Submarine Phosphorites: The Deposits of the Chatham Rise, New Zealand, off Namibia and Baja California, Mexico—Origin, Exploration, Mining, and Environmental Issues

Abstract

Rising consumption and increasing prices of phosphate fertilizers initiated the commercial exploration of offshore phosphorite deposits. Origin of the phosphorite, state of exploration, and mining concepts are described for three of the most advanced projects. The phosphorites of the Chatham Rise/New Zealand are located in 400 m water depth, about 500 km offshore and originated by Late Miocene phosphatization of a hemipelagic chalk. The gravel-sized phosphorites form a residual decimeter-thick layer with an inferred resource of 24 million tons. The authigenic, sand-sized phosphorites at the outer shelf of Namibia form a meter-thick layer with estimated resources of 60 million tons. The sand-sized phosphorites on the mid-shelf off the Baja California/Mexico contain an estimated resource of about 300 million tons. In all three projects, the phosphorites are planned to be recovered by a trailing suction hopper dredge and enriched by onboard size screening. Mining licenses were granted, but environmental considerations have delayed the economically feasible exploitation of these marine phosphorites.

Abstract

Onshore phosphorite reserves are well-explored and can supply the worldwide consumption of phosphate fertilizer for the next hundred years (Orris and Chernoff 2002). But the deposits are not evenly distributed around the world and not always close to agriculture centers. The continents on the southern hemisphere are especially lacking in significant production of phosphorite. The main phosphorite resources are the widespread deposits of northern Africa and the Nubian Shield, which originated in shallow-water along the former southern margin of the Late Cretaceous Tethys Ocean and the Paleocene to Eocene Atlantic Ocean (Soudry et al. 2007). Other economically important deposits are the Miocene phosphorites of Florida and the Permian Phosphoria Formation, both of which mainly serve the North American fertilizer markets. Sedimentary deposits from the Late Proterozoic in China produce exclusively for local consumption (Li 1986).