Diagenetic recrystallization and neoformation of apatite in the Triassic phosphogenic facies in Svalvard

Phosphate accumulations in the Middle Triassic phosphogenic facies in Svalbard exhibit locally features of diagenetic recrystallization and neoformation of apatite. These features, seen on a microscale, reflect the development of hexagonal apatite crystal habit that replaced amorphous-like and globular micromorphologies of the original phosphatic fabric. The neoformed apatite crystals are commonly skeletal, either containing organic inclusions in crystal cores (Botneheia Formation in central and eastern Svalbard) or showing hollow or sieved cores with complex internal structure (Bravaisberget Formation in western and southwestern Spitsbergen). Detailed petrographic analysis (TLM, RLM, SEM, BEI, and XRD) of the apatite and associated diagenetic minerals, supported by the stable carbon and sulphur isotopic data (d13C and d34S apatite, d34S pyrite, d13C carbonate) suggest that the development of conspicuous skeletal apatite structure was a two-stage process. The first and major stage of neoformation of apatite (carbonate fluorapatite, CFA) was associated with early diagenetic processes in the anoxic sulphidic zone in organic-rich sediment column, reflecting maintenance of phosphatic bodies at shallow depths in the subsurface environment and recrystalli- zation of original phosphatic fabric at margins exposed towards the primary and mouldic pore space. The recrystallization was preceded by early diagenetic dissolution of biogenic silica (radiolaria, sponge) and followed by precipitation of carbonate cements that originated in deeper parts of the anoxic sulphidic zone. The second stage of apatite neoformation was confined to phosphate accumu- lations occurring in the West Spitsbergen Fold Belt, and related to burial processes and thermal degradation of kerogen. Thermal degradation of organic inclusions in apatite crystal cores led to the formation of conspicuous skeletal crystals and was associated with supplementary recrystallization. The well-defined neoformed apatitic fabric is seldom observed in the Botneheia and Bravaisberget formations, suggesting that the recrystallization processes were of negligible importance during diagenesis and burial of the Middle Triassic phosphogenic sequence in Svalbard.