Djelika Sogoba, MSc

D. Sogoba¹, M. Schindler¹, S. Brueckner²

¹Earth Science, University of Manitoba,Winnipeg, Manitoba, Canada

²Harquail School of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada

The Cobalt Embayment in northwestern Ontario, Canada, is a historic mining district, famous for its five-element veins that were previously mined for the precious metal Ag and critical metals Co and Bi. Numerous studies examined the genesis and fluid composition of these veins. This led to multiple models and hypotheses that remain strongly debated to this day. However, there has been no nanoscale investigation to characterize the underlying mechanisms of silver (Ag) enrichment toward massive Ag. This study explores these mechanisms using a combination of scanning electron microscopy and transmission electron microscopy to characterize mineralized samples from the now-abandoned Cobalt and Hudson Bay mines in the Cobalt Embayment, ON. These investigations show that silver occurs as Ag nanoparticles within dolomite and as secondary As-Sb minerals, including stibioclaudetite (antimonate), Fe-rich clay, and organic matter. The Ag nanoparticles recorded within stibioclaudetite preserved traces of growth mechanisms, including polycrystalline aggregates formed by non-oriented attachment and monocrystalline aggregates formed by oriented attachment. Based on these results, we developed a “dendritic texture model of formation”. Furthermore, the association of Ag with As-Sb phases and carbonates provides evidence supporting mineral replacement reactions and fluid transport as enrichment mechanisms for silver. The mineral replacement reaction model describes processes in which  Ag-bearing parent phases such as breithauptite, dissolves and reprecipitates, forming Ag nanoparticles. The hydrothermal fluid transport model is supported by the occurrence of Ag nanoparticles finely distributed in carbonates. The current results of this study provide an additional mechanism for enriching Ag-bearing phases in five-element veins and contribute to our understanding of how high-grade silver vein deposits form.