Geology and hydrothermal alteration of the Sierra Inesperada lithocap, Guanaco District, Chile

PERMUY VIDAL, C.; GUIDO, D.; MAYDAGAN, L.; OSORIO, J.; PAEZ, G.

Salta

Simposio; 15th Quadrennial IAGOD Symposium; 2018

Association on the Genesis of Ore Deposits Symposium

INTRODUCTIONThe Sierra Inesperada hill is situated about 7 km southwestward direction of the Guanaco Mine, a high-sulfidation 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 (1km2). Lithocaps are volumetrically significant domains of hypogene silicic (>1km thick), advanced argillic and argillic-altered rocks that can form above and to the side of shallow intrusions. Lithocaps can be particularly challenging 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 vectoring 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 vectorize towards mineralized structures within the lithocap.METHODOLOGYGeologic and hydrothermal alteration mapping, core logging of diamond drill holes from the area was integrated with available geochronological determinations to establish the volcanic stratigraphy and temporal framework of volcanism and alteration within the district in Sierra Inesperada (Figure 1). A total of 41 chip rock samples were collected along a 5-km transverse of the advance argillic alteration zones for portable SWIR analyzes to complement the mapping. Spectral analysis of all samples were made using a TerraSpec 4 Hi-Res Mineral Spectrometer (Analytical Spectral Devices, Inc.) provided by Austral Gold Ltd. The spectral range varies from 350?2,500 nm of both visible-to-near infrared electromagnetic radiation. At least two analyzes were performed for each sample to avoid differences in the intensity of absorption features and heterogeneities within the rock. Spectra and spectral parameters were obtained and calculated using The Spectral Geologist (TSG) software (www.thespectralgeologist.com.au). Spectral parameters corresponding such as wavelength position of an absorption feature were calculated using scalers in TSG for both reflectance and hull quotient corrected spectra. RESULTS AND CONCLUSIONSThe geology of the Sierra Inesperada is mostly composed of Paleocene volcanic rocks assigned to the Chile-Alemania Formation. Basal units are andesitic-dacitic porphyritic lava flows overlied by a thick stratified pyroclastic sequence. The last includes several pyroclastic surge deposits intercalated with massive pumice ignimbrites and lenses of lithic-rich breccias with can reach up to 1 m diameter clasts at the northern part of the hill. Fine grained lapilli size surge deposits show high flow regime structures such as cross-stratified stratification, antidunes, and ballistic impacts. These features accompanied with accretional lapilli and slumping structures are interpreted to be formed as a consequence of water-magma interaction (Páez et al. 2015). At the northern side of the hill, two intrusive breccia bodies (Brecha Inesperada and Brecha Medialuna) were mapped. These are 1.5 to 0.5 km diameter outcrops composed of massive to stratified matrix supported breccias with polymictic porphyritic clasts of rounded to subangular shape with up to ten meters diameter. The matrix is fine grained with cavities. The sequence is topped by a rhyodacitic flow dome cropped in the Cerro Chancho, in the southern Sierra Inesperada. Páez et al. (2015) interpreted that the pyroclastic sequence and the two intrusive bodies are part of a maar-diatreme volcanic edifice. The geology of Sierra Inesperada is competed by small dioritic stocks of Eocene age intruding paleocene units and late olivine basaltic lava flows of the Catalina Formation (Espinoza et al. 2011). The Sierra Inesperada has prominent advance argillic alteration affecting the paleocene units, mapped as pervasive silicification blankets, quartz alunite + argillic zones which give place to an extensive lithocap of 1km2 (Figure 1). Pervasive silicification occurs in the south and eastern part of the hill typified by strong silica replacements of volcanic rocks that preserve primary volcanic textures. NNO-trending silicified structures are preserved with locally residual quartz and vuggy silica textures. These structures locally host strongly silicified matrix supported breccias with fluidized matrix interpreted as phreatic breccias. Late faults are filled with silica and hematite.In the northern part of the Sierra Inesperada, volcanic units show a bleached appearance 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 and pyrite 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 feature at 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 from Mina 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 and corresponding cropping structures represent the conduits or feeder zones of the lithocap and display the same ENE-WSW orientation that mineralized structures in Guanaco Mine which makes them attractive for encourage exploration.