Summary:
Deposit types
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.
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.
Localised high-grade gold mineralisation (>10 g/t Au) is related to a late pulse of chalcedonic cream to grey-coloured silicification, which overprints a background of earlier disseminated low-grade mineralisation (from 0.5 to 10 g/t Au and 10 to 50 g/t Ag. Gold-mineralised, zinc-rich, intermediate sulphidation-style quartz veinlets tend to occur as a halo around alunite alteration, but so far appear to be of minor economic significance.
At BP, the main breccia bodies are elongated to strongly elongated, with a northeast–southwest to east–west orientation. The BP body is observed at surface to a depth of 450 m in drill core and remains open at depth. The shape of the breccia, as defined by current drilling, is like an inverted cone.
The AA body is strongly elongated northwest–southeast and is characterised by stacked, tabular, flat-lying polymictic breccia bodies surrounded by monomictic breccia. Drilling to date has not identified a root zone that extends to depth.
Surface and downhole structural studies confirm that steep northwest, east-northeast, and east–west-trending structures control brecciation, fluid flow and mineralisation. Oxidation is locally deep, attaining 300 m depth along permeable lithologies and faults (i.e., breccia). More than 90 % of the defined economic gold mineralisation is in oxide material.
Structural framework
Four main groups of faults and lineaments are mapped in the deposit area. The first group trends northwest-southeast and is indicated by drainage orientations and by the distribution of steam-heated alteration and silicified, structurally controlled breccias (ledges) which are exposed on the dacitic dome.
At BP, the Salares NW Fault and Salares SW Fault are the two main northwest-southeast trending structures. At least three sub-parallel splays have been mapped to the east of the Salares NW Fault. The most easterly Lagartija Fault appears to define the eastern boundary of mineralisation.
At AA, the mineralised breccia system trend northwest-southeast and is also associated with a group of sub-parallel northwest-southeast trending structures (Domo and Agua Amarga NW Faults), which are steeply southwest dipping to sub-vertical. Both the Domo and Agua Amarga NW Faults are important as they constrain the alteration and mineralisation. The Isla, Jardin, and Inca NW Faults are interpreted conduits for the alteration and mineralisation.
The northwest-southeast faults are intersected by a second group of northeast–southwest lineaments. The Salares NE Fault is partially defined by the main drainage along the northern contact of the dacite dome and marks the boundary between BP and AA.
A third group of east-west trending structures are only observed in the BP area. A fourth group of north-south trending structures are interpreted from AA modelling.