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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