Distribution of gold and silver and its relation with hypogene ore minerals in the Esperanza porphyry deposit, Antofagasta Region, Chile

Modes of occurrence of Au- and Ag-bearing phases and their relation with associated hypogene ore minerals were examined with the objective to elucidate Au-Ag distribution at the Esperanza porphyry deposit in the Eocene Centinela copper belt, using ore-microscope modal analysis, semi-quantitative analyses by automated mineralogy, electron probe microanalysis, and secondary ion mass spectrometer. The Esperanza hypogene mineralization is characterized by early-stage chalcopyrite-rich veinlets in the potassic alteration zone and later polymetallic stage with tennantite and galena in the chlorite-sericitic alteration zone. Only the early-stage chalcopyrite contains fine-grained electrum (Au₆₈Ag₃₂ - Au₈₁Ag₁₉) and hessite (Ag₂Te), and thus yields positive correlations in Cu vs. Au and Cu vs. Ag grades that are clearly recognized in the hypogene sulfide zone. The early-stage chalcopyrite grains frequently exhibit polysynthetic twinning suggestive of inversion from intermediate solid solution. These features suggest that the fine-grained electrum and hessite are products exsolved in the cooling process with the intermediate solid solution to chalcopyrite inversion. In contrast, tennantite and galena of the later-stage mineralization contain no detectable Ag, and it is thus proposed that the early-stage inverted chalcopyrite is the principal storage of economically important precious metals.