Heavy-Mineral Potential of Shelf Areas: A Review

ABSTRACT After a discussion of the principles of heavy mineral enrichment and the sedimentary history of shelf sediments, four models are described which can explain the genesis of heavy-mineral deposits in shelf areas. On the basis of these models, the heavy-mineral potential of most shelf areas is estimated to be significantly lower than the potential of their neighboring coastal plains. INTRODUCTION Heavy-mineral deposits in coastal sediments are exploited for a variety of commodities. About half of the world's titanium production comes from rutile- and ilmenite-bearing beach placer deposits. As a by-product of these mining activities the minerals zircon and monazite are recovered, which are used either as refractory sands or as raw material for rare earth elements. Gold, diamonds, chromite, and Ti-bearing magnetite are other valuable minerals from coastal beach placer deposits. Fluvial sediments in coastal lowlands are the predominant source for the main Sn-producing mineral, cassiterite, which is also recovered from several shelf areas. Despite the high economic value of these deposits and despite the decline of proven reserves, the submarine extension of these deposits onto the shelf are poorly explored. On' the basis of a literature review and the results of several exploration campaigns this paper describes sedimentary models to estimate the heavy-mineral potential of shelf areas. PRINCIPLES OF HEAVY-MINERAL ENRICHMENT Heavy minerals become enriched in eluvial, fluvial, shallow-marine and eolian sediments because of their greater chemical resistivity, mechanical stability and density than most other common light minerals. Heavy minerals from placer deposits are not destroyed by the weathering process disintegrating their igneous or metamorphic source rock. Their mechanical stability allows long-distance transport without much abrasion by different sedimentary processes. The greater density of heavy minerals (2.8 – 19 g/cm3) causes their hydraulic properties to be different from those of light minerals, mostly quartz and feldspar (2.7 g/cm3). During the final enrichment, these differences in density playa decisive roll in the formation of placer deposits, which occur in different environments.2 Heavy minerals with a high density (>6.8 g/cm3, e.g. cassiterite, gold, and platinum, called "heavy heavy-minerals) and large primary grain size are very slowly' transported or are almost immobile in the fluvial transport system.3 These minerals are enriched by eluvial processes removing the weathered part of the primary rock or by being trapped near their source rocks at 'the base of river beds. Even during extreme flood conditions they settle in potholes and boulder beds (shear sorting and kinetic sieving:4,5). Lighter heavy minerals with a density of 4.0 to 6.0 g/cm3 (called "light heavy-minerals") are usually not enriched by fluvial transport. These usually fine-grained « 200 pm) minerals (rutile, ilmenite, leucoxene, zircon, monazite, magnetite, garnet, chromite) are commonly enriched in the surf zone of beaches.