Characterisation of sericitic alteration at the Taca Taca Bajo porphyry Cu deposit, Argentina

The Taca Taca Bajo porphyry Cu deposit contains 2,165 Mt at 0.57% Cu Eq. It is located in the province of Salta, in north-western Argentina. It was emplaced in the back-arc of the Chilean Eocene-Oligocene porphyry belt, and localised by WNW trending cross-arc lithospheric structures. Multiple phases of syn-mineralization Oligocene (29.30 ¬¨¬± 0.57 Ma) rhyodacite porphyritic intrusives have been recognized, although most of the mineralisation is hosted in the Taca Taca Silurian granite batholith host. Green sericite has been observed at Taca Taca Bajo to be spatially associated with Cu mineralisation and thought to be part of the phyllic assemblage, occurring together with white sericite. A combination of petrography, spectrometry, scanning electron microprobe (SEM) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), has shown that the green sericite in Taca Taca Bajo is phengite, and occurs as fine grained, shreddy replacements of pre-existing secondary biotite and orthoclase. White sericite, on the other hand, occurs as a late, pervasive alteration of coarse, bladed muscovite which overprints and destroys all pre-existing alteration and mineralisation. Spectral data suggests that phengite crystallised at higher temperature and pH than muscovite, and is interpreted to be a temporal transition stage between potassic and phyllic alteration. Chalcopyrite was introduced during the early potassic and phengite alteration phases. The later muscovitic stages were ore-destructive, with chalcopyrite replaced by pyrite and, as the system cooled, a pyrite ‚ÄövÑv¨ bornite ‚ÄövÑv¨ chalcocite ‚ÄövÑv¨ covellite high sulfidation state assemblage. Phengite has been reported previously in porphyry Cu deposits, typically as an outer, lower temperature alteration mineral. In the case of Taca Taca Bajo, phengite appears to be analogous to sericite ‚ÄövÑv¨ chlorite ‚ÄövÑv¨ clay (SCC) alteration documented elsewhere, with a compositional variation potentially due to the felsic nature of the host rocks. Using systematic spectral measurements and whole rock assay data, it is possible to map the area where phengite has overprinted pre-existing potassic alteration.