Sebastian Avalos , Nicolas Avalos

Porphyry ore deposits are the largest source of copper, molybdenum, gold, and silver in the world. Various exploration techniques have been developed due to their importance as a source of metals. A special focus has been devoted to understanding the processes of hydrothermal fluids as mineralization drivers. Fluid inclusions provide direct information about the fluids involved in the hydrothermal genesis of the ore deposit. They convey temperature and mineral composition information conditioned to the salinity of the fluid system. Thus, fluid inclusion analysis works as an additional tool for porphyry ore deposit exploration.

We present a new exploration technique based on the properties of PVTX fluids to find porphyry copper. The Collahuasi district located in the North of Chile has 2 currently exploited and known porphyry: Rosario and Ujina porphyries. The investigation was carried out around the Rosario porphyry, which has reserves estimated at 2.18 BTon (0.91% Cu). Potassium, propylitic, quartz-sericite, argillic and advanced argillic alteration have been identified in this area. The latter was associated with high sulfidation veins, possibly related to a second unknown intrusion, named the Rosario Oeste zone.

The petrographic study of fluid inclusions was carried out on more than 300 drilling samples from Rosario and Rosario Oeste. 7 classifications of the types of FI that occur, these are: liquid-rich fluid inclusions, FI steam-rich, FI with coexistence of liquid and vapour, FI rich in liquid with halite, FI liquid-rich with halite coexisting with vapour, FI rich in liquid with chalcopyrite and FI with solids. The veins were classified with the support of the Collahuasi geology team, as veins type A, B, C, D, E, TES, and only quartz. The main alterations defined to use were: propylitic and argillic.

This information, an exploration model called the Copper Porphyry Probability Index (CPPI) is generated. All this information was integrated into a 2D and 3D model to study the spatial distribution of these parameters. It is concluded that boiling is present throughout the system in the upper and lower zones. Higher salinity and metal content in the fluid are recognized, positively correlating with the presence of type B and D veins and with high CPPI values, showing these in the direction of Rosario Oeste.

These similarities in shallow and deep parts, which correlate with two high-grade Cu-Mo zones, suggest the presence of two simultaneous mineralizing centers or the possibility of recurrent mineralization by a low-angle structural process.