Tourmaline, scheelite, and magnetite compositions from orogenic gold deposits and glacial sediments of the Val-d’Or district (Québec, Canada) : applications to mineral exploration

Tourmaline, scheelite, and magnetite from orogenic gold deposits (n = 22) and glacial sediments (n = 5) of the Val-d'Or mining district (Québec, Canada) were investigated by Electron Probe Micro-Analyzer (EPMA) and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) in order to determine their chemical signature and to assess their potential as indicator minerals for gold exploration. Three types of tourmaline are recognized. Type I tourmaline from deposits hosted in felsic and intermediate-composition calc-alkaline rocks has low contents of V, Cr, Mn, Fe, Co, Ni, Zn, and Sn and a high content of Mg compared to Type II tourmaline from deposits hosted in mafic tholeiitic rocks. Type III tourmaline from deposits located at the contact between mafic metavolcanic and metasedimentary rocks shows a chemistry similar to Type I tourmaline with slightly higher Li, Mn, and Pb contents. Tourmaline from orogenic gold deposits is characterized by lower contents of Zn, Cu, Sn, and Pb compared to data from tourmaline associated with Cu-Zn, Pb-Zn-Cu, and Sn mineralization. Till tourmaline carries the chemical signature of that from orogenic gold deposits with a majority having the signature of Type I tourmaline. Orogenic gold deposits scheelite from the Val-d'Or district hosted in calc-alkaline intrusions of intermediate composition displays high Na, REE, and Y contents compared to scheelite from sediment- and mafic-hosted gold deposits. Till scheelite carries the chemical signature of that from orogenic gold deposits. Although rare in orogenic gold deposits of the Val-d'Or district, hydrothermal magnetite in the gold veins is characterized by higher Cr, Zn, Mn, K, Ca, Ti, and Al than magmatic magnetite found in dioritic and gabbroic host rocks. Magnetite associated with gold mineralization forms fine, disseminated grains, suggesting that the coarse-grained magnetite recovered in till does not originate from the gold veins. Till tourmaline and scheelite carry the signature of the orogenic gold deposits and can thus be used for exploration in overburden sediments

Multi-method 2D and 3D reconstruction of gold grains morphology in alluvial deposits : a review and application to the Rivière du Moulin (Québec, Canada)

The aim of this paper is to document and compare the 2D qualitative and semi-quantitative methods currently used to describe the shape of gold grains in fluvial environments with the 3D quantitative methods using X-ray microtomography and SEM photogrammetry. These 3D methods are used to compute flatness, roundness, convexity, sphericity and ellipticity shape descriptors of 13 gold grains from the Rivière du Moulin (Québec, Canada) in order to quantify the morphological change along 9 km of fluvial transport. Gold grains have moderate to high values of flatness, compactness, sphericity and ellipticity indices that do not change significantly with distance of transport, whereas the roundness increases during transport. Gold grains are used to compare 2D and 3D methods, and the results show small differences (<8%) when shape descriptors are computed using image analysis software, whereas the difference (up to 70%) is more important for 2D measurements performed by a human operator. For application and characterization on a large set of gold grains, the 2D methods offer the advantage of speed, whereas, for a more detailed study on a limited number of gold grains, 3D methods enable estimation of the volume and yield more detailed shape descriptor changes during fluvial transport

Quantification of the morphology of gold grains in 3D using X-ray microscopy and SEM photogrammetry

The shape of gold is widely used in mineral exploration and in sedimentology to estimate the distance of transport from the source to the site of deposition. However, estimation of the morphology is based on qualitative observations or on the quantification of shape in 2D. The 3D analysis of grain shape is useful for accurate morphometric quantification and to evaluate its volume, which is related to particle size. This study compares X-ray 3D microscope and 3D SEM photogrammetry to reconstruct the shape of gold particles. These new methods are exploited to quantify the shape of gold grains 85 to 300 μm in size. The shape parameters, such as axial lengths, surface area, volume, diameter of curvature of all corners, and diameter of the largest inscribed sphere and smallest circumscribed sphere are measured on a particle in order to estimate shape factors such as flatness ratios, shape indices, sphericity, and roundness. Most shape parameters and shape factors estimated on the same gold grain with simple geometry are similar between the two approaches. This result validates these methods for the 3D description of gold particles with simple morphology, while providing a methodology for describing grains with more complex geometry

Atmospheric carbon sequestration in ultramafic mining residues and impacts on leachate water chemistry at the Dumont Nickel Project, Quebec, Canada

Passive carbon mineralization in ultramafic mining residues, which allows the sequestration of CO2 through carbonate precipitation, is one of the options being considered to limit the accumulation of anthropogenic CO2 in the atmosphere. The Dumont Nickel Project (DNP) will generate approximately 1.7 Gt of utramafic mining residues over 33 years of production and the mine will release about 127,700 tonnes of CO2 each year. Using two experimental cells filled with ultramafic waste rock (EC-1) and milling residues (EC-2), the impacts of carbon mineralization on leachate water quality were studied and the quantity of sequestered carbon was estimated.Hydrotalcite supergroup minerals, aragonite, artinite, nesquehonite, dypingite and hydromagnesite precipitated through atmospheric weathering, while the inorganic carbon content of the weathered mining waste increased from 0.1 wt% to 4.0 wt% which indicate active CO2 sequestration. The leachate water, sampled at the bottom of the experimental cells, is characterized by an alkaline pH (~9.5), a high alkalinity (~90 to ~750 mg/L) and a high concentration of magnesium (~50 - ~750 mg/L), which is typical from weathering of ultramafic rocks in a system open to CO2. Since 2012, the chemical composition of the leachate water has evolved seasonally. These seasonal variations are best explained by: (1) climatic variations over the year and, (2) increased carbonate precipitation between May and July. Increased carbonate precipitation decreased the alkalinity and magnesium concentrations in the leachate water and produced pore waters which were undersaturated with respect to carbonate minerals such as artinite and hydromagnesite. *Revised manuscript with no changes marked Click here to view linked References Carbonate precipitation thus self-limits carbon sequestration through a negative feed-back loop. The carbon sequestration potential of the DNP residues is also influenced by the hydrogeological properties of the residues. In cell EC-2, a high liquid/solid ratio, which limits carbonate precipitation, was maintained by the hydrogeological properties. Since 2011, an estimate of 13 kg of atmospheric CO2 has been sequestered in the milling residues (EC-2), which corresponds to a mean rate of 1.4 (+/- 0.3) kgCO2/tonne/year. Using this mean rate, the 15 Mt of tailings produced each year, during the planned 33 years of mining operation, could potentially sequester 21,000 tonne of CO2 per year by passive carbon mineralization, about 16% of the 127,700 tonnes of CO2 annually emitted by the planned mining operation

Trace element composition of scheelite

Scheelite from 25 representative orogenic gold deposits from various geological settings was investigated by EPMA (electron probe micro-analyzer) and LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometer) to establish discriminant geochemical features to constrain indicator mineral surveys for gold exploration. Scheelite from orogenic gold deposits displays five REE patterns including a bell-shaped pattern with a (i) positive or (ii) negative Eu anomaly; (iii) a flat pattern with a positive Eu anomaly and, less commonly, (iv) a LREE-enriched pattern, and (v) a HREE-enriched pattern. The REE patterns are interpreted to reflect the source of the auriferous hydrothermal fluids and, perhaps, co-precipitating mineral phases. Scheelite from deposits formed in rocks metamorphosed at upper greenschist to lower amphibolite facies have low contents in REE, Y, and Sr, and high contents in Mn, Nb, Ta, and V, compared to scheelite formed in rocks metamorphosed below the middle greenschist facies. Scheelite from deposits hosted in sedimentary rocks has high Sr, Pb, U, and Th, and low Na, REE, and Y, compared to that hosted in felsic to intermediate rocks. Statistical analysis including elemental plots and multivariate statistics with PLS-DA (partial least square-discriminant analysis) reveal that the metamorphic facies of the host rocks as well as the regional host rock composition exert a strong control on scheelite composition. This is a result of fluid-rock exchange during fluid flow to gold deposition site. PLS-DA and elemental ratio plots show that scheelite from orogenic gold deposits have distinct Sr, Mo, Eu, As, and Sr/Mo, but indistinguishable REE signatures, compared to scheelite from other deposit types

Trace element composition of igneous and hydrothermal magnetite from porphyry deposits : relationship to deposit subtypes and magmatic affinity

Trace element compositions of igneous and hydrothermal magnetite from nineteen well-studied porphyry Cu ± Au ± Mo, Mo, and W-Mo deposits, combined with partial least squares-discriminant analysis (PLS-DA), were used to investigate the factors controlling magnetite chemistry during igneous and hydrothermal processes, as divided by magmatic affinity and porphyry deposit subtypes. Igneous magnetite can be discriminated by relatively high P, Ti, V, Mn, Zr, Nb, Hf, and Ta contents but low Mg, Si, Co, Ni, Ge, Sb, W, and Pb contents, in contrast to hydrothermal magnetite. Compositional differences between igneous and hydrothermal magnetite are mainly controlled by the temperature, oxygen fugacity, co-crystallized sulfides, and element solubility/mobility that significantly affect the partition coefficients between magnetite and melt/fluids. Binary diagrams based on Ti, V, and Cr contents are not enough to discriminate igneous and hydrothermal magnetite in porphyry deposits. Relatively high Si and Al contents discriminate porphyry W-Mo hydrothermal magnetite, probably reflecting the control by high Si, highly differentiated, granitic intrusions for this deposit type. Relatively high Mg, Mn, Zr, Nb, Sn, and Hf, but low Ti and V contents, discriminate porphyry Au-Cu hydrothermal magnetite, most likely resulting from a combination of mafic to intermediate intrusion composition, high chlorine in fluids, relatively high oxygen fugacity, and low temperature conditions. Igneous or hydrothermal magnetite from Cu-Mo, Cu-Au, and Cu-Mo-Au deposits cannot be discriminated from each other probably due to similar intermediate to felsic intrusion composition, melt/fluid composition, and conditions such as temperature and oxygen fugacity for the formation of these deposits. The magmatic affinity of porphyritic intrusions exerts some control on the chemical composition of igneous and hydrothermal magnetite in porphyry system. Igneous and hydrothermal magnetite related to alkaline magma is relatively rich in Mg, Mn, Co, Mo, Sn, and high field strength elements (HFSE), perhaps due to high concentrations of chlorine and fluorine in magma and exsolved fluids, whereas those related to calc-alkaline magma are relatively rich in Ca but depleted in HFSE, consistent with the high Ca but low HFSE magma composition. Igneous and hydrothermal magnetite related to high-K calc-alkaline magma is relatively rich in Al, Ti, Sc, and Ta, due to a higher temperature of formation or enrichment of these elements in melt/fluids. PLS-DA on hydrothermal magnetite compositions from worldwide porphyry Cu, iron oxide-copper-gold (IOCG), Kiruna-type iron oxide-apatite (IOA), and skarn deposits identify important discriminant elements for these deposit types. Magnetite from porphyry Cu deposits is characterized by relatively high Ti, V, Zn, and Al contents, whereas that from IOCG deposits can be discriminated from other types of magnetite by its relatively high V, Ni, Ti, and Al contents. IOA magnetite is discriminated by higher V, Ti, and Mg but lower Al contents, whereas skarn magnetite can be separated from magnetite from other deposit types by higher Mn, Mg, Ca, and Zn contents. Decreased Ti and V contents in hydrothermal magnetite from porphyry Cu and IOA, to IOCG, and to skarn deposits may be related to decreasing temperature and increasing oxygen fugacity. The relative depletion of Al in IOA magnetite is due to its low magnetite-silicate melt partition coefficient, immobility of Al in fluids, and earlier, higher-temperature magmatic or magmatic-hydrothermal formation of IOA deposits. The relative enrichment of Ni in IOCG magnetite reflects more mafic magmatic composition and less competition with sulfide, whereas elevated Mn, Mg, Ca, and Zn in skarn magnetite results from enrichment of these elements in fluids via more intensive fluid-carbonate rock interaction

PCA of Fe-oxides MLA data as an advanced tool in provenance discrimination and indicator mineral exploration : case study from bedrock and till from the Kiggavik U deposits area (Nunavut, Canada)

Magnetite and hematite grains from the 0.25–0.5 mm and 0.5–2.0 mm ferromagnetic fractions of ten till samples collected up-ice, overlying and down-ice of the Kiggavik U deposits (Nunavut, Canada), as well as eight bedrock samples from Kiggavik igneous and metasedimentary basement and overlying sedimentary rocks were characterized for their grain size and mineral association using optical microscopy, scanning electron microscopy (SEM) and mineral liberation analysis (MLA). Principal component analysis (PCA) was used to evaluate the MLA data for Fe-oxide mineral association and grain size distribution. PCA shows that mineralogical and granulometric differences in Fe-oxides from Kiggavik igneous rocks distinguish them from that of Kiggavik metasedimentary and sedimentary rocks. In addition, The PCA results indicate that the composition and abundance of minerals associated/intergrown with Fe-oxides are not only different in various till samples, but also in different size fractions of the same sample. Higher proportions of hornblende, quartz, gahnite, grunerite, apatite, chromite and sulfides are intergrown with Fe-oxides in the 0.5–2.0 mm till fraction, as compared to the 0.25–0.5 mm fraction in which Fe-oxides are mostly associated with pyroxene, titanite, rutile, feldspars, calcite and zircon. The mineral associations and grain sizes of proximal bedrocks are reflected in smaller size fractions of Kiggavik till, whereas detrital grains in the 0.5–2.0 mm fraction of Kiggavik till may have originated from distal sources. PCA also shows that Fe-oxides from the Kiggavik bedrock and till can be discriminated from those of volcanogenic massive sulfide (VMS) deposits because of smaller grain sizes and higher abundances of sulfides, gahnite, axinite, corundum, hypersthene and pyroxene intergrown with VMS Fe-oxides. This study emphasizes the importance of selecting suitable representative grain size fractions of till, or other sediments, when using indicator minerals for exploration. The results of PCA of Fe-oxides MLA data are consistent with the results of using Fe-oxides geochemical data in provenance discrimination of Kiggavik till

Metal mobility during hydrothermal breakdown of Fe-Ti oxides : insights from Sb-Au mineralizing event (Variscan Armorican Massif, France)

Hydrothermal alteration related to Sb-Au mineralization is widespread in the Variscan Armorican Massif, but mineral replacement reactions are not well characterized, in particular the hydrothermal breakdown of ilmenite-titanohematite. Based on petrography, electron probe micro-analyzer and laser ablation-inductively coupled plasma-mass spectrometer analyses, we document mineralogical change at rock- and mineral-scale and the redistribution of Sb and others trace elements during the recrystallization of ilmenite-titanohematite to hydrothermal rutile. Hydrothermal alteration is mainly potassic with associated carbonation. The replacement mechanism is interpreted to be an interface-coupled dissolution-reprecipitation process. Results show that Mn, Zn, Co, Ni, Sn, Mo and U are released during hydrothermal alteration, whereas Sb and W are incorporated in newly-formed hydrothermal rutile from the hydrothermal fluid. Furthermore, the concentration of Sb evolves through time suggesting a change in fluid composition likely related to an enrichment of fluid in Sb during rutile crystallization. Considering that Fe-Ti oxides breakdown during hydrothermal alteration is common within epithermal and mesothermal/orogenic Au-Sb mineralizing systems, results report in this study yield important constraints about metal mobility and exchanges in hydrothermal gold systems

Trace element composition of iron oxides from IOCG and IOA deposits : relationship to hydrothermal alteration and deposit subtypes

Trace element compositions of magnetite and hematite from 16 well-studied iron oxide–copper–gold (IOCG) and iron oxide apatite (IOA) deposits, combined with partial least squares-discriminant analysis (PLS-DA), were used to investigate the factors controlling the iron oxide chemistry and the links between the chemical composition of iron oxides and hydrothermal processes, as divided by alteration types and IOCG and IOA deposit subtypes. Chemical compositions of iron oxides are controlled by oxygen fugacity, temperature, co-precipitating sulfides, and host rocks. Iron oxides from hematite IOCG deposits show relatively high Nb, Cu, Mo, W, and Sn contents, and can be discriminated from those from magnetite + hematite and magnetite IOA deposits. Magnetite IOCG deposits show a compositional diversity and overlap with the three other types, which may be due to the incremental development of high-temperature Ca–Fe and K–Fe alteration. Iron oxides from the high-temperature Ca–Fe alteration can be discriminated from those from high- and low-temperature K–Fe alteration by higher Mg and V contents. Iron oxides from low-temperature K–Fe alteration can be discriminated from those from high-temperature K–Fe alteration by higher Si, Ca, Zr, W, Nb, and Mo contents. Iron oxides from IOA deposits can be discriminated from those from IOCG deposits by higher Mg, Ti, V, Pb, and Sc contents. The composition of IOCG and IOA iron oxides can be discriminated from those from porphyry Cu, Ni–Cu, and volcanogenic massive sulfide deposits