Abstract
Abstract. This study presents and discusses infrared spectroscopic data of well characterised, naturally occurring trioctahedral layer silicates of the serpentine (Srp), talc (Tlc), and sepiolite (Sep) mineral groups, which are found in reactivated faults and sequences of white and green clay veins (deweylite and garnierite) of the New Caledonian Ni-silicate ores. Bands assigned to the OH stretching vibrations of these 1:1 and 2:1 layer silicates in both the fundamental and first overtone regions of mid- and near-infrared (MIR and NIR) spectra, respectively, are compared to those reported in the literature for synthetic Mg–Ni series of the Srp and Tlc mineral groups. They are also presented according to the sequences of infillings recognised in the white and green veins of the Ni-silicate ores. The study reveals that serpentine-like (SL) minerals of the first sequences of clay infillings are residues of larger crystals of serpentines (lizardite, chrysotile, and antigorite) and that the newly formed talc-like (TL) minerals and Sep are the main Ni-bearing carriers of the Ni-silicate ores. Decreasing crystal size and order in serpentine species have major effects on vibrational bands. They favour the broadening of the OH stretching bands, the degradation of the signals assigned to the interlayer OH, and the enhancement of the signal related to weakly bound water molecules. The replacement of Mg by Ni in octahedral sites of the 2:1 layer silicates (TL, Sep) of the greenish clay infillings can be traced by specific OH stretching bands related to the Mg3OH, Mg2NiOH, MgNi2OH, and Ni3OH configurations in the fundamental (MIR) and first overtone (NIR) regions of the spectra. The dominance of the Mg3OH and Ni3OH configurations with respect to mixed configurations in the Mg–Ni mineral series of the clay infillings (mostly in the dominant TL minerals) suggests that Mg and Ni segregation is related to separate Mg-rich and Ni-rich mineral phases rather than to a cationic clustering within the individual particles. This segregation of Mg and Ni in discrete mineral phases is related to Mg–Ni oscillatory zoning patterns (banded patterns) and is reproduced at the scale of the Ni-silicate ores between the white (deweylite) and greenish (garnierite) veins of the reactivated faults.
Highlights
White and green (Mg-rich and Ni-rich) clay infillings of the New Caledonian Ni-silicate ore deposits are commonly found at depth in crosscutting veins (“minerai quadrillé”) from peridotite saprock of thick lateritic profiles (Cluzel and Vigier, 2008; Wells et al, 2009; Fritsch et al, 2016, 2019; Cathelineau et al, 2017; Muñoz et al, 2019)
Bulk samples selected for this study belong to the large set of serpentine veins and clay infillings collected by Fritsch et al (2016) in the saprock of thick Ni-laterite deposits of New Caledonia (Fig. 1a, b)
For the Srp mineral group, we will mostly rely on the synthetic Mg–Ni series investigated by Baron and Petit (2016), as OH stretching modes involving Ni could not be related to well-defined peaks from our set of samples containing SL minerals (i.e. SL and STL infillings)
Summary
White and green (Mg-rich and Ni-rich) clay infillings (deweylite and garnierite) of the New Caledonian Ni-silicate ore deposits are commonly found at depth in crosscutting veins (“minerai quadrillé”) from peridotite saprock of thick lateritic profiles (Cluzel and Vigier, 2008; Wells et al, 2009; Fritsch et al, 2016, 2019; Cathelineau et al, 2017; Muñoz et al, 2019). In New Caledonia, two sequences of clay infillings have been recognised in the crosscutting veins of the saprock (Fritsch et al, 2016, 2019) They were attributed to two major periods of tectonic activity, following the dismantling and cooling of the ophiolite nappe and related to the early alteration of the serpentine (Srp) network of the peridotites (Mg−Fe 1 : 1 layer silicates) into fine-grained and ill-ordered SL and TL minerals (locally Sep) (Fritsch et al, 2016). Modification of the spectroscopic signals for each mineral series and sequence of infillings is used to better decrypt the mechanisms involved in the formation of the Ni-silicate ores
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