Summary:
Salares Norte is in Chile’s Western Cordillera, at the northern edge of the Maricunga Belt and near the Central Volcanic Zone. The deposit is hosted in Neogene volcanic rocks, including breccias and domes, with mineralisation occurring in a high-sulphidation epithermal system. The arid, high-altitude setting has preserved volcanic landforms, and the lack of significant post-mineral faulting differentiates it from more segmented terrains further south. Gold and silver are associated with vuggy silica and advanced argillic alteration, localised at the contact of andesitic and dacitic flows.
Mineral Resources and Mineral Reserves have been delineated by drilling in two separate deposits, Brecha Principal and Agua Amarga, which are located ~500m apart. Most of the mineralisation known to date is oxidised. The sulphide mineralisation contains mainly pyrite, is generally lower grade and lower volume, and has lower processing recovery than the oxide material.
Two broad stages of mineralisation are recognised at Salares Norte: an early porphyry mineralisation/barren epithermal lithocap stage and a later fertile high-sulphidation epithermal stage with evidence of overprinting of the mineralising events. The main (fertile) epithermal mineralisation stage, related to high-sulphidation fluids, is the most important stage given that it is associated with the gold and silver mineralisation discovered to date. This main mineralisation stage is subdivided into three discrete events, which have been determined based on cross-cutting relationships and mineralogical observations, followed by supergene oxidation (Baumgartner and Benn, 2013).
Event 1: High-sulphidation gold mineralisation likely to be associated with the main advanced argillic alteration. It is characterised by the presence of pyrite, rutile, scarce enargite, and covellite (where not oxidised). The mineralisation is disseminated and the gold occurs as minute grains in quartz, pyrite, as liberated grains and probably as solid solution in pyrite.
Event 2: Intermediate-sulphidation mineralisation (gold-silver±lead episode) that is structurally controlled and occurs as cement in the breccia and veins and veinlets. It is characterised by silver sulphosalts including proustite, smithite, pyrargyrite, stephanite, acanthite, argento-tennantite, and is accompanied by boulangerite, pyrite, and ironpoor sphalerite.
Event 3: High-grade gold-silver event characterised by areas with a fine-grained silica infill cement, pink and grey coloured rock in the sulphide zone and a cream colour in the oxide zone. The texture appears as flooding and contains mainly fine-grained quartz accompanied by alunite and pyrite (± barite) in the sulphide zone and alunite and jarosite in the oxide zone. Gold is difficult to observe even in some high-grade samples from the oxide or sulphide zones due to its small size.
Event 4: Late-stage supergene oxidation event affecting a large part of the deposit, oxidising parts of the sulphide mineralisation. This event is characterised by the development of plumbo-jarosite, argento-jarosite, chlorargyrite, iodargyrite, and capgaronnite. Silver and gold were probably recrystallised, as some textures evidence.
Alterations and Mineralisation
The hydrothermal alteration and mineralisation styles observed at Salares Norte are typical of high-sulphidation epithermal deposits. Most low-grade gold mineralisation is related to alunite-quartz alteration. High-grade mineralisation is hosted in rocks with strong silica alteration, predominantly in polymictic breccia interpreted to have formed by phreatomagmatic explosions with overprinting of phreatic/hydrothermal breccias. A smaller portion of the mineralisation is hosted by fine-grained fragmental units including laminated facies. These were originally interpreted as tuffs but are now thought to represent, at least in part, fine-grained micro-breccia sills.
The high gold grade-related advanced argillic alteration (mainly alunite-quartz alteration with minor development of strong silica alteration and residual vuggy silica) is surrounded by zones of intermediate argillic alteration (i.e., illitesmectite) and overlain by remnants of steam-heated alteration (alteration zones preserved in the highest level of the deposit). The distribution of the breccia, hydrothermal alteration and the mineralisation is spatially related to the emplacement of a series of andesitic and dacitic domes.
Two separate mineralised zones have been defined by drilling: BP and AA. These zones are about 500 m apart and on the southwest and northwest margin of a dacite dome respectively. The upper limit of mineralisation at BP is about 4,370 m above sea level, while AA is about 4,300 m.
At BP, the structural model speculates that the Salares NW Fault and Salares SW Fault are master faults that formed a structural jog in sinistral movement, which created several tensional east–west faults generating zones of opening to the emplacement of breccia bodies. Following the breccia body emplacement and overprint by intense hydrothermal activity, the northwest–southeast and the east–west dilational faults may have reactivated to generate a reopening that controlled the late high-grade gold and silver mineralisation.
At AA, the structural controls on mineralisation are mainly the northwest corridor and northeast secondary corridor; however, the intersection of these structural corridors allowed the concentration of hydrothermal fluids to generate alteration and mineralisation. The lithostratigraphic control also produces an outline of the lower grade mineralisation, which is a tabular, sub-horizontal body that trends northwest–southeast, sub-parallel to an array of faults mapped at the surface.
As in most epithermal deposits, the location of the paleo-groundwater table and its vertical variations are considered to have played an important role on the gold and silver distribution.