From bacterial to thermochemical sulfate reduction: Sulfur isotope constraints on the genesis of hyper-enriched black shales

Hyper-enriched black shales (HEBS) are thin stratiform and stratabound sulfide-rich (~70 vol%) units that have percentage level concentrations of metals such as Ni, Mo and Zn. In the Peel River area of Yukon, Canada, HEBS are highly enriched in Ni (~3 wt%), Zn (~0.2 wt%), and Mo (0.2 wt%) hosted mainly by millerite (NiS), sphalerite (ZnS) and pyrite (FeS 2 ). The processes that control millerite-sphalerite precipitation in HEBS are poorly constrained. Here, we report the first in-situ sulfur isotope measurements of sulfide minerals in the Peel River HEBS. Early diagenetic pyrite (δ 34 S -31.9 ± 9.0 ‰) formed in response to open-system bacterial sulfate reduction (BSR), whereas later sphalerite (16.5 ± 5.1 ‰) and millerite (+10.0 ± 1.4 ‰) crystallized via thermochemical sulfate reduction. We have developed a three-stage model for HEBS formation. Stage 1 involves high primary productivity, a redox-stratified water column, a low sedimentation rate, and an active Fe-oxide shuttle, which promoted BSR-driven pyrite precipitation. Organic matter scavenged Ni and Zn from the water column and pore-water, producing metal-rich kerogen. Stages 2 and 3 occur during late diagenesis, with the infiltration of an acidic, oxidized, metal-poor, sulfate-bearing brine that interacted with the metal-bearing kerogen, triggering thermochemical sulfate reduction and millerite-sphalerite precipitation. Subsequent fracturing and increased fluid-flow during peak hydrocarbon generation caused acid-driven replacement of sphalerite by a second generation of millerite. This model accounts for HEBS formation in both Canada and China, demonstrating that, although late-stage fluids did not transport the metals, brines were essential to their genesis. • SIMS δ 34 S data resolve the timing of sulfide formation in Peel River HEBS. • Pyrite formed by open-system BSR during early diagenesis. • Millerite and sphalerite formed by TSR during late diagenesis. • Ni and Zn were stored in organic matter prior to sulfide precipitation. • Sulfate-bearing brines triggered TSR but did not transport metals.