PHOSPHORITE AND IRON FORMATION DEPOSITED DURING THE MARINOAN SNOWBALL EARTH ICE AGE, MATO GROSSO, BRAZIL: PALEOCEANOGRAPHY OF A GLACIOMARINE BIOCHEMICAL SYSTEM

The Neoproterozoic Snowball Earth ice ages were among the most extreme climate events in Earth history. These were associated with ferruginous deep oceans and the return of iron formation deposition after a one-billion-year absence. Phosphorite, a sedimentary rock rich in the highly-substituted carbonate-fluorapatite mineral francolite, is largely a Phanerozoic phenomenon, rarely occurs with iron formation, and is a signpost to biochemical activity. The concentration of phosphate in Precambrian seawater is controversial, but it is adsorbed onto iron oxide particles, which may have inhibited phosphogenesis. Phosphorite and iron formation do occur together, however, in the Serra do Caeté Formation, Mato Grosso, Brazil. This relatively undeformed succession is located on the southern edge of the Amazonian craton and is bordered to the east by the Paraguay Fold Belt.

Based on study of drill core, francolite occurs in microbially laminated facies composed of siderite layers up to 1 cm thick and laminated siltstone to sandstone. Ripple cross-laminated sandstone, punctuated by dropstones, also contains francolite. This phosphatic interval is sandwiched between a glaciomarine diamictite below and the major ice-contact diamictite of the Puga Formation above. Siderite is an authigenic precipitate composed of 1-10 µm crystals that preserves microbial and peloidal textures. Some of the francolite that was originally precipitated in microbial laminae was reworked forming peloids; these are partially replaced by diagenetic apatite cement. Mobilization of authigenic phosphate to form apatite cement occurred during burial diagenesis, but before later hydrothermal alteration. Iron occurs in siderite and hematite; although much of the hematite is due to late oxidative weathering of siderite and hydrothermal pyrite, some hematite mud beds are primary and represent suspension settling of oxidized iron.

Unlike most other Precambrian phosphorites, this glaciomarine example suggests that phosphogenesis occurred under fluctuating redox conditions that were, at least periodically, associated with sea-ice cover and ice-margin upwelling. The near absence of pyrite and abundance of siderite suggest that melt water may have limited sulfate levels, which may have restricted the efficiency of phosphogenesis.