ABSTRACT

   The Arizaro and Lindero prospects are possibly the youngest recognized examples of the hydrothermal iron oxide-copper-gold class of deposits in the world. These two prospects, and numerous other iron-oxide manifestations, form a newly discovered district within northwest Argentina. The Arizaro and Lindero prospects are hosted within mid-Miocene, calc-alkaline, dominantly andesitic, volcanic complexes that formed within a convergent plate margin tectonic setting. This descriptive paper presents observations from surface geology and petrographic studies.
   Alteration at the Arizaro prospect is zoned from distal calcite-epidote-chlorite alteration (propylitic) inwards to actinolite-potassium feldspar-magnetite-biotite±diopside-albite alteration (calcic-potassic) and finally to a central core of intense potassium feldspar-magnetite-biotite alteration (potassic). Weak hydrolytic alteration (quartz-sericite-clay) occurs on the periphery of the system and overprints other alteration types. Alteration at Lindero forms near-complete, concentric rings that are zoned from a central chlorite-calcite core, outwards into potassic alteration (potassium feldspar-magnetite-biotite), into hydrolytic alteration (quartz-sericite-clay-limonite), and finally to an outer propylitic alteration zone (epidote-calcite). Both prospects contain magnetite-rich, pyrite-absent assemblages with Cu-Au and elevated Pd and REE concentrations (La-Ce-Nd). Sulphides are largely restricted to zones of strong to intense potassic alteration and consists of disseminated chalcopyrite and minor bornite.
    The interplay between extensional tectonics, magmatism, and thermal convection of saline basinal brines is probably critical for the development of the Arizaro-Lindero alteration systems.

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