Laterite resource definition through geophysics

The Ravensthorpe nickel laterite deposits in Western Australia are defined by different regolith layers that must be mined and processed separately. A surface caprock layer is waste material (and used as road base), beneath which are limonite and saprolite layers that are sent to separate processing streams. As the regolith grades into saprock below this, the mineral resource falls below an economic level. Within the limonite and saprolite, further variations in material properties affect processing results, including during the beneficiation, or physical upgrading stage. The boundaries between these layers are geometrically complicated, such that very close-spaced drilling would be required to define them to an accuracy relevant to mining. In an effort to mitigate wider drillhole spacing, different geophysical techniques have been trialled to trace boundaries between drill holes. Any technique must be rapid, with high spatial resolution, and preferably continuous, in order to cover the many kilometres of resource drilling and eventually be deployed in a mine pit during operation. Petrophysical logging serves as both a check on the surface geophysics and a potential avenue for predicting material behaviour where multi-element geochemical assays alone have failed. Conductivity and gamma logging, for example, characterise discrete zones that are not represented in the geochemistry or visual logging. Active and passive seismic, electromagnetics, and ground-penetrating radar show variable success in mapping different interfaces. Ground EM demonstrates the most promise, mapping both resistive caprock and a conductive, clay-rich, upper limonite layer that is also indicated by the petrophysical logging.