Research on nanoscale processes is expanding in many scientific and technical disciplines, and economic geology is no exception. The idea that colloidal gold particles (or nanoparticles) may have played a significant role in ore formation was initially based on textural evidence from high-grade epithermal veins. More recently, the discovery of gold nanoparticles in some present-day geothermal systems and several orogenic gold deposits suggests that their formation could be a common process in gold-supersaturated hydrothermal solutions. Rare laboratory studies indicate that gold nanoparticles typically aggregate to form dendritic clusters. Over time, these dendrites perhaps can evolve to form coarser-grained crystals as subsequent annealing and recrystallization occurs. Due to the ephemeral nature and later recrystallization of dendrites, evidence of their former existence is commonly obscured. However, newer nanoscale imaging technologies have resulted in an increased recognition of their presence in hydrothermal gold ores, and thus their role in ore-forming processes merits further research. In particular, does their nucleation and deposition lead to forming higher-grade ores?

Pumping groundwater from arsenic (As)-contaminated aquifers exposes millions of people, especially those in developing countries, to high doses of the toxic contaminant. Previous studies have investigated cost-effective techniques to remove groundwater arsenic by stimulating sulfate-reducing bacteria (SRB) to form biogenic arsenian pyrite. This study intends to improve upon these past methods to demonstrate the effectiveness of SRB arsenic remediation at an industrial site in Florida. This study developed a ferrous sulfate and molasses mixture to sequester groundwater arsenic in arsenian pyrite over nine months. The optimal dosage of the remediating mixture consisted of 5 kg of ferrous sulfate, ~27 kg (60 lbs) of molasses, and ~1 kg (2 lbs) of fertilizer per 3785.4 L (1000 gallons) of water. The remediating mixture was injected into 11 wells hydrologically upgradient of the arsenic plume in an attempt to obtain full-scale remediation. Groundwater samples and precipitated biominerals were collected from June 2018 to March 2019. X-ray diffraction (XRD), X-ray fluorescence (XRF), electron microprobe (EMP), and scanning electron microscope (SEM) analyses determined that As has been sequestered mainly in the form of arsenian pyrite, which rapidly precipitated as euhedral crystals and spherical aggregates (framboids) 1–30 μm in diameter within two weeks of the injection. The analyses confirmed that the remediating mixture and injection scheme reduced As concentrations to near or below the site’s clean-up standard of 0.05 mg/L over the nine months. Moreover, the arsenian pyrite contained 0.03–0.89 weight percentage (wt%) of sequestered arsenic, with >80% of groundwater arsenic removed by SRB biomineralization. Considering these promising findings, the study is close to optimizing an affordable procedure for sequestrating dissolved As in industry settings.