In the Kalgoorlie area of Western Australia, significant deposits of Ni-Co laterite are developed on large ultramafic complexes at Siberia and Bulong. The ultramafic rocks form part of extensive Archean greenstone belts in the granitoid terrain of the southeastern Yilgarn Block, and were lateritized during Late Cretaceous to early Tertiary times.The laterite profile developed over serpentinized dunite and peridotite is divided into four zones on the basis of morphology, mineralogy, and chemical composition. In the oxidized bedrock zone, Ni-rich "saprolitic serpentine" and smectite are developed in joints and fractures in the bedrock. The overlying saprolite zone consists predominantly of "saprolitic serpentine" and may retain bedrock structures and textures. It is characterized by a decrease of Mg and an increase in Ni upward. A well-defined clay zone is commonly developed over the Bulong saprolite zone. The clay zone is composed essentially of nontronite and quartz, and commonly contains Ni values of 1 to 2 percent. The limohire zone occurs at the top of the profile and may consist of an upper hematite-rich section and a lower goethite-rich section. The limonite zone is rich in Fe, Al, and Cr and is formed by residual concentration of stable oxides. A concentration of Mn oxides, commonly containing high Co and Ni grades, occurs at the base of the limonite zone. Where sufficiently thick, Co and Ni-rich Mn concentrations have been mined for direct smelter feed to augment Co and Ni production from Kambalda Ni sulfides. Physical and chemical characteristics of the underlying bed rock are important controls on the distribution of Ni and Co accumulations.The mode of occurrence of Ni and the presence of Co are significant factors in the economics of exploiting these deposits.