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Per-descensum formation
For the New Caledonian Ni laterite deposits, as well as for other laterites worldwide, a model of downward fluid circulation is classically applied to explain the distribution of elements and geometry of the lateritic profiles (e.g. Avias, 1969; Trescases, 1975). This model is based on general observations of the vertical weathered profiles showing an increase in Ni concentra-tions from the top to the bottom of the lateritic regoliths and developed ferruginous residual zone where Si, Mg, and Ni are leached (Fig. 1.6). The process that is generally proposed to interpret the distribution of nickel comprises (i) progressive release of the elements during dissolution of the primary silicates of parent rock and (ii) a per-descensum migration of these elements in a soluble form, followed by trapping of Ni in adsorbed or precipitated form by underlying rock. The most soluble minerals, olivine and pyroxene, should remain in the freshest part of the profile, while the least soluble goethite constitutes its most developed part (Fig. 1.8).
The export of silica and nickel appears to happen on a short distance. Leached nickel is reincorporated into goethite and garnierite veins, and silica is reincorporated into both – garnierite and silica veins within the saprolitic level. The magnesium mobility is more prob-lematic. Indeed, given the high intensity of its leaching the erratic occurrences of garnierite are not sufficient to explain the fate of the whole leached magnesium. In New Caledonia it is found in a form of veins at the basis of the peridotite nappe as well as in the form of nodules in a soil (Quesnel et al., 2015). It is still not understood whether this magnesite is a by-product of supergene alteration or its formation is governed by other processes.
Relations of the Deposits to Structures
Geological structures, as well as fractures and faults, can play a key role in the development of Ni laterite deposits. Indeed, the degree of permeability of the parent material together with hydraulic heads are being defining factors in directing the cations and anions released due to hydrolysis of initial phase-olivine, or the secondary precipitating minerals. The process of weathering itself proceeds more rapidly in fractured zones. Fluid follows fractures and complexities of the cracked protolith along a path of least resistance causing enrichment along fissures, settling of boulders and rim enrichment of the protolith fragments (Norton, 1973; De Vletter, 1955, 1978). Moreover, it causes highly irregular boundaries between the weathering layers. The tectonic trends also constitute preferential drainage-axes and are the seat of the most important absolute accumulations, such as garnierites (e.g Trescases, 1975; Cluzel and Vigier, 2008).
Up-to-date mineralogical observations of Koniambo massif revealed two main types of local mineralizing distributions of Ni silicate ore that occur in the saprolite level and formed under the tight structural control. The first (type 1) is represented by mineralized frac-tures with kerolite(talc-like) infillings of cm to dcm width and typically correspond to those described elsewhere in New Caledonia (Fig. 1.9). This infillings occur in the sets of discon-tinuities. The second (type 2) is shown by concentric zonation of Ni-bearing silicates, green (pimelite) and white (Mg-kerolite) coatings in ”target-like” joints that occur always close to the type 1 (Cathelineau et al., 2016b) (Fig. 1.9).
Modelling the hydrodynamic system
A reactive multicomponent 1-D transport model of supergene enrichment of lateritic Ni deposits has been simulated by assuming the weathering of a one-dimensional vertically oriented column of serpentinized olivine due to steady state meteoric water flow. The code used for the simulations is PHREEQC (V 3.1.4) (Parkhurst and Appelo, 2013). The 1-D column is defined by a series of 40 cells of 0.5 m. in length. The velocity of water in each cell is determined by the length of the cell, porosity and amount of annual rain precipitation. Solute concentrations at some point on a flowline may change by i) advection of concentration gradients, ii) reactions with the solid material, and iii) dispersion and diffusion. The Advection-Reaction-Dispersion equation that describes these changes along the flowline and implemented on Phreeqc is the following: ( ∂t ) x = −ν ( ∂t ) t − ( ∂t ) x + DL ( ∂x2 )t (1).
where c is the solute concentration (mol.l−1), v is pore water flow velocity (m.s−1), DL is hydrodynamic dispersion coefficient (m2.s−1) and q is the concentration in the solid (mol.l −1 of pore water). In our calculations we neglect dispersion and set DL to 0. According to in situ observations, indeed, transport of the chemical components is controlled by advection and high reaction rates. Neglecting dispersion will cause sharper reaction front and higher concentration peaks. In other words, spreading of the concentration front should be slightly underestimated. Flux boundary conditions (Cauchy) were defined for the first and last cell. A slightly acidic tropical rainwater with pH=5.6 due to its equilibrium with atmospheric CO2 repre-sents an incoming solution and is injected in the first cell (Fig.3). Regarding separately the values of the experimental rates of kinetic dissolution of the parent rock constituents (olivine, serpentine, enstatite), presented in the literature (e.g. Brantley and Chen (1995); Pokrovsky and Schott (2000); Wilson (2004); Thom et al. (2013)), one may note that olivine has a highest rate of the dissolution process among them. The rates of enstatite and ser-pentine are extremely sluggish compared to those of olivine, being one to a few orders of magnitude lower depending on pH. This makes olivine the main initial mineral in a sys-tem that provides nickel and other elements (Mg, Si and Fe). In this way, the numerical modelling was performed assuming olivine in each cell of the 1-D column at initial state (Fig.2). The New Caledonia is situated in seasonally humid wet savannas, thus, characterized by summer rainfall of 900-1800 mm and a 2-5 months winter dry season (Butt and Cluzel, 2013). Nevertheless, these values are related to the current climatic circumstances and most likely were not the same during all 10 Ma of laterite formation. According to calculations of Thorne et al. (2012) Ni laterites develop where rainfall exceeds 1000 mm/y and mean monthly temperatures range between 22-31◦C (summer) and 15-27◦C (winter). For our simulations the value of 2000 mm/y was chosen as acceptable.
Effect of pimelite/kerolite being present as solid solution
The numerical simulations presented above consider the Mg-Ni-phyllosilicates to be pure end-members although in reality they are being members of solid-solution series extending from Mg and Ni end-members (e.g. Springer (1974, 1976); Reddy et al. (2009); Villanova-de-Benavent et al. (2014); Cathelineau et al. (2015)). Usually solid solutions show deviations from ideal behavior and the activity coefficients, that account for non-ideality, become a function of the excess free energy of mixing. The free energy of mixing might be obtained from different non-ideal thermodynamic properties such as variations in fractionation factor, miscibility gaps etc. (Glynn, 2000). Despite the fact that many researchers describe Mg-Ni-phyllosilicate members of solid solutions, the possibilities of modelling such a system remain poor due to the lack of thermodynamic data, thus, limiting simulations by assumption of ideality. In this way, for example, Galí et al. (2012) have calculated Lippmann diagrams for the garnierite solid solutions. This subchapter aims at understanding the effect of e.g. pimelite and kerolite being present as a solid solution instead of separate mineral phases. Linear combination of reactions for kerolite and pimelite, presented in Table 1 results in Equation (5): Mg3S i4O10(OH)2 : H2O + 3Ni2+ = Ni3S i4O10(OH)2 : H2O + 3Mg2+ (5).
Competition of elements for sorption sites
According to the results of the 1-D numerical transport modelling, presented in chapter (3.1), adsorption of nickel strongly depends on pH (Fig.5 and 7(B)) and does not appear in limonite zone due to full dissolution of silicates and decrease of pH to the value of rainwater (i.e. 5.6). Indeed, the surfaces of oxides carry a charge that enhances sorption of metals and depends on pH and composition of the solution. Figure 8 represents a batch calculation of nickel distribution among the aqueous phase and strong/weak sorption sites of 0.09 g of goethite, while Figure 9 shows the shape of the sorption edge of Ni2+ as a function of pH. These calculations are based on the Gouy-Chapman model from Dzombak and Morel (1990) and show the results of distribution for low (10−8 M) and high (10−4 M) nickel concentrations. Concentrations of Ni, representative in our modelling, generally lie in this interval, being lower than 10−6 M.
As one can see from both Figures 8 and 9, nickel is more strongly sorbed at high pH values than at low pH values. The shape of the sorption edge is shown in Figure 9 and actually depends on the Ni concentration. At low concentrations (e.g. 10−8 M) the pH domain where Ni is sorbed to the surface of goethite increases from pH 8 to 10.5 while at higher concentrations it reduces, general shape of the Ni sorption edge smooths, and the acid branch shifts to the higher pH.
Table of contents :
1 Geological context and formation of Ni laterite deposits in New Caledonia
1.1 Overview
1.2 Geological formation
1.2.1 Geodynamic evolution of the SW Pacific
1.2.2 Peridotite nappe
1.3 Nickel laterite formation
1.3.1 Typical laterite profile in New Caledonia
1.3.2 Direct laterite formation
1.3.3 Multi-stage formation
1.4 Deposit model
1.4.1 Per-descensum formation
1.4.2 Relations of the Deposits to Structures
1.5 Description and objectives of the thesis
2 Reactive geochemical transport model of the formation of nickel laterite profile
2.1 Introduction
2.2 Article 1. Revealing the conditions of Ni mineralization in laterite profile of New Caledonia: insights from reactive geochemical transport modelling
2.3 Conditions for precipitation of talc-like and sepiolite-like minerals
3 Reactive Transport Modelling applied to Ni ore deposits in New Caledonia: Role of hydrodynamic factors and geological structures on Ni mineralization.
3.1 Introduction
3.2 Materials and methods
3.2.1 Conceptual model of Saprolitic nickel-ore formation in New Caledonia
3.2.2 Physical assumptions and equations governing hydrodynamic system
3.2.3 Geochemical system
3.3 Numerical model and Validation
3.4 Results and discussion
3.4.1 2D reactive transport model of saprolitic deposits formation
3.4.2 Impact of fractures on redistribution of ore deposits
3.4.3 Weathering of peridotite corestone within the set of fractures. Target-like ore
3.5 Conclusions
4 Conceptual model of multistage fracture filling due to the fluid overpressure. Evidences of low-to-medium-temperature hydrothermal fluid circulation during the formation of the Ni silicate veins
4.1 Article 3. Multistage crack seal vein and hydrothermal Ni enrichment in serpentinized ultramafic rocks (Koniambo massif, New Caledonia)
Bibliography
