Silica- and sulfate-bearing rock coatings in smelter areas: Part II. Forensic tools for atmospheric metal(loid)- and sulfur-isotope compositions

Detection and Mapping of Black Rock Coatings Using Hyperion Images: Sudbury, Ontario, Canada

“…In the Sudbury region, smelter-derived particulate matter trapped in the silica-rich matrix consists largely of nanometer-to micrometer-sized spherical particulates composed of carbon soot, minerals of the spinel group, and high-temperature silicates such as minerals of the olivine and pyroxene groups [2,3,15,16]. Coatings here are found up to tens of kilometers from the sites of past smelting activities [8].…”

“…Base metal smelting activities can result in the release of particulate matter and of sulfuric acid and SO2 gas, all of which can be entrained by winds and distributed far beyond smelter stacks [2][3][4][12][13][14]. Sulfuric acid and oxidation of SO2 can result in precipitation events characterized by very low pH (<3).…”

“…Black rock coatings in the Sudbury region ( Figures 2 and 3) are generally less than 200 µm in thickness and typically consist of: (1) a lower zone that consists of variably-weathered underlying rock; (2) a main zone that consists of opal-like amorphous SiO2 that acts as a matrix for both locally-derived and smelter-derived particulates; and (3) metal-sulfate-enriched zones that typically occur as thin layers either at the upper surface of the coating or between the underlying rock and the coating [2,8]. In the Sudbury region, smelter-derived particulate matter trapped in the silica-rich matrix consists largely of nanometer-to micrometer-sized spherical particulates composed of carbon soot, minerals of the spinel group, and high-temperature silicates such as minerals of the olivine and pyroxene groups [2,3,15,16].…”

“…Black rock coatings can form in the vicinities of base metal smelters as a result of the interaction of smelter emissions with rock surfaces [1][2][3][4]. Though characterized by thicknesses no greater than several hundred micrometers, black rock coatings can persist for decades and are among the most prominent long-term vestiges of past smelting activities.…”

“…Remote sensing techniques are potentially well suited for discrimination and mapping of exposed black rock coatings, and correspondingly may be helpful in highlighting areas most affected by past or ongoing environmental degradation resulting from smelting activities. The formation mechanisms of black rock coatings are similar to acid-sulfate processes that operate under natural conditions in regions of volcanic and hydrothermal activity [2,[5][6][7], and the application of remote sensing techniques toward the discrimination of smelter-related coatings is thus relevant to the broader topic of the remote detection of acid-sulfate deposits. Problematically, discrimination of smelter-related black rock coatings can prove challenging as a result of the absence of distinctive features in the reflectance spectra of coatings [8].…”