1 Mineralogical and Geochemical Studies – Key Elements in Evaluating Nickel Laterites Phillip L Hellman Hellman & Schofield Pty Ltd Sydney office: Suite 6, 3 Trelawney St, Eastwood NSW 2122. Perth office: 36 / 4 Ventnor Ave, W Perth WA 6005 Brisbane office: 2 / 1 Swann Road, Taringa, QLD, 4068 Ph 61 2 9858 3863 Fax 61 2 9858 4057 e-mail plh@hellscho.com.au 1. INTRODUCTION An electron micro-probe study of limonite and saprolite samples from a South-West Pacific nickel-cobalt laterite deposit was completed to provide an insight into its geochemistry and mineralogy. This work was completed in 1991, with further studies in 1995, and was designed to provide data relevant for metallurgical testwork and the various processing options under consideration. The current owner of the deposit has provided permission for the publication of the results of the work but has requested that the deposit not be named. One option being considered at the time was the possibility of producing a “lateritic concentrate” by various flotation strategies. The success of any lateritic beneficiation strategy is dependent upon the distribution of the target elements (viz Ni, Co and possibly Cr) within mineral phases and their intergrowths. Another processing option also under consideration was a pressure acid leach (“PAL”) followed by a low temperature neutralization leach of acid-consuming saprolite resulting in atmospheric leach residues (“ALR”). The absence of detailed geochemical and mineralogical studies prompted the first study in 1991 and the success of that work resulted in a further mineralogical study of PAL metallurgical products in 1995. 2. GEOLOGICAL SETTING AND LATERITE PROFILE The deposit (termed the “SWP” deposit) is typical of the many wet tropical laterites found in Indonesia, Papua New Guinea and the Philippines. The laterite is developed over an ophiolite 1 consisting of gabbros 2 and dunites 2 with minor pyroxenites 2 and harzburgites 2 . Chromite layers are widespread but rarely exceed 50 mm in thickness and have contributed to local alluvial concentrations. A summary of the laterite profile is given in Figure 1, below: Laterite Layer Average Thickness (m) Comments Overburden Humic layer Red limonite ~2.5 ~0 – 1 ~1.5 (0 – 30) Soft, free dig Rainforest litter Leached, with low Ni & Co Yellow limonite ~7.5 (0 – 30) Bulk of known Ni & Co resource Saprolite Low rock Rocky Bedrock ~6 ~2 (0 – 20) ~3.5 (0 – 30) Enriched in Ni & Co and Mg Contains variable amounts of dunite boulders within a saprolite host Typically dunite Figure 1. Laterite Profile Summary 1 An upthrust slice of the oceanic crust. 2 Gabbro is a coarse grained igneous rock consisting of Ca-felspar (Ca, Al & Silica), pyroxene (Ca, Mg,Fe, Al & silica) and olivine (Mg & silica); dunite is An igneous rock consisting almost wholly of Mg-olivine (with some chromite and pyroxene); pyroxenite is consists almost wholly of pyroxene; harzburgite consists of olivine and minor orthopyroxene (Mg & silica)