AUTHIGENIC AND DIAGENETIC PROCESSES IN THE PERU SHELF PHOSPHORITES: FUNDAMENTAL PROCESSES IN THE DEVELOPMENT OF ECONOMIC PHOSPHORITE AND IMPLICATIONS FOR ANCIENT ANALOGS

Phosphorites are phosphate-rich sedimentary rocks and are the major source of phosphate for fertilizer. Phanerozoic phosphorites form in oceanic upwelling systems where nutrient-rich water enters the photic zone which results in high productivity. High accumulation rates of organic matter and low oxygen levels characterize such environments and establish biogeochemical redox reactions that precipitate highly substituted carbonate-fluoro-apatite (francolite), pyrite, and dolomite below the seafloor.

We analyzed drill core samples collected from ODP Leg 112 on Peru's continental shelf. This phosphogenic system ranges from the Eocene to present, and has never experienced deep burial or weathering that is characteristic of ancient analogs, so it provides an excellent opportunity to study biochemical processes associated with authigenesis and early diagenesis.

We used petrographic and scanning electron microscopy to determine paragenetic relationships with setting, age, and depth of burial. Francolite formed in situ within organic-rich muds that is reworked into grain-rich lags. Grains often contain multiple generations of coatings typical of simple and compound grains, and along with abundant micro-borings, reflect multiple cycles of in situ francolite growth and reworking. Pyrite framboids are associated with the earliest stage of francolite precipitation. There are two distinct francolite cement phases. One comprises small rod-shaped prisms that average 3µm by 0.7µm in size, and occur as euhedral to subhedral hexagons, and rounded rods. These small prisms partially fill intragranular pore space and replace (phosphatize) pre-existing carbonate grains. The second phase is composed of an isopachous grain-rimming cement that consists of elongate-fibrous francolite crystal bundles that radiate from grains. This is followed by pore-filling dolomite cement and continued pyrite precipitation. Most major ancient phosphorites have similar mineral assemblages and textures, but the analog early-formed rod-shaped francolite phase is usually not apparent due to infilling of intercrystal space by francolite precipitated during burial. These recent phosphorites provide an excellent model for understanding compositional and textural relationships seen in ancient analogs.