Page 6 - i1052-5173-29-5_flip-compressed (1)
P. 6

major Ni-producing countries, but it is     Lithology         Ore    Terminology
          recognized by metallogenists (Laznicka,
          2006) as one of the largest and richest
          Ni-providing areas in the world. Many
          scientific aspects are addressed by             0 m
                                                          0
          Ni-laterite deposits, including resources,                           Duricrust
          processes, environment, and societal
          impact (CNRT, 2017). In this paper, we
          present some recent advances in knowl-          10                   Plasmic   Laterite residuum
                                                                               horizon
          edge of geological controls and Ni specia-
          tion of these deposits in some emblematic                                            Figure 2. Weathering pro-
                                                                                               files on ultramafic proto-
          regions of the world and the contribution                                            lith. Lithology: common
          of New Caledonia in this respect.               20                                   terminology  of the differ-
                                                                                               ent horizons of the weath-
                                                                                               ering profile and reaction
          ORE TYPOLOGY                                                                         fronts  (after  Eggleton,
                                                                                               2001; Freyssinet  et  al.,
            Three types of Ni-laterite ore types,         30                   Ferruginous     2005; Butt and Cluzel,
          and hence deposits, are known worldwide                       Oxide  enrichment  Residual  saprolite  2013). Ore: enrichment
          (Brand et al., 1998; Freyssinet et al., 2005;                                        and ore types.
          Golightly, 2010; Butt and Cluzel, 2013),                                       Saprolite
          although mixed-type deposits are frequent:      40                   Mg
                                                          4
          (i) hydrous Mg-Ni silicate (garnieritic) ore                         discon nui y
          has the highest Ni grade, commonly ≥2                                Saprolite
          wt% Ni with very low Co; (ii) oxide ore is                   Mg-Ni silicate  Hydrous  enrichment  Absolute  Saprock
          dominated by iron oxy-hydroxides, with          5 5 50
          1.5 wt% Ni on average and generally
          recoverable Co up to 0.3 wt%; (iii) clay                             Weathering
                                                                               front
          deposits are dominated by Ni-bearing                                 Bedrock
          swelling clays (nontronite), containing         6 60
          Ni in the same range as oxide deposits.
            Garnierite was first discovered by Jules
          Garnier in New Caledonia (Garnier, 1867).   where the topographic gradient is low,   Southeast Asia, and the Caribbean region,
          Garnierite is actually a field term not    internal drainage is weak, the water table   all within ~25° of the equator (Fig. 1).
          recognized by the International Mineral   is high, leaching and precipitation are lim-  These countries hold more than 50% of
          Association (IMA), referring to a rather   ited, and Ni enrichment is mainly residual,   the global Ni laterite and were in the top
          complex mixture of poorly crystalized   located in the saprolite horizon; con-  five Ni producers for the past decade.
          phyllosilicate phases composed mainly of   versely, hydrous Mg-Ni silicate dominant   They present a similar geological setting,
          serpentinite-like, talc-like, chlorite-like,   ore deposits are formed where the topo-  as islands emerging from a series of sinu-
          and clay-like phases (Fritsch et al., 2016).   graphic gradient is important, internal   ous eastern-facing belts, corresponding to
          The appended “-like” refers to a certain   drainage is free, the water table is low,   complex ocean-ocean or continent-ocean
          weakness in the crystallinity of these   leaching and precipitation are optimal,    convergence zones, distributed along the
          phases and stacking disorder. Most of the   and Ni enrichment is mainly absolute and   western parts of the Pacific and Atlantic
          types individualized so far belong to a    located deeper in the saprock.  Oceans. They are separated from the con-
          continuous solid-solution from Mg to Ni   Theoretically, calculated as purely   tinent to the west by marginal basins and
          end-members. These highly variable,   “residual enrichment,” a simple removal    from the ocean to the east by island arcs
          poorly crystallized minerals can have up    of mobile elements would result in 0.6–1   and active subduction or transcurrent
          to 27% Ni (Freyssinet et al., 2005).  wt% Ni (Brand and Butt, 2001). Therefore,   fault zones. On land, most of these belts
            Clay deposits are typical of large, stable   higher grades recorded in saprock of   include one or several ultramafic terranes,
          continental, or cratonic, areas, formed on   hydrous Mg-Ni deposits must be explained   remnants of short-lived (~50 m.y.) mar-
          Archean to Proterozoic ultramafic layered   by other factors and are referred to as   ginal basins opened above successive
          intrusions or komatiite, further weathered   “absolute enrichment.” The total mass loss   subduction planes and then accreted dur-
          in the Mesozoic, continuing to the present   and dissolved phase may reach 80% of the   ing repeated convergence periods. Most
          (e.g., in Australia, Brazil, and Africa),   fresh rock (Trescases, 1975; Brand and   of these ophiolites were emplaced during
          whereas hydrous Mg-Ni silicate formed   Butt, 2001; Freyssinet et al., 2005).  the (Late) Cretaceous to Paleogene peri-
          mostly on ophiolites, eventually weathered                            ods. In this accretionary context, lying in
          in the Late Mesozoic to Cenozoic. Oxide   GEOLOGY AND METALLOGENY     the intertropical zone, weathering started
          deposits are present in both contexts. For   The four highest-ranked countries by   as soon as the Oligocene. Hydrous Mg-Ni
          many authors (Elias, 2002; Freyssinet et   Ni-laterite resource (New Caledonia,   silicate deposits are dominant, and oxide
          al., 2005; Golightly, 1981; Trescases, 1975),   Philippines, Indonesia, and Cuba) are   or clay deposits are subordinate. A major-
          oxide-dominant ore deposits are formed   located in the Southwest Pacific,   ity of these countries are tectonically still

       6  GSA Today  |  May 2019
   1   2   3   4   5   6   7   8   9   10   11