Geochemistry and mineralogy of rare earth elements in high-phosphorus ooidal ironstones: A case study of the Kamysh-Burun deposit (Azov–Black Sea iron Province)

Abstract

This study is aimed at characterizing the distribution and speciation of the rare earth elements and yttrium (REE + Y) in ooidal ironstones from the Kamysh-Burun deposit (Kerch Peninsula), with implications for the depositional environments and contributions of different REE + Y carriers to the ore budget. The Lower Pliocene ooidal ironstone sequence of the so-called Kerch ores, up to 15 m thick, occupies an area of 28 km2. The ore sequence lies between Miocene – Lower Pliocene shell limestones and Upper Pliocene – Pleistocene sandy and clayey sediments and consists of horizontal ooidal ironstone beds composed mainly of goethite and X-ray amorphous Fe3+-(oxy)hydroxides intercalated with siderite and/or rhodochrosite beds. Iron is a main component in ooidal ironstones (50.03–66.19 wt% Fe2O3 and ≤1.61 wt% FeO), as well as in fresh carbonate ores (~34 wt% FeO and ≤14.70 wt% Fe2O3). The Kerch ores contain up to 4.59 wt% P2O5 and show significant positive correlation between P and Fe (r = 0.75). The bulk ore samples and their coarse (1–10 mm) fractions are goethite-dominated and have similar phase, major- and trace-element, and REE compositions. The fine (<1 mm) fractions are poorer in goethite and P2O5 but enriched in SiO2, Al2O3, Ti, Zr, Y, and Th due to greater percentages of Fe-saponite and fine detrital matter (including very scarce rutile, zircon, monazite, and xenotime). The average ΣREE values of 400 ppm in bulk ooidal ironstones and 405 ppm in the coarse fractions markedly exceed those in the Post Archean Australian Shale (PAAS). They have similar PAAS-normalized REE + Y patterns, with moderate enrichment in middle REE (MREE), minor Ce* (0.67–1.09), and moderate negative Y* anomalies (0.57–0.70). The fine ore fractions are markedly enriched in ΣREE (Xav = 858 ppm) mainly due to the presence of authigenic light REE phosphates of rhabdophane-tristramite series with Ce > La ≈ Nd ≈ Ca > Pr > Sm. The ΣREE content in fresh carbonate ores is below that in PAAS (Xav = 103 ppm), but approaches the latter in oxidized crusts (Xav = 178 ppm). The Kerch ironstones also contain REE-poor early-diagenetic Fe2+ phosphate vivianite (ΣREE Xav = 0.93 ppm) and Ca-Fe2+ phosphate anapaite (ΣREE Xav = 6.95 ppm) with typical seawater REE + Y distributions. The ooidal ironstones show MREE-enriched patterns with distinct negative Y* anomalies, which reveals Fe3+-(oxy)hydroxides as chief carriers of adsorbed REE (mainly MREE). The Kerch ironstones accumulated REE progressively due to hysteretic dissolution and precipitation of Fe3+-(oxy)hydroxides in oscillating redox conditions. The diagenetic source of REE stored in ooidal goethite ironstones was inferred from the PAAS-normalized values: CeN/CeN* vs Nd and CeN/CeN* vs YN/HoN discrimination diagrams. In general, the REE + Y budget of the Kerch ooidal ironstones mainly formed during early diagenesis and scavenging from pore water, with a minor contribution of siliciclastic inputs, in the absence of hydrothermalism.