CONICET | Buscador de Institutos y Recursos Humanos

Altar (31° 29? S, 70° 28? W) is a porphyry Cu-(Au-Mo) deposit with associated epithermal Au- (Ag-Cu) veins located in the Cordillera Principal of San Juan province (Argentina). Mineralization in Altar (995 Mt, 0.35% Cu, 0.083 g/t Au; Marek, 2014) is mainly hosted in a complex stockwork of quartz veinlets and disseminations.Previous studies (e.g., Cohen, 2011) demonstrated that the wavelength of the 2,200-nm absorption feature corresponding to the Al-OH bond energy of white mica-bearing samples is critical to determinate the pH of hydrothermal fluids in porphyry systems. The wavelength of the 2,200-nm feature of white mica (including both muscovite and illite), shifts from 2,195 nm in muscovite toward 2,220 nm in phengite, as Al is replaced by (Fe, Mg)+Si. We performed Short Wave Infrared (SWIR) spectroscopy analyzes on 85 samples from three W-E sections that cut the Altar Central deposit. The objective of this contribution is to identify new mineral vectors towards Cu-richcores in the deposit and compare the new data with previous microprobe and XRD analyses.The equipment used for the measurements was an Analytical Spectral Devices, Inc. TerraSpec? short-wave infrared (SWIR) spectroscopic mineral analyzer. Alteration zones based on the position of the wavelength of Al-OH absorption (ca. 2200 nm) feature in white micas (WM) were mapped in a section of Altar Central. The results revealed a peripheral zone with phengitic white mica(>2209nm) that encompasses an internal zone of normal potassic WM (2200 ? 2209nm). At depths between 3550 and 3200 m.a.s.l within the normal potassic white mica zone, a hanging level with absorption features 0.7%) correlate to potassic white mica even to the transition to phengitic type. In the caseof gold, the medium-grade zone (0.1 - 0.2 g/t) correlates with potassic white mica and part of the phengitic type to the west. Furthermore, higher gold grades (>0.3 g/t) observed at hole ALD-49, related to D type veins coincides with the transition zone from potassic to paragonitic white mica located at shallower levels.Muscovitic white mica (including illite) composition reflects an acidic environment whereas a phengitic composition indicates a more neutral environment (e.g., Cohen, 2011).The normal potassic white mica zone recognized herein reflects the main channel of acid fluids during the phyllic stage. The external halo with phengitic white mica (+ chlorite) indicates peripheral more neutral fluids. At depth, phengitic white mica + chlorite overprints early potassic (biotitic) alteration. The shallower zone characterized by white mica paragonitic tendency (+ kaolinite) represent more acidic conditions, suggesting a transition to the epithermal environment (base of lithocap).Previous studies of the Altar deposit (Maydagán et al., 2016) analyzed the Cu and Au distribution, demonstrating the existence of remobilization and hypogene enrichment zones related to phyllic stage alteration.In this research, we found that the zonation of white mica generated during phyllic-stage overprint, determined by drifting in 2200nm wavelength feature, is correlated with Cu and Au grades in the hypogene environment.Finally, the results obtained support the use of white micas composition by reflectance spectroscopy as a vectoring tool to high-grade mineralization in porphyry systems like the Altar project.

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