Okorusu tailings – Part I: General characterisation of superficial tailings

Abstract

Former open-pit fluorite mining and processing at the Okorusu carbonatite deposit, Namibia, resulted in a large tailings dump comprising an old (~553 kt) and a new tailings part (~3.8 Mt). We characterise dump tailings mineralogically and geochemically in order to evaluate the potential for processing of relict fluorite and recovery of other commodities of interest. The tailings comprise largely quartz, K-feldspar, different carbonates (ankerite, calcite, dolomite, siderite and manganese carbonates), fluorite, apatite, pyroxenes, hornblende and Fe (hydro)oxides, whereas all other mineral phases such as baryte, pyrite, pyrochlore and REE fluorcarbonates occur in traces. The largest proportional difference between samples from the surface of the new and old tailings pertains to fluorite (average old dump: ~12 vol.%; average new dump: ~5 vol.%) and apatite proportions (average old dump: ~8 vol.%; average new dump: ~5 vol.%). Mineralogical contrasts between both tailings parts result largely from fluorite, apatite, quartz and Fe (hydro)oxides being more abundant in old tailings and carbonates being more abundant in new tailings. Geochemically, these contrasts are reflected in the major element composition, while variances in trace element compositions are mostly small. The mineral proportions clearly point out considerable fluorite (≥804 kt) and apatite (≥742 kt) resources, with potential by-products of Nb and REE based on drilling data. The readily available material in combination with a high proportion (about 80%) of liberated fluorite are favourable for re-processing by locally available flotation technology. Therefore, the tailings dump can be considered a valuable resource for the critical raw material fluorite and the fertiliser raw material apatite, which are wanted materials on international and national markets.