"Invisible" silver and gold in supergene digenite (Cu1.8S)

Abstract

Despite its potential economic and environmental importance, the study of trace metals in supergene (secondary) Cu-sulfides has been seriously overlooked in the past decades. In this study, the concentration and mineralogical form of “invisible” precious metals (Ag, Au) and metalloids (As, Sb, Se, Te) in supergene digenite (Cu1.8S) from various Cu deposits in the Atacama Desert of northern Chile, the world’s premier Cu province, were determined in detail using a combination of microanalytical techniques. Secondary ion mass spectrometry (SIMS) and electron microprobe analyzer (EMPA) measurements reveal that, apart from hosting up to 11,000 ppm Ag, supergene digenite can incorporate up to part-per-million contents of Au ( 6 ppm) and associated metalloids such as As ( 300 ppm), Sb ( 60 ppm), Se ( 96 ppm) and Te ( 18 ppm). SIMS analyses of trace metals show that Ag and Au concentrations strongly correlate with As in supergene digenite, defining wedgeshaped zones in Ag–As and Au–As log–log spaces. SIMS depth profiling and high-resolution transmission electron microscopy (HRTEM) observations reveal that samples with anomalously high Ag/As (> 30) and Au/As (> 0.03) ratios plot above the wedge zones and contain nanoparticles of metallic Ag and Au, while samples with lower ratios contain Ag and Au that is structurally bound to the Cu-sulfide matrix. The Ag–Au–As relations reported in this study strongly suggest that the incorporation of precious metals in Cu-sulfides formed under supergene, low-temperature conditions respond to the incorporation of a minor component, in this case As. Therefore, As might play a significant role by increasing the solubility of Ag and Au in supergene digenite and controlling the formation and occurrence of Ag and Au nanoparticles. Considering the fact that processes of supergene enrichment in Cu deposits can be active from tens of millions of years (e.g. Atacama Desert), we conclude that supergene digenite may play a previously unforeseen role in scavenging precious metals from undersaturated (or locally slightly supersaturated) solutions in near-surface environments.