Quantitative micro-PIXE analysis of heavy-metal-rich framboidal pyrite

Framboidal pyrite in mudstone is known to be a reservoir of arsenic, yet few studies have conducted quantitative analyses of framboidal pyrite with ppm-level sensitivity. In this study, we used micro-PIXE to measure the concentrations of heavy metals in framboidal pyrite from drill-core samples of mudstone from the Late Cretaceous Hakobuchi Group, central Hokkaido, Japan. Pyrite framboids that occur in polyframboids can be divided into two types based on framboid diameter (D) and the average diameter of microcrystals within framboids (d): type 1, being relatively small grains; and type 2, being relatively large grains. Higher concentrations of As are observed in most type 2 framboids than in type 1 framboids. High concentrations of Pb generally occur as isolated islands within type 1 framboids, concentrated along the rims of polyframboids. It could be concluded that contrasting formation processes among different framboids led to the observed changes in heavy metal concentrations.