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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
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