Supergene Alteration of Primary Ore Assemblages from Low-Sulfidation Au-Ag Epithermal Deposits at Pongkor, Indonesia, and Nazareño, Perú

Abstract

The textures and mineralogical assemblages of high-grade ore samples from the low-sulfidation Au-Ag epithermal deposits of Pongkor (2 Ma; West Java, Indonesia) and Nazareno (10 Ma; Cordillera Shila, southern Peru) were studied to investigate processes related to post-depositional alteration of primary mineral parageneses. Both deposits are characterized by a late primary high-grade base and precious metal stage composed of chalcopyrite-sphalerite-galena-pyrite–polybasite-pearceite–tennantite-tetrahedrite–electrum. Minerals of the polybasite-pearceite series, the main Ag carriers, were found to be transformed into acanthite. Acanthite was also found locally in association with native silver, as a replacement product of base metal sulfides, as veinlets, alternating rhythmically with supergene iron oxyhydroxides replacing pyrite, as well as associated with barite, iron oxyhydroxides and covellite as boxwork fillings. Primary electrum grains with a composition of about Au0.6Ag0.4 are partially replaced by Au-Ag sulfide, locally identified as uytenbogaardtite (Ag3AuS2), and/or by acanthite, with both of the replacing minerals commonly associated with vermicular electrum of a greater fineness (up to Au0.8Ag0.2). The occurrence of the replacement textures near the present-day surface and the associated secondary minerals, strongly suggests a supergene origin for the Au and Ag sulfide phases. The comparison of Pongkor and Nazareno also suggests that supergene remobilization of Au and Ag is geochemically significant and can occur in widely differing morphological and climatic contexts.