A geological factory powered by weather
The nickel ore deposits in Sekong Province are a type of mineral accumulation known as a laterite. These deposits form from the intense, prolonged chemical weathering of specific parent rocks, in this case, ultramafic rocks rich in the mineral olivine. This region of Laos is part of an ancient block of continental crust called the Kontum Massif, which contains rocks that are hundreds of millions of years old. Over millions of years, the consistent heavy rainfall of a tropical climate has slowly broken down these parent rocks.
The process is a massive natural filtration. Rainwater, which is slightly acidic, percolates through the ground and dissolves the more mobile elements in the rock, primarily magnesium and silica. These elements are carried away by groundwater, leaving behind a thick, residual layer enriched in less-soluble elements like iron and aluminum. Nickel, originally present in the parent rock's olivine at concentrations around 0.3%, is also released. The nickel is not washed away. Instead, it is redeposited and concentrated at specific depths within the weathered rock profile, creating an economically valuable ore body. This entire process can take as little as one million years to develop a complete laterite profile.
Anatomy of the ore deposit
A vertical cross-section of a nickel laterite deposit reveals distinct layers created by the weathering process. The uppermost layer is often a hard, iron-rich cap called ferricrete. Below this lies the limonite zone, a reddish-brown layer composed mainly of the iron-oxide mineral goethite. In this zone, nickel concentrations can range from 0.8% to 1.5%, with the nickel ions incorporated directly into the crystal structure of the goethite.
The most valuable part of the deposit is typically found deeper, in the saprolite zone. This layer is essentially soft, decomposed parent rock with a clay-like consistency. Here, the highest concentrations of nickel are found, often exceeding 1.5%. The nickel precipitates from the percolating water to form secondary silicate minerals. These minerals are collectively known as garnierite, which is not a single mineral but a general term for a mixture of green, nickel-rich hydrous silicates. Pockets of garnierite can contain very high nickel contents, sometimes between 20% and 40%. The deposits are mined using open-pit methods, and the ore is often processed using techniques like high-pressure acid leaching (HPAL) to extract the nickel for use in batteries and stainless steel.