This study explores the integration of petrophysical and geochemical data to characterize hydrothermal alteration in mafic and ultramafic rocks, with a focus on the Augmitto-Bouzan orogenic gold deposit in the Abitibi Greenstone Belt. Employing established mass balance techniques using zirconium (Zr) as the immobile reference element, we quantified element mobility during alteration processes. Due to the absence of original komatiite protolith data, we compiled geochemical compositions from published sources across the Superior Province and applied bootstrapping to derive a representative baseline for mass balance calculations, which were then correlated with petrophysical logs from multiple boreholes to identify modification signatures. The analysis reveals robust correlations: potassium enrichment aligns with elevated gamma-ray responses, carbonate alteration is marked by increased CaO and reduced density, and magnetic susceptibility decreases correspond to Fe-Mg depletion and sulfide mineralization. These observations demonstrate systematic links between geochemical changes and petrophysical data, showing that petrophysical logs can serve as high-resolution, cost-effective proxies for alteration mapping, offering a scalable framework for exploration targeting in orogenic gold systems hosted in ultramafic rocks. The methodology bridges traditional geochemistry and in-situ measurements of physical properties, improving subsurface characterization and supporting the development of data-driven exploration tools while advancing our understanding of processes associated with gold mineralization.

The Archean Val-d’Or orogenic gold vein field (Abitibi, Canada) is characterized by quartz-carbonate-tourmaline veins in which at least two temporally distinct Au deposition events are commonly recognized, including Au-Ag-Te-Bi inclusions in pyrite formed during the main quartz mineralizing event, and Au-Te-Bi-Cu-Pb minerals in late quartz ± carbonate brittle veinlets. SEM-CL imaging combined with in situ SIMS oxygen isotope of quartz in four orogenic Au deposits (Beaufor, Lac Herbin, Sigma-2, Triangle) of the Val-d’Or vein field reveal similar quartz vein generations and relative timing. In SEM-CL images, early idiomorphic oscillatory-zoned Qz1 is light grey and formed in vein open space. Qz2, dominant in volume, forms light to medium grey sub-equant grains overprinting Qz1. Later Qz3 forms irregular medium grey bands with sharp or gradual contacts with Qz1 and Qz2, indicating a second recrystallization event. Qz4, minor in volume, forms brittle thin veinlets (< 400 μm in width) and stockworks characterized by dark grey luminescence, cutting Qz1, 2 and 3. Qz4 veinlets host Au- Ag-Te-Bi-Cu-Pb minerals. In situ SIMS oxygen isotope analyses of the successive quartz types show a similar, small progressive increase of 𝛅18Oquartz values, typically by 1‰ between Qz1 and Qz2, and by up to 3.3‰, from Qz1 to the later quartz generations. The 𝛅18Oquartz values at each deposit follow the 𝛅18Oquartz regional variation recorded in previous studies, suggesting that later Qz2 and Qz3 are a product of local dissolution and reprecipitation of Qz1. The higher 𝛅18Oquartz values in Qz2 and Qz3 are interpreted to result from pressure solution/dissolution of Qz1 and reprecipitation either in equilibrium with a fluid with a higher proportion of high 𝛅18O metamorphic fluids, or from a small amount of cooling. The Qz4 veinlets with Au-Ag-Te-Bi-Cu-Pb minerals result from vein-scale quartz and metal (including gold) dissolution and precipitation into brittle fractures during cooling triggered by a regional thermal event.

The Augmitto-Bouzan deposit is a 12 km long segment of the Larder Lake-Cadillac Deformation Zone (LLCDz) south of Rouyn-Noranda (Québec, Canada) that is characterized by an uneven gold distribution hosted in quartz-carbonate ± tourmaline veins within Piché Group ultramafic rocks. This study compares the fluid flow conditions between the variable gold-endowed sectors to identify deposit-scale processes responsible for gold endowment. Stable isotopes indicate that quartz and tourmaline have equilibrium temperatures (228–⁠420 °C) that likely define a high vertical thermal gradient ( 30 °C/100 m) along the LLCDz. Covariation between temperature and computed δ18OH2O and δDH2O is interpreted to result from mixing between a high temperature (> 420 °C), high δ18O (> 10.8‰), and low δD (< –29‰) deep-seated metamorphic fluid, and a low temperature (< 230 °C), low δ18O (< 4‰) and high δD ( 0‰) upper crustal pore fluid. Local upwelling of auriferous deep-seated fluid, shown by interpolation of δ18OH2O in the gold-endowed Augmitto-Cinderella and Astoria segments, was likely focused along higher permeability deformation-related pathways. Sectors of low gold endowment have lower δ18OH2O and fluid/rock ratios, likely reflecting a larger proportion of upper crustal fluid and differences in fluid-flow behavior. Modeling of fluid flow shows that this is due to 1) weaker metamorphic fluid flux in the thinner band of Piché Group rocks and 2) more porous volcanic rocks north of the LLCDz, drawing more pore fluid into the fault. We suggest that most of the variation of gold endowment is related to variations in advection of auriferous metamorphic fluid along the segment, whereby a weaker metamorphic fluid flux or increased admixture of upper crustal fluids decrease the gold potential along the LLCDz.

The Rouyn-Noranda mining district of Quebec contains 20 Cu-Zn (+/- Au +/- Ag) volcanogenic massive sulfide (VMS) deposits, including the giant and gold-rich Quemont and Horne deposits. Mineralized epigenetic veins are also present, but their origin and relative timing remain enigmatic. The nature and extent of their alteration signatures and the effect of their superposition on district-scale alteration patterns is unknown. The VMSrelated quartz-sulfide Cu-Zn-Ag veins have delta O-18(quartz) values of 8.5 +/- 0.8 parts per thousand, reflecting delta O-18(fluid) compositions of -0.4 to 3.1 parts per thousand (250 degrees-350 degrees C) that are typical of Archean seawater. They are associated with a proximal Fe-rich chlorite alteration and marginal spotted sericite-chlorite alteration with whole-rock delta O-18 values of 2.9 to 5.9 parts per thousand and are interpreted to have formed within the structurally controlled discordant upflow zones of a VMS hydrothermal system. Younger gold-bearing quartz-carbonate veins were emplaced along mechanical anisotropies created by mafic dikes during north-south compression and the formation of regional E-trending faults, folds, and cleavage. They are characterized by delta O-18(quartz) values of 11.3 +/- 0.8 parts per thousand, reflecting delta O-18(fluid) compositions of 2.4 to 5.9 parts per thousand (250 degrees-350 degrees C), typical of a metamorphic fluid, possibly mixed with a lower delta O-18 upper crustal fluid. They are associated with ankerite, calcite, muscovite, chlorite, albite, and quartz +/- hematite alteration with whole-rock delta O-18 values of 5.8 to 10.3 parts per thousand. Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon ages for two tonalite intrusions constrain the maximum age of the Cu-Zn-Ag veins to 2697.6 +/- 0.7 Ma and the minimum age to 2695.3 +/- 1.0 Ma, which is also the maximum age of the gold quartz-carbonate veins. Superposition of alteration related to the gold quartz-carbonate veins on previously chlorite- and sericite-altered rocks has resulted in mixed alteration signals with whole-rock delta O-18 values of similar to 6 to 8 parts per thousand that have perturbed and masked regional alteration patterns related to older VMS mineralization, such as those found in the Quemont and Horne deposits. These results indicate that defining alteration vectors in camps that have superimposed hydrothermal systems requires full consideration of the hydrothermal history of the camp, and if such constraints are lacking, whole-rock delta O-18 values should not be used as a stand-alone exploration method.

The O-H isotope composition of orogenic gold-bearing veins along the Cadillac Larder Lake Fault Zone (CLLFZ) between Val-d'Or and Kirkland Lake is documented in detail to unravel the fluid sources of orogenic gold deposits. Coexisting vein minerals show common oxygen isotope equilibrium, which yields temperatures between similar to 250 and similar to 550 degrees C. Temperature covariation with fluid O and H isotope compositions demonstrates mixing between a higher temperature (> 500 degrees C), deep-seated metamorphic fluids with high delta(OH2O)-O-18 (>9 parts per thousand), low delta D-H2O (<-40%), with lower temperature (<250 degrees C) upper crustal fluids with lower delta O-18(H2O) (<4 parts per thousand), and higher delta D-H2O that ranges from 0 to 30%. Along the Augmitto-Bouzan segment (Rouyn-Noranda), decreasing temperatures from 420 degrees C at 700 m depth, to 230 degrees C at 100 m depth, show the vertical ascent of the deep-seated metamorphic fluids along the CLLFZ. In the eastern part of the CLLFZ (Val-d'Or to Malartic) the metamorphic fluid has an end-member O isotope composition of 9-10%, whereas to the west from Malartic to Kirkland Lake, it has a delta O-18(H2O) between 11-13%. The switch in metamorphic fluid reservoirs occurs where the CLLFZ has an inflection in strike. The upper crustal fluids, that cannot be distinguished despite differences in the composition of the country rocks hosting the orogenic gold deposits.

Le gisement de talc-chlorites de Trimouns résulte de l’altération métasomatique de métasédiments paléozoïques, par des fluides aqueux et salés. Ce processus s’effectue entre 250 et 300 ºC à une profondeur inférieure à 5 km. Daté entre 120 et 90 millions d’années, il est lié à l’amincissement de la croûte continentale lors de l'hyper-extension provoquée par l’ouverture du Golfe de Gascogne.

AbstractOrogenic gold deposits formed in various terranes of most ages since the Paleoarchean and generally consist of quartz veins hosted in shear zones formed at the ductile brittle transition under greenschist to lower amphibolite metamorphic conditions. Vein mineralogy is dominated by quartz with various amounts of silicates, carbonates, phyllosilicates, borates, tungstates, sulfides, and oxides. The isotopic composition of these minerals and fluid inclusions has been investigated since the 1960s to constrain the characteristics of orogenic fluid systems involved in the formation of gold deposits worldwide. This review is based on 8580 stable isotope analyses, including δ18O, δD, δ13C, δ34S δ15N, δ11B, and δ30Si values, from 5478 samples from 558 orogenic gold deposits reported in the literature from 1960 to 2010. This contribution describes the variability of the light stable isotopic systems as function of the minerals, the age of the deposits, their regional setting, and their country rocks. The temperature of isotopic equilibrium of orogenic gold veins is estimated from mineral pairs for oxygen and sulfur isotopes. Based on these temperatures, and on fractionation between mineral and fluid components (H2O, CO2 and H2S), the isotopic composition of fluids is estimated to better constrain the main parameters shared by most of auriferous orogenic fluid systems. Orogenic gold deposits display similar isotopic features through time, suggesting that fluid conditions and sources leading to the formation of orogenic gold deposits did not change significantly from the Archean to the Cenozoic. No consistent secular variations of mineral isotope composition for oxygen (−8.1‰ ≤ δ18O ≤ 33‰, n = 4011), hydrogen (−187‰ ≤ δD ≤ −4‰, n = 246), carbon (−26.7‰ ≤ δ13C ≤ 12.3‰, n = 1179), boron (−21.6‰ ≤ δ11B ≤ 9‰, n = 119), and silicon (−0.5‰ ≤ δ30Si ≤ 0.8‰, n = 33) are documented. Only nitrogen (1.6‰ ≤ δ15N ≤ 23.7‰, n = 258) and sulfide sulfur from deposits hosted in sedimentary rocks (−27.2‰ ≤ δ34S ≤ 25‰, n = 717) display secular variations. For nitrogen, the change in composition is interpreted to record the variation of δ15N values of sediments devolatilized during metamorphism. For sulfur, secular variations reflect incorporation of local sedimentary sulfur of ultimate seawater origin. No significant variation of temperature of vein formation is documented for orogenic gold deposits of different ages. Quartz-silicate, quartz-carbonate and sulfide-sulfide mineral pairs display consistent temperatures of 360 ± 76 °C (1σ; n = 332), in agreement with the more common greenschist facies hostrocks and fluid inclusion microthermometry. Fluid sources for orogenic gold deposits are complex but the isotopic systems (hydrogen, boron, carbon, nitrogen, oxygen, sulfur) are most consistent with contributions from metamorphic fluids released by devolatilization of igneous, volcano-sedimentary and/or sedimentary rocks. The contribution of magmatic water exsolved from magma during crystallization is not a necessary component, even if permissible in specific cases. Isotopic data arrays can be interpreted as the result of fluid mixing between a high T (~550 °C)—high δ18O (~10‰)—low δD (~−60‰) deep-seated (metamorphic) fluid reservoir and a low T (~200 °C)—low δ18O (~2‰)—high δD (~0‰) upper crustal fluid reservoir in a number of orogenic gold deposits. The origin of the upper crustal fluid is most likely sea- or meteoric water filling the host rock porosity, with a long history of water–rock isotope exchange. Mixing of deep-seated and upper crustal fluids also explains the large variation of tourmaline δ11B values from orogenic gold veins. Regional spatial variations of oxygen and hydrogen isotope compositions of deep-seated fluid reservoirs are documented between orogenic gold districts. This is the case for the Val-d’Or (Abitibi), Coolgardie and Kalgoorlie (Yilgarn) where the oxygen isotope composition of the deep-seated fluid end-member is 4‰ lower compared to that from the Timmins, Larder Lake, and Kirkland Lake districts (Abitibi). However, both mixing trends converge towards a common, low δ18O upper crustal fluid end-member. Such variations cannot be related to fluid buffering at the site of deposition and suggest provinciality of the fluid source. The contribution of meteoric water is mainly recorded by fluid inclusions from Mesozoic and Cenozoic age deposits, but micas are not systematically in isotopic equilibrium with fluid inclusions trapped in quartz from the same vein. This suggests late involvement of meteoric water unrelated to deposit formation. Yet, a number of deposits with low δD mica may record infiltration of meteoric water in orogenic gold deposits. Isotope exchange between mineralizing fluid and country rocks is documented for oxygen, carbon, sulfur and silicon isotopes. Large variations (> 10‰) of sulfide δ34S values at the deposit scale are likely related to evolving redox conditions of the mineralizing fluid during reaction with country rocks. Deposits hosted in sedimentary rocks show a shift to higher δ18O values as a result of fluid/rock oxygen exchange with the regional sedimentary country rocks.

This study evaluates the applicability of the clumped isotope thermometry to mesothermal hydrothermal systems (5-10 km depth; 250-450 degrees C). We measured Delta(47), delta O-18 and delta C-13 of calcite as well as delta O-18 of cogenetic minerals from typical quartz-calcite +/- tourmaline +/- chlorite orogenic veins from the Neoarchean Augmitto-Bouzan orogenic gold deposits (Abitibi, Canada). Our findings show that caution is required when utilizing the clumped isotope thermometry in the study of old mesothermal deposits. Temperatures calculated from Delta(47) values are systematically and significantly shifted to low temperature, i.e., similar to 150 degrees C rather than the similar to 350 +/- 50 degrees C expected for orogenic gold deposit formation and documented using oxygen isotope equilibrium between mineral pairs. We show that the low temperatures estimates resulted from solid-state reordering that occurred in calcite grains during the cooling history of the vein-hosting rocks. Because systems are geologically unrealistic, we suggest that refractory minerals (i.e., minerals with higher blocking temperature such as magnesite, dolomite, ankerite) should be investigated to apply clumped isotope thermometry in such context.

In orogenic gold systems, the source of the fluids and the processes leading to mobilization, transport, and deposition of gold remain debated. Most studies focus on endowed rather than on gold-poor orogenic systems to unravel the “key” parameters of gold mineralizing processes. Here, we present stable isotope (O, H) data from the Moly-Desgagné–Guercheville fault system (gold-free to low gold endowment) in the Chibougamau area, Abitibi greenstone belt, Canada. The gold-free Moly-Desgagné showing and nearby poorly endowed Hazeur showing and Monster Lake deposit share similar features typical of orogenic systems. The stable isotope compositions of tourmaline and quartz from the Moly-Desgagné–Guercheville fault system also display similar characteristics, such as (1) temperature of vein formation of 345 ± 86 °C (1σ); (2) fluid mixing between an upper crustal reservoir (low T – high δD – low δ18O) and a metamorphic water reservoir (high T – low δD – high δ18O); and (3) positive δDfluid values consistent with multiple boiling-condensation cycles related to fault-valve processes. These characteristics are similar to those from the gold-endowed Val-d’Or vein field. The difference in gold endowment between the Moly-Desgagné–Guercheville and Val-d’Or vein fields may be related to the nature of the fault system (i.e., terrane-bounding or “intragreenstone belt”) and the volume and (or) composition (i.e., gold, its ligands, CO2) of the fluid source rocks.

Phyllosilicates may trap hydrogen (H-2) in the crust, but they may also produce it through various processes, including oxidative dehydrogenation. The dehydrogenation temperature depends on the type and composition of the phyllosilicates considered, but it may be as low as 300 degrees C. Here, we document the release of H-2 and CO during thermal treatment of chloritite (300 degrees C) and talc (500 degrees C) from the Trimouns deposits (Eastern Pyrenees, France). Thermal release of gases coupled to stable isotope analysis has been used to recover and characterize H-2 and CO, the two detected gases. Hydrogen content may be as high as 7 ppm with delta D-H2 values ranging from -258 parts per thousand to - 224 parts per thousand for sub-pure chloritite and - 140%0 for pure talc. CO content ranges between 3 ppm and 35.3 ppm with very homogeneous delta C-13(CO) values between -27.6 parts per thousand and - 25.7 parts per thousand. This study supports the idea that H-2 was produced during experiments by dehydrogenation. The origin of CO remains enigmatic, but its carbon isotope composition suggests a link to the few amounts of graphite documented in chloritite and talc from the deposit. This work also reports extensive hydrogen isotope fractionation between H-2 produced by dehydrogenation and both talc and chloritite. Dehydrogenation of phyllosilicates is a potential source term of H-2 in numerous magmatic-hydrothermal settings and must thus be accounted for in the budget of the H-2 geochemical cycle.

The North Pyrenean Zone corresponds to the palaeopassive margin of the North Iberia plate, at the foot of which subcontinental mantle was exhumed during Albian times. Rare bodies of exhumed mantle rocks associated with strongly sheared lenses of continental crust are scattered among the North Pyrenean Zone metasediments. Significant fluid flow occurred along a major decollement at the basement-Trias interface in the Urdach massif (Chainons Bearnais). Fluids with a broad range of salinity (10-38 wt.% NaCl equiv.), indicative of mixing between brines and more dilute waters, produced strong silicification of breccias. The brines circulated at c. 240-280 degrees C under lithostatic pressures at c. 6 +/- 1 km depth. The fluids became increasingly saline towards the final stages. The syndeposition of Cenomano-Turonian flysch layers then progressively isolated the lower aquifers close to the decollement where Triassic brines were predominant. The release and migration of significant volumes of brines during stretching and squeezing of the Triassic evaporites played a crucial part in the mineralogical and rheological transformations that occurred during the Pyrenean Cretaceous rifting event.

In this report, partial results from fieldwork of a Ph.D. project conducted by the lead author in summer 2019 are presented. The study area (Figure 1) is located approximately 9 km south of the city of Rouyn-Noranda, Québec. The aims of the project were to constrain the extent to which the fluids released during Barrovian metamorphism (from the biotite to the sillimanite zone) of the northwestern Pontiac Subprovince contributed to the significant endowment in gold of the Cadillac–Larder Lake fault zone (CLLFZ)—a world-renowned gold district in the Abitibi greenstone belt. Further constraining the metamorphic evolution of the Pontiac Subprovince will also provide an opportunity to better understand its geodynamic significance.

The Regional Isotopic Survey Systematics (RISS) is part of a Metal Earth thematic project, which focuses on the characterization through time and space of the auriferous fluid-flow system(s) as ‘sourceto-sink’ systems. The objective of the RISS is to characterize the spatial variation of auriferous fluid flows and their isotopic composition (O, C, H, S) along the main deformation zones where the orogenic gold deposits are localized. Indeed, the general orogenic gold model of fluid circulation consists in the circulation of auriferous fluid along a major lithospheric-scale shear zone and in the connected anastomosed network of lower-order shear zones, in which orogenic gold deposits occur (Robert et al., 1995, 2005). This spatial distribution of gold deposits could suggest that spatial variation of fluid-flow and related variation of fluid–rock interaction are important parameters in their formation.

During the Albian, the hyperextension of the Pyrenean passive margin led to a hyperthinning of the continental crust and the subsequent subcontinental mantle exhumation. The giant Trimouns talc-chlorite deposit represents the most prominent occurrence of Albian metasomatism in the Pyrenees, with the occurrence of the largest talc deposit worldwide. Consequently, this deposit, which is located on a fault zone and a lithological contact, represents one of the major drains at the scale of the Pyrenees and one of the best geological targets in order to determine the origin(s) of the fluid(s) that circulated during this period. Talc-chlorite ore is characterized by the presence of brines trapped in dolomite, quartz, and calcite fluid inclusions in the vicinity of the talc-rich zone. Considered as being responsible for the formation of talc, these fluids may be interpreted in several ways: (i) primary brines expelled from Triassic evaporites, (ii) secondary brines produced through halite leaching by diagenetic/metamorphic fluids, and (iii) brines derived from seawater serpentinization of mantle rocks. Stable isotope analyses (δ13C, δ18O, δD, and δ37Cl) and Cl/Br ratio measurements in fluid inclusions and their host minerals were carried out in order to determine the origin of the fluid(s) involved in the formation of the ore deposit. The data are consistent with a primary brine origin for the mineralizing fluid, which could have been expelled from the Triassic levels. Other hypotheses have been tested, for example, the production of brines via the seawater concentration during serpentinization. The geochemical proxies used in this study provide equivocal results. The first hypothesis is by far the most realistic one considering the numerous occurrences of Trias formations nearby, their deformation during the extension, and the drainage of the expulsed brines as evidenced by the high-salinity fluid inclusions found all around the deposit. Alternatively, the exhumation of the mantle is considered as a major source of heat and stress that favored brine migration along the major shear zones. Our results fit well with brine circulation in a hyperextensional geodynamic context, which is related to the formation of the talc-chlorite ore, the thinning of the continental crust, and the exhumation of the subcontinental mantle, in accordance with recent works.

Resulting from the weathering of the Peridotite Nappe, laterites are abundant in New Caledonia and host one of the largest nickel deposits worldwide. This work presents a 3D model of the Koniambo nickel laterite ore deposit. It shows that the laterites are located along the ridges of the massif and organized as hectometric-sized patches obliquely cut by the topography and distributed at various elevations. Three kinds of geometry were observed: (i) a thick laterite cover (between 20 and 40 m) overlying saprolite and mainly localized on topographic highs, (ii) a thin laterite cover (from a few meters to 20 m) mainly localized on areas with gentle slopes, and (iii) exposure of saprolite without laterite cover. Our data show that Ni-rich and Ni-poor areas are organized as hectometric-sized patches which broadly correlate with the distribution of the laterite thickness. The highest Ni areas are localized on slopes where laterite cover is thin or absent. The areas with lowest Ni are located in topographic highs under the thickest laterite cover. The vertical Ni mass balance for each borehole shows that, in areas with thick laterite cover, Ni is sub-equilibrated to slightly depleted whereas in areas with thin laterite cover, Ni is enriched. This suggests the existence of lateral infiltration of water rich in dissolved Ni, from areas such as topographic highs to downstream slope areas, in a process leading to enrichment of saprolite in Ni in slope areas. Mechanical transport and leaching of laterite material on slopes, including Ni-bearing material, could also contribute to local enrichment of Ni in the saprolite.

The Peridotite Nappe of New Caledonia is one of the few ophiolites worldwide that escaped collisional orogeny after obduction. Here we describe the deformation associated with serpentinization in two klippes of the nappe in northwestern New Caledonia. The klippes are flat lying and involve S/SW vergent reverse-slip shear zones which are true compressional structures in origin. Further northeast, the nappe is folded in association with the development of a steep schistosity in low-grade metasediments. This difference in structural style indicates that the Peridotite Nappe experienced compression at greater depths toward its root zone, suggesting a "push from the rear" mechanism of emplacement. This supports the view that the nappe has been emplaced through horizontal contraction sustained by plate convergence. We establish a crustal-scale cross section at the end of the obduction event, before Neogene extension. This involves a large fold nappe of high-pressure rocks bounded from below by a major thrust. Furthermore, we show that obduction in New Caledonia occurred through dextral oblique convergence. Oblique convergence probably resulted from the initial obliquity between the subduction trench and the continental ribbon that became incorporated in it. This obliquity can solve the paradox of the Peridotite Nappe seemingly being emplaced at the same time the high-pressure rocks were exhumed. Oblique convergence together with focused erosional denudation on the northeastern flank of the island led to exhumation of the metamorphic rocks in a steep fold nappe rising through the rear part of the orogen.

Sets of fractures and breccia sealed by Ni-rich silicates and quartz occur within saprock of the New Caledonian regolith developed over ultramafic rocks. The crystallization sequence in fractures is as follows: (1) serpentine stage: lizardite > polygonal serpentine > white lizardite; (2) Ni stage: Ni-Mg kerolite followed by red-brown microcrystalline quartz; and (3) supergene stages. The red-brown microcrystalline quartz corresponds to the very last stage of the Ni sequence and is inferred to have precipitated within the 50–95 °C temperature range. It constitutes also the main cement of breccia that has all the typical features of hydraulic fracturing. The whole sequence is therefore interpreted as the result of hydrothermal fluid circulation under medium to low temperature and fluctuating fluid pressure. Although frequently described as the result of a single downward redistribution of Ni and Mg leached in the upper part of the regolith under ambient temperature, the Ni silicate veins thus appear as the result of recurrent crack and seal process, corresponding to upward medium temperature fluid convection, hydraulic fracturing and subsequent fluid mixing, and mineral deposition.

The stable isotope compositions of veins provide information on the conditions of fluid-rock interaction and on the origin of fluids and temperatures. In New Caledonia, magnesite and silica veins occur throughout the Peridotite Nappe. In this work, we present stable isotope and clumped isotope data in order to constrain the conditions of fluid circulation and the relationship between fluid circulation and nickel ore-forming laterization focusing on the Koniambo Massif. For magnesite veins occurring at the base of the nappe, the high delta O-18 values between 27.8% and 29.5% attest to a low temperature formation. Clumped isotope analyses on magnesite give temperatures between 26 degrees C and 42 degrees C that are consistent with amorphous silica-magnesite oxygen isotope equilibrium. The meteoric origin of the fluid is indicated by calculated delta O-18(water) values between -3.4% to +1.5%. Amorphous silica associated with magnesite or occurring in the coarse saprolite level displays a narrow range of delta O-18 values between 29.7% and 35.3%. For quartz veins occurring at the top of the bedrock and at the saprolite level, commonly in association with Ni-talc-like minerals, the delta O-18 values are lower, between 21.8% and 29.0% and suggest low-temperature hydrothermal conditions (similar to 40-95 degrees C). Thermal equilibration of the fluid along the geothermic gradient before upward flow through the nappe and/or influence of exothermic reactions of serpentinization could be the source(s) of heat needed to form quartz veins under such conditions. (C) 2016 Elsevier Ltd. All rights reserved.