Abstract

In this study, we present a detailed investigation of C-O-S-Sr isotope systematics and elemental analysis of secondary sulfates and associated host-rock carbonates of Cambrian to Devonian sedimentary successions along the eastern flank of the Michigan Basin, Ontario. This study evaluates the diagenetic evolution of pore fluids and their sources in fracture-fill and replacement sulfate minerals in low-permeability carbonate units in the Michigan Basin. Secondary sulfates, represented by gypsum and anhydrite, contain various petrographic types, represented by vug- and fracture-filling fibrous anhydrite in the Cambrian (δ¹⁸O vary between 16.8 to 17.6‰ VPDB and δ³⁴S 28.3 to 29.0‰ CDT, ⁸⁷Sr/⁸⁶Sr ratios vary from 0.70834 to 0.70991, respectively) and Ordovician fibrous anhydrite (δ¹⁸O 16.8‰, δ³⁴S 28.2‰, ⁸⁷Sr/⁸⁶Sr ratios 0.70829). These phases display mainly uniform REE patterns with Y/Ho and Zr/Hf ratios, flat La*, Gd*, and Y*, and enriched LREEs.

In the Silurian strata, idiotopic satin-spar δ¹⁸O values vary between 4.0 and 8.3‰, δ³⁴S 23.4 and 31.4‰ with ⁸⁷Sr/⁸⁶Sr ratios 0.70816–0.70866, xenotopic porphyroblast gypsum δ¹⁸O values vary between 4.5 and 13.3‰, δ³⁴S 22.6 to 33.1‰, with ⁸⁷Sr/⁸⁶Sr ratios of 0.70850–0.70880, alabastrine gypsum δ¹⁸O values vary between 4.2 and 11.7‰, δ³⁴S 23.1 to 26.9‰, with ⁸⁷Sr/⁸⁶Sr ratios of 0.70816–0.70876 and felted anhydrite δ¹⁸O values of 11.1‰, δ³⁴S 27.4‰, with a ⁸⁷Sr/⁸⁶Sr ratio of 0.70849. These geochemical proxies suggest a comparable sulfate-rich source for both age groups under similar geochemical conditions. Gypsum in the Silurian Salina Group displays a wide range of REE values with Y/Ho, and Zr/Hf ratios, Nd/Yb_N and Pr/Yb_N, flat to strong Ce/Ce*, and a flat to strong Eu/Eu* anomaly, Gd/La_N, Tb/La_N, and Sm/La_N. These sulfates formed at burial from fluids of variable isotope compositions, chemical compositions, and temperatures. The sources of these fluids range from brines migrating from a deeper part of the basin forming anhydrite in the Cambrian and Ordovician sequences which were also affected by dolomitization and later hydrothermal fluid influx, to rehydration of gypsum at shallower burial depths and affected by salt dissolution and incursion of meteoric water during and following the Alleghenian orogeny.