Evolution of the Post-Mineralization Marianas Hydrothermal Eruption Breccia, Cerro Negro District, Patagonia, Argentina

Abstract

Cerro Negro project is one of the largest low sulfidation epithermal systems under exploration in the world. The deposit has a total resource estimation of ~6.7 Moz Au equivalent at 6 vein systems (www.goldcorp.com). The two Marianas veins (Mariana Central and Mariana Norte) represent about ~50% (3.3 Moz Au equiv.) of these resources, and they have the distinction of being discovered under a ~50 m thick post-mineralization hydrothermal eruption breccia event. The Marianas hydrothermal eruption breccia deposit extends over an area of about 0.5 km2 and comprises a series of successive hydrothermal eruption breccia deposits interbedded with reworked volcaniclastic material of fluvio-lacustrine origin. These postmineralization units were described in detail from selected drill-hole cores and correlated in two integrated cross sections in order to make an interpretation of the history of eruptive and sedimentary processes leading to concealment of the high grade precious metal veins. The main hydrothermal eruptive event is assigned to the latest mineralization stage. It is recorded in the breccia bodies with quartz-bearing banded fragments located just above dilatational structures. The fragments, up to 1 meter in diameter, were found as vein floats in the surface deposits and led to the discovery of the precious metal-rich veins. The reconstructed geometry of these breccias indicates that the eruption vents were located on the major dilatational faults hosting the epithermal veins. Most hydrothermal eruption breccias have been described in relatively young, poorly eroded terrains and active geothermal environments, where surficial and near-surficial features are preserved. Such shallow environment features are also preserved in the Jurassic rocks of the Cerro Negro district and consist of Au- Ag high grade epithermal veins, hydrothermal eruption breccias, and hot spring deposits. These rock types allow the reconstruction of entire volcanic–geothermal–epithermal system providing a unique model for the epithermal Deseado Massif region.