Rare Earth Element Mineralogy and its Recovery from the Neoproterozoic Riviera W–Mo Deposit, South Africa

Abstract

The Riviera W–Mo deposit is hosted by I- and A-type granitoids of the Neoproterozoic to Palaeozoic Cape Granite Suite that intruded the meta-volcano-sedimentary Malmesbury Group of the Saldania Belt. Scheelite-powellite and molybdenite are the principal ore minerals and are associated with greisen-, reduced endoskarn- and vein-type pervasive alteration of the granitoids. Exoskarn ore makes an insignificant contribution to the total resources. The mineralised body is dome-shaped and located in the roof or cupola of the granitoid pluton. The deposit has an outlined resource of 46 million tons at a grade of 0.216% tungsten (WO3) and 0.02% molybdenum (Mo) with a high grade zone of 7 million tons of 0.30% WO3 and 0.02% Mo. Significant concentrations of Ce–La-enriched allanite, a light rare earth-enriched mineral of the epidote group, were recently discovered in the high tungsten grade, endoskarn part of the Riviera pluton. A resource of some 20 million tons at a tentative average grade of 0.35% ΣREE and has been indicated for this zone. Considering the high demand for rare earth elements this could constitute an economically important by-product and contribute towards the viability of a presently marginal W–Mo deposit. The textural and late stage alteration features of allanite are however complex and could inhibit its recovery and the production of a high grade allanite concentrate. Poikilitic intergrowths of apatite, zircon, monazite, kalkowskite and beckelite, variable chemistry indicated by zoning, metamictization as well as alteration of allanite to parasite and bastnasite will result in poor recovery and present a formidable geometallurgical challenge. Although allanite is not the rare earth mineral of choice by the international metals market the anticipated increased demand for light rare earth elements could change this situation in the future.