of 2.2 wt% Ni, 0.4% Cu, 0.05% Co and 1.2 ppm PGE+Au, and comprises (i) a main orebody, which is metamorphosed, deformed and transposed along a regional shear zone, consisting mainly of disseminated, brecciated and stringer sulfide ores that are interpreted to be of early magmatic origin, and (ii) PGE-rich discordant veins that are hosted in N–S- and NE–SW-trending late faults that cross-cut the main orebody. The discordant PGE-rich ore (up to 4 ppm total PGE) is characterized by thin, discontinuous and irregular veins and lenses of massive sulfides hosted by serpentinite and talc schist, and is relatively undeformed if compared with the early types of ore. It is composed mainly of pyrrhotite, pentlandite, chalcopyrite, magnetite, carbonates, and amphiboles, with minor cobaltite–gersdorffite, sphalerite, ilmenite, and quartz, and rarely maucherite (Ni11As8), tellurides and platinum-group minerals (PGM). Omeeite, irarsite, sperrylite, and Ni-bearing merenskyite are the main PGM, followed by minor amounts of testibiopalladite and an unknown phase containing Ru, Te, and As. The PGM occur either included in, or at the margins of, sulfides, sulfarsenides, silicates and oxides, or filling fractures in pyrrhotite, pentlandite, and chalcopyrite, suggesting that they started to precipitate with these minerals and continued to precipitate after the sulfides were formed. The mantle-normalized metal distribution of the two samples of discordant veins shows distinct patterns: one richer in Ni–Pd–Ir–Rh–Ru–Os and another with higher amounts of Cu–Pt–Bi. Both are strongly depleted in Cr if compared with the metamorphosed magmatic ore of this deposit, which follows the general Kambalda-type magmatic trend. On the basis of structural, mineralogical and geochemical evidence, we propose that the PGE-rich discordant ore may have formed by remobilization of metals from the deformed, metamorphosed magmatic orebody (which shows a depleted pattern in these elements) by reduced (pyrrhotite – pentlandite – pyrite are stable), neutral to alkaline and carbonic fluids (carbonate-stable). The PGE may have been transported as bisulfide complexes, and precipitated as tellurides (mainly Pd) and arsenides (Pt, Rh, Ru, Os, Ir) in the late N–S and NE–SW-trending faults owing to a decrease in the activity of S caused by the precipitation of sulfides in the veins.

The Siscoe mine was one of the richest in the Val d’Or mining camp, producing 27.5 tonnes (t) of gold and 9.5 t of silver from 1929 to 1949. The C quartz-tourmaline vein at the Siscoe mine, which is the youngest auriferous vein on the property, is an excellent example of a high-grade Archean lode gold deposit; it contains an average of 45 g/t Au and locally up to 221 g/t Au. This vein cuts all dikes, the main regional foliation (S2), and several auriferous quartz-carbonate veins, and is interpreted to be a shear structure that formed during the development of a late- to post-D2 reverse fault. The vein comprises mainly alternating tourmaline- and quartz-rich layers, and gold occurs commonly in late fractures cutting the vein. The visible wall-rock alteration is characterized by the occurrence of tourmaline, pyrite, and calcite. In this study, we report the Pb isotope compositions of hydrothermal pyrite separates taken from the C quartz-tourmaline vein and its wall rock. Pyrite is abundant in the wall rock, where it occurs as grains with inclusions of chalcopyrite, gold, tetradymite, pyrrhotite, rutile, silicates, and carbonates. In the vein, it forms euhedra generally devoid of inclusions. Locally, the euhedra are fractured, and the fractures are filled with native gold, calcite, and tetradymite. Textural relationships suggest that pyrite precipitated synchronously with and/or after gold in the wall rock, but predated gold in the vein. Pyrite separates from the vein and its wall rock yielded similar 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios (13.63–13.83, 14.58–14.62, and 33.42–33.76, respectively), which are more radiogenic than those reported for volcanic supracrustal and syntectonic plutonic rocks of the southern Abitibi subprovince, but are very similar to those of the late- to posttectonic S-type granites in the Pontiac subprovince and Lacorne block. This suggests that the Pb originated from a reservoir similar to that of the magmas which produced these granites, or alternatively, that it was leached from the latter. The Pb ratios yield a model age of 2.57 ± 0.07 Ga, which is interpreted to date the cooling of the hydrothermal system that introduced and/or remobilized gold into the C quartz-tourmaline vein. These results, when considered in the context of the evolution of the southern Abitibi, indicate that widespread hydrothermal circulation occurred between 2.64 and 2.55 Ga, after the collision of the Pontiac and Abitibi subprovinces, and was caused by dewatering and S-type magmatism in the thickened unstable continental crust. This long-lived hydrothermal system promoted remobilization of gold from earlier veins and possibly introduction of gold from deeper zones, enhancing gold resources in the Siscoe deposit.

The Otago and Alpine schists of the South Island of New Zealand form a young ( A large suite of samples representative of the lithologic and metamorphic variation in the Otago and Alpine schists was collected and analyzed for a comprehensive suite of elements. The aim was to identify any rock type or metamorphic setting that may be depleted in the suite of ore-forming elements (Au, Ag, As, Sb, Hg, Mo, and W) relative to unmetamorphosed protoliths, perhaps representing the source for the enrichments observed in the Otago ore deposits. Gold, Ag, As, Sb, Hg, Mo, and W were found to have significantly lower concentrations in higher grade metamorphic rocks compared to unmetamorphosed protolith samples. These were the only elements in a suite of 12 major and 50 trace elements to show systematic depletions with metamorphic grade. Investigation of the trace element chemistry of sulfide minerals indicates that the whole-rock depletions are caused by the disappearance between greenschist and amphibolite facies conditions of pyrite, galena, sphalerite, and cobaltite, the major host phases for the ore-forming elements. More than 95 percent of upper greenschist and amphibolite facies samples are significantly depleted in the ore-forming elements. Such regional-scale depletions require pervasive, grain-boundary fluid flow throughout these rocks. The leaching is most likely to have been caused by metamorphic fluid produced by dehydration reactions at the greenschist-amphibolite boundary. The suite of elements depleted in mid- to high-grade Otago and Alpine schists is almost identical to those enriched in the orogenic gold deposits in Otago. Furthermore, the vertical zonation in depletions is similar to the vertical zonation in enrichments that occurs in the Otago deposits. Mass-balance calculations suggest that 2 metric tons (t) Au and 24,000 t As was leached from 1 km3 of amphibolite facies rock and that the Macraes deposit could have been formed by leaching of a 5- × 5- × 5-km cube of amphibolite facies rock. We propose that the orogenic gold deposits in Otago, such as Macraes, were formed directly from metal-rich metamorphic fluid produced during prograde metamorphism at depth. The contribution of other fluid and metal sources in the formation of these deposits, such as magmatic fluids, cannot be ruled out, but there is no direct evidence to support their involvement. Infiltration of meteoric water, such as occurs currently in the Southern Alps of New Zealand may have contributed to the formation of the late-stage deposits that formed at shallow level during uplift of the Otago schists.

The Serrinha gold deposit in the Juruena-Teles Pires gold province is spatially and genetically related to the hydrothermal alteration of the I-type calc-alkaline 1872 ± 12 Ma Matupa monzogranite in the south-central Amazonian craton. The mineralized areas are characterized by intense hydrothermal alteration of the monzogranite, comprising incipient autometasomatism, K silicate, sodic, Mn chlorite, phyllic, carbonate, and microcline alteration, with partial overprinting of the early alteration by later stages. A distal barren propylitic zone is interpreted from drill core. The gold mineralization is disseminated in the most altered samples and genetically related to Mn chlorite, K silicate, and phyllic hydrothermal alteration types. Hydrothermal magnetite and rutile are ubiquitous within pyrite. The early Serrinha gold mineralization stage is characterized by Ag-poor gold included in pyrite (Au/Ag ~7‐15), associated with pyrrhotite, cubanite, and galena. The fluids interpreted to be associated with this early auriferous stage were found only in quartz from the Mn chlorite alteration and are typically saline (45‐57 wt % NaCl equiv) and high-temperature (375° and 480°C) H2O-NaCl-KCl fluids entrapped at pressures of at least 1.3 kbars. The late-stage gold mineralization, which is coeval with the phyllic alteration, occurs either as isolated gold grains or as fracture-fillings and/or inclusions within pyrite associated with tetradymite, galena, tsumoite (BiTe), hessite (Ag2Te), and aikinite (PbCuBiS3). Gold grains associated with this late stage are richer in Ag (Au/Ag ~2‐5). Data from chlorite geothermometry and aqueous-carbonic and saline (NaCl-KCl) fluid inclusions associated with this second auriferous event indicate P-T conditions of 1.5 to 2.4 kbars and 293° to 365°C. The occurrence of lower temperature coeval aqueous, aqueous-carbonic, and carbonic fluid inclusions with no postentrapment modification in the K silicate assemblage, partially overprinted by phyllic alteration and in phyllic assemblages, suggests that fluid immiscibility and/or mixing with meteoric water occurred in the hydrothermal system. H2O-NaCl-CaCl2 fluids contemporaneous with carbonate and late microcline alteration, which postdated gold mineralization, were entrapped at lower temperatures (172°‐200°C). Calculated δ18O and δD values are 8.2 and ‐37 per mil, respectively, for the early saline fluids in equilibrium with the Mn chlorite assemblage and 1.7 to 4.7 and ‐20 to ‐15 per mil for the late fluids in equilibrium with the phyllic assemblage. These data in conjunction with fluid inclusion results suggest that early fluids exsolved from granitic melts and later mixed with meteoric water. Gold is interpreted to have been initially transported from the crystallizing magma as chlorine complexes in a hot, saline, acidic, and oxidized fluid. Decrease in temperature during fluid ascent, immiscibility, or pH increase is interpreted to have caused gold precipitation. Subsequent dilution of the saline fluid could have been responsible for the deposition of late gold in pyrite fractures. Based on field, petrological, mineralogical, fluid inclusion, and isotopic evidence, we propose that Serrinha is a typical proximal intrusion-related gold deposit, similar to porphyry-style gold deposits.

The Sigma-Lamaque mine (>100 metric tons (t) of gold) is an example of an Archean greenstone belt gold deposit formed by processes related to multistage hydrothermal fluid circulation. The earliest recognized vein systems are steeply to moderately dipping fault-fill veins within shear zones and contemporaneous subhorizontal veins, which were overprinted by the late North Dipper vein system. The latter strikes east-west, dips moderately to the north, and exhibits complex internal geometry with features similar to fault-fill and extensional veins, and therefore is interpreted as extensional shear veins. This late vein system was considered one of the major sources of gold in the shallow underground and open-pit ore reserve estimations. These late North Dipper veins were studied in detail in the Sigma pit where they contain ore-grade gold and are hosted by the calc-alkaline tuffs of the Val d’Or Formation and in the North zone where they are barren and hosted by the tholeiitic volcanic rocks of the Jacola Formation. In the Sigma pit, these veins have apparent thicknesses of 0.8 to 3 m and very irregular contacts with the vein walls. They comprise irregular zones of milky quartz filling, disrupted blocks of layered quartz and tourmaline and slabs of foliated pyrite-rich wall rock. Layers with tourmaline and quartz crystals oriented perpendicular to the vein walls are observed close to the vein walls. The veins contain minor amounts of calcite, pyrite, rutile, and traces of chlorite, muscovite, pyrrhotite, chalcopyrite, galena, bismuth tellurides, and gold. Locally, the sulfide content can attain 10 modal percent of the vein mineralogy. In the veins, native gold (with 6 wt % Ag) occurs rarely associated with the main vein-filling minerals, commonly as inclusions in pyrite (main vein-filling stage), and most commonly filling fractures in pyrite, tourmaline, and quartz, where it is alloyed with 8 to 17 wt percent Ag and is associated with calcite and Bi tellurides (late auriferous stage). The proximal wall-rock alteration related to the auriferous North Dipper veins overprints the metamorphic assemblage and is characterized by abundant fine-grained plagioclase, calcite, quartz and minor chlorite, tourmaline, and pyrite in the tuffs and more abundant muscovite in the intrusive feldspar porphyry. In the wall rock, gold with 6 to 28 wt percent Ag is found locally filling fractures in pyrite, where it is associated with a late generation of calcite. The barren North Dipper vein also comprises mainly quartz and tourmaline but lacks calcite and gold, has a very low content of sulfides ( Aqueous-carbonic fluid inclusions with salinities less than 8 wt percent (NaCl equiv) and homogenization temperatures between 228° and 440°C, and carbonic fluid inclusions with less than 0.2 mol percent CH 4 coexisting in the same healed fractures, are common in the auriferous North Dipper veins but very rare in the barren vein where aqueous fluid inclusions have lower temperatures (dominantly 125°–225°C). Crush leaching and laser ablation time-of-flight inductively coupled mass spectrometry (LA-TOF-ICPMS) of fluids in the inclusions indicate elevated concentrations of Ni, Cu, Sb, Pb, and Ag in the fluids in the auriferous veins compared to the aqueous fluids in the barren veins. This suggests different sources for the metals in the fluids associated with the gold, possibly including both igneous and sedimentary rocks. Calculated oxygen isotope equilibration temperatures for quartz-tourmaline pairs from the auriferous main vein-filling stage are 323° to 370°C. This is within the range documented in fluid inclusions related to the late auriferous stage, indicating that the temperature of the auriferous hydrothermal fluids did not change appreciably during the evolution of the system. Calculated δ 18 O H 2 O values for the mineralizing fluids are 9.5 per mil, consistent with either low water/rock ratio systems associated with high-grade metamorphism or a magmatic origin for the fluids. Gold included in pyrite may have precipitated during the main vein-filling stage due to the decrease of H 2 Sconcentration in the hydrothermal fluid following the deposition of sulfides. However, gold in fractures in the main vein-filling minerals precipitated later, due probably to dilution of the auriferous aqueous-carbonic fluids by mixing with aqueous fluids. The absence of ore-grade gold in the barren North zone vein is probably due to the lack of circulation of auriferous aqueous-carbonic fluids in the North zone, located distal to the first-order Larder-Lake Cadillac fault, which may have served as the main regional conduit for the auriferous aqueous carbonic fluids.

L'indice mineralise en Au et en elements du groupe du platine de McBratney (jusqu'a 31 g/t Pd et 9 g/t Pt) est situe dans des roches mafiques et ultramafiques brechifiees de l'unite mafique de Bear Lake,pres des massifs intrusifs de pyroxenite et de gabbro de la ceinture de roches vertes proterozoiques de Flin Flon, au Manitoba. A l'indice McBratney, des veines de sulfures massifs contiennent des proportions variables de quartz, carbonate, clinochlore, chamosite, biotite, muscovite, et une variete de mineraux du groupe du platine. Les sulfures principaux sont chalcopyrite, pyrrhotite, et pyrite, avec violarite, pentlandite, pyrite nickelifere, marcasite, sphalerite, galene, millerite et cobaltite-gersdorffite. Magnetite, coloradoite, et un alliage Au-Ag sont d'importance locale. Les mineraux du groupe du platine comprennent, en ordre d'importance, borovskite [(Pd 2 . 6 Ni 0 . 3 5 Fe 0 . 1 2 ) Σ 3 . 0 7 (Sb 1 . 0 6 Βi 0 . 0 6 ) Σ 1 . 1 2 Te 3 . 8 1 )], sudburyite tellurifere [(Pd 0 . 9 7 Ni 0 . 0 1 Fe 0 . 0 2 Hg 0 . 0 1 ) Σ 1 . 0 1 (Sb 0 . 7 8 Te 0 . 2 1 ) Σ 0 . 9 9 ], un tellurure-antimoniure de Pd meconnu [(Pd 1 . 7 2 Ni 0 . 2 3 Fe 0 . 0 7 ) Σ 2 . 0 2 (Te 1 . 9 4 Sb 1 As 0 . 0 2 )Σ 2 . 9 4 ], sudburyite [(Pd 0 . 9 4 Ni 0 . 0 1 Fe 0 . 0 1 Hg 0 . 0 3 ) Σ 0 . 9 9 (Sb 0 . 9 5 Te 0 . 0 6 ) Σ 1 . 0 1 ] sperrylite [(Pt 0 . 9 4 Ag 0 . 0 1 Ni 0 . 0 1 Fe 0 . 0 9 )Σ 1 . 0 5 As 1 . 9 5 ], temagamite [(Pd 2 . 8 4 Ni 0 . 1 5 Fe 0 . 1 2 ) Σ 3 . 1 1 Hg 0 . 9 3 (Te 2 . 6 0 Sb 0 . 3 5 ) Σ 2 . 9 5 ] et merenskyite [(Pd 0 . 9 7 Ni 0 . 0 1 Co 0 . 0 1 Fe 0 . 0 7 ) Σ 1 . 0 6 (Te 1 . 8 5 Sb 0 . 0 8 Bi 0 . 0 1 ) Σ 1 . 9 4 ]. On les trouve generalement en inclusions dans la chalcopyrite, la pyrite et le carbonate, et ils sont partiellement remplaces par cobaltite-gersdorffite. D'apres les assemblages de mineraux et les textures, la mineralisation serait hydrothermale, et tardive par rapport a l'evenement de metamorphisme regional. Les mineraux du groupe du platine ont ete formes en meme temps que leurs hotes, les sulfures Ni-Fe-Cu, chlorite-chamosite, biotite, carbonate et quartz a une temperature inferieure a 500°-550°C.

The Miocene, epithermal, Ag-rich polymetallic Santo Toribio deposit is hosted by the volcanics of the Quiruvilva-Pierina subbelt, Northern Peru, which also comprises the world-class, high sulphidation Pierina deposit. The Ag-rich, alabandite-bearing veins of the Santo Toribio deposit formed during two major stages. The early stage is characterized by deposition of arsenopyrite, pyrite, quartz, Mn- sphalerite, stannite, alabandite and minor miargyrite. Sphalerite associated with this stage is exceptionally enriched in Mn (up to 14.5 wt.%) and alabandite is optically and mineralogically zoned. Its brown zones have greater Fe+Sb and smaller Mn contents than the green zones and Fe+Sb replaces Mn in its structure. During this early stage, fs2 must have been high to allow the stabilization of alabandite relative to rhodochrosite. In the second stage, the physicochemical conditions changed and the CO2/S ratio increased, causing dissolution of alabandite and the deposition of abundant rhodochrosite and a second generation of arsenopyrite, pyrite and quartz, sphalerite with chalcopyrite inclusions, mirargyrite, ramdohrite, and finally stibnite. This polymetallic ore probably formed due to an abrupt decrease in H+ and/or Cl- concentration caused by boiling or dilution of the high-salinity hydrothermal fluids and constitutes an example of “intermediate sulfidation-state” epithermal deposits.

The C quartz-tourmaline vein of the Siscoe deposit is a classic example of a high-grade Archean gold lode and averages 45 g/t Au, locally attaining 221 g/t Au. This vein cuts all dikes, the main regional foliation (S2) and several auriferous quartz-carbonate veins and is interpreted to be a shear vein, which formed during the development of a late to post-D2 reverse fault. The vein is composed of alternating tourmaline- and quartz-rich layers with minor amounts of rutile, scheelite, apatite, chlorite, muscovite, carbonate (mainly calcite), pyrite, chalcopyrite, tetradymite, and native gold. Vein filling occurred during the development of a reverse fault through a process involving repeated episodes of opening, mineral deposition, and plastic and brittle deformation. The vein geometry was controlled by local rheologic heterogeneities or by a west-northwest–east-southeast compressive event. The wall-rock alteration is characterized by the occurrence of tourmaline, pyrite, and calcite. The timing of gold deposition and the gold fineness in the C quartz-tourmaline vein differ from those of the wall rock. In the vein, native gold (4.7–7.3 wt % Ag) was the latest phase to deposit and commonly occurs in fractures that cut across the quartz and tourmaline layers; whereas in the wall rock, gold (7.9–13.2 wt % Ag) started to precipitate just before or synchronously with pyrite. Consequently, the textural and mineral chemical evidence suggests that the fluids in the vein and wall rock were not in equilibrium with each other during gold deposition and that the mechanisms of precious metal precipitation were different in the two environments. In the wall rock, gold started to precipitate synchronously with sulfides due to reaction of hydrothermal fluids with magmatic ilmenite. This reaction caused the destabilization of gold bisulfide complexes and prompted gold deposition; however, sulfides are very rare in the C quartz-tourmaline vein, and gold deposited after the main episode of vein filling, suggesting that its precipitation was not controlled by pyrite deposition. The occurrence of aqueous-carbonic (AC) fluid inclusions in healed fractures that radiate from gold grains suggests that these fluids were associated with the ore-forming process. Two types of aqueous-carbonic fluid inclusions were identified: H2O-NaCl-CO2-bearing (salinities between 4 and 7 wt % NaCl equiv) and H2O-NaCl-CO2-CH4-bearing (salinities varying between 7 and 9 wt % NaCl equiv). Both types homogenize at T > 190°C. Based on structural, textural, and fluid inclusion evidence, we propose that gold deposition in the C quartz-tourmaline vein took place after the main episode of vein filling as a result of continued fracturing and mixing of these two fluids.

The Conceicao mine, which is located in the Itabira district, southern Sao Francisco craton, is a rare example of gold and palladium mineralization hosted by jacutinga, an altered Lake Superior-type iron-formation. The mine sequence consists of three metamorphosed lithostratigraphic units, which are from the oldest to the youngest: (1) an Archean volcano-sedimentary sequence, (2) a Paleoproterozoic iron-formation, and (3) a Paleoproterozoic quartzite. These units are cut by mafic intrusive rocks and, with the exception of intrusive contacts, all contacts between the various units are structural and interpreted as thrust faults. The rocks were subjected to three phases of folding: (1) D1, characterized by tight asymmetric and isoclinal folds, which develop locally into sheath folds (the D1 fabrics are the S1 foliation and elongation lineation); (2) D2, represented by tight, large-scale folds and associated parasitic folds; and (3) D3, characterized by open folds with an associated crenulation cleavage. The D1 and D2 deformation are interpreted to be products of simple shear with a transport direction consistently east over west. These rocks were metamorphosed to typical mineralogy of amphibolite facies conditions during the D1 phase of folding. The Conceicao orebody consists of a single quartz vein surrounded by jacutinga. The vein is parallel to the S1 foliation, which is locally oriented west-southwest and dips 55° to 75° NW, and was folded by D2 deformation. It extends 100 m downdip in the direction of the Le elongation lineation and 400 m along strike. The jacutinga consists of centimeter-scale layers of muscovite, specular hematite (hematite I), and octahedral hematite grains (hematite II) with remnants of magnetite, and a finely laminated envelope around the quartz vein, which is composed mainly of Fe hydroxides, hematite (hematite III), kaolinite, muscovite, and tourmaline. The quartz vein is dismembered and quartz grains are partially corroded and filled by Fe hydroxides and kaolinite. The ore minerals, interpreted to have formed during the main hydrothermal event, are palladian gold (up to 3.2 wt % Pd) and an unknown Pd-Sb-As mineral (tentatively identified as mertierite II), which are typically included in hematite II octahedra. A few grains of palladian gold (up to 0.9 wt % Pd) were found mantling hematite II grains. Native gold is commonly associated with Fe hydroxide in the finely laminated alteration envelope around the quartz vein and is interpreted to have precipitated during weathering. The similarities between the Au-Pd orebodies at the Conceicao and Caue deposits (referred to as "jacutinga-type" deposits) suggest that they were formed by the same processes and their characteristics favor an epigenetic model for gold and palladium concentration in the iron-formation. Since the Cretaceous, these Archean and Proterozoic rocks have been weathered, and the hydrothermal silicates and oxides have been replaced by kaolinite and Fe(Mn) hydroxides; gold and palladium have been remobilized and reprecipitated as pure metals or Pd oxides.

Breccia-hosted fluorite-bastnaesite deposits associated with epizonal quartz syenite intrusions in the Gallinas Mountains, New Mexico, provide an important example of hydrothermal rare earth element (REE) mineralization, and an excellent opportunity to investigate the transport and deposition of the REE under hydrothermal conditions. Fluid inclusion studies show that mineralization commenced with the deposition of quartz at a temperature of approximately 400°C from sulfate-rich NaCl-KCl brines having a salinity of ~15 wt percent NaCl equiv. These fluids are interpreted to have been of orthomagmatic origin. Fluorite and bastnaesite deposition occurred at slightly lower temperatures, and coincided with the introduction of separate CO2-bearing and sulfate-poor NaCl brines into the system. These fluids are considered to have been of external origin, the latter being a formation water that circulated in surrounding Permian sandstones and limestones. Evaluation of the available thermodynamic data on the aqueous speciation of REE, and the preliminary data on the chemistry of the fluids obtained in this study, suggest that the REE were dissolved primarily as fluoride complexes. A model is proposed in which the REE were transported by the orthomagmatic fluids, and deposited as bastnaesite owing to destabilization of REE-fluoride complexes. The latter occurred in response to the sharp reduction in F– activity that accompanied large-scale deposition of fluorite as a result of mixing of the orthomagmatic fluids and formation waters. This mixing also led to the deposition of anhydrite and barite owing to the interaction of externally introduced calcium and barium ions with orthomagmatic sulfate ions. The later stages in the evolution of the Gallinas Mountains hydrothermal system were marked by increased incursion of external fluids, and the alteration of bastnaesite to Ca bastnaesite and minor deposition of Ca bastnaesite and parisite owing to lowered F–/\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(CO_{3}^{2{\mbox{--}}}\) \end{document} ratios and/or increased Ca2+ activity. The study suggests that Ca-free, REE fluoride-bearing fluids are an important ingredient for REE mineralization, that mixing of such fluids with Ca-, carbonate-bearing fluids or interaction with carbonate rocks is a major control of REE-fluorocarbonate mineral deposition, and that the occurrence of fluorite could provide an important guide in the exploration for economic REE deposits.

Pb isotopic results are reported for mineral separates from iron formations that host Pd-Au mineralization at the Caue Mine, in the Itabira District of the Southern Sao Francisco Craton, as well as for an amphibolite that has been tectonically imbricated with the iron formations during the earliest D1 tectonic event in this area. Pb isotopic results were obtained for hornblende and ilmenite that were rotated within the regional S1 foliation of an amphibolite. These minerals yielded a metamorphic age of 2.66 ± 0.27 Ga and a time-integrated μ1 value of 8.35. This shows, for the first time, the involvement of late Archean to Paleoproterozoic basement rocks in the tectonic history of the Itabira District; this unit is tentatively correlated with the Archean Rio das Velhas Supergroup in the Quadrilatero Ferrifero. Pb isotopic results were also obtained for quartz, gold and specular hematite that were formed during the transformation of the iron formations into itabirite and jacutinga which occurred during the D1 event. These minerals, which are stretched within the S1 foliation plane, yielded an age 1.83 ± 0.10 Ga which corresponds to the Tranzamazonian Orogenesis in the southern part of the Sao Francisco Craton. Other Pd-bearing gold deposits hosted in Minas Supergroup iron-formations of the southern Sao Francisco Craton have many similarities with the Caue mine, suggesting that Pd-Au mineralization may have been a widespread process associated with the tectonic transport of the Minas sequence during the Transamazonian orogenesis.

Palladium-bearing minerals from the Cauê iron mine, Itabira District, Minas Gerais, Brazil, are found in gold-rich jacutinga, a hydrothermally-altered Lake Superior-type carbonate-bearing oxide facies iron-formation. Palladium occurs as: native palladium with trace contents of Au, Fe and Cu; palladseite ((Pd,Cu,Hg)17Se15), which was found in the core of a grain of palladium; palladium–copper oxide ((Pd,Cu)O); and arsenopalladinite (Pd8(As,Sb)3), with inclusions of palladium–copper oxide. The palladium and palladium–copper oxide grains are coated with films of gold and commonly do not exceed 100 µm in width. These palladium minerals occur in hematite bands and in boudinaged bands of quartz and white phyllosilicate parallel to the S1 mylonitic foliation. Palladium-copper oxide also occurs as inclusions in gold grains which are strongly to weakly stretched parallel to S1.Palladium mineralization is interpreted as synchronous with intense D1 shearing and contemporaneous with the peak of thermal metamorphism. At high oxygen fugacities and high temperatures (up to 600°C), Pd may have been transported as chloride complexes and deposited following changes in pH caused by mineralizing fluids reacting with jacutinga. Deposition may also have been prompted by the formation of insoluble selenide and arsenide–antimonide minerals and by the dilution of C1 concentrations in the mineralizing fluid. Textural studies, and the zonation observed in palladium and other hydrothermal minerals, suggest that oscillations in the physico-chemical conditions of hydrothermal fluids occurred during the mineralizing event.

The Caue iron mine is located in the Itabira district in the southern Sao Francisco craton. Three geologic units metamorphosed to amphibolite facies have been identified: (1) an Archean volcano- sedimentary sequence; (2) a Proterozoic iron-formation unit; and (3) a Proterozoic quartzite unit. The iron-formation unit is cut by amphibolite dikes and tectonically imbricated with talc schists and amphibolites of the volcano-sedimentary sequence. All the rocks were affected by three phases of folding and associated thrusting and boudinage. In the iron-formation unit, the D 1 and D 2 structures have been generated by a progressive simple shear regime with a transport direction consistently east-over-west. D 3 structures are characterized by open folds with an associated crenulation cleavage. The Caue palladium-bearing deposit comprises five orebodies hosted by jacutinga in the iron-formation unit. Jacutinga is a highly sheared, metamorphosed, hydrothermally altered Lake Superior-type iron-formation composed of hematite, quartz, talc, phlogopite, with minor amounts of tourmaline, apatite, and monazite. In the Corpo Y orebody, the highest gold concentrations occur in quartz veins, whereas in the Corpo X, Central, Aba Leste, and Aba Notre orebodies, the highest gold grades occur in hematite veins with local concentrations of quartz. The gold-rich veins are parallel to S 1 foliation in all orebodies except Aba Norte. In the Aba Norte, the gold-rich boudinaged hematite vein is parallel to S 2 . The ore minerals are palladian gold, gold, palladium, palladium oxide, and palladseite. The main mineralizating event was synchronous with the peak of thermal metamorphism (T = 600 degrees C) and with D 1 shearing and thrusting. The hydrothermal alteration associated with this event is characterized by the formation of tale, phlogopite, and tourmaline, and hematite and quartz veins, generating the jacutinga rock type. Weathering of the jacutinga resulted in the alteration of silicates to kaolin and oxides to goethite...

Palladian gold from the Cauê iron mine, Itabira District, Minas Gerais, Brazil is found in horizons of hydrothermally-altered Lake Superior-type oxide-facies iron-formation locally called jacutinga. Palladium content of the gold grains varies from 1% to 20%, high palladium values being associated with palladium-copper-oxide inclusions and showing island-mainland and replacement (relict) textures in the Corpo Y and Corpo X orebodies. In the Aba Leste orebody, palladium is homogeneously distributed throughout the gold grains.Palladium and gold mineralization was synchronous with intense D1 shearing. Palladium was deposited early during the generation of S1-mylonitic foliation, and was replaced by gold during progressive deformation.At high oxygen fugacities (hematite stability field) and high temperatures (up to 600°C) Pd and Au may have been transported as chloride complexes, and their deposition may have occurred mainly as a result of changes in pH, but dilution of Cl concentrations may have also caused Pd and Au precipitation. Deposition of Pd as selenide may have taken place early during the creation of S1-fabrics.

The Santa Rita gold deposit (Central Goias, Brazil) is hosted by Middle to Upper Proterozoic carbonate-pelite sequences of the Paranoa Group that have been metamorphosed in the greenschist facies. The ore is contained in pyrite-bearing quartz-carbonate veins. The mineralization is structurally controlled by WNW-ESE high-angle faults and fractures resulting from the reactivation of older NE-SW lineaments. Pyrite is the sole sulphide and it shows growth zones enriched in Co, Ni and As (up to 4 wt%). Hydrothermal alteration zones are enriched in Co, Ni and As and are characterized by diffuse albitization, carbonatization, silicification and pyritization. A fluid inclusion study on quartz from pyrite-bearing quartz-carbonate veins led to the identification of two fluids: (1) a highly saline CO 2 N 2 -rich aqueous fluid with halite and ± sylvite daughter minerals, and (2) a CO 2 N 2 rich aqueous fluid with moderate salinity. The two fluid types occur in the same quartz domain and display great variation in the degree of filling and notable dispersion of the microthermometric data. On heating, all the inclusions decrepitate between 200° and 300°C. Raman spectrometry detected high concentrations of N 2 in the gas phase, with a CO 2 N 2 molar ratio between 1 and 19 and a small proportion of CH 4 (up to 2 mole %). The simultaneous entrapment of compositionally variable fluids in the system H 2 OCO 2 N 2 NaClKCl allow us to propose a mechanism of heterogeneous trapping. The entrapment may result from the mixing of a high-salinity fluid (H 2 ONaClKCl system) with a carbonic fluid (H 2 OCO 2 N 2 system) produced by the devolatilization process of carbonate and phyllitic host rocks. Considering the absence of spatially and temporally related igneous activity and the low P - T regional metamorphism in the Paranoa Group, the brines are inferred to result from leaching of evaporites occurring in the lower part of the Paranoa lithostratigraphic column. Gold was probably initially transported as an AuCl 2 -complex ( T >300°C, low pH, moderate ƒ O 2 - pyrite field stability). As temperature decreased below 290°C, the “switch-over” process would lead to the predominance of Au(HS) 2 − in the fluid. Pyrite precipitated in this temperature interval. The oscillatory zoning of the AsCoNi-bearing pyrites indicates episodic fluctuation of the fluid composition. Such changes in fluid composition are favoured by a mechanism of fluid mixing by intermittent supplies in the hydrothermal system. The proposed mechanism of heterogeneous trapping of two separate fluids in the system H 2 OCO 2 N 2 NaClKCl and the resulting changes in the physicochemical conditions caused by the fluid mixing appears as a conspicuous process for the Santa Rita hydrothermal fluid evolution. A model based on the existence of a Proterozoic geothermal system involving the regional thermal gradient is proposed.

The Millennium uranium basement-hosted unconformity-related deposit is one of the most recent discoveries of basement-hosted deposits, and may hold as much as 46.8 million pounds (18,000 metric tonnes U) at an average grade of 4.53 percent U3O8. The alteration zone associated with the deposit is atypical, as it consists predominantly of muscovite rather than the chlorite and muscovite commonly found in the other basement-hosted deposits of the Athabasca basin. The preore stage consists of early minor clinochlore alteration at temperatures of about 275°C, interpreted to result from retrograde metamorphism of the basement rocks prior to the deposition of the Athabasca basin. This clinochlore event is followed by an extensive 1Mc muscovite alteration of the basement rocks by Na-K-Fe-rich basinal brines at temperatures of about 250°C, creating up to 25 percent void space in the basement rocks. Subsequently, a weak chamoisite alteration resulted from Fe-Mg-Ca-rich fluids with temperatures of about 300°C that evolved from reactions between the basinal brines and the basement rocks. Ore stage uraninite has an extrapolated chemical age of 1590 Ma and is contemporaneous with synore 1Mc muscovite and iron oxide deposited at temperatures near 250°C from fluids with similar δ 18O but lower δ D values to preore basinal fluids. The low δ D values indicate postdepositional exchange of H isotopes with Cenozoic fluids. Postore alteration consists of dravite and sudoite precipitated from Mg-Ca rich fluids, from the same evolved basinal brines that precipitated preore chamoisite, at temperatures of about 185°C, filling voids created during the preore stage. Our results indicate that the pre- and synore minerals were formed dominantly by basinal fluids, with no contribution from basement fluids, which differ from some other basement-hosted deposits in the Athabasca basin where basement-derived fluid influx was more significant. Several U-Pb and Ar-Ar resetting events recorded on preore muscovite and on uraninite are coincident with fluid flow induced by distal orogenies, remobilizing radiogenic Pb from the deposit. The radiogenic Pb remobilization events were not widespread as the presence of radiogenic Pb is restricted to basement rocks proximal to the deposit and along the unconformity.

Previous studies on Athabasca basin unconformity-related uranium deposits have focused on major deposits and have not investigated sites with barren alteration systems that could clarify some of the critical factors controlling mineralization processes. A paragenetic study of the Wheeler River area reveals the presence of minerals that formed during the diagenetic, the main hydrothermal, which is subdivided into early, mid-, and late hydrothermal substages, and the late alteration stages. The diagenetic stage consists of early quartz overgrowths, siderite, rutile, hematite, and abundant dickite in the pore spaces of the Manitou Falls Formation. The early hydrothermal alteration substage is characterized by pervasive 1Mc muscovite alteration and minor goyazite clusters, which formed from oxidizing basinal fluids at temperatures around 240°C prior to 1550 Ma, based on Ar-Ar dates. The mid-hydrothermal alteration substage comprises dravite and sudoite in the basal 200 m of the Manitou Falls Formation, which are interpreted to have formed at temperatures around 175°C from fluids chemically distinct but isotopically similar to the basinal fluids involved during the early hydrothermal alteration substage. The late hydrothermal substage was observed only at zone K of the Wheeler River area and is characterized by the precipitation of clinochlore, copper sulfides, and florencite from reducing basement fluids emerging into the Manitou Falls Formation at temperatures around 230°C, creating a ~250-m-high by ~250-m-wide reducing halo. Oxidized uranium-bearing basinal fluids interacted with the Manitou Falls Formation during the early hydrothermal substage prior to the arrival of the reducing fluids during the mid- and late hydrothermal substages and this precluded uranium precipitation. The post-hydrothermal alteration stage is characterized by formation of kaolinite after late hydrothermal clinochlore near fractures by meteoric waters. A minimal amount of leachable radiogenic Pb, with a Pb-Pb model age of 1907 Ma that is older age than both the Athabasca basin and the main mineralization event of 1590 Ma, was encountered at zone K, indicating low probability of this area to host uranium mineralization. However, areas of possible unconformity-related uranium deposits were identified outside zone K wherein significant amounts leachable radiogenic Pb were observed.