Investigating nanoparticle pathfinder geochemistry in stream sediments using single particle inductively coupled plasma mass spectrometry: a case study at the Sundance gold mineralization, Wyoming, USA

In this study we use single particle inductively coupled plasma mass spectrometry (spICP-MS) to investigate the presence, composition, concentrations, size distributions and dispersal of nanoparticulate forms of gold pathfinder elements in stream sediments. We tested this method in Lytle Creek, which drains from the Sundance gold mineralization in Wyoming, USA. Within this system, spICP-MS analysis of stream sediments provides new insights into the downstream dispersal of pathfinder elements including Ag, Sb, Bi, Te and Tl. Nanoparticulate forms of these pathfinder elements are highly abundant in the sediments, with 10⁴–10⁷ particles detected per gram of sediment. Despite significant gold anomalies detected by aqua regia/ICP-MS, nanoparticulate Au was not detected, suggesting it is held in larger particles. Nanoparticulate concentrations of Ag, Sb, Bi, Te and Tl reflected the exponential decay down drainage seen in the bulk concentrations. A significant fraction of nanoparticulate mass of these pathfinder elements is held in particles below the spICP-MS size detection limits. We found that among the five pathfinders tested, Ag behaved uniquely, with respect to both the fraction of its total concentration present as nanoparticles and its logarithmic particle size distribution. Both observations may indicate dissolution/reprecipitation of Ag, or size-dependent transport mechanisms for its dispersal. This study provides insight into the role of nanoparticles in forming stream sediment geochemical anomalies, and suggests that spICP-MS may be a useful complement to conventional techniques for future mineral exploration.