HIGH-SULFIDATION DEPOSIT, CHILE–ARGENTINA

The distribution of gold and other trace elements in the ore-stage pyrite from the high-sulfi dation Au–Ag–Cu Pascua deposit in the El Indio belt in north-central Chile was studied using electron-microprobe and secondary-ion mass spectrometry (SIMS) imaging and analysis. The images show that concentrations of Au and Cu are concentrically zoned, and those of Ag, As, Se and Te are zoned both concentrically and sectorally. The principal trace-element associations are As–Ag, Au–Cu and Se–Te. We propose that the fi rst two associations, As–Ag and Au–Cu, refl ect coupled substitutions for Fe, whereas Te and Se replace S by direct-ion exchange. The near-identical patterns of sector zoning of Ag and As suggest that silver enters the structure of pyrite via a coupled substitution with As, in which one atom of Ag and one of As substitute for two Fe atoms, yielding (Ag+0.5As3+0.5)S2. Gold and copper were excluded during an interval in which growth conditions promoted the development of | hk0 | sectors at the expense of | 111 | sectors; their enrichment patterns show only slight evidence of sectoral preferences. With the analytical methods employed, we could not distinguish unequivocally between incorporation of gold as minute inclusions of the native metal or a coupled substitution of gold as an Au3+ ion and copper as a Cu+ ion for two Fe2+ ions, yielding (Au3+0.5Cu+0.5)S2. However, we favor the latter interpretation. Incorporation of gold as Au3+ is also consistent with evidence of unusually oxidizing conditions during the formation of the Pascua deposit. The zonation of invisible gold and associated trace elements in the ore-stage pyrite at Pascua is the fi rst documented example of crystallographic structural control of the surface on the incorporation of multipl