Revisiting the phosphorite deposit of Fontanarejo (central Spain): new window into the early Cambrian evolution of sponges and the microbial origin of phosphorites

Fossils within early Cambrian phosphorites worldwide are often well preserved due to early diagenetic permineralization. Here, we examine the fossil record contained within phosphorites of the Lower Cambrian Pusa Formation (late Fortunian to Cambrian Stage 2) in Fontanarejo, central Spain. The sedimentology and age of these phosphorites have been controversial and are here reviewed and discussed, providing also an updated geological map. The Pusa Formation is composed of fine clastic sediments that are partly turbiditic, with channels of quartz-rich conglomerates and abundant phosphorites in the upper part of the succession. The microfacies and mineralogy of these channel deposits are studied here for the first time in detail, showing that they are mainly composed of subspherical apatite clasts, with minor mudstone intraclasts, quartzite and mica grains. Numerous sponge spicules, as well as entirely preserved hexactinellid sponges and demosponges, were collected within these phosphorites and likely represent stem groups. In addition to sponges, other fossils, such as small shelly fossils (SSF) of the mollusc Anabarella sp., were found. The phosphorites exhibit multiple evidence of intense microbial activity, including diverse fabrics (phosphatic oncoidal-like microbialites, thrombolites, stromatolites and cements) and abundant fossils of filamentous microbes that strongly resemble extant sulphur-oxidizing bacteria. Our findings strongly suggest that microbial processes mediated the rapid formation of most of the Fontanarejo apatite, probably accounting for the exceptional preservation of fragile fossils such as sponge skeletons. The apparent presence of taxonomically diverse hexactinellid and demosponge communities at the lowermost Cambrian further corroborates a Precambrian origin of the phylum Porifera.

Epigenetic-Hydrothermal Fluorite Veins in a Phosphorite Deposit from Balaton Highland (Pannonian Basin, Hungary): Signatures of a Regional Fluid Flow System in an Alpine Triassic Platform

The middle Anisian extensional tectonics of the Neotethyan realm developed a small, isolated carbonate platform in the middle part of the Balaton Highland (western Hungary), resulted in the deposition of uranium-bearing seamount phosphorite on the top of the drowned platform and produced some epigenetic fluorite veins in the Middle Triassic sequence. The stable C-O isotope data of carbonates are shifted from the typical Triassic carbonate ranges, confirming the epigenetic-hydrothermal origin of veining. Primary fluid inclusions in fluorite indicate that these veins were formed from low temperature (85–169 °C) and high salinity NaCl + CaCl2 + H2O type (apparent total salinity: 15.91–22.46 NaCl wt%) hydrothermal fluids, similar to parent fluids of the Alpine-type Pb-Zn deposits. These findings indicate that the Triassic regional fluid circulation systems in the Alpine platform carbonates also affected the area of the Balaton Highland. This is also in agreement with the previously established palinspatic tectonic reconstructions indicating that the Triassic carbonate and basement units in the Balaton Highland area were a part of the Southern Alpine. Similar fluorite veining in phosphorite deposits is also known in the Southern Alpine areas (e.g., Monte San Giorgi, Italy). Raman spectroscopic analyses detected H2 gas in the vapor phase of the fluid inclusions and a defect-rich fluorite structure in violet to black colored growth zones. This unique phenomenon is assumed to be the result of interaction between the uranium-rich phosphorite and the parent fluids of the epigenetic fluorite veins.

Origin and Evolution of the Late Cretaceous Reworked Phosphorite in the Sirhan-Turayf Basin, Northern Saudi Arabia

The redeposition of pristine phosphorite plays an important role in phosphorus accumulation, which created reworked phosphorite extensively on the continental shelf. This paper, using geochemical analysis combined with data from petrology and diagenesis, focuses on the reconstruction of the formation processes of the Late Cretaceous Thaniyat phosphorite deposition in northwestern Saudi Arabia, which is a part of the famous large Neo-Tethys Ocean’s phosphorite deposit. The results of our study illustrate that the phosphorites represent the reworked products from the north, close to the edge of the Neo-Tethys Ocean’s shelf, where upwelling had accreted the pristine phosphorite. The reworked phosphatic grains were redeposited near the shore in sandstone, forming sandy phosphorite and on a carbonate platform and creating calcareous phosphorite. The microscale sedimentological and geochemical information hosted in the eroded phosphorite grains indicates that the source sediment, pristine phosphorite, occurred under a fluctuating geophysical condition and in a relatively limited geochemical environment. They were physically crushed and transported landward and deposited under oxic conditions, forming the Thaniyat phosphorites. Early diagenesis in the Thaniyat phosphorite was evidenced by recrystallization of the phosphate minerals, geochemical depletion, and C and O isotope excursion.

Revisiting the phosphorite deposit of Fontanarejo (central Spain): new window into the early Cambrian evolution of sponges and into the microbial origin of phosphorites

Fossils within early Cambrian phosphorites worldwide are often well preserved due to early diagenetic permineralization. Here, we examine the fossil record contained within phosphorites of the Lower Cambrian Pusa Formation (late Fortunian to Cambrian Stage 2) in Fontanarejo, central Spain. The sedimentology and age of these phosphorites have been controversial and are here reviewed and discussed, providing also a updated geological map. The Pusa Formation is composed of fine clastic sediments that are partly turbiditic, with channels of quartz-rich conglomerates and abundant phosphorites in the upper part of the succession. The microfacies and mineralogy of these channel deposits are studied here for the first time in detail, showing that they are mainly composed of subspherical apatite clasts, with minor mudstone intraclasts, quartzite and mica grains. Numerous sponge spicules, as well as entirely preserved hexactinellid sponges and demosponges, were collected within these phosphorites and likely represent stem groups. In addition to sponges, other fossils, such as small shelly fossils (SSF) of the mollusk Anabarella sp., were found. The phosphorites exhibit multiple evidence of intense microbial activity, including diverse fabrics (phosphatic oncoidal-like microbialites, thrombolites, stromatolites, and cements) and abundant fossils of filamentous microbes that strongly resemble sulfur oxidizing bacteria. Our findings strongly suggest that microbial processes mediated the rapid formation of most of the Fontanarejo apatite, probably accounting for the exceptional preservation of fragile fossils such as sponge skeletons. The apparent presence of taxonomically diverse hexactinellid and demosponge communities by the lowermost Cambrian further corroborates a Precambrian origin of the phylum Porifera

REE Geochemical Characteristic of Apatite: Implications for Ore Genesis of the Zhijin Phosphorite

Phosphorite-type rare earth deposits, which are one of the important types of rare earth elements (REE) ore deposits, have attracted increasing attention because of the extreme enrichments in heavy rare earth elements (HREE), including Yttrium (Y). In this study, in situ geochemical analyses of apatite grains from Zhijin phosphorites were conducted using electron probe microanalysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Based on EPMA mapping analysis, we show that rare earth elements and Yttrium (REY) entering into the crystal lattice by isomorphism rather than by inclusions of REY-bearing accessory minerals. The post-Archean Australian Shales (PAAS)-normalized REY patterns of the apatite grains are characterized by hat-shaped MREE-enriched patterns. We interpret that this pattern may reflect the REE distribution of seawater at that time. We propose that in a local, reducing environment, dramatically increased the concentration of REY in seawater, and resulted in the MREE-enriched patterns in the ancient ocean. The main mechanism for the genesis of the Zhijin phosphorite deposit is the apatite crystallizes during the mixing process of REY- and P-rich fluid and oxidizing seawater.