Original paper
An alternative model for the formation of hydrous Mg/Ni layer silicates (“deweylite”/“garnierite”) in faulted peridotites of New Caledonia: II. Petrography and chemistry of white and green clay infillings
Fritsch, Emmanuel Jean Charles; Juillot, Farid; Dublet, Gabrielle; Fonteneau, Lionel; Fandeur, Dik; Martin, Etienne; Caner, Laurent; Auzende, Anne-Line; Beaufort, Daniel
European Journal of Mineralogy Volume 31 Number 5-6 (2019), p. 945 - 962
53 references
published: Dec 20, 2019
published online: Aug 21, 2019
manuscript accepted: May 31, 2019
manuscript revision received: May 2, 2019
manuscript received: Jul 28, 2018
Abstract
This study, which complements a first mineralogical work, presents detailed petrographic and chemical data on the sequences of clay infillings commonly found in serpentine veins of reactivated faults from the New Caledonian peridotite formation. Chemical trends and transfers established from the outer serpentine fringe to the inner clay infilling, as well as from the white (deweylite) to the greenish (garnierite) parts of the veins, enable us to decipher the processes and conditions involved in the redistribution of Mg and Ni along reactivated faults. As commonly reported from studies of peridotite formations worldwide, two main chemical trends are distinguished. In New Caledonia, these trends belong to distinct periods of tectonic activities associated with the dislocation and early alteration of the ophiolite nappe. They result from two kinds of Ni-ore-forming processes and reveal a significant decrease in the mobility of Ni over time. The first process relates the step-by-step alteration of serpentine species into talc-like (TL) minerals to the sequential leaching of Mg and Fe, together with their local replacement by Ni in octahedral sites of the newly formed TL minerals. The TL minerals are thus considered as the main Ni-bearing phases (pimelite) of the ore. The large-scale redistribution of Mg and Ni during a first period of tectonic activity and alteration leads to the differentiation of white (Ni-free) and olive-green (Ni-rich) clay infillings along the reactivated faults. The second process belongs to a second period of tectonic activity and alteration where most of the serpentine species have been converted into TL minerals without major Ni enrichment (formation of a second sequence of milky white and turquoise clay infillings). Redistribution of Mg and Ni occurs over shorter distances from the transition of Mg-rich to Ni-rich clay infillings (or breccias). It results from oscillatory phenomena and self-organised precipitation processes leading to cyclic and inverse Mg and Ni variations in alternating bands of contrasted optical anisotropy. The early redistribution of Ni and the significant decrease in the mobility of this element from the first to the second period of tectonic activity provide better constraints on an alternative model for the genesis of the Ni-silicate ore.
Keywords
nickel • faulted peridotites • serpentine veins • white (deweylite) and green (garnierite) clay infillings • 1:1 and 2:1 hydrous Mg/Ni layer silicates • kerolite • pimelite • népouite • nickel-ore-forming processes