Uncovering pathways of metal contamination with microscopic techniques and lead isotopic tracing

Over several decades, there has been considerable disagreement about heavy-metal contamination in mining communities as to whether the contamination arose from natural processes such as oxidation and weathering over millennia or from mining activities. More sophisticated geochemical methods and isotopic tracing especially for lead (Pb) may or may not provide definitive answers. We suggest that relatively simple approaches using the microscopic techniques of optical microscopy complemented by scanning electron microscopy and energy-dispersive analyses along with X-ray diffraction can provide definitive answers to the conundrum. Heavy liquid and –53 + 48 µm fractions of soil, pavement and gutter sweepings, dust depositional gauges, long-term dust accumulation, vacuum cleaner dust and ceiling dust were investigated using the above methods. The most common Pb-bearing particles consisted of Pb, Fe, Mn, Al, Si and O. The majority of grains were rounded with cavities and overgrowths, and showed evidence of transport and recrystallisation, probably deriving from post-mining activities and/or earlier geological processes. A small number of samples from ceiling dust, pavement sweepings, vacuum cleaner dust and long-term dust accumulation contained galena with a high degree of crystallinity suggesting a derivation from recent mining and ore concentration activities. High-precision Pb isotopic analyses showed that some samples with extensive oxidation and weathering had absorbed Pb from sources other than mine Pb, and these could be from gasoline, paint or weathering of the country rocks.