Fluid evolution of the Lindero porphyry gold deposit, NW Argentina: the critical role of salt melts in ore formation

The Lindero deposit is located in the Puna plateau, northwest Argentina, at the southern end of the Central Volcanic Zone of the Central Andes. The high-K calc-alkaline dioritic composition of the subvolcanic intrusions, the shallow emplacement depth (< 1.5 km), and the gold-rich and copper-depleted mineralization style suggest that the Lindero deposit is a porphyry gold deposit. Porphyry gold deposits are scarce worldwide and the factors controlling their formation are still poorly known. Here we present a detailed study of fluid inclusions in order to characterize the mineralizing fluids that precipitated the Au mineralization at Lindero. Different types of fluid inclusions in quartz veins (A-type and banded quartz), which are associated with the K-silicate alteration, were analyzed using Raman spectroscopy, microthermometry, and LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry). Four inclusion types can be recognized in quartz veins: (i) Salt melt inclusions, which are characterized by a dense packing of daughter minerals (mainly Fe-chloride, sylvite, halite, anhydrite, and hematite), by a distorted vapor bubble, and by the lack of liquid phase; (ii) Halite-bearing inclusions which contain liquid, vapor, and halite; (iii) Two-phase aqueous inclusions that contain liquid and vapor; (iv) Vapor-rich inclusions containing only vapor. The inclusion types are related to different stages of hydrothermal evolution. Stage 1 is the main mineralization stage, characterized by vapor-rich inclusions coexisting with salt melt inclusions. Salt melt inclusions commonly show total homogenization temperature (Th_L) > 1000 °C. This Na-K-Fe-Cl-rich highly saline brine (~ 90 wt% NaCl eq.) was of magmatic origin and responsible for the Au mineralization. Two later stages involving cooler fluids (Th_L < 300 °C) and gradually lower salinities (from 36.1 to 0.2 wt% NaCl eq.) trapped by halite-bearing and two-phase aqueous inclusions during stages 2 and 3, respectively, correspond to a late magmatic-hydrothermal system, that is probably related to a deep supercritical fluid exsolution. Salt melt inclusions represent the most likely parental fluid of K-silicate alteration and associated Au mineralization at Lindero. This uncommon type of fluid must have played an important role in Au transport and precipitation in shallow porphyry gold deposits.