Phyllosilicates and pyrite compositions in the shallow porphyry environment of the Quebrada de la Mina (QDM) Cu-Au deposit, Argentina: their application as proximity indicators to ore

MAYDAGÁN, LAURA; FRANCHINI, MARTA; LENTZ, DAVID; IMPICCINI, AGNES; ROMERO, MARÍA ISABEL; ROBETO, JAVIER

Santiago

Congreso; South American Metallogeny 2019; 2019

Society of Economic Geologists

The Quebrada de la Mina (QDM) porphyry Cu-Au deposit (31°29′S, 70°30′W) is located in the SW of San Juan Province (Argentina) in the Andean Main Cordillera, close to the Argentina-Chile border. Early Miocene volcanic rocks in the area were intruded by a circular dacite porphyry stock with a U-Pb zircon age of 11.91 ± 0.33 Ma (Maydagán et al., 2017). On the surface, A veins were not observed and the presence of scarce D veins reflect a low degree of erosion of the porphyry system. The aim of this study is to document the compositional variations of phyllosilicates, clays and pyrite of the shallow porphyry environment as monitors of proximity to mineralized cores at depth.The mineralogy and chemical composition of sulfides, mica-like phyllosilicates (> 2 μm) and clay fraction (< 2 μm) of the shallow hydrothermal alteration zones were constrained by XRD at Universidad Nacional del Comahue (Argentina), and electron microprobe analyzes were performed at Oregon State University (USA). Pyrite mappings were performed at the LA-ICPMS laboratory of the University of New Brunswick.The hydrothermal assemblages recognized at shallow levels were the following: (1) relict patches of quartz+K-feldspar ± rutile (potassic) alteration, (2) a peripheral chlorite-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 QDM deposit coincides with the phyllic halo. The distribution of Au, Ag, Zn, Pb, and As forms partially overlapping haloes toward shallower levels.Chlorites from QDM have higher contents of Mn (0.17?0.25 apfu) and Fe (2.70?3.59 apfu) compared to deeper chlorites from the nearby Altar Cu deposit associated with Cu mineralization. When compared to the chlorites from the Altar propylitic alteration, QDM chlorites have similar Fe contents and are enriched in Mn and impoverished in Mg. Wilkinson et al. (2015) proposed that Mg in chlorites decreases exponentially in concentration with increasing distance from the mineralized center, whereas Mn and Fe show evidence for an enrichment halo, with peak concentrations at a distance of ~1 km from a deeper porphyry Cu-Au mineralized center. Thus, high Mn and Fe contents observed in chlorites from the propylitic zone of the QDM could be indicative of the Mn and Fe enrichment halo. Recent drill-holes performed by the mining company have found a Cu-Au rich core at depth (bornite+chalcopyrite+magnetite), which corroborates this hypothesis.At shallow levels, cooling of the hydrothermal fluids may have produced the pervasive phyllic alteration that overprinted the early hydrothermal assemblages and filled D veins with phyllic halos and straight-walls. White micas (> 2 μm) in QDM 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 QDM, 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). 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. Tourmaline increases in phyllic zones located above and around the potassic zones, thus could also be used as an indicator to locate Cu cores at depth.Most of the gold mineralization at shallow levels of QDM is linked to pyrite disseminated in phyllic zones. LA-ICPMS mappings were performed on pyrite samples from a relatively deeper potassic zone and from the shallow phyllic halo. Pyrite in equilibrium with the shallow phyllic alteration presents higher contents of Au (up to 100 ppm) and Ag (up to 500 ppm), in addition to contents of Pb, Mn, As, Zn, Sb, Cu, Te, and Bi between 1 and 2 orders of magnitude higher than pyrite from potassic alteration. Au occurs as micro-inclusions that correlate with Ag, Pb, Te, Zn, Cu, and Pb (pyrite mapping of sample QDM-06-52 m, Fig. 1). Zonation patterns in pyrite are shown by Co, Ni, and As with distributions that don?t correlate with Au. The textures recognized in the pyrite maps and their high contents of trace elements are indicative of the overlapping of lower temperature fluids (low temperature phyllic to epithermal stage) that affect the pyrite crystals precipitated in an early higher temperature stage (porphyry stage, e.g., Sykora et al., 2018).The new data indicate that in the shallow porphyry enviroment of the QDM deposit the compositional variations in pyrite, chlorite, white micas, and the illite crystallinity could be used as a guide for exploration of copper-gold mineralization at depth.