Stable isotope (C, O, S, and Zn) geochemistry of marble-hosted exhalative zinc deposits in the Central Metasedimentary Belt, Grenville Province, Canada: Insights into ore deposition and tectonic setting

Abstract

Mesoproterozoic marbles of the southwestern Grenville Province contain numerous stratiform zinc deposits, which include the world-class Balmat-Edwards Zn-Pb district in the Adirondack Lowlands and the Franklin-Sterling Zn-Fe-Mn district in the New Jersey Highlands. Ten small deposits in the Central Metasedimentary Belt (CMB) of Ontario and Quebec were selected for stable isotope study (C and O in carbonates and S and Zn in sulfides) to help constrain depositional conditions, ore genesis, and tectonic setting. These deposits and the Balmat-Edwards and Franklin-Sterling districts are interpreted to have formed in contemporaneous back-arc basins associated with rifting of the Andean-style margin of Laurentia. Stable isotope systematics and the geology of these deposits indicate two distinct sedimentary exhalative (Sedex) settings: marble-hosted deposits associated with volcanic rocks, and marble-hosted deposits that are not associated with volcanic rocks and sometimes occur with evaporites. Volcanic-associated Sedex deposits (the Long Lake, Slave Lake, Wilkinson, Lafontaine, Bouchette, and Leitch deposits, and the Franklin-Sterling district) have COS isotope systematics that are consistent with water/rock interaction between marine carbonate protoliths and fluids having an igneous component at ca. 150 ± 25 °C. In contrast, Sedex deposits not associated with volcanic rocks (the Cadieux, Spry, Thirty Island Lake, and Salerno Lake deposits, and the Balmat-Edwards district) have C and O isotope ratios that are consistent with formation at ambient or low (<80 °C) temperatures. Sulfur isotopes of sulfides in some evaporite-associated deposits are similar to those of other Proterozoic Sedex deposits (δ34S = 5–15 ‰ VCDT), with the Cadieux and Spry having lower values (δ34S ≈ 0‰) which might reflect differences in depositional settings. Zinc isotope ratios of CMB Zn deposits are relatively constrained (δ66Zn = 0.17 ± 0.04 ), and are similar to other seafloor deposits. An exception to this is the Thirty Island Lake deposit in the Frontenac terrane, which has δ66Zn = 0.46 ± 0.05 ‰, which may indicate an exotic origin relative to other CMB deposits. The recognition of the volcanic association for some Grenville Sedex deposits and not for others (and the relationship with evaporites for this group) has implications for Zn exploration programs in this region.