TRACE-ELEMENT INVESTIGATION OF CARBONATE MINERALS IN THE GUNFLINT BANDED IRON FORMATION

Terrestrial banded iron formations (BIF) preserve some of the fragile remnants of the earliest life on Earth. Hematite deposits have been located on the martian surface so BIFs may serve as analogs for identifying both conditions of hematite deposition and evidence of primitive biologic activity in martian rocks. We report on the petrography, mineralogy and trace-element abundances of individual carbonate grains in the least metamorphosed Gunflint BIF of Ontario, Canada.

Petrographic studies reveal that the dominant carbonates are siderite, ankerite, and calcite and provide a textural context for microbeam analyses. Carbonates are found as primary phases, cement and secondary replacement minerals. Major- and minor-element compositions were determined using an electron microprobe. Trace elements were measured using the Cameca IMS 4f ion microprobe (UNM/SNL consortium), using carbonate mineral standards.

Major- and minor- element compositions vary for the different types of carbonates. Siderite ooids range from 78-92 mole % FeCO₃ within the magnesite-siderite solid solution. Siderite cement around granules range from 76-86 mole % FeCO₃. Siderite rhombs range from 71-94 mole % FeCO3 and are compositionally distinct from ooids. Ankerite averages 18 mole % FeCO₃ and 29 mole % MgCO₃. Calcite spar as replacement within granules has 94 mole % CaCO₃ and is compositionally distinct from calcite between granules with 97 mole % CaCO₃.

Trace-elements also vary among the carbonates. Strontium abundances range from 4 - 617 ppm, increasing with increasing Ca content. In general, all REE patterns show LREE > HREE with no Ce anomalies. Siderite ooids and isopachous bladed cement are essentially indistinguishable in major-element and REEs and likely reflect original marine precipitates. REE and Sr data suggest that siderite rhombs and calcite spar were precipitated from different fluids and that calcite was likely meteoric cement. Variations of the REE patterns may reflect a changing fluid composition between primary precipitated carbonates and those formed in a diagenetic environment. This suggests that REEs may record the geochemical environments of the precipitating carbonate phases. We are measuring isotopic abundances of Gunflint carbonates and will test for correlations between REE and isotope signatures.