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Location: 147 km NW from Salta, Argentina
Alvarado 1185 1° pisoSaltaArgentina4400
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The geology of the El Quevar project is characterized by silver-rich veins and disseminations in Tertiary volcanic rocks that are part of an eroded stratovolcano. Silver mineralization at El Quevar is hosted within a broad, generally east-west-trending structural zone and occurs as a series of north- dipping parallel sheeted vein zones, breccias and mineralized faults situated within an envelope of pervasively silicified brecciated volcanic rocks. There are at least three sub-parallel structures that extend for an aggregate length of approximately 6.5 kilometers. Several volcanic domes (small intrusive bodies) have been identified and mineralization is also found in breccias associated with these domes, especially where they are intersected by the structures. The silver mineralization at the Yaxtché zone is of epithermal origin. The cross-cutting nature of the mineralization, the assemblage of sulfide and alteration minerals, and the presence of open spaces with euhedral minerals, all point to an origin at shallow to moderate depths (a few hundred meters below surface) from hydrothermal solutions. The Yaxtché deposit shows alteration assemblages typical of high sulfidation epithermal deposits whereas the metal content and sulfide assemblages are characteristic of mineralizing fluids with an intermediate sulfidation state. The transition from high- to intermediate-sulfidation state is thought to define an evolving epithermal system as high-sulfidation state metal-bearing fluids cooled and interacted with host rocks as they moved vertically and laterally though the Yaxtché structure. This is depicted with three stages of primary fluid evolution: • Alteration and gangue mineral assemblages related to acidic magmatic–hydrothermal fluids created permeability through acid leaching (i.e. vuggy silica) • High-sulfidation state mineral assemblages (namely enargite–luzonite–famatinite) and metal contents (copper–gold dominant) formed at lower elevations within the Yaxtché structure • Transition of high- to intermediate-sulfidation state as metal-bearing fluids ascended and further interacted with host rocks. The final phase of fluid evolution was critical for precipitation of silver-bearing minerals as tennantite–tetrahedrite became stable. Sillitoe and Hedenquist (2003) defined the following key features of intermediatesulfidation systems: • Intermediate-sulfidation deposits occur in calc-alkaline andesitic–dacitic arcs, although more felsic rocks can locally act as mineralization hosts • The arcs typically display neutral to mildly extensional stress states • Deposits form under acidic, oxidizing conditions within 1 km of the surface and between temperatures of 150º and 250ºC • Deposits show a large range in metal content and characteristics and can vary along the spectrum from gold-dominant to silver-dominant mineralization • Although there is a large range of sulfide and sulfosalt minerals, these are dominated by sphalerite with low FeS content, and include galena, tetrahedrite–tennantite, and chalcopyrite. Sulfide abundance can vary from 5–20 vol% • Mineral assemblages typically contain Ag ± Pb, Zn (Au) • The typical Ag:Au ratio is > 20:1 • Minor mineral associations can include Mo, As, Sb; may have associated tellurides • Silica alteration can include vein-filling crustiform- and comb-textured veins • Typical alteration assemblages include advanced argillic, alunite and kaolinite with pyrophyllite deeper in the system; the proximal alteration mineral is often sericite. Mineralization at Yaxtché consists of fine-grained black sulfides and sulfosalts that are difficult to identify in hand specimens. The mineralization occurs variously as disseminations, open-space filling, and in massive veinlets or clots. The identified mineralogy is consistent with that expected within a high- to intermediate- sulfidation epithermal deposit. Coote (2010) observed: • Tennantite–tetrahedrite is both intergrown with and overgrowing/replacing enargite–luzonite defining a trend of progressively decreasing sulfidation state of acid hydrothermal fluids with time at any given location within the hydrothermal system. The association of minor amounts of very fine-grained chalcopyrite with tennantite–tetrahedrite as overgrowths to or replacement of enargite–luzonite is consistent with the interpreted decreasing hydrothermal fluid sulfidation state. Sphalerite, locally abundant in association with the tennantite–tetrahedrite, formed about or after luzonite–enargite, also formed as a component of the physio-chemically evolving acid hydrothermal system • Silver is mostly identified (from electron microprobe analyses and reflected light optical properties) as a component of the complex antimony- and lead-bearing and bismuth-rich sulfosalts which span the enargite–luzonite through to predominant tennantite–tetrahedrite paragenesis. It would appear that silver is poor in early bismuth-rich sulfosalts and rich in the later bismuth-rich sulfosalts that are mostly associated with tennantite/tetrahedrite. Silver mineralization therefore is also genetically associated with the evolving high-sulfidation system. Only minor to trace amounts of argentite are associated with tennantite–tetrahedrite and sphalerite. Distinctive metal zonation patterns are recognized at Yaxtché. Patterns are broadly defined as a copper–gold assemblage at lower elevations, transitioning upwards into a silver–lead–zinc– barium–antimony metal assemblage at higher elevations. These zonation patterns suggest that physio-chemical gradients had a significant control on localization of silver bearing mineral assemblages. Corbett (2012) proposed that sites of bonanza grade silver mineralization may be a product of fluid mixing along structures as silver-bearing fluids mixed with low pH steam heated waters collapsing down faults.
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