SILICON ISOTOPE STRATIGRAPHY OF 2.728 TO 2.722 GA IRON FORMATION AND ASSOCIATED SILICIFIED ROCKS FROM THE ABITIBI GREENSTONE BELT, CANADA

The ~2.7 Ga Abitibi Greenstone Belt (AGB), Canada represents a well-preserved composite volcanic arc terrane comprised of multiple volcano-sedimentary sequences dominated by classic greenstone lithologies. We measured silicon isotope values (via SIMS at the NORDSIM facility) of coeval samples of iron formation (IF) and silicified volcanic rocks from three separate locations within the belt: (1) the Temagami iron formation, near Temagami, ON, (2) the Deloro Assemblage, near Timmins, ON, and (3) the Hunter Mine Group (HMG) near Rouyn-Noranda, QC. At Temagami, we measured single quartz crystals from three samples of Algoma-type IF, which range from δ³⁰Si = -2.2 ± 0.4 ‰ to +0.7 ± 0.4 ‰. Within the Deloro Assemblage, we analyzed one sample from each of three stratigraphically continuous units of iron formation (lower, middle, and upper), and an interbedded silicified volcanic unit. In total, single quartz crystals of IF possess a large range of ³⁰Si-depleted values (δ³⁰Si = -3.9 ± 0.2 ‰ to +0.2 ± 0.2 ‰), while the silicified volcanic sample is more homogeneous and ³⁰Si-enriched (δ³⁰Si = -0.8 ± 0.2 ‰ to +1.0 ± 0.1 ‰). We also analyzed a silicified volcanic rock sample from the Hunter Mine Group, which overlapped in δ³⁰Si values with the Deloro Assemblage sample, ranging from δ³⁰Si = -0.7 ± 0.4 ‰ to +0.8 ± 0.3 ‰. The overwhelming majority of δ³⁰Si values (93%) for quartz precipitates (iron formation) fall less than 0 ‰, while 79% of δ³⁰Si values for silicified volcanic rocks plot greater than 0 ‰. Experimental and natural data demonstrate that silicon isotopes fractionate during precipitation and adsorption due to a kinetic isotope effect. Based on geologic context, comparison with experimental data and estimation of initial δ³⁰Si source fluid values, we interpret ³⁰Si-depleted values within precipitates to result from rate-dependent fractionation during rapid precipitation induced by conductive cooling and potential adsorption on to Fe/Al phases in a hydrothermally dominated setting. We interpret ³⁰Si-enriched values of silicified volcanic rocks to result from slower precipitation induced by limited fluid circulation during progressive silicification and quartz precipitation in fractures and pore spaces below the water-rock interface potentially during a higher-temperature stage of hydrothermal activity.