ALBERT

Description: 〈span〉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.〈/span〉

Description: New electron-microprobe analyses of ‘tsavorites’ from the Neoproterozoic Metamorphic Mozambique Belt deposits allow the characterization of green grossular according to its trace-element chemistry (V, Cr, Mn). Five chemical types are defined: type 1, vanadian grossular with V 〉Cr 〉Mn (in atoms per formula unit); type 2, vanadian grossular with V 〉Mn 〉Cr; type 3, Mn-bearing vanadian grossular with Mn 〉 V 〉 Cr; type 4, Mn-bearing chromian grossular with Mn 〉 Cr 〉 V; and type 5, Cr- and Mn-bearing grossular with Cr 〉 Mn 〉 V. These types are also characterized by different absorption spectra in the ultraviolet–visible–near infrared. Type 1 tsavorite spectra show a total absorption below 430 nm due to the high vanadium contents. Type 2 tsavorite spectra present the classical absorption bands of V. Types 3 and 4 tsavorite spectra display additional shoulders at 407 and 408 nm due to Mn 2+ , whereas spectra of Cr-bearing types 4 and 5 tsavorite show the two additional bands of Cr 3+ at 697 and 701 nm. The different absorption spectra also indicate Fe 2+ -Ti 4+ charge transfer. We measured OH – equivalent to 0.08 to 0.38 wt% eq. H 2 O within the structure. Concentrations of vanadium, chromium and manganese are good chemical "fingerprints" for determining the geographic provenance of economic tsavorite from Kenya, Tanzania and Madagascar.

Description: 〈span〉Mineralogy, chemistry and spatial distribution of phyllomanganates, found in abundance at the bottom of thick Ni-laterite deposits, were established on dislocated vein-infillings showing banded and fibrous patterns (〈span〉i.e.〈/span〉 colloforms with rows of tiny boxworks) to decipher their conditions of formation in intensively faulted and Al-poor regoliths of the New Caledonian ophiolite (Koniambo klippe). Phyllomanganates of the vein-infillings belong to a Ni-rich, 9.5 Å type (Ni-asbolane) and two 7 Å types of contrasted alkali (K and Na) contents (Alk–birnessite and H–birnessite). Periodic development of radial cracks and differentiation of sequences of phyllomanganates enabled distinguishing up to five episodes of infillings. The first outer infilling of a botryoidal shape is devoid of Ni–asbolane and made of flakes of Alk–birnessite of different lengths and thicknesses, mixed and then replaced toward the inner section of the infilling by fine-grained H–birnessite. For the other infillings, the sequence of phyllomanganates is invariably the same from their outer to inner parts: (1) laths of Ni–asbolane in sealed cracks, (2) flakes of Alk–birnessite at the margin and extremity of the cracks or isolated in (3) a continuous groundmass of fine-grained H–birnessite. The relative proportions of these three phyllomanganate species can change drastically from one infilling to another. That of Ni–asbolane is closely related to the development of cracks, which are both important in the last two infillings. Thermal effects (〈span〉e.g.〈/span〉 cooling of a hot fluid on an older surface deposit) likely contributed to the development of radial cracks and, by self-organized diffusion and precipitation processes, to the sequential production of phyllomanganates (Ni–asbolane, Alk–birnessite and then H–birnessite) of decreasing particle sizes. As for the Mg/Ni phyllosilicate (“garnierite”) ore found at greater depth in the vein-infillings of the saprolite, the Ni/Co phyllomanganate (“chocolate”) ore observed abundantly as relics (dislocated veins) at the transition with the overlying laterite could also result from the early hydrothermal alteration of Al-poor peridotites following the emergence, dislocation and cooling of the ophiolite nappe onto the New Caledonian basement.〈/span〉

Description: Detailed textural and mineralogical investigations carried out in mineralized veins of the New Caledonian peridotites reveal three major periods of infilling and alteration closely linked to post-obduction tectonic activity. The first two periods of infillings are related to the alteration of hydrothermal serpentines, mostly found in the thick serpentine network of the peridotites, into fine-grained serpentine-like residues and newly formed talc-like minerals of weak but variable swelling capacity. The alteration of serpentine into talc-like minerals is limited during the first period of infilling and almost completed during the second one. In some fault zones of the New Caledonian peridotites, talc-like minerals are replaced by sepiolite. Such alterations are reported in both the Ni-free and Ni-rich zones of the infillings ( i.e. in the white ‘deweylite’ and greenish ‘garnierite’ of the fault zones, respectively). They led to the individualization of the hydrous Mg/Ni silicate ore, which is nowadays found in fractures of the saprock and saprolite units of thick lateritic profiles. The third and last period of infilling is assigned to the accumulation of silica and crystallization of quartz. This succession of clay minerals (serpentine-like and talc-like minerals, sepiolite) and quartz in the infillings is interpreted as the result of sequential exportation of Mg and redistribution of Ni along the reactivated faults, generating periods of increasing Si activity in solutions. In this paragenetic model, the meteoric water infiltrating repeatedly the permeable network created by post-obduction tectonic activity would have interacted with a low-temperature hydrothermal field following the exhumation and cooling of the ophiolite nappe. In the less permeable parts of the fault network, restricted leaching conditions would have generated greater alkalinity in solutions and therefore favored the crystallization of sepiolite instead of finely divided talc-like minerals.

Description: 〈span〉Mineralogy, chemistry and spatial distribution of phyllomanganates, found in abundance at the bottom of thick Ni-laterite deposits, were established on dislocated vein-infillings showing banded and fibrous patterns (〈span〉i.e.〈/span〉 colloforms with rows of tiny boxworks) to decipher their conditions of formation in intensively faulted and Al-poor regoliths of the New Caledonian ophiolite (Koniambo klippe). Phyllomanganates of the vein-infillings belong to a Ni-rich, 9.5 Å type (Ni-asbolane) and two 7 Å types of contrasted alkali (K and Na) contents (Alk–birnessite and H–birnessite). Periodic development of radial cracks and differentiation of sequences of phyllomanganates enabled distinguishing up to five episodes of infillings. The first outer infilling of a botryoidal shape is devoid of Ni–asbolane and made of flakes of Alk–birnessite of different lengths and thicknesses, mixed and then replaced toward the inner section of the infilling by fine-grained H–birnessite. For the other infillings, the sequence of phyllomanganates is invariably the same from their outer to inner parts: (1) laths of Ni–asbolane in sealed cracks, (2) flakes of Alk–birnessite at the margin and extremity of the cracks or isolated in (3) a continuous groundmass of fine-grained H–birnessite. The relative proportions of these three phyllomanganate species can change drastically from one infilling to another. That of Ni–asbolane is closely related to the development of cracks, which are both important in the last two infillings. Thermal effects (〈span〉e.g.〈/span〉 cooling of a hot fluid on an older surface deposit) likely contributed to the development of radial cracks and, by self-organized diffusion and precipitation processes, to the sequential production of phyllomanganates (Ni–asbolane, Alk–birnessite and then H–birnessite) of decreasing particle sizes. As for the Mg/Ni phyllosilicate (“garnierite”) ore found at greater depth in the vein-infillings of the saprolite, the Ni/Co phyllomanganate (“chocolate”) ore observed abundantly as relics (dislocated veins) at the transition with the overlying laterite could also result from the early hydrothermal alteration of Al-poor peridotites following the emergence, dislocation and cooling of the ophiolite nappe onto the New Caledonian basement.〈/span〉

Description: 〈span〉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.〈/span〉