Origin and redistribution of metals in sedimentary rocks

Abstract

Integrated analysis of whole-rock and mineral geochemical data from black shale and metamorphosed black shale demonstrates the importance of sulfides as trace-element hosts and reveals the influence of depositional environment on whole-rock and mineral chemistries. Detailed analysis of Fe-sulfide contents provides evidence of considerable elemental redistribution during diagenesis and metamorphism, indicating that such sulfides plausibly serve as a source of metals and other elements to epigenetic fluids. Spatial and temporal analysis of the distribution of Phanerozoic black shale, phosphorite, and ironstone suggests a related geologic origin coupled to marine upwelling. Organic-rich sedimentary rocks from diverse geologic settings in China, Canada, US Great Basin, and US Mid-Continent were studied along with regionally metamorphosed black shale (Maine) and contact metamorphosed carbonaceous argillite (Nevada). Assessment of whole-rock geochemistry in these samples was supplemented with analysis of a large geochemical database compiled from published sources. Both the studied sample suite and literature database reveal systematic elemental enrichments and depletions that can be related to depositional environment. Regionally metamorphosed black shale demonstrates trace-element loss with metamorphism (Au, As, and Sb), an observation that fits well with models proposing black-shale-derived mineralizing fluids. Compositional and textural features of sedimentary and metamorphic sulfides were investigated utilizing petrographic and in situ analysis methods. Concentrations of As, Se, Ni, Co, Cu, Zn, V, Cr, and Mo in sulfides were acquired by electron microprobe. Au, Pt, and sulfur isotope contents were measured in Fe-sulfide by SIMS. Fe-sulfide data demonstrate a complex history of pyrite growth due to diagenetic, metamorphic and hydrothermal processes. Sulfur isotopes reflect bacterially mediated growth of framboidal pyrite and redistribution of this sulfur with advanced diagenesis and metamorphism. Early framboidal pyrites are enriched in many trace metals. Later diagenetic and metamorphic pyrites are typically lower in trace metal content and demonstrate recrystallization to coarser crystals. Comparison of trace elements in framboidal and coarse overgrowth pyrites shows systematic elemental changes (losses: Au, Pt, Ni, Cu, Zn, V and Cr; gains: Se; variable: As and Mo). This pattern is found in many black shales, regardless of bulk chemistry or depositional environment. Metamorphic replacement of pyrite by pyrrhotite results in similar losses from the sulfide fraction of the rock. Mass balance of the whole-rock trace inventory indicates that Fe-sulfide is an important host for As, Se, Ni, Co, and Au. These mass balance results, combined with evidence of trace element redistribution during diagenesis and metamorphism, point to a possible sourcing of metals, sulfur and other elements from black shale.