Evolution of the post-mineralization Marianas hydrothermal eruption breccia, Cerro Negro District, Patagonia, Argentina

PERMUY VIDAL, C.; ZALAZAR, M.; GUIDO, D.; BROWN, G.

Workshop; 35th New Zealand Geothermal Workshop; 2013

Cerro Negro project is one of the most relevant low sulphidation 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 particularity of being discovered under a ~50 m thick postmineral hydrothermal eruption breccia event. The Marianas hydrothermal eruption breccia deposit extends over an area of about 0.5 km2 and comprises series of successive hydrothermal eruption breccia deposits interbedded with reworked volcaniclastic material of fluvio-lacustrine origin. All these postmineral units were described in detail at selected drill-holes and correlated in two integrated cross sections in order to make an interpretation of the evolution of the explosive and sedimentary processes that were hiding the high grade precious metal veins. The main hydrothermal explosive event is assigned to the latest mineralization stage, and it is recorded in the breccia bodies with quartz-bearing banded fragments located just above main dilatational structures. Those fragments, of up to 1 meter in diameter, were found as vein floats in the surface and allowed the discovery. The reconstructed geometry of these breccias indicates that the eruption vents were located at the major dilatational faults hosting the epithermal veins. Most hydrothermal eruption breccias were described in relatively young, poorly eroded terrains and active geothermal environment, where surficial and near-surficial features are preserved. Such shallow environment features were recorded in the Jurassic rocks of the Cerro Negro district comprising Au-Ag high grade epithermal veins, hydrothermal eruption breccias, and hot spring deposits. These allow the reconstruction of entire volcanic?geothermal?epithermal system providing a unique model for the epithermal Deseado Massif region.