Geochemistry and mineralogy of the regolith profile over the Aries kimberlite pipe, Western Australia

Residual regolith over the Aries kimberlite pipe, situated in the central Kimberley Block of Western Australia, consists of lateritic duricrust, mottled zone, upper saprolite, lower saprolite and saprock. Duricrust and mottled zone are dominated by goethite and kaolinite while saprolite consists mostly of smectite with minor amounts of mica and serpentine. Most kimberlite indicator minerals except chromite have been weathered. Kimberlitic rock fabric is preserved in the regolith up to the top of saprolite. Chromium, Ni, Co, Cu, Nb, Zr, Ti, P and rare earth elements (REE) are significantly enriched in the mottled zone and duricrust relative to the saprolite and fresh kimberlite. Vanadium, Sb, As, Mo and U, derived from the sediments and basaltic rocks around the pipe, are also enriched. Goethite partly hosts Cr, Ni, and Co, and Nb and REE are associated with anatase. The clay fractions of the mottled zone and saprolite contain significant quantities of Cr, Co, Cu, Ni, Zr, Ga, Ti, Nb, Zn and REE. A suite of six trace elements comprising those associated with ultramafic rocks (Cr and Co) and felsic rocks (Nb, La, Sm and P) can readily distinguish the kimberlitic laterite from that derived from ultramafic or felsic rocks.