Oxide deposits are present in both contexts. For many authors Elias, ; Freyssinet et al. They present a similar geological setting, as islands emerging from a series of sinuous eastern-facing belts, corresponding to complex ocean-ocean or continent-ocean convergence zones, distributed along the western parts of the Pacific and Atlantic Oceans. They are separated from the continent to the west by marginal basins and from the ocean to the east by island arcs and active subduction or transcurrent fault zones.
Most of these ophiolites were emplaced during the Late Cretaceous to Paleogene periods. In this accretionary context, lying in the intertropical zone, weathering started as soon as the Oligocene.
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Hydrous Mg-Ni silicate deposits are dominant, and oxide or clay deposits are subordinate. A majority of these countries are tectonically still active, in the sense that they are under the influence of an active plate boundary. In a comparable accretionary context, New Caledonia is the emerged part of the Norfolk Ridge in the northern part of what has been recently referred to as Zealandia, a mostly submerged continent Mortimer et al.
In this area of interaction between the Australian and Pacific plates, the Eastern Gondwana margin has been fragmented into several thinned continental ribbons and extensional basins, some of which are oceanic e.
Future Sources of Industrial Metals
Figure 3. The terrane was exposed to emergence, dismantling, weathering, and erosion during the Neogene post-obduction period. The oldest weathering profiles have been dated indirectly by paleomagnetism methods at 25 Ma Sevin et al. Worldwide, Ni-Co—laterite deposits are controlled by a wide range of interacting factors Freyssinet et al. Weathering tends to accumulate laterite and thus nickel; tectonic activity or eustatic variations tend to erode and destroy, or cover up and conceal, these accumulations. If the rate of chemical weathering of a peridotite is greater than the rate of physical erosion, a deep weathering profile may be preserved.
Conversely, it is destroyed. Mantle rocks dunite, harzburgite, lherzolite , which have a very low hydraulic conductivity, are, however, densely fractured as a result of their long evolution in oceanic domains and their eventual obduction.
The Genesis and Nature of Silver Ores
Serpentinization of the protolith, diffuse or closely associated with fracturing, is a common alteration, which, on the whole, corresponds to hydration and cooling of the lithospheric mantle rocks. Once exposed on land, above sea level, the fracture network allows pervasive underground water circulation and setting up of a water table.
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Through internal water fluctuations and circulation, fluids in the ultramafic protolith propagate weathering, leading to a general saprolitization, karstification with sinkholes, underground conduits, and lapies. A complete and differentiated weathering profile may reach 50 m in thickness. The youngest and active front is at the base, the oldest at the top. It comprises, from top to base Eggleton, ; Fig. The highest concentration of nickel is at the base of the weathering profile, in the typical high-grade hydrous Mg-Ni silicate ore type, where its concentration may reach several tenths of a percent of Ni.
The degree of serpentinization of the protolith is therefore an important factor of mineralization, which greatly contributes to Ni grade, more than the degree of weathering Orloff, ; Pelletier, ; Trotet et al. When early serpentine is saturated in Ni, the typical green-colored garnierite precipitates commonly as a stockwork of veins. Garnierite occurrences are generally associated with other supergene precipitates, including variously crystalized silica chalcedony, quartz , as brecciated veins, sigmoidal fillings, and slickenside coating on serpentinite fault planes, indicating a complex supergene syn-tectonic evolution Cluzel and Vigier, ; Genna et al.
Higher in the profile, the Mg discontinuity is characterized by a drastic change in the Mg, Si, and Fe proportion.
Life Cycle Assessment of Metals: A Scientific Synthesis
A number of dark Mn- and Co-rich concretions appear that include phyllomanganate family minerals, which are the main Co and Ni carriers Llorca, ; Freyssinet et al. In the rest of the profile, Ni is mainly hosted by iron oxy-hydroxides dominant goethite, subordinate hematite in the typical low-grade oxide ore type. Nickel content is correlated with goethite crystallinity and anti-correlated with hematite proportion.
Nickel content thus decreases, while hematite increases upward Dublet et al. Owing to its importance regarding ore processing, detailed publications on the mineralogy and geochemistry of deposits worldwide are generally available. However, the shape of deposits and distribution of ore grades are rarely documented. The experience and knowledge gained from research on deposits in New Caledonia can be applied to exploration in accretionary terranes elsewhere.
At a large scale, the regolith surfaces topping the Peridotite Nappe are enclosed in a broad, shallow-dipping envelope, delineating a conspicuous longitudinal up warp Fig. Although disrupted at smaller scale by many discontinuities and associated with complex sets of stepped benches, this bulge shape is interpreted as the result of the long-term post-obduction isostatic re-equilibrium.
It is worth noting that this uplift, which started as soon as 25 Ma Sevin et al. In New Caledonia, mining geologists commonly distinguish deposit types according to their geomorphological context Fig. Basin deposits host large areas of dominant oxide ore e.
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Mountain deposits typically host smaller-sized deposits, with typical high-grade hydrous Mg-Ni silicate ore. Plateau deposits are intermediate with both types of ore, including even clay mineralogy e. This typology of deposit and geomorphological features can be associated in a consistent model Maurizot et al. Basins, plateaus which are inverted basins , and mountainous areas which are dissected plateaus may be considered as different steps of a continuum of geomorphological evolution on which the nickel factory has evolved through time Fig.
The rate of chemical weathering versus the rate of physical erosion driven by post-obduction deformation is therefore crucial in the constitution of Ni resources. Where subsidence prevails and internal drainage is impeded, Ni concentrations will cease to increase and will be buried under sediments e. When the rate of uplift compensates for the rate of weathering, large contiguous basin deposits can form, with dominant residual enrichment and oxide type ore e. Where the rate of uplift is higher than the rate of weathering, basins are inverted to plateaus, and absolute enrichment prevails, forming the typical high-grade hydrous Mg-Ni ore deeper in the profile e.
Beyond that, in mountainous areas, the trend of enrichment is enforced by more uplift, to the expense, however, of a partial destruction and fragmentation of the deposits, which are dissected by erosion. This last stage before complete destruction of deposits is exemplified by many small high-grade deposits disseminated in New Caledonia, which were mined in the early time of Ni exploitation. The importance of exceptionally large, or rich, deposits has greatly increased in the age of globalization when a small number of international corporations dominate the metals market, based on few very large ore deposits, practically anywhere in the world.
Search for giant orebodies thus drives the exploration industry: not only the in-house teams of large internationals, but also hundreds of junior companies hoping to sell their significant discoveries to the "big boys". The book will provide abundant material for comparative research in metallogeny, practical information for the explorationists as to where to look for the "elephants", and some inspiration for commodity investors. While every attempt has been made to ensure stock availability, occasionally we do run out of stock at our stores.
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