Summary:
The Los Azules porphyry copper deposit is in western San Juan Province, Argentina, within the Cordillera Principal, the highest-altitude section of the Andean Cordillera along the Argentina-Chile border. The deposit sits at approximately 3,600 mASL in a region dominated by north-south mountain ranges that increase in elevation from east to west.
Los Azules exhibits characteristics typical of Andean-style porphyry copper deposits. It consists of a barren leached zone, an underlying supergene enrichment zone, and hypogene mineralization extending at least 1,000 m in depth. The hydrothermal system spans 5 km by 4 km NNW along a structural corridor, with its ultimate extent concealed beneath volcanic cover to the north. Mineralization limits along strike and depth remain undefined, with some drill holes terminating in mineralized zones above the resource cut-off grade.
Key minerals include chalcopyrite, bornite, chalcocite-digenite and idaite. Copper sulfides rarely exceed 2% to 3% of rock volume, with hypogene copper grades typically ranging from 0.1% to 0.35%. Silver is present at approximately 1 gram/tonne, along with trace amounts of gold and molybdenum.
Supergene enrichment resulted from meteoric groundwater leaching primary sulfides and redepositing copper below the water table as chalcocite and covellite. The enrichment zone transitions into hypogene mineralization at depth, with intensity diminishing below major structures.
Geological models by Sillitoe (2014) and Vázquez (2015) highlight similarities between Los Azules and Miocene-Pliocene porphyry systems like Río Blanco-Los Bronces and Los Pelambres in Chile. Sillitoe identified an early mineralized porphyry dike (EMP) phase responsible for much of the hypogene and supergene mineralization and less mineralized “inter-mineral” dikes. Vázquez redefined the chronological sequence and spatial distribution of igneous and hydrothermal events, which includes intrusion of a dioritic stock, pervasive alteration, and supergene enrichment.
Lithology
Volcanic Country Rocks (host) Los Azules is hosted within volcanic lithologies of the Choyoi group, believed to be of Triassic age. These rocks include rhyolite and crudely bedded pyroclastics, ranging from fine-grained tuffs to coarse breccias (Rojas, 2008; Pratt, 2010).
Magmatic – Hydrothermal Breccias
Breccias at Los Azules are predominantly magmatic-hydrothermal in origin and are associated with both the EMP and IMP. They occur as two types: early breccias related to the EMP and later breccias associated with the IMP. Both breccia types host hypogene mineralization, with early breccias generally exhibiting higher copper grades. Early breccias are composed of fragments of porphyry and diorite in a quartz-cemented porphyritic matrix. They are generally characterized by potassic alteration with secondary biotite, potassium feldspar, magnetite, and anhydrite. The early breccias are found as small, high-grade zones along the edges and cupola zones of the EMP.
Later breccias related to the IMP are composed of crackle breccias with IMP or diorite in a tourmaline-rich matrix containing quartz, pyrite, chalcopyrite, and minor bornite. The alteration is dominantly sericite-quartz, with early type-A quartz veinlets restricted to clasts, and later veinlets of type C and D cutting both clasts and the matrix. These breccias occur primarily in the western deposit and are less abundant than early breccias.
Supergene Enrichment
The supergene mineralization at Los Azules consists of a sub-horizontal chalcocite-covellite blanket that overlays hypogene sulfide mineralization. This enriched zone is capped by a leached oxidized layer with minimal copper content (<0.10%). The leached cap, ranging from 0 m to 260 m thick, is characterized by spots of jarosite, goethite, and hematite. Beneath this, a mixed sulfide-oxide zone transitions to the supergene blanket, where hypogene sulfides are replaced by chalcocite and minor covellite.
Potential gold-silver mineralization near Los Azules includes late-stage, intermediate-sulfidation epithermal quartz veins, with minor sphalerite and galena, as described by Pratt (2010). Although precious metal deposits often occur around porphyry copper systems, the district remains largely unexplored for this style of mineralization.
At Los Azules, a leached cap and supergene chalcocite blanket indicate copper oxidation, dissolution, and vertical transportation, with subsequent redeposition within the system. Copper may have also been transported laterally and redeposited as “exotic” copper mineralization nearby (Sillitoe, 2010). However, no exploration for this style of mineralization has yet been undertaken in the vicinity of Los Azules.
Deposit Type
Los Azules is located within the Central Chile segment (400 km-long) of the Miocene-Early Pliocene porphyry copper belt (6,000 km-long) of the north and Central Andes.
Mineralization at Los Azules is Andean-Cordilleran, late Miocene, (quartz-) diorite-hosted, oxidized porphyry copper style with a well-developed leached cap and supergene chalcocite-covellite blanket. Los Azules displays numerous features in common with other porphyry deposits.