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- Ferruginous saprolite (or yellow A MODEL FOR Ni-LATERITE that this uplift, which started as soon as
limonite), where the inherited bed- DEPOSITS IN ACCRETIONARY 25 Ma (Sevin et al., 2014), is still active as
rock structure is retained but trans- TERRANES recorded in the construction of the
formation to a soft material is almost Owing to its importance regarding ore Pleistocene and Holocene reef barrier that
complete, correlative of an important processing, detailed publications on the rims Grande Terre (Cabioch, 2003).
mass loss. mineralogy and geochemistry of deposits In New Caledonia, mining geologists
• Saprolite divided into: worldwide are generally available. commonly distinguish deposit types
- Saprolite (more than 20% of weath- However, the shape of deposits and distri- according to their geomorphological
ered rock), in which concentric sphe- bution of ore grades are rarely docu- context (Fig. 4); namely, basin, plateau,
roidal weathering preserves blocks or mented. The strategy of exploration for and mountain (crest or slope) deposits
boulders of fresh rocks (corestones); many mining companies is commonly (Trotet et al., 2015). Basin deposits host
- Saprock (less than 20% of weathered very basic, consisting of expensive and large areas of dominant oxide ore (e.g.,
rock). Saprock may be the Ni-richest environmentally harmful systematic grid Goro, Prony). Mountain deposits typically
horizon. drillings with a mesh adapted to the high host smaller-sized deposits, with typical
• Protolith fresh peridotite. variability of the Ni content (Ni semi- high-grade hydrous Mg-Ni silicate ore.
The highest concentration of nickel is variogram ranges are commonly <20 m). Plateau deposits are intermediate with
at the base of the weathering profile, in The experience and knowledge gained both types of ore, including even clay
the typical high-grade hydrous Mg-Ni from research on deposits in New mineralogy (e.g., Tiébaghi). This typol-
silicate ore type, where its concentration Caledonia can be applied to exploration ogy of deposit and geomorphological
may reach several tenths of a percent of in accretionary terranes elsewhere. features can be associated in a consistent
Ni. In the saprock and above, early- At a large scale, the regolith surfaces model (Maurizot et al., 2019). Basins,
formed serpentinite can be secondarily topping the Peridotite Nappe are enclosed plateaus (which are inverted basins), and
enriched up to 3–6 wt% by Ni-Mg sub- in a broad, shallow-dipping envelope, mountainous areas (which are dissected
stitution. The degree of serpentinization delineating a conspicuous longitudinal up plateaus) may be considered as different
of the protolith is therefore an important warp (Fig. 3; Sevin et al., 2012). At both steps of a continuum of geomorphological
factor of mineralization, which greatly ends of Grande Terre (Iles Béleps to the evolution on which the nickel factory has
contributes to Ni grade, more than the north and Ile des Pins to the south), the evolved through time (Fig. 4).
degree of weathering (Orloff, 1968; major duricrust surfaces plunge under sea The rate of chemical weathering versus
Pelletier, 1996; Trotet et al., 2015). When level, whereas in the middle of the island, the rate of physical erosion (driven by
early serpentine is saturated in Ni, the dismantled regolith surfaces culminate at post-obduction deformation) is therefore
typical green-colored garnierite precipi- more than 1000 m. Although disrupted at crucial in the constitution of Ni resources.
tates commonly as a stockwork of veins. smaller scale by many discontinuities and Where subsidence prevails and internal
Garnierite occurrences are generally associated with complex sets of stepped drainage is impeded, Ni concentrations
associated with other supergene precipi- benches, this bulge shape is interpreted as will cease to increase and will be buried
tates, including variously crystalized the result of the long-term post-obduction under sediments (e.g., Fluvio-lacustrine
silica (chalcedony, quartz), as brecciated isostatic re-equilibrium. It is worth noting Formation, near the Goro deposit; Folcher
veins, sigmoidal fillings, and slickenside
coating on serpentinite fault planes, indi-
cating a complex supergene syn-tectonic Economic window
evolution (Cluzel and Vigier, 2008; Size Large size (world class) Small size
Genna et al., 2005; Iseppi et al., 2018). Grade Low grade (Ni <= 1 wt %) High grade (Ni > 1.5 wt %)
Higher in the profile, the Mg disconti- Deposit Ore main Laterite>saprock Saprock>laterite
regolith
horizon
Oxide
nuity is characterized by a drastic change Ore type Co by-product Hydrous Mg-Ni
silicate
in the Mg, Si, and Fe proportion. A num- Enrichment Residual Absolute
1 000 m
ber of dark Mn- and Co-rich concretions
appear that include phyllomanganate fam-
ily minerals, which are the main Co (and Al tude Low Water table
Ni) carriers (Llorca, 1993; Freyssinet et Sea level High
al., 2005; Roqué-Rosell et al., 2010; 0
Fritsch et al., 2014). In the rest of the
profile, Ni is mainly hosted by iron oxy-
hydroxides (dominant goethite, subordi- Morphology Drowned offshore Basin Plateau Mountain
nate hematite) in the typical low-grade Dominant external Subsidence Upli
oxide ore type. Nickel content is corre- forcing factor Burial Weathering Erosion
lated with goethite crystallinity and anti- Tectonics Weak structural control High structural control
correlated with hematite proportion.
Nickel content thus decreases, while
hematite increases upward (Dublet et al., Figure 4. A model for Ni-laterite deposits in New Caledonia. Typology, based upon geomorphology,
geology, and economic factors of Ni deposits, is a function of altitude, which in turn is a function
2015, 2012). of time.

8 GSA Today | May 2019