The Miocene aged polymetallic (Ag-Zn-Sn-Pb) Pirquitas deposit of NW Argentina, hosted in an uplifted horst of Ordovician metaturbiditic rocks, defines the southern extent of the Andean Tin Belt. The nearby and recently discovered Cortaderas breccia-hosted Ag-Zn deposit contrasts markedly with the main Pirquitas deposit. To address these differences, a comprehensive geological, mineralogical, and geochemical study was undertaken and here reported. Ore formation commenced with brecciation and clay alteration of host rocks followed by six polymetallic ore stages and a waning stage. The complex mineralization (Ag, Sb, As, Sn, Fe, Zn, Cu, Pb, and Bi) consists of pyrite, marcasite, sphalerite, arsenopyrite, galena, cassiterite, quartz, dickite, siderite, hydrous phosphate minerals, and various sulfosalts and sulfides; metal assays indicate a telescoped system. Sphalerite-rich domains are characterized by massive, colloform, dendritic, and cockade textures. Silver and Sn principally occur in various sulfosalts (tetrahedrite, pyrargyrite, miargyrite, pirquitasite-hocartite, and canfieldite). Fluid inclusions reflect mixing of a ca. 5 wt% eq. NaCl fluid having a maximum Th value of 340 °C with heated meteoric water; in addition, fluid unmixing and flashing are documented to have accompanied ore formation. The δ34S values for pyrite and sphalerite constrain δ34SH2S to ≈ 5‰ which suggests a magmatic reservoir. Collectively the data support a model, whereby the Cortaderas breccia reflects focusing of magmatic-sourced mineralizing fluids along an episodically dilating fault zone. Fluids ascended and depressurized following brecciation of the host rocks which initiated fast and slow boiling. The latter processes accounts for the intimate association of colloform- and dendritic-textured sphalerite. Subsequent sealing of the system allowed temperature gradients to rise which resulted in telescoping of the system and generation of this polymetallic ore zone.
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