MORPHOLOGICAL AND STRUCTURAL CONTROLS ON THE DISTRIBUTION OF ARSENIC, SELENIUM, AND MERCURY IN HIGH PYRITE APPALACHIAN COALS

Pennsylvanian coals in the Appalachian basin host pyrite that is locally enriched in potentially toxic trace elements such as As, Se, and Hg. Trace element enrichment is the result of migrating metal-rich hydrothermal fluids generated during the late Paleozoic Allegheny orogeny. Metal contents vary over microscopic to mine scales and over local to regional scales because As, Se, and Hg are inhomogeneously distributed in different morphological forms of pyrite, and in multiple generations of sulfides in structural elements such as crosscutting veins and cell lumens.

Pyrite occurs as framboids, dendrites, or in massive form in cells or veins. Early diagenetic framboidal pyrite is usually As, Se, and Hg poor. In dendritic pyrite, maps of As intensity show a chemical gradient from As-rich centers to As-poor distal branches. Se concentrations are highest at the distal edges of the branches.

Pyrite-filled veins consist of several generations of sulfide precipitation. Pyrite in late-stage veins commonly exhibits As-rich growth zones, indicating an epigenetic hydrothermal origin. Se is concentrated at the distal edges of veins. A positive correlation of As and Se, and As and Hg, in Kentucky and Alabama pyrite-filled veins, respectively, indicates precipitation of these elements from the same fluid source.

In Kentucky coal samples (n = 18), As and Se contents of pyrite-filled veins average 4200 ppm and 200 ppm, respectively. In Alabama coals, As in pyrite-filled veins averages 2700 ppm (n = 34), whereas As in pyrite-filled cellular structures averages 6470 ppm (n = 35). Se averages 80 ppm in pyrite-filled veins but was not detected in cell structures. In West Virginia samples of massive pyrite (n = 24), As averages 1700 ppm, and Se averages 270 ppm. The highest concentration of Hg (≤102 ppm) is in Alabama veins.

Characterizing morphology, pyrite generations, and trace element content contributes to a better understanding of the progression of weathering of pyrite in coals and release of trace elements into the environment. Trace element-rich pyrite is demonstrated to be susceptible to dissolution, and fragile forms such as dendrites are susceptible to disaggregation. Therefore, identifying the morphological residence of As, Se, and Hg aids in predicting the mobility of these hazardous elements and developing methods of removal.