San Juan, Argentina
Super Porphyry Cu and Au|
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The Los Azules porphyry copper deposit is located ~80 km NW of the town of Calingasta in San Juan Province, Argentina and ~6 km east of the border with Chile. It is ~95 km north to NNE of the similarly aged Los Pelambres and El Pachón porphyry Cu deposit cluster that straddles the Chile-Argentina border (#Location: 31° 6' 40"S, 70° 13' 13"W).
The Los Azules deposit is situated within the Principal Cordillera of the Andean Orogen, the central and most elevated of a series of parallel Andean ranges, the crest of which forms the Chile-Argentina border. The Principal Cordillera is composed of strongly folded, faulted and uplifted Palaeozoic and early Mesozoic sedimentary and volcanic lithologies deformed by the Gondwana and Pre-Andean tectonic cycles. These sequences are overlain by mid- to late Mesozoic and Eocene to early Miocene volcaniclastic strata and by extensive Upper Miocene ignimbrites of the Andean Tectonic Cycle.
The Eocene to early Miocene volcaniclastic strata were deposited in an extensional basin followed by plutonic intrusion and contractional deformation from 19 to 16 Ma. These rocks were overlain and intruded by 16 to 7 Ma volcanic flows and pyroclastic units with comagmatic 12 to 8 Ma plutons and porphyry systems. The succeeding 8 to 5 Ma compressional event resulted in crustal shortening, thickening and regional uplift (Sillitoe and Perello, 2005).
For more detail of the regional tectonic, geological amd metallogenic settin, see the Central Andes and Bolivian Orocline record.
The principal lithologies of the deposit area may be summarised as follows:
Volcanic Country Rocks
The country rocks surrounding the Los Azules deposit belong to the Triassic Choyoi Group (or equivalents) and include rhyolitic crystal lapilli tuffs, rhyolitic flow domes which are commonly autobrecciated, rhyolitic to andesitic lapilli tuffs and rhyolitic pyroclastic breccia. These rocks range from fine-grained tuffs to coarse breccias and include rhyolite intrusions and crudely-bedded pyroclastics (Rojas, 2010; Pratt, 2010).
The intrusive complex at Las Azules includes up to twelve different lithologies, although only the following are common. They comprise, from oldest to youngest:
• Quartz Diorite - which occurs as a pre-mineral, multi-phase, hypabyssal, quartz diorite pluton that is several kilometers in diameter that hosts the porphyry system. It consists of fine- to medium-grained, equigranular, holocrystalline, hornblende-biotite-magnetite quartz diorite, tonalite and granodiorite. Whilst a medium-grained granodiorite occurs in the southern and southwestern parts of the deposit area, a finer-grained quartz diorite or tonalite phase is widespread in the east, NE and north. This intrusion is variably altered but often exhibits moderate to strong biotite alteration of primary mafic minerals.
• Porphyritic Quartz Diorite occurs as frequent dykes intruding the quartz diorite pluton in the deposit area with strikes of 300 to 310° and 330 to 340°. They constitute up to ~35% of the total rock volume, and appear to decrease in thickness from ~30 m in the south from the south, to ~5 to 15 m in the north. It comprises >50% feldspar phenocrysts with scarce (1 to 3%) quartz eyes set in a finer-grained, holocrystalline groundmass. The grain size apparently increases with depth (Rojas, 2010). Myrmekitic-textured feldspar-quartz occurs at the contact with younger 'crowded' porphyry dykes. These dykes are strongly affected by all styles of alteration and appears to be spatially related to the bulk of the mineralisation (Zurcher, 2008).
• 'Crowded' Quartz Diorite or Tonalite Porphyry also occurs as dykes that intrude the quartz diorite pluton and porphyritic quartz diorite dykes with 330 to 340° strikes and, less commonly, follow 300 to 310° structures (Zurcher, 2008). These younger dykes are numerous but are generally <5 m thick and constitute up to ~10% of the total rock volume, defining a NNW-striking zone that is up to 1 km wide. Their texture is phaneritic-aphanitic with hornblende, biotite and abundant, typically broken, feldspar phenocrysts and uncommon (1 to 5%) resorbed and cracked quartz eyes. Phenocrysts areoften grain-supported or set in an aplitic groundmass. These dykes are weakly- to strongly-overprinted by all alteration styles.
• Intrusive Breccias are found in the north, and occur as dyke-like tabular masses that are up to a few metres thick with a preferential trend direction of ~75° and 285°. They are composed of clasts of quartz diorite, porphyritic quartz diorite and, less commonly, 'crowded' porphyry clasts in a porphyry matrix, and are variably mineralised. Timing of the brecciation is poorly constrained, although it appears most were emplaced contemporaneously with the 'crowded' porphyry. Magmatic breccias are also reported from the western part of the Los Azules system. Hydrothermal breccias are also known at Los Azules.
• 'Open' Granodiorite or Dacite Porphyry which comprises common quartz, feldspar and lesser hornblende and biotite phenocrysts set in a very fine-grained aphanitic groundmass that locally has a vitreous appearance. It occurs as dykes on the eastern and western flanks of the deposit area, and in the northernmost part of the mineralised zone where it expands to form larger dome-like masses. This lithology contains pyrite but is otherwise relatively less affected by alteration than the previous intrusive units, and as such is probably late mineral.
U-Pb zircon dating (Zurcher, 2008) yielded the following ages - quartz diorite 10.6±0.2 Ma; feldspar porphyry 10.7±0.2 Ma;
andesite porphyry 9.2±0.2; quartz diorite 8.2±0.3 Ma.
The Triassic volcanic country rocks at Los Azules are folded into an anticlinal/monoclinal structure with a flat eastern and steep western limb and an axis that strikes ~north-south. It is interpreted to overlap a NNW-striking 'structural corridor' that apparrently controlled the
location of Upper Miocene volcanic/intrusive centres in the region (Rojas, 2010). The same structural corridor also appears to control the locations of hydrothermal alteration and mineralisation over a 7 km strike length, including the Los Azules porphyry system (Rojas,
2010). The porphyry system at Los Azules follows a broad NNW trending composite valley that also coincides with this structural corridor. A prominent topographic whaleback ridge, named 'La Ballena', rises above the floor of this composite valley separating it into the Piuquenes and Vegas valleys, to the east and west respectively. To the NNW the valley abruptly curves to the west due to an upfaulted block to the north as Los Azules Valley.
The porphyritic and 'crowded' porphyry dykes were emplaced along numerous, strong, pre- and syn-ore, NNW and NW striking faults with significant strike-slip displacement. Mineralisation and alteration zones similar structural trends and apparent strtural conrol. Post-mineral north to NNE striking reverse faults (the 'Las Lagunas' and 'Diagonal' sets) juxtapose diverse structural blocks, each of which is characterised by contrasting alteration and mineralization characteristics.
Hydrothermal alteration at Los Azules defines a NNW elongated, 5 x 2 km elliptical zone that has a spatial correspondance with the occurrence of porphyry and 'crowded' diorite porphyry dykes. The principal alteration assemblages are as follows:
• Potassic - the earliest and most extensive alteration, characterised by secondary K feldspar (pink) and biotite (black) varieties, apparrently most intensely developed in spatial association with porphyritic quartz diorite dykes. K feldspa alteration is characterised by pink orthoclase in veins/veinlets/stockworks and vein envelopes and as pervasive replacements of plagioclase phenocrysts and/or the matrix of diorite porphyry host rocks. Biotite alteration occurs as veins and pervasive- to selective-replacement of igneous biotite and hornblende in diorite porphyry host rocks. It usually occurs as a zone peripheral to, and coeval with the main K feldspar alteration. Potassic-altered zones are commonly accompanied by anhydrite/gypsum veins and veinlets and by disseminated magnetite.
• Propylitic alteration is peripheral to and coevcal with the potassic zone, and primrily consists of chlorite
replacing ferromagnesian minerals, as well as minor epidote and calcite within veins.
• Sodic-calcic alteration, composed of albite and chlorite with minor epidote, actinolite and tourmaline. Diorite porphyry is altered to a bleached (white and green) porous rock with numerous druses lined with chlorite and crystalline quartz. The porous character is interpreted to be the result of dissolution of anhydrite/gypsum and removal of iron oxide, quartz, feldspars and ferromagnesian minerals (Zurcher, 2008). Sodic-calcic alteration post-dates the potassic and propytlitic phases, overprinting the flanks of the potassic core of the system. It is best developed in the NNWt sector where it is apparrently spatially and temporally associated with development of quartz-sericite alteration. Sodic-calcic zones are often accompanied by well-crystallised chalcopyrite interpreted to be due to remobilisation and redeposition during overprinting by late-stage, highly saline brines. At Los Azules this mineralisation lacks the minerals more characteristic of this style of alteration (i.e., significant actinolite and scapolite) and may instead be a variation of chlorite alteration.
• Chloritic alteration is commonly found peripheral to the potassic alteration zone, or along its contact with sodic-calcic, biotite and/or phyllic alteration. It is interpreted to represent an intermediate assemblage where ferromagnesian minerals are altered but feldspars are only weakly corroded. Hornblende, biotite and secondary biotite are variably replaced by chlorite, and is more pervasively developed in the northernmost and southernmost fringes of the deposit. Chlorite also variably affects feldspar and sericite, suggesting it was probably introduced at several stages during the life of the hydrothermal system.
• Phyllic (quartz-sericite-pyrite) alteration is the results of moderate to intense sericitisation of feldspars and primary and secondary biotite, and is associated with moderately well-developed quartz stockworks. It is also associated with silica flooding and development of sericite-(tourmaline-)quartz breccias. It is usually developed vertically above, and broadly overprints potassic and calcic-sodic
alteration at shallow depths, but also extends down structures that host 'crowded' porphyry dykes within the NNW trending axis of the potassic core. The most intense development of phyllic alteration surrounds and penetrates zones of potassic-, calcic-sodic and chloritic alteration. In its extremities it diminishes in intensity and gradually grades into weakly propylitic and fresh rock.
• Silicification locally occurs as massive and pervasive quartz peripheral to potassic and phyllic alteration zones. Quartz 'A-vein stockwork arrays occur in phyllic and potassic alteration zones while 'D veins' are developed in the most intense phyllic alteration zones. Silicification occurs in association with both potassic and sericite alteration. Silicification and tourmaline breccias also locally occur together at deep levels in the northern part of the deposit, and to the east of the Ballena zone. Tourmaline also occurs in veinlets and as disseminated traces in phyllic alteration.
• Argillic alteration is usually restricted to faults and structures and to the peripheries of the quartz-sericite/phyllic zone. It is composed of illite, kaolinite and montmorillonitic clays pervasively replacing sericite at shallow levels. Modest sericite and strong argillic alteration occur in association with latestage 'Open' Granodiorite-Dacite porphyry dykes.
• Advanced Argillic alteration is predominantly localized along narrow, sub-vertical structures, most of which strike NW, and are localised peripheral to the porphyry system, characterised by an assemblage that includes alunite, vuggy silica and kaolinite/dickite. At least some of the alunite is very coarse grained and clearly hypogene.
Porphyry-style copper mineralisation at Los Azules is spatially, temporally and genetically related to a suite of NNW-striking, porphyritic quartz diorite-dacite and 'Crowded' Porphyry dykes that cut a pre-mineral diorite porphyry pluton. Only the overlying leached capping of the deposit is exposed, principally along the Ballena ridge and below the fringing Piuquenes and Vegas valleys. Within the deposit, both hypogene porphyry-style and supergene chalcocite blanket mineralisation are represented.
The mineralisation has been dated at 7.84 Ma±0.04 Ma (Re-Os molybdenite; Zurcher, 2008).
This mineralisation is best developed at depth below the Ballena ridge, and in the upfaulted Cerro Este block north of the Lagunas fault system. The main hypogene sulphide mineralisation at Los Azules is predominantly disseminated and includes chalcopyrite with lesser bornite, chalcocite-digenite and idaite, with trace molybdenite, magnetite and hematite, usually deposited on igneous mafic minerals. Copper sulphides rarely exceed 2 to 3 vol.%, although unusual concentrations of often well-crystallised chalcopyrite are locally associated with zones of sodic-calcic alteration. The most important bodies of hypogene copper mineralisation occur within the zones of potassic alteration, comprising magnetite-pyrite-chalcopyrite-digenite in the south, and specularite-pyrite-chalcopyrite-bornite-digenite in northern parts of the deposit (Zurcher, 2008).
Where overprinted by phyllic alteration, the hypogene sulphide assemblage is similar, but it is usually richer in pyrite (1 to 5%), which occurs as disseminations and in sericite-quartz veins, with or without tourmaline. Quartz-sulphide stockworks and sericite alteration are apparently best developed in and around 'Crowded' porphyry dykes, although significant high-grade copper intersections are less common than those in potassic alteration where a phyllic overprint is absent. Sparse molybdenite is associated with some sericite-quartz veins.
Stockwork vein/veinlet mineralisation is widespread, although not strongly developed at Los Azules. A number of vein/veinlet styles and mineral assemblages have been identifiable within the deposit (Pratt, 2010; Almandoz, 2010) and include the following, from earliest to latest, based largely from cross-cutting relationships.
Stage 1 veining, accompanying early K feldspar alteration, comprising:
• Narrow magnetite veinlets ('M' veins);
• Chalcopyrite-molybdenite-magnetite veinlets;
• Biotite-magnetite-bornite veinlets;
• Biotite (chlorite-sericite)-pyrite-chalcopyrite veinlets.
Stage 2 veining, accompanying late K feldspar alteration, comprising:
• Anhydrite/gypsum-chalcopyrite-molybdenite veins/veinlets;
• Granular quartz K feldspar-biotite-chlorite-anhydrite-chalcopyrite-bornite-chalcocite-molybdenite ('A' veins);
• Grey-pink, quartz-chalcopyrite-bornite-chalcocite±molybdenite±magnetite-hematite veins/veinlets,
that are fine-grained, granular and homogeneous, and are cut by Group 3 veins;
• Quartz-hematite (after magnetite) in central suture ('B' veins).
Stage 3 veining, accompanying sodic-calcic alteration, comprising:
• Quartz-epidote veinlets;
• Chlorite-chalcopyrite filled fractures;
• Quartz-chlorite-illite/sericite-chalcopyrite-pyrite-molybdenite veinlets;
• Chlorite-illite/sericite-pyrite-chalcopyrite-chalcocite veinlets;
• Crystalline quartz-chalcopyrite-chalcocite pyrite-molybdenite veinlets (also in phyllic zones);
• Drusy quartz-chalcopyrite-chalcocite veins;
• Opaline silica with milky to blue fracture coatings;
• Molybdenite veinlets.
Stage 4 veining, accompanying phyllic alteration, comprising:
• Crystalline quartz-pyrite with sericite halos ('D' veins).
The main Los Azules deposit expands to the north, from ~350 m wide and 350 m vertical thickness in the south, to 550 and 800 m respectively, 1 km to the north.
The separate Southwest Zone of deep hypogene mineralisation also appears to trend NNW, parallel to the main Los Azules deposit over a strike length of >1 km, and at depth, typically ~300 to 400 m below the surface (at an elevation of ~3400 m asl). It appears to have a horizontal width of 400 to 800 m and vertical extent of 250 to 600 m, increasing in thickness and width to the south, as the main deposit tapers in the same disrection. The mineralisation comprises hypogene chalcocite, chalcopyrite, bornite and pyrite, predominately associated with potassic alteration, and is characterised by relatively high-grade intervals of hydrothermal-magmatic breccias. There is no supergene enrichment in the deep Southwest Zone.
Supergene sulphide mineralisation is thickest and best developed beneath the Piuquenes Valley floor, between the northern end of the Ballena ridge and the southern limit of the down faulted Cerro Oeste block to its SW. It extends to the SSE along the Ballena ridge, but diminishing in grade and extent southward. A second zone of high grade supergene mineralisation immediately to the west of the main Los Azules deposit, the 'Western Supergene Zone' appears to be developed above the deep Southwest Zone hypogene mineralisation.
The supergene mineralisation underlies an oxidised leached cap and comprises an enriched, sub-horizontal chalcocite-covellite blanket which grades downwards through a partially enriched mixed hypogene-supergene sulphide zone of incomplete replacement, into the underlying hypogene sulphides. The preserved leached cap varies from 30 to 180 m in thickness and is argillic-altered, with limonitic boxworks and common, disseminated spots of jarosite, goethite and hematite. Hematite is more abundant to the south, whilst jarosite is best developed over the central section, and goethite is apparently relatively more widespread in the northern segment (Zurcher, 2008). Copper oxides are reported from the margins of the leached zone and include brochantite, copper pitch and copper wad. Grades in the leached cap range from 0.01 to ~0.1% Cu.
A zone of mixed oxide-hypogene pyrite mineralistion occurs at depth, where primary magnetite is oxidised to hematite and ferrimolybdite after molybdenite occurs, although copper minerals and sulphides are mostly absent. This zone is below the leached cap where the underlying supergene enriched zone is largely absent (Rojas, 2010).
A narrow mixed sulphide-oxide zone beneath the leached cap gives way to the main supergene sulphide zone where hypogene sulphides are replaced by chalcocite and minor covellite. The supergene copper blanket is best developed in the central and central-northern sectors, where the overlying leached cap is characteristically more jarositic and developed in the pyritic phyllic-altered zone located directly above the potassic alteration zone. Earthy supergene chalcocite and minor covellite partially, or rarely, completely replace hypogene sulphides, although pyrite usually survives. Traces of native copper and gypsum after anhydrite occur into the underlying potassic alteration zone. The thickness of the supergene sulphide blanket ranges from ~60 to 250 m, but can penetrate to depths of >400 m down fault zones. Grades in the supergene enriched blanket vary from ~0.4 to >1% Cu in the north-central part of the system and decrease to the south and the peripheries to 0.2 to 0.4% Cu.
The resource estimation the supergene domain is based on a cyanide soluble copper to total copper ratio of 50%. No 'partially enriched' mineralisation is included in the resource model. For geological interpretation, mineralisation is classified as 'enriched' where 100 to 70% of the total copper is leachable and 'partially enriched' where 70 to 30% of total copper is leachable (Rojas, 2010).
Published Mineral Resources (McEwen Mining Mineral Reserve and Resource statement, 2017) at a 0.2% Cu cut-off, were:
Indicated Mineral Resource - 962 Mt @ 0.48% Cu, 0.06 g/t Au, 30 ppm Mo, 1.8 g/t Ag;
Inferred Mineral Resource - 2666 Mt @ 0.33% Cu, 0.04 g/t Au, 30 ppm Mo, 1.6 g/t Ag,
At a 0.35% Cu cut-off (Kunter et al., 2013 for McEwen Mining):
Indicated Mineral Resource - 389 Mt @ 0.63% Cu, 0.07 g/t Au, 30 ppm Mo, 1.8 g/t Ag;
Inferred Mineral Resource - 1397 Mt @ 0.46% Cu, 0.06 g/t Au, 40 ppm Mo, 1.9 g/t Ag.
The information in this summary is drawn from: Kunter, R., Davis, B.M., Rose, W.L., Sim, R., Duff, J.K., Elfen, S.C. and Pozder, S.A., 2013 - Los Azules porphyry copper project San Juan Province, Argentina; An NI 43-101 Technical Report prepared for McEwen Mining Inc., by Samuel Engineering Inc., 216p.
The most recent source geological information used to prepare this summary was dated: 2013.
This description is a summary from published sources, the chief of which are listed below.
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