REGIONAL GEOLOGY – Abstracted from Giacosa (2011)

The Andacollo district is located in the north-eastern part of the Neuquén Province in the extra-andine sector of the Neuquén Basin. The Cordillera del Viento forms the western boundary of the Chos Malal fold and thrust belt, which in turn is the northern continuation of the Agrio fold and thrust belt. The Cordillera del Viento stands out sharply from the rest of the geology of northern Neuquén because it constitutes a structural high exposing pre-Jurassic basement rocks of

the Neuquén Basin. Paleozoic rocks are exposed mainly in the western sector whereas Mesozoic rocks constitute both the basement and the filling of the Neuquina Basin along its eastern slope (see Figure 4). Some authors suggest that this mountain range is the most important post-Albian structure in the Neuquén Basin. Chauveau et al. (1996) suggests that the Cordillera del Viento is an anticline on the ramp of an east verging blind thrust, which generated

a pop-up structure. Koslowski et al. (1996) define it as a brachianticlinal 90 km in length that integrates the western part of the Chos Malal Folded Belt. Based on a seismic line located on its eastern flank, the anticline is interpreted as a fault flexure fold of large dimensions associated with a basal shift with plane-ramp geometry, whose take-off zone would be about 12 km deep in the Paleozoic basement.

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The oldest rocks in the district are Lower Carboniferous silicic volcanic and sub-volcanic rocks (Arroyo del Toreón Fm.) uncomformably overlain by Upper Carboniferous clastic marine sediments (Huaraco Fm.), collectively termed the Andacollo Group. The former consists of 1,200 – 1,800 m of tuffs, ignimbrites, and rhyolite-dacite flows and domes inter-bedded with sandstones, black shales and well-rounded, clast-supported conglomerates. The latter consists of 700 m of mainly shale and dark green to black siltstones with inter-bedded sandstone and conglomerate. These Carboniferous units are, in turn, unconformably overlain by 1,000 m of Permian ignimbrites with subordinated

conglomerates and sandstones (La Premia Fm.) that correlate with Choiyoi Group rocks to the north. All of these rocks were intruded by Permian granodiorite (287 ± 9 Ma) and minor rhyolite domes (260 ± 10 Ma) then covered unconformably by Permian to Lower Triassic silicic volcanic rocks, which are exposed south and east of the district.

The Paleozoic rocks were deformed and folded during the Chanic (Neoproterozoic–early Carboniferous) and Gondwanan (early Carboniferous–early Permian) orogenic cycles. Together with the overlying Permian to Lower Triassic rocks, these rocks were cut by later normal faults associated with

the development of an extensional basin that controlled volcanic deposition of the La Premia Formation. In the Andacollo Mining District, subsequent E–W to ENE–WSW trending normal faults with bookshelf geometry deformed the Huaraco and the La Premia formations and provided conduits for subsequent mineralizing fluids during the Late Cretaceous to Paleogene age (Giacosa et al, 2014). During the Andean Orogeny between Late Cretaceous and Miocene, the region was uplifted and eroded prior to deposition of Eocene andesitic rocks of the Cayanta Formation. Post-depositional faulting has tilted the entire sequence 15-20° to the west.

Geological map of the central Andacollo District (Giacosa, et al., 2014)

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Mineral deposits in the Andacollo district are hosted principally in silicified volcanic rocks of the Arroyo del Toreón, Huaraco and La Premia formations. With exception of the Fortuna, Erika, Santa Elena and Sofía-Julia veins that occupy low-angle fault planes, that are presumed thrust faults, most of the veins in the district occupy high-angle normal faults. In addition to the principal vein deposits, historic placer workings are evident in many of the drainage areas and, 10 km south of Cerro Minas at the Primavera prospect, “bulk mineable”

resources (currently uneconomic) have been delimited within permeable ignimbrites.

The two types of vein deposits are:
• Intermediate sulfidation saccharoidal quartz±chlorite veins containing abundant pyrite with minor chalcopyrite, argentite and trace gold (Au:Ag=1:100).

• Shear hosted polymetallic quartz-carbonate veins with pyrite, chalcopyrite, galena, sphalerite and high gold usually hosted in a chlorite-graphite gangue (Au:Ag=1:5).

The saccharoidal quartz±chlorite veins typically are hosted in silicified volcanic rocks that are susceptible to brittle fracturing and open-spaced filling whereas the polymetallic veins, which are notably thinner, occur in less competent sedimentary rocks that are more susceptible to ductile shearing. In both cases, the width and extension of the veins are variable. They are believed to have formed at intermediate (mesothermal)

depths given that all of the veins lack typical shallow-level textures and have average homogenization temperatures between 210°C and 309°C. Based on whole-rock geochemistry, trace element associations and relatively high Au:Ag ratios, the Andacollo district appears to be a metaluminous calc-alkaline intermediate sulfidation gold system.