Alunite and high sulfidation gold-silver-copper mineralization in the El Indio-Pascua Belt, Chile-Argentina

• Main Author: Deyell, Cari L.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/12943

High sulfidation precious-metal deposits, one of the principal styles of epithermal mineralization, are characterized by minerals diagnostic of a high sulfidation state (pyrite, enargite) and acidic hydrothermal conditions, particularly alunite (KAl₃(SO₄)₂(OH)₆) and alunite-group minerals. One of the world's foremost high sulfidation districts, the El Indio- Pascua Belt, straddles the Chile-Argentina border in the Cordillera Principal of the Andes. The region contains the world-class El Indio mine, the extensive Pascua-Lama and Veladero exploration projects, and several smaller prospects and mineralized alteration zones. This belt has been the subject of numerous studies at both a regional- and deposit-scale, and provides an excellent site to constrain genetic and exploration models for high sulfidation deposits. The genesis and timing of high sulfidation deposits in the El Indio-Pascua Belt has been constrained by detailed field studies, supplemented by geochemical, stable isotopic, and ⁴⁰Ar- ³⁹AT analyses of alunite, a critical mineral throughout the paragenesis of most systems in this region. Widespread barren, pre-mineral alteration containing abundant alunite may have provided permeability controls and metals for subsequent mineralization. Economic mineralization in the district formed between ca. 6 to 9.5 Ma, and is associated commonly with syn-mineral alunite. This alunite has textural, chemical and isotopic characteristics consistent with a magmatic-hydrothermal origin and requires the presence of relatively oxidizing, acidic, and sulfur-rich mineralizing fluids. Data suggest that these fluids were derived from the condensation of a magmatic vapor plume, further supported by the presence of syn-ore magmatic steam alunite at the Tambo deposit. This conclusion is consistent with emerging evidence from other systems indicating the importance of vapor transport for metals in high sulfidation systems. Alteration and mineralization in the El Indio-Pascua Belt were dominated by magmatic fluids, even in the near-surface environment. The negligible contribution of meteoric fluids is attributed to prolonged magmatic activity, an arid climate, and depressed water tables resulting from tectonic uplift and repeated incision. These features provide new constraints for the genetic model for high sulfidation deposits, although the specific setting of the El Indio-Pascua Belt may have provided a unique combination of processes explaining the formation and preservation of these prolific mineralizing systems.