The Brolga deposit is a Ni-Co enriched laterite, formed over intensly
serpentinised peridotites of the Marlborough Block. Basement lithologies
at Brolga consist predominantly of serpentinised dunite, with lesser
massive serpentinite shear, gabbro and harzburgite. Investigation of the
deposit has defined 17 different varieties of saprolite, ferralite (limonite),
and ferricrete. Different combinations of these varieties form three
distinct lateritic associations, the Hard, Soft and Dyke Associations.
Recognition of key textural and mineralogical features in each
association, an understanding of basement geology, and an awareness of
current climatic conditions, suggest that: 1. both the Hard and Soft Associations have formed under relatively
dry climatic conditions; 2. the Soft Association reflects much poorer groundwater drainage
than the Hard Association; and 3. the Dyke Association has formed from a gabbroic basement, and
has also been affected by restricted groundwater drainage, caused
by poorly permeable marginal serpentinite shears.