Environmental Conditions of Shelly Phosphorite Accumulation in the Rakvere Phosphorite Region, Northern Estonia

Heterogeneous REE + Y distribution in Early Paleozoic shelly phosphorites: Implications for enrichment mechanisms

“…Furongian-Tremadocian phosphorite of the Kallavere Formation is made up of yellow to light grey cross-bedded quartz sandstones with abundant phosphatic bioclasts and occasional carbonaceous mudstone interbeds (Sturesson et al, 2005), with a maximum thickness of up to 17 m (Heinsalu and Viira, 1997). The main apatite-containing particles in the Kallavere Formation are lingulate brachiopods, dominantly represented by Ungula ingrica species (Heinsalu et al, 1994;Puura, 1996). Brachiopod valves occur as intact shells or fragments of varying sizes, forming lenses and coquinas throughout the formation.…”

“…Sedimentary apatite (Ca-phosphate) and phosphate rocks in general are prominent sinks for the REE+Y in marine settings and are a potential source for the world's growing REE demand (Emsbo et al, 2015). Phosphate phases in sediments can be broadly divided into two groups: (i) biogenic apatite composed of skeletal remnants of apatite-secreting animals, such as brachiopods (e.g., Heinsalu et al, 1994;Lécuyer et al, 2004) vertebrates, and coprolites (e.g., Garnit et al, 2017;Lécuyer et al, 2004); and (ii) microbially mediated authigenic sedimentary apatite (Arning et al, 2009b;Compton and Bergh, 2016;Föllmi, 1996). Both biogenic and sedimentary apatite incorporate low amounts of REE+Y during their formation (Lécuyer et al, 1998;Lumiste et al, 2019).…”

Early diagenetic transformation stages revealed by micro-analytical studies of shelly phosphorites, Rakvere region

“…Post-mortem, during the diagenesis, the biogenic phosphatic matter chemically evolves toward CAF-apatite composition through various substitutions, together with an increase in crystallite size and formation of authigenic apatite phases (Trappe 1998). However, such alteration pathways are still only fragmentally understood (Ferretti et al 2021).The shelly phosphorites found in northern Estonia were deposited during the Cambrian-Ordovician transition in the coastal zone of a shallow, epicontinental sea (Heinsalu and Viira 1997;Nielsen and Schovsbo 2011). They belong to the Kallavere Formation, which spreads over most of northern Estonia and parts of the Leningrad region in northwestern Russia (Fig.…”

“…The shelly phosphorites found in northern Estonia were deposited during the Cambrian-Ordovician transition in the coastal zone of a shallow, epicontinental sea (Heinsalu and Viira 1997;Nielsen and Schovsbo 2011). They belong to the Kallavere Formation, which spreads over most of northern Estonia and parts of the Leningrad region in northwestern Russia (Fig.…”