ORE-HOSTING BRECCIAS AT THE VELADERO HIGH-SULFIDATION EPITHERMAL DEPOSIT, EL INDIO–PASCUA BELT, ARGENTINA

The Veladero high-sulfidation epithermal deposit is located near the northern end of the El Indio-Pascua belt in the Argentinean Cordillera. The El Indio-Pascua belt hosts numerous epithermal Au, Au-Ag, and Au-Ag-Cu deposits and prospects including the 12.2 M oz world-class Veladero deposit.

At Veladero, gold is primarily hosted in polymict breccias of the Miocene Cerro de las Tórtolas Formation. Two contrasting genetic models have been proposed. Early research suggested that mineralization occurred contemporaneously with breccia emplacement in a diatreme-maar setting. However, more recent work indicated that the ca. 16 Ma host rock succession represents a dome-vent complex. As preliminary ⁴⁰Ar-³⁹Ar dating of alunite contained in zones of advanced argillic alteration yielded ages between 10.7 and 10.9 Ma, it was proposed that the mineralization is about 5 Ma younger than the volcanic host rocks. The present research focuses on the characterization of the ore-hosting breccias and the development of a paragenetic sequence to unravel the relative age relationships between mineralization and the different types of alunite recognized at Veladero.

Field work revealed that the fine-grained disseminated gold is hosted in a subhorizontal ore body striking north-northwest over a length of 3 km. Alteration of the host rocks is characterized by intense silicification, accompanied by hypogene crystalline alunite. Stratified breccia is interpreted to record clast deposition in a sedimentary environment. These rocks were subjected to at least two episodes of post-depositional hydrothermal alteration and incorporation into pipe-like zones of secondary breccia, suggesting that initial clast formation and deposition pre-dated mineralization. Silicified breccias are cross-cut by mineralized alunite veinlets, indicating that a second phase of alunite alteration occurred after silicification. Jarosite is widespread, but sulfides are rare as the ore body is affected by intense supergene alteration. The initial field observations corroborate the above assumption of an age difference of about 5 Ma between the deposition of the stratified host breccia and mineralization. Hydrothermal activity at Veladero may, however, been more prolonged than previously suggested.