• journal_title：The Canadian Mineralogist
• Contributor：Annick Chouinard ; Jeanne Paquette ; Anthony E. Williams-Jones
• Publisher：Mineralogical Association of Canada
• Date：2005-
• Format：text/html
• Language：en
• Identifier：10.2113/gscanmin.43.3.951
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：43
• issue：3
• firstpage：951
• section：Articles

The distribution of gold and other trace elements in the ore-stage pyrite from the high-sulfidation 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 first two associations, As–Ag and Au–Cu, reflect 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 first documented example of crystallographic structural control of the surface on the incorporation of multiple trace elements in pyrite. Moreover, the nature of this zonation provides clear evidence that incorporation of gold in pyrite does not necessarily involve coupled substitution with arsenic, as proposed for many other deposits. Not only can arsenic behave as a metal in the structure of pyrite, but its sectoral pattern of incorporation is a potentially sensitive indicator of redox conditions during mineralization.