GEOLOGY AND HYDROTHERMAL ALTERATION OF THE SIERRA INESPERADA LITHOCAP, GUANACO DISTRICT, CHILE

PERMUY VIDAL, CONRADO; MAYDAGÁN, LAURA; PÁEZ, GERARDO; GUIDO, DIEGO; OSORIO, JORGE

Ciudad de Salta

Simposio; 15th Quadrennial International Association on the Genesis of Ore Deposits Symposium; 2018

IAGOD

The Sierra Inesperada hill is situated about 7 km southwestward direction of the Guanaco Mine, a highsulfidation structurally controlled deposit formed during the Paleocene to early Eocene (~45 Ma; Puig et al., 1988; Guido et al., 2014). The mineralization at Guanaco is hosted in ENE-WSW to E-W trending structures composed of gold-bearing vuggy-silica with minor quartz-enargite veins (Guido et al., 2014). The Sierra Inesperada is a N-S elongated topographic high composed of Paleocene volcanic rocks interpreted as part of a maar- diatreme complex (Páez et al., 2015). The whole volcanic pile is affected by prominent advanced argillic alteration corresponding to an extensive lithocap (1 km2). Lithocaps are volumetrically significant domains of hypogene silicic (>1 km thick), advanced argillic and argillic-altered rocks that can form above and to the side of shallow intrusions. Lithocaps can be particularlychallenging for exploration because may host high-sulfidation epithermal mineralization in their fracture-controlled roots (feeders) and some overlie and partially overprint porphyry deposits (Sillitoe, 1995, 2010). Chang et al. (2011) demonstrated that a combination of SWIR analyses, focusing on spatial changes in the alunite 1,480 nm peak positions, coupled with whole rock geochemistry, can provide effective tools for vectorization towards the heat and fluid source responsible for lithocap formation. This works shows preliminary exploration results on geological and alteration features in the Sierra Inesperada for vectorization towards mineralized structures within the lithocap.In the northern part of the Sierra Inesperada, volcanic units show a bleachedappearance due to kaolinite, "limonite", goethite and a regular pattern of jarosite veinlets. Discontinuous outcrops of NNE trending silicified structures can be traced along 1 km length. These are composed of coarse grained alunite fillings with locally residual quartz, and vuggy silica textures andpyrite boxworks. Some structures are affected by late barite breccias and some of them contain Cu oxides. In the western portion of the hill, a vertical shaft is placed with significant dumps of the Mina Inesperada old gold mine. Sulphide-bearing (pyrite and enargite) breccias with intense altered porphyritic volcanic clasts are observed. Breccias with Cu oxide minerals (brochantite-atacamite) seems to be the main ore minerals for miners.Results of collected samples for SWIR analyses drops alunite, jarosite, gypsum, kaolinite, goethite, dickite, pyrophyllite, siderite, illite-smectite and muscovite. In SWIR spectra, K-Na alunites have a strong absorption featureat about 1,480 nm wavelength. The position of this feature shifts related to the Na/(Na+K) ratio (Chang et al., 2011).For the Sierra Inesperada alunite samples, the 1,480 peak reveals that the highest Na contents (~1,490-1,495) drop in the center-northern portion of the hill within quartz-alunite zone and related to NNE corridors. The sample fromMina Inesperada shielded the highest 1,499 value corresponding to calcic (huangite) alunite composition. This sample also registered pyrophyllite representing the highest temperature within NNE corridors. These corridors andcorresponding cropping structures represent the conduits or feeder zones of the lithocap and display the same ENEWSW orientation that mineralized structures in Guanaco Mine which makes them attractive for encouragingexploration.