A scar from a vanished ocean
The chromite deposits near the Dawei River are not native to the continental crust of Southeast Asia. They are part of an ophiolite, a term for a complete section of oceanic crust and the underlying upper mantle that has been violently shoved onto a continent. These rocks are the geological remnants of the Neotethys Ocean, which once separated the ancient supercontinents of Gondwana and Laurasia. The process, known as obduction, occurred as tectonic plates converged, closing the ocean basin and scraping up sections of the seafloor.
The rocks hosting the chromite are ultramafic, meaning they are very low in silica and high in magnesium and iron. Specifically, they include peridotite and dunite, rock types characteristic of the Earth's mantle. Within these dark mantle rocks, the chromite forms as irregular, lens-shaped bodies called podiform deposits. This style of deposit is exclusive to ophiolites and is a direct physical record of magmatic processes that occur deep beneath the ocean floor. Geologists study these "out-of-place" rocks to understand the composition of the mantle and the dynamics of seafloor spreading centers.
A continental collision zone
The geology of Myanmar is the product of immense tectonic forces, as several continental blocks and island arcs drifted north and collided with the main Asian continent. The Dawei deposits are located within a suture zone that marks the collision between the West Burma Block and the Shan-Thai Block (also called Sibumasu). This collision was part of the final closure of the Neotethys Ocean during the Mesozoic and Cenozoic eras.
The chromite itself is an oxide mineral with the chemical formula (Fe, Mg)Cr₂O₄. It crystallizes early from cooling magma deep within the mantle. The deposits in this region are part of what is known as the Western Ophiolite Belt, a discontinuous line of oceanic fragments that runs north-south through western Myanmar. This belt represents the suture where the West Burma island arc terrane was welded onto the larger continental block to its east. The intense pressure and heat from these tectonic events folded and faulted the rocks, creating the complex geological structure seen today. Analysis of these ophiolites allows geologists to reconstruct the paleogeography of the Tethyan field and pinpoint the timing of continental assembly in Southeast Asia.