Summary:
The Chinchillas and Pirquitas deposits are considered part of the Bolivian tin-silver–zinc belt that extends from the San Rafael tin-copper deposit in southern Peru into the Puna region of Jujuy. Deposits with similar styles of mineralization include San Cristóbal, Potosí, and Pulacayo. These deposits are characterized by dacite dome complexes with mineralization hosted in shears and breccias within the dacite domes or within shears and breccias within the host rocks. More rarely, as in the case of the Chinchillas Property and San Cristóbal, the deposits morphology includes flat-lying manto bodies within sedimentary and pyroclastic rocks. All the deposits have large vertical extents.
Chinchillas Mineralization
Significant silver-lead-zinc mineralization occurs in four main areas at Chinchillas: the Silver Mantos and Basement Mantos zones in the west part of the caldera and the Socavon del Diablo and Socavon Basement/Melina zones in the east part.
Northeast trending faults within the metasedimentary rocks are interpreted to control the location of the Basement Mantos deposit, the distribution of high grade mineralization in the Silver Mantos deposit, and, potentially, the Socavon del Diablo deposit. Mineralization is dominated by silver, with lesser amounts of lead and zinc. Mineralization occurs as disseminated sulfides, matrix infilling within the volcanic tuffs, and as matrix and fracture filling in breccias within the metasedimentary rocks. Dacite volcanic rocks are rarely mineralized in shear zones, veinlets, or vein-like structures. Within the metasedimentary lithologies, shear zones and faults are more commonly mineralized. The depth of oxidation is several meters within the volcanic rocks and is insignificant within the metasedimentary rocks. Silver, lead, and zinc bearing minerals include silver sulfosalts, freibergite, boulangerite, tetrahedrite, schalenblende, sphalerite, and galena. Main mineral associations include chalcopyrite, quartz, pyrite, siderite, limonites, manganese oxides, cerussite, smithsonite, anglesite, and malachite (Marshall and Mustard, 2012 and Coira et al., 1993).
Within the Silver Mantos zone, mineralization is disseminated throughout several shallow (approximately 5°), east dipping layers hosted within clay altered pyroclastic tuffs and breccias. The mineralization occurs between surface and 100 m depth with a north-south extension of 150 m that ranges between two meters and 60 m thick and averaging greater than 20 m. The mineralization is very finely disseminated in the tuff matrix, and commonly consists of galena and sphalerite. Located below the Silver Mantos, the Basement Mantos comprises an area 600 m wide and up to 210 m thick, with an average thickness of 80 m, dipping at approximately 40° to the east. The zone has been traced down dip for approximately 350 m. The Basement Mantos is hosted entirely within Acoite Formation metasedimentary rocks and is comprised predominantly of breccias with small veinlets and fracture fill mineralization. The Socavon del Diablo zone is located in the eastern central area of the caldera. Mineralization is dominated by mantos style disseminated sulfides within shallow, west dipping volcanic tuff horizons. Mineral occurrences, textures, alteration, and ore types within the volcaniclastic lithologies are similar to those described for the Silver Mantos area, however, the mineralization is thought to be related to a different fluid event based on compositional differences. There may have been a different vent source within the volcanic center, as the Socavon del Diablo mineralization is generally lower in silver and higher in zinc content. The Socavon Basement zone is mainly hosted within the Acoite Formation metasedimentary rocks located along the northern and eastern rim of the volcanic center. The eastern limit of the Socavon del Diablo zone is a dacitic dome intruded in the tuff units and flowed over the tuff at surface. Immediately to the east of the dacite dome, biotitic, sub-horizontal tuff layers up to 80 m thick cover the Socavon Basement zone. Here, the mineralization is hosted in breccias filled with argentiferous galena and a stockwork of sphalerite-siderite-galena within a halo of low grade zinc.
The Melina target is the most continuous zone of mineralization located in the northeast sector of the caldera at the contact between Miocene tuff units and the Acoite Formation. Here, there is a large volume of hydraulic breccia and sulfide filling open spaces. This volume extends discontinuously west-northwest to east-southeast with a thickness between 20 m and 70 m and a length of more than 500 m.
Pirquitas Mineralization
There are two types of mineralization at Pirquitas: (1) polymetallic veins with peripheral disseminated mineralization; and (2) mineralized hydrothermal breccia. Vein type is the dominant mineralization style and has been the main source of extracted ore. This mineralization type is characterized by quartz and massive sulfides (pyrite, sphalerite, galena, or wurtzite) in association with a wide variety of Ag-Sn-As-Sb-Pb-Cu-Bi sulfosalts (freibergite, pyrargyrite, miargyrite, and polybasite) and rare oxides (cassiterite or wolframite). Hydrothermal breccia bodies, found in various parts of the area, were formed concomitantly with veins. They host a similar assemblage of pyrite cassiterite, sphalerite, arsenopyrite, galena, and Ag-Sn-AsSb-Pb-Cu-Bi sulfosalts, except that the abundance of galena is greater (Malvicini, 1978; Paar et al., 1996).
The San Miguel open pit exploited portions of the Potosí, San Miguel, and Chocoya vein systems. Sheeted sulfide bearing quartz veins and associated disseminated mineralization of the San Miguel system occur in a swarm that is 160 m wide in the north-south direction and up to 400 m along strike in the east-west direction. Veins within the San Miguel pit have a strike of west-northwest (azimuth 285°) and are generally sub-vertical. Veins with this orientation include the Potosi, San Miguel, Chocoya, Oploca, San Pedro, Llalagua, Chicharron, and Colquiri. The Potosi vein is the largest known single vein on the property, with a strike length of approximately 500 m and a maximum thickness of 3.0 m. This vein has an average dip of 80º to the northeast. Other veins of this orientation typically have a strike length of 50 m to 150 m, with average widths of 30 cm to 50 cm. A set of secondary veins is represented by the Veta Blanca and Colquechaca veins, located north of the Potosi vein; and narrow veins (50 cm to 2 m) in the Oploca zone. The secondary veins strike southeast (130°) and dip steeply to moderately to the southwest (Board et al., 2011). The Crucero vein corresponds to a series of fractures that follow the axial plane of the anticline (striking north-northeast) in the center of the San Miguel pit. Sulfide mineralization within the Crucero vein develops irregularly along fractures within undeformed white crystalline quartz.
In addition to veins, zinc-rich mineralization is hosted within pipe-like breccia bodies interpreted as breccia diatremes (Board et al., 2011). North of the San Miguel open pit is the Cortaderas breccia which trends east-southeast (110° to 120°) and dips 75° to the southwest. Its thickness varies from 0.5 m to 7.0 m and the overall strike length is 500 m. The Cortaderas body is related to a regional west-northwest striking fault system. No appreciable offset is observed along the structural zone. It is interpreted from field mapping that the zone is occurring in a relay/accommodation zone where all appreciable offset is being accommodated along multiple fault strands.