TRACE ELEMENTS IN BLACK SHALE FE-SULFIDES DURING DIAGENESIS AND METAMORPHISM

New electron microprobe (EMP) and SIMS analyses of Fe sulfides in black shale and metamorphosed equivalents demonstrate considerable elemental redistribution during diagenesis and metamorphism, indicating that such sulfides plausibly serve as a source of metals and other elements to fluids in post-depositional processes.

Concentrations of V, Cr, Co, Ni, Cu, Zn, As, Se and Mo in sedimentary pyrite and metamorphic pyrite and pyrrhotite were determined by EMP at 20 kV, 200 nA, 60-180 s, with detection limits (DL) of 20-170 ppm. Au and Pt were acquired by SIMS, with 20 ppb DL. Metal-rich and typical black shales reflecting a diverse range of geologic environments include samples from China, Canada and the US Great Basin and mid-continent. Sulfide-bearing graphitic schists from Maine provide a suite of low and high grade metamorphic equivalents.

Data indicate a complex history of pyrite growth due to diagenetic, metamorphic and hydrothermal processes. Early diagenetic framboidal pyrites are enriched in many trace metals (c.f. py separates, Raiswell & Plant, 1980, Econ Geol, 75, 684-699). Later diagenetic and metamorphic pyrites have mostly lower trace metal contents and show recrystallization to euhedral crystals, some with oscillatory zoning. Comparison of trace elements in framboidal pyrite with coarse overgrowth pyrite shows systematic changes in trace elements (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 growth of pyrrhotite at the expense of pyrite results in similar losses from the sulfide fraction of the rock. Hydrothermal pyrites localized in post-sedimentary veins are chaotically zoned and display discrete enrichments in Ni and As.

Our Fe sulfide compositions record a complex, yet systematic, redistribution and dispersion of many trace elements during black shale diagenesis and metamorphism. These processes and the sulfides they produce can provide metals, sulfur and other elements to surficial, sedimentary and metamorphic fluids.