Source:Department of Earth Sciences, Laurentian University, Volume MSc, p.220 (2011)
Black coatings on rock surfaces have been observed in the Sudbury, Canada region since the advent of Ni-Cu sulfide ore smelting operations in the early 1900's. Coating samples were collected from the Sudbury region and in the following work, are characterized using Scanning electron microscopy (SEM), Transmission electron microscope (TEM), Laser ablation inductively coupled mass-spectrometry (LA ICP-MS), X-ray diffraction (XRD), Electron microprobe analysis (EMPA), S-stable isotope measurements, Infrared spectroscopy and Micro X-ray fluorescence (XRF) mapping<br/>employing synchrotron radiation. The amorphous silica coatings vary in thickness from <5 um to a maximum of 200 um with metal-sulfate- and Si-rich layers. The metal-sulfate-rich layers contain finely distributed, nanometer aggregates of synthetic Fe-sulfate-hydroxide (Fe3+4(SO4)(OH)i0, goldichite (KFe3+(S04)2 • 4H20), mereiterite (K2Fe2+(S04)2 • 4H20), guildite (Cu2+Fe3+(S04)2(OH) • 4H20) and butlerite (Fe3+(S04)(OH) • 2H20). The 834S values of the black coatings indicate that the bulk (S04)2" incorporated within coatings is the result of the aqueous oxidation of emitted S02 from local smelting operations.<br/>Particles of 1 nm up to 50 urn in diameter in the coating are a mix of detrital, mineral grains (i.e. feldspars and quartz grains) and, angular or spherical metal-oxide and -silicate particles which may originate from smelting processes (i.e. hematite and spinel-type minerals). Clay minerals such as chlorite and montmorrillonite are also embedded within coatings. The apparent disequilibrium between these clays and the sulfate phasessuggest that the clay particles are from local soil environments and sulfates may have a mixed origin: emitted by smelters and/or secondary minerals formed in-situ.<br/>The comparison of trace metal concentrations in coatings and local soils shows that trace metal concentrations are more elevated in coatings relative to soils. The enrichment of metals in the coatings with regard to the soils increases in the sequence Mn —> Cr —> Co —• Ni —»• Fe —> Cu —• Se —> As —• Pb. Sulfates identified in coatings have not been reported in soils. As a result, the trace metal composition of the coatings is controlled by metal-bearing smelter particulates and secondary metal-bearing sulfates whereas in the soils, they are mainly governed by the persistence of less-soluble metalbearing particulates.<br/>Coating formation is no longer observable, indicating that former and extinct weathering conditions must have been present to promote the growth of amorphous silica rock coatings. A dissolution-evaporation-precipitation model is proposed to describe the formation of the black coatings. Acidic S02 fumigations from smelting and acid rain increased chemical weathering rates of silicates in the Sudbury region producing acidic water films at the rock-atmosphere interface. Water films containing dissolved silicic acid (FLi.Si04) became supersaturated with respect to amorphous silica after partial evaporation. Amorphous silica precipitated in the form of a silica gel and accumulated a variety of detrital particles and dissolved ions, and promoted the nucleation of secondary minerals. Dehydration of silica-gel coatings effectively limited further incorporation of particulates and the diffusion of ions in solution. The result is a stable coating which encapsulates and protects soluble secondary mineral phases and detrital grains available in the local environment at the time of coating formation.