Fluid sources and hydrothermal architecture of the Sudbury Structure: Constraints from femtosecond LA-MC-ICP-MS Sr isotopic analysis of hydrothermal epidote and calcite

This study presents new Sr isotopic data on hydrothermal mineral assemblages from different stratigraphic levels within the Sudbury impact structure, determined using femtosecond LA-MC-ICP-MS. These data provide new insights into the origin of the hydrothermal fluids that have circulated through and around the Sudbury Igneous Complex (SIC), and further define the overall hydrothermal architecture of the structure. Early, blocky miarolitic epidote from the SIC has a narrow range of initial 87Sr/86Sr (0.70710 to 0.70742), similar to the SIC (~ 0.70648 to 0.70736) that is consistent with the precipitation from orthomagmatic fluids derived from the SIC. The 87Sr/86Sr values of later, replacement miarolitic and vein epidote from different stratigraphic levels of the SIC, and of replacement and amygdule epidote from the overlying Onaping Formation are distinctly higher (up to 0.71753) than those of the blocky miarolitic epidote. These data suggest that the fluids responsible for this late epidote were not derived from the SIC, and more likely represent basement-derived fluids that passed through the SIC and Onaping Formation. The Sr isotopic composition of syngenetic, laminated calcite in the post-impact Vermillion Formation suggests precipitation from Proterozoic seawater (87Sr/86Sr = ~ 0.70295), whereas some Vermilion Formation-related syngenetic laminated calcite and epigenetic (discordant and replacement) mineralized calcite with more radiogenic values suggest precipitation from seawater modified through interaction with the vitric and lithic fragments present in the Onaping Formation. The formation of some replacement and amygdule carbonate in the Onaping Formation also involved mixing between seawater and basement-derived fluids of similar chemical and isotopic composition to those responsible for the replacement epidote hosted by the SIC and Onaping Formation. The Sr isotopic composition of hydrothermal epidote from throughout the Sudbury Structure indicates a fluid connection between the footwall and hanging wall of the Sudbury Igneous Complex.

Research Highlights
► First 87Sr/86Sr study of epidote and calcite (Sudbury Structure) by (Fs) -LA-MC-ICP-MS.
► Early blocky miarolitic epidote (0.7071 to 0.7074) precipitated from magmatic fluids.
► Vein and late replacement epidote in the SIC represent basement-derived fluids.
► White, laminated Vermilion Fm calcite (0.7030) precipitated from Proterozoic seawater.
► A fluid connection between the footwall and hanging wall of the SIC is established.