Distribution of the hydrothermal alteration at the Guanaco Au-Cu district mine, Northern Chile.

PERMUY VIDAL C.; PAÉZ G.; GALINA M.; JOVIC S.; GUIDO D.; MOREIRA PILAR; LÓPEZ L.

Santiago

Conferencia; SEG 2019 Conference South American Metallogeny: Sierra to Craton; 2019

SEG

The El Guanaco mining district islocated at 195 km to the south-southeast of Antofagasta city, northern Chile.Available production data reveals a minimum production of 1.32 Moz, that couldeven reach 3 MOz, during more than 130 years of mining history (Galina et al.,2014). The El Guanaco HS deposit is hosted in the Paleocene-Eocene volcanicbelt (Espinoza et al., 2011). This volcanism is part of a widespread volcanicevent developed along the present-day Longitudinal Valley and ChileanPrecordillera. Large alteration zones, epithermal and minor porphyry systemsare spatial and genetically associated within this volcanism (Davidson andMpodozis, 1991). Paez et al. (2015) and Permuy Vidal et al. (2015) describedthe Guanaco Volcanic Complex (GVC) of Paleocene age as a succession ofphreatomagmatic lapilli-tuffs, diatreme breccias, dacitic domes and subvolcanicintrusions that lays unconformably over the Cretaceous pyroclastic rocks of theAugusta Victoria Formation. The El Guanaco is a gold, silver and copper-rich HSepithermal system located within a broad lithocap of >30 km2. At surface, a large advanceargillic alteration zone is located in the highest hills (above 2800 m). Themineralization is hosted in a series subparallel ENE-WSW to E-W trending?ledges? of gold-bearing vuggy- silica and quartz-enargite veins. Galina etal., (2015) describes a paragenetic sequence with 4 main stages: 1)hydrothermal alteration; 2) mineralization; 3) remobilization and 4) oxidation.This work summarizes the surface distribution of the alteration zones in themine and its distribution in depth at the Dumbo open-pit. Geological andalteration mapping at 1:1000 scale of the operative zone and open- pits wereintegrated with RC logging (Figs. 1 and 2). A total of 36 samples werecollected in different levels for portable spectrometry analysis (ASDTerraspec) and RX diffraction. The early stage of advance argillic alterationflares upwards from structurally controlled feeders to form a subhorizontalcontrolled lithocap coincident with a pumice-rich tuff on the top of thevolcanic sequence. The shallower parts are dominated by quartz-aluniteand kaolinite-alunite assemblages characterized by apervasive replacement throughout much of the pumice-rich tuffs that caps CerroEstrella (Fig. 1). It is characterized by coarsely crystalline pink K-aluniteand silica that replaces welded pumice fragments. In these shallower parts,this alteration is typically barren. Deeper, steep ledges show an alterationpattern with an outward zonation which can be traced in depth up to 300 metersbelow the surface (Fig. 2). The innermost zone is composed of residualquartz and vuggy silica reaching up to 50m in thickness.In this zone, the rock textures are mostly obliterated, but it is possible torecognize some diagnostic textural features of the volcanic sequence (e.g.fine-grained quartz phenocrysts, lithophysae, lithics vugs of the tuff). Thesilicified and residual quartz zone commonly display a sharp boundary with a quartz-alunite-kaolinite zone that contain quartz, finegrained Na-alunite, kaolinite ± dickite and can reach tens of meters inthickness. At the Dumbo Pit this alteration zone is also controlled by a sub-horizontal to slightly tilted pumice-rich pyroclastic layer toward the middleparts of the volcanic pile (Fig 2 A&B). Galina et al., (2015) also reportsAPS minerals for this zone. This advanced argillic assemblage grades into asecond envelope of argillic alterationimmediately adjacent to/or intermixed withsilicified zones, that is characterized by an assemblage of kaolinite-alunite and white clays(illite, smectite) ± hematite. This zone of generallylaterally grades into zones of pale green clays chlorite + carbonate ± pyrite and hematite which can range in thickness fromcentimeters up to many tens of meters distributed in the tuffs andandesite/basalt sills units peripherical to the argillized zones. Thisassemblage is barren of gold, but locally it is stained by copper andmanganese oxides. The Au-Cu mineralization is related to a later and multi stage event ofsilicification hosted within the residual quartz and quartz-alunite-kaoliniteassemblages. At deeper levels, high Cu grades are related to fine grainedquartz + pyrite + enargite and locally tennnatite-tetredrite. Gold-silvermineralization is dominant up to 150 meters in deep controlled by theandesitic/basaltic sill position. Au is related with massive quartz + jarosite+ barite veins and/or vug fillings. Leaching and oxidation products such aschenevixite, brochantite and atacamite are commonly distributed up to 250meters below the surface. The almost 300 m deep surface mine exposures at ElGuanaco mine offers a unique opportunity to explore the three-dimensionalarrangement of hydrothermal alteration assemblages of a HS deposit. The lateraland vertical alteration zoning that characterized HS deposits reflects the reactionand neutralization of early and barren high- temperature acidic fluids as theyinteract with the host rocks. The detailed recognition of volcanic units withinthe volcanic pile helps to understand the distribution of the later high Au-Cu mineralizationevent.