Chlorite, white mica and clay minerals as proximity indicators to ore in the shallow porphyry environment of Quebrada de la Mina deposit, Argentina

Abstract

The mineralogy and chemical composition of mica-like phyllosilicates (>2 µm) and clay fraction (<2 µm) of the shallow hydrothermal alteration zones of the Quebrada de la Mina Cu-Au deposit (San Juan Province, Argentina) were constrained by petrography, XRD, electronic microprobe and IR. The results allow us to identify: (1) relict patches of quartz + K-feldspar ± rutile (potassic) alteration, (2) a peripheral clinochlore-rich propylitic halo, and (3) a central white mica (>2 µm) and illite halo associated with phyllic and tourmaline-rich phyllic assemblages. The subhorizontal, shallow level Au-rich (>0.5 ppm Au) zone at the Quebrada de la Mina deposit coincides with the phyllic halo. The distribution of Au, Ag, Zn, Pb and As forms partially overlapping haloes toward shallower levels. Weak Cu (Mo) anomalies were found in potassic patches below the Au-rich zone. High Mn and Fe contents in clinochlore from the peripheral propylitic halo suggest a distance of ∼1 km to a porphyry Cu-(Au) mineralized core at deeper levels. White micas (>2 µm) in Quebrada de la Mina show a range of compositions between muscovite and illite. In the propylitic halo and in the patches of potassic alteration, white micas (>2 µm) show the highest values of Fe + Mg + Mn (0.40–0.55 apfu) and the lowest of Al (2.40–2.55 apfu). In the phyllic alteration zone of Quebrada de la Mina, white micas (>2 µm) with Al contents between 2.45 and 2.70 apfu and Fe + Mg + Mn between 0.35 and 0.15 apfu match the areas with high gold content (>0.5 ppm Au). The lowest values of Fe + Mg + Mn (0.11–0.05 apfu) and the highest Al contents (2.80–2.85 apfu) occur in white micas from the tourmaline-rich phyllic zone. Values of the Kübler index of illite of 0.19–0.1 2θ in the clay fraction of deep samples are indicative of higher temperature and/or higher fluid/rock ratios during illite formation, compared to values of the Kübler index > 0.2 2θ recognized in shallow samples. Thus, both the compositional variations in chlorite and white micas and the illite crystallinity could be used as a guide for exploration of copper-gold mineralization in shallow levels of porphyry systems.