PARAGENESIS OF EFFLORESCENT SULFATE MINERALS

Efflorescent sulfate minerals associated with oxidizing sulfide deposits can rapidly dissolve during rain events causing large excursions in the pH and trace metal content of near by receiving waters so local ecosystems are severely degraded. Despite their environmental importance, the stability and formation mechanism of efflorescent sulfate minerals remains largely unknown. This study documents the paragenesis and environmental stability of sulfate minerals at two sites at an abandoned massive sulfide mine site near Galax, Virginia. Site I is near an adit wall and site II is a sheltered overhang along the side wall of an open pit. Site II is warmer and dryer than site I. Representative samples, sample depth, and pile temperature were collected from trenches created in each of the piles. The equilibrium relative humidity of the samples was determined in the laboratory using a modified isopiestic method. Minerals were identified using light microscopy, x-ray diffraction, and scanning electron microscopy with energy dispersive spectroscopy. The chemical composition of the soluble minerals was determined with inductively coupled plasma atomic emission spectroscopy. Field observations and mineralogical identifications suggest that the minerals from the site evolve in the following manner. Melanterite, and some sulfuric acid, forms directly from oxidizing pyrrhotite and is extruded from the walls into piles on the floor due to the increase in volume. The melanterite undergoes further oxidation and dehydration to other sulfate phases depending on relative humidity. Under moist conditions, first fibroferrite, then copiapite forms in efflorescent blooms on the surface of the piles. Under dryer conditions, melanterite dehydrates to rozenite that oxidizes to copiapite and halotrichite that make hard sulfate crusts at the surface. Although these are predominantly iron sulfate minerals, interaction of the sulfuric acid with the silicate gangue minerals releases Ca, Mg, and Al that are incorporated in some of these phases.