A Mantle on the Surface
The story of New Caledonia's nickel deposits begins with a colossal geological event. During the Eocene epoch, around 37 million years ago, a vast slab of the Earth's upper mantle, called an ophiolite or peridotite nappe, was thrust up and over the island's existing crust. This event, known as obduction, placed dense, ultramafic rocks—primarily peridotite—onto the surface, a material normally found deep within the planet. This single ophiolite sheet covers over a third of Grande Terre, the main island of New Caledonia.
Over millions of years, the island's tropical climate went to work on these exposed mantle rocks. Intense rainfall and warm temperatures drove a chemical weathering process called laterization. Rainwater percolated through the peridotite, leaching away soluble elements like magnesium and silica and leaving behind a thick soil profile rich in less soluble elements, particularly iron and nickel. This natural enrichment process concentrated nickel from an average of 0.2% in the parent rock to economically viable grades of 2-5% or higher in the resulting laterite soil. The resulting area is a reddish-brown expanse, caused by the high concentration of iron oxides.
The Chemistry of Green Gold
While the soil is predominantly red from iron, the main resource is the nickel, which often appears as bright green veins and coatings. This green mineralization is primarily garnierite, a general term for a mix of hydrous nickel-magnesium silicates. Discovered in 1864 by French geologist Jules Garnier, this "green gold" can contain a nickel content of 20-30%. Mining began in earnest after 1873, transforming the island's economy and making it one of the world's top nickel producers. New Caledonia is estimated to hold about 10% of the world's total nickel reserves.
The high nickel concentration makes the soil toxic to most plants. A unique suite of flora has adapted to these extreme conditions. Some species are hyperaccumulators, plants that actively absorb and store metals in their tissues. The most extreme example is Pycnandra acuminata, a rainforest tree whose latex can contain up to 25% nickel by dry weight, the highest concentration of nickel found in any living organism. This nickel-rich sap is a startling turquoise-green or blue-green color, sometimes called "sève bleue" (blue sap). Scientists believe this adaptation may deter herbivores or protect the plant from the toxic effects of the metal in the soil.