Weathering of a mined quartz-carbonate, galena-sphalerite ore and release and transport of nanophase zinc carbonate in circumneutral drainage

The formation and transport of geogenic metal nanoparticles in mining-impacted environments is a developing concern because of their potential for greater distribution compared to larger particles. Discharge from the abandoned Gem Mine in the Coeur d'Alene Mining District of northern Idaho was examined for the presence of metal nanoparticles from weathering of an ore body of galena [PbS] and sphalerite [(Zn,Fe)S] with associated carbonate zones of siderite [FeCO3] and ankerite [Ca(Fe,Mg,Mn)(CO3)2] in intruded quartz veins. Analysis of this circumneutral discharge from the abandoned mine and groundwater in the receiving shallow aquifer indicate poor-quality mine drainage containing nanophase Zn-CO3 form(s) that dissociate into smaller particles or ions with release into the new geochemical environment of the aquifer. The nanoparticles were identified through acid titrations and dynamic light scattering analysis of 450-nm-filtered mine water. The stability of the nanoparticles was estimated through ζ potential analysis of mine water and groundwater, which indicated limited stability of the nanoparticles that was sufficient for transport in the mine drainage but insufficient for transport in the aquifer. The release of the nanophase Zn-CO3 form(s) likely occurs through weathering of secondary carbonate minerals in the mined areas of the Gem-Gold Hunter deposit through water-rock interaction → crystal repulsion → particle detachment → solution entrainment → and limited dissociation during transport as opposed to crystal formation in solution with mineral-phase saturation.