MODULE 1 - SOUTH AMERICA, Chile & Peru
Field Workshop ...................... Sunday 12 October, 2003.
A full day geological traverse across the central Andes near Santiago was conducted by well known Chilean expert Estanislao Godoy of Sernageomin-Chile to illustrate the geological and tectonic setting of the segment of the Cordillera hosting super porphyries in central Chile and provide a regional context to the deposits to be visited in the following days.
Return to top
Los Bronces - Rio Blanco ...................... Monday 13 October, 2003.
Los Bronces-Rio Blanco is one of three giant Cu-Mo breccia/porphyry copper deposits in central Chile. It is located some 70 km northeast of Santiago and is known to contain more than 50 Mt of economically recoverable copper metal. The deposit is exploited by the Disputada Los Bronces mine (0.225 Mt of Cu pa capacity) and by the Codelco Andina Division (0.25 Mt Cu pa).
Return to top More detailed description
Mineralisation is generally contained in the matrix of mineralised breccia pipes and in the host Miocene to early Pliocene porphyry intrusions and Oligocene to Miocene andesite lavas surrounding the breccias which are located on the western margin of the composite 20.1 to 7.4 Ma San Francisco Batholith.
Approximately 50% of the copper ore in the deposit is present as breccia matrix, stockwork and disseminated mineralisation in a zone of potassic alteration associated with the emplacement of biotite rich breccias of the 7.4 to 5.4 Ma Rio Blanco Breccia Complex and quartz monzonite porphyry intrusions. This zone of strong potassic alteration is surrounded by a concentric propylitic shell and/or patches of sericitisation, which with the potassic alteration post dates an early actinolite-magnetite phase.
A second pulse of mineralisation followed a period of uplift and erosion and comprised younger, tourmaline rich breccia pipes which contained the remaining 50% of the copper in the deposit. The most notable of these are the Sur Sur and Donoso breccias which have been dated at between 5.9 and 5.1 Ma.
The combined Los Bronces-Rio Blanco deposit is estimated to contain more than 5 Gt @ 1% Cu, 0.02% Mo. In 2001 the declared reserve at the Los Bronces mine was 475 Mt @ 1.03% Cu + 741 Mt @ 0.47% Cu leach ore with an additional resource of 200 Mt @ 0.95% Cu.
Los Pelambres ...................... Tuesday 14 October, 2003.
The Los Pelambres and satellite Frontera porphyry style copper-molybdenum ore deposits are located some 200 km north of Santiago in Chile, and immediately west of the border with Argentina (#Location: 31° 43' 01"S, 70° 29' 28"W).
Return to top More detailed description
The deposit contains more than 20 Mt of copper, and in 2010 produced around 0.325 Mt of copper per annum. It is the northern-most of the trio of giant Miocene to Pliocene deposits (with El Teniente and Los Bronces-Rio Blanco) formed near the boundary between two major Andean segments, the flat slab segment to the north and the steeper subduction angle Southern Volcanic zone to the south.
Some 5 km to the south-east of Los Pelambres the El Pachon porphyry copper deposit (880 Mt @ 0.62% Cu, 0.01% Mo, 0.02 g/t Au) in Argentina is embraced by a continuation of the same alteration zone/mineralised system responsible for Los Pelambres.
The regional basement to the Los Pelambres-El Pachon mineralised system is composed of a Palaeozoic sequence commencing with marine shelf sediments, grading up into continental clastics and volcanics, and cut by common Palaeozoic granitoid batholiths. These were overlain by Jurassic to Cretaceous marine strata and andesitic lavas and flow breccias (with associated batholiths), discordantly followed by Paleocene to Oligocene continental volcanics and lesser continental sediments, which following deformation, uplift and erosion were overlain by Oligocene to Miocene andesite flows and pyroclastics, and interbedded continental sediments.
The dominant rocks within the Los Pelambres district comprise andesitic lavas, flow breccias and volcaniclastic sediments which have been intruded by dioritic and granodioritic masses, including the mineralised stock at Los Pelambres. This local andesitic sequence is subdivided into three units, commencing with the early Cretaceous Los Pelambres Formation (hosting the main mineralised intrusive at Los Pelambres) which is composed mainly of thinly bedded andesitic lava flows, which are generally each only a few metres thick, with some flow breccias and volcaniclastics. These are overlain by the Viñita Formation lava flows, flow breccias and volcaniclastics of pre 24.9 Ma age, followed by the third unit, equivalents of the Farellones Formation, which comprise reddish andesite and andesitic breccia, tuffs and volcaniclastics.
These andesites are intruded by small irregular dioritic to granodioritic plutons and by the 5 x 1 to 2 km composite mineralised stock at Los Pelambres. In the mine area the following phases have been recognised in the latter:
i). A quartz-feldspar porphyry (a granite porphyry) is the oldest intrusive, which occurs as a steeply dipping sill like mass and is believed to predate the main mineralised stock.
ii). Quartz diorite (or tonalite) which comprises 90% of the stock and is the principal host, comprises a light to medium grey, sub-equigranular rock with prominent subhedral plagioclase and has disseminated biotite and biotitised hornblende and disseminated bornite, chalcopyrite and pyrite with accessory magnetite.
iii). Porphyritic quartz diorite (granite porphyry), occurring as a 500 m wide irregular plug to the north and two dykes to the south-east, similar in composition to the quartz-diorite, but with plagioclase and quartz phenocrysts.
iv). Porphyry A (a quartz-diorite porphyry) present as sparse, scattered small dykes and irregular bodies, commonly of less than 5 m in width, concentrated in a north-south band just to the centre of the main stock. It is generally dark grey with plagioclase phenocrysts and a biotite altered mineralogy.
v). Porphyry B (quartz-monzodiorite porphyry) which is more widespread than Porphyry A, occurring as dykes in the central part of the deposit and is of light to medium grey to brown colour with a crowded mass of predominantly plagioclase phenocrysts.
vi). The late porphyry, which closely resembles Porphyry A and is found as a small number of short, thick dykes and plugs in the east and centre of the deposit. It cuts the main mineralised veins but in turn hosts some quartz-chalcopyrite-molybdenite veins and late veins with sericitic halos.
Dating of these porphyries has yielded an average Miocene age of 9.9 ±1 Ma.
Hypogene mineralisation commenced prior to the cessation of magmatism, taking the form of quartz stockwork veining, potassic alteration and breccia pipes. Late mineralisation occurs as pyrite veining with sericite halos. Mineralisation in both types is present in veins, with the only disseminated ore being in the alteration halos of these veins. Vein types include:
i). Granular quartz - with sparse chalcopyrite, minor bornite and traces of molybdenite, but common magnetite. These are the first to appear and are 2 to 10 mm thick with little evidence of an alteration halo.
ii). Green mica - with accompanying biotite and chlorite, while quartz is largely absent. These veins are an insignificant copper source, although they commonly contain chalcopyrite and bornite with local magnetite and are 1 to 5 mm thick with K feldspar envelopes.
iii). Type 4 copper rich - which are complex mineralogically and are responsible for the bulk of the copper ore. They contain quartz and variable amounts of green mica, biotite, K feldspar, magnetite, pyrite, molybdenite, tourmaline and anhydrite and are planar to irregular in shape with widths of millimetres to several centimetres. The bulk of the associated mineralisation is contained within the 1 mm to 1 m (averaging 5 cm) envelopes of the veins which comprises a dark grey, fine grained mixture of green mica, biotite, quartz, K feldspar and copper sulphides, with local magnetite. The copper sulphides are present as disseminated bornite-chalcopyrite, with bornite deceasing in abundance outwards from the centre of the deposit.
iv). K feldspar halo veins and fractures - are abundant in areas of K feldspar alteration, enveloping granular quartz veins a few mm wide and fractures with sparse associated chalcopyrite and bornite with local molybdenite, both within the veins and the potassic halo. They contribute 5 to 10% of the orebodies copper content.
v). Comb quartz veins - which are smooth and planar without a pronounced alteration halo. They only have a small copper content but represent the main molybdenum source being most common in the core of the deposit.
vi). Sericite halo veins - are abundant around the margin of the orebody but contain mainly pyrite with common quartz and overprint the previous vein types.
Two types of breccia are recognised at Los Pelambres, igneous and a hydrothermal. A large, 600 m across triangular shaped breccia pipe occurs near the centre of the quartz-diorite stock and at the centre of the mineralisation, while several 50 to 200 m diameter breccia pipes are found on the eastern side of the stock. Mineralised breccias also flank some of the dykes. The igneous breccias are not enhanced in copper or molybdenum. The hydrothermal breccias however make an important contribution to the copper grade. They are composed of angular to subrounded clasts of quartz-diorite, porphyritic quartz-diorite, fine grained quartz-diorite, andesite and porphyry A and B set in a matrix of quartz, with abundant tourmaline and common biotite, K feldspar, magnetite and sulphides (pyrite, chalcopyrite, bornite and molybdenite).
Supergene enrichment is important at Los Pelambres, with 5 blanket like zones formed vertically as follows, from the surface down:
i). Complete oxidation and leaching from the surface to between 10 and 200 m and carrying <0.05% Cu accompanied by variable to totally leached limonites (jarosite, goethite and exotic varieties).
ii). Partial leaching and oxidation.
iii). Sulphide enrichment - where chalcopyrite and bornite are partially replaced by grey chalcocite and covellite, while pyrite has thin chalcocite film coatings, decreasing downwards,
iv). Hypogene mineralisation with anhydrite leached, and
v). Hypogene mineralisation with anhydrite sealing fractures. Hypogene mineralisation shows a general concentration of bornite in the centre of the deposit, extending to the north-east. A second bornite rich, but low grade zone occurs to the south-east. Pyrite increase to the west from the limit of bornite development and appears to have been superimposed on the earlier bornite-chalcopyrite (and local bornite-chalcopyrite-magnetite) zoning and is associated with the younger sericite halo veining.
Prior to mining, geological reserves/resources at various cut-off grades had been estimated as follows:
3.311 Gt @ 0.63% Cu, 0.016% Mo (0.4% Cu cut-off),
1.699 Gt @ 0.76% Cu, 0.018% Mo (0.6% Cu cut-off),
471 Mt @ 0.95% Cu, 0.022% Mo (0.8% Cu cut-off),
236 Mt @ 1.06% Cu, 0.023% Mo (0.9% Cu cut-off).
Published ore reserve and mineral resource estimates at 31 December, 2006 were as follows (Antofagasta PLC website):
Measured + indicated + inferred resource - 2.939 Gt @ 0.61% Cu, 0.016% Mo, 0.033 g/t Au, 0.80 g/t Ag; including
Proved + probable reserve - 2.1246 Gt @ 0.64% Cu, 0.018% Mo, 0.033 g/t Au, 0.92 g/t Ag.
Published ore reserve and mineral resource estimates at 31 December, 2010 were as follows (Antofagasta PLC website):
Measured + indicated + inferred resource - 5.8184 Gt @ 0.53% Cu, 0.010% Mo, 0.04 g/t Au; including
Proved + probable reserve - 1.4330 Gt @ 0.64% Cu, 0.021% Mo, 0.030 g/t Au.
Published ore reserve and mineral resource estimates at December 31, 2015 were as follows (Antofagasta PLC Annual Report for 2015):
Proved + probable reserves - 1.3087 Gt @ 0.61% Cu, 0.019% Mo, 0.05 g/t Au;
Measured + indicated resource - 3.414 Gt @ 0.55% Cu, 0.018% Mo, 0.05 g/t Au; plus
Inferred resource - 2.6901 Gt @ 0.46% Cu, 0.015% Mo, 0.06 g/t Au;
TOTAL resource - 6.1041 Gt @ 0.51% Cu, 0.016% Mo, 0.06 g/t Au (inclusive of reserves).
Note: The ore reserve and mineral resource cut-offs in 2015 were 0.42% and 0.35% Cu respectively.
The Los Pelambres operation is 60% owned by Antofagata PLC, and 40% by a Japanese consortium.
The Frontera deposit is ~1.5 km south east of the Los Pelambres pit, and is centred on a quartz diorite porphyry outlier of the Los Pelambres intrusive complex, emplaced into early Miocene andesites. It comprises a northwest-trending zone of magnetite-rich potassic alteration that is characterised by hydrothermal biotite and an intense quartz-magnetite stockwork.
The mineralised system comprises an upper, oxidised zone containing erratic green and black copper oxides plus pitch limonite, underlain by a weakly developed supergene enrichment blanket of chalcocite and covellite, draping hypogene mineralisation dominated by bornite and chalcopyrite. A phyllic sericite-pyrite assemblage forms a poorly defined annulus between the central potassic and external propylitic zones. Molydenite is locally present within the ore zone, although the average Mo content of the deposit of ~60 ppm is only half that of at its neighbor at Los Pelambres.
Published resources at Frontera comprise (Perello et al., 2011):
700 Mt @ 0.52% Cu, 0.1 g/t Au using a 0.4% Cu cutoff.
For more detail consult the reference(s) listed below which were the principal source of the information on which this summary was based.
El Teniente ...................... Wednesday 15 October, 2003.
The El Teniente porphyry/breccia style copper-molybdenum ore deposit is located some 80 km to the southeast of Santiago in Chile and supports the world's largest underground mine which in 1999 produced 35 Mt of ore averaging 1.16% Cu, yielding 0.346 Mt of fine copper. It is also possibly the largest copper deposit in the world containing more than 75 Mt of recoverable copper in ores with a grade in excess of 0.67% Cu. The mine is operated by Codelco Chile, El Teniente Division.
Return to top More detailed description
El Teniente is the southernmost of the Andean "super porphyries" and was emplaced in multiple telescoped late Miocene and Pliocene magmatic-hydrothermal biotite, anhydrite and tourmaline breccias hosted by pervasively biotite altered middle Miocene mafic intrusive and extrusive rocks comprising gabbros, dolerites (diabases), porphyritic basalts and basaltic andesites. These are developed within a semi-regional sequence of Cretaceous to Quaternary volcanics, continental sediments and felsic to intermediate intrusives. The breccias and mafic intrusive complex are cut by the three felsic intrusives, a 5.67 to 5.46 ±0.19 Ma quartz-diorite to tonalite, the 5.28 ±0.10 Ma Teniente (dacite) Porphyry, and the central 4.82 ±0.09 Ma latite ring dykes with a related cataclysmic explosive diatreme breccia pipe in the centre of the deposit, the 1200 m diameter Braden Breccia Pipe.
The majority of the high grade (>1% Cu) hypogene copper ore is found within and is encompassed by the first stage multiple magmatic-hydrothermal breccia pipes of the mafic complex, with vertical extents of more than 1.5 km. These breccias include copper and sulphide rich igneous, tourmaline, anhydrite, magnetite and rock flour breccias. They are surrounded by a dense stockwork of biotite veining that coalesces to form the pervasive biotite alteration. The copper mineralisation is characterised by dominant chalcopyrite with lesser bornite and pyrite. Later veins with various amounts of quartz, anhydrite, sericite, chlorite, tourmaline, feldspars and copper sulphides accompanied the felsic intrusions and the Braden Breccia, generating a sericitic zone in the upper parts of the deposit and mostly contributing additional copper.
Overall the orebody has a gross triangular shape in plan view, elongated to the north where it is centred on a lower grade core corresponding to the Teniente Porphyry and the Braden Breccia Pipe. It is implied that the deposit was formed over a period of approximately 2 Ma from before 6.4 Ma to 4.3 Ma, towards the end of a 10 m.y. episode of Miocene and Pliocene magmatic activity. Vertically the orebody comprises an around 100 m thick oxidation-leaching zone, a supergene enrichment interval that extends to a depth of 500 m in the north with hypogene mineralisation below this to as deep as 1600 m below the surface.
The total reserve + resource at El Teniente has been estimated at 11.845 Gt @ 0.92% Cu, 0.02% Mo.
Escondida & Zaldivar ...................... Thursday 16 October, 2003.
Escondida is a giant supergene enriched open pit porphyry copper deposit located in northern Chile, some 160 km to the east-south-east of Antofagasta. It is currently one of the largest copper producing mines in the world with an annual output of more than 0.8 Mt of fine copper, and capacity of 1.2 Mt pa. The deposit contains in excess of 30 Mt of copper metal and is operated by Minera Escondida, principally owned by BHP Billiton 57.5% and Rio Tinto 30%.
Return to top More detailed description
Escondida, like Chuquicamata (200 km to the north), lies within the regional Domeyko fault system. It is the largest of six known porphyry deposits in the district, including Escondida Norte, Zaldivar, Pinta Verde, Carmen and Ricardo, all of which are within a localised zone of north-south striking, sinistral strike-slip faulting producing a cymoidal shape. Regionally the basement Palaeozoic to Mesozoic sequences are overlain and cut by Cretaceous to late Cainozoic continental margin Andean-type magmatic arcs, with the associated large porphyry deposits being formed mainly between 42 and 31 Ma, the Eocene-Oligocene phase. From 31 Ma onwards uplift, climatic change and low rates of denudation have favoured the development and preservation of supergene enriched blankets over the porphyries of the Escondida district.
Hypogene mineralisation at Escondida is closely associated with a NW-SE trending, late Eocene to Oligocene, three stage dioritic to quartz-monzonite to granodiorite stock complex, the third of which is a porphyry breccia which contains mineralised clasts but is cut by copper rich veins. Intruded country rock and hosts include Palaeozoic andesite and 265 to 270 Ma rhyolites, marine-continental Mesozoic calcareous and red sandstone and conglomerate and unconformably overlying late Cretaceous to Paleocene andesites.
Early potassic, biotite rich, alteration grades out into a propylitic periphery and a silicified shell to the intrusive, with associated magnetite, chalcopyrite and bornite. This was followed by sericite-chlorite and then to quartz-sericite alteration with chalcopyrite, pyrite and molybdenite. The third and final phase was an acid sulphate-advanced argillic alteration episode and a range of sulphides including bornite, chalcopyrite, pyrite, chalcocite, covellite, enargite, sphalerite, tennantite and galena. Supergene enriched ore constitutes 65% of the Escondida resource occurring as a sub-horizontal chalcocite blanket, varying from a few metres to 400 m in thickness. The leached cap varies from a few metres to around 200 m over the blanket with local copper oxide zones representing around 5% of the resource.
The total reserve + resource at Escondida and its contiguous mineralised systems amounted to approximately 2.62 Gt @ 1.15% Cu in 2000.
Zaldivar was visited to study ore in the mine as safety considerations precluded entry to the Escondida pit. The total reserve at Zaldivar amounted to approximately 400 Mt @ 0.71% Cu in 2003. Zaldiva is owned and operated by Compania Minera Zaldivar, a subsidiary of Placer Dome Inc.
Chuquicamata ...................... Friday 17 October, 2003.
The main Chuquicamata and associated Radomiro Tomic and MM porphyry copper-molybdenum deposits and the Mina Sur (Exotica) exotic copper orebody are located in northern Chile, some 240 km north-east of Antofagasta and 1690 km north of Santiago. These mines are developed in an elongate zone of alteration and mineralisation that extends for 30 km along the north-south trending transcurrent West Fissure zone. Together they contain (production + reserve/resource) a total of 86 Mt of copper metal. Production from the complex in 1999 amounted to 0.82 Mt of copper from 100 Mt of ore. Chuquicamata, Mina Sur and Radomiro Tomic are operated by the Codelco Division Norte.
The country rock comprises Palaeozoic meta-granitoids overlain by Mesozoic volcanics and sediments. The geology and structure of the Chuquicamata district is dominated by the West Fissure zone which juxtaposes barren rocks to the west with the mineralised units to the east. It also controlled the emplacement of the host porphyry and the density and orientation of vein filling fractures and faults in the ore deposit. It is part of the regional Domeyko fault system that passes through Escondida 200 km to the south, and the Collahuasi deposits, 200 km to the north.
Virtually the entire ore deposit at Chuquicamata is hosted by and related to the 36 to 33 Ma Chuqui Porphyry Complex which comprises four main phases, only one of which has sharp contacts with some of its neighbours. Where unaltered, all are composed of plagioclase, quartz, K-feldspar, biotite and hornblende and all are modified by the same stages of alteration and mineralisation.
An early linear, north-south zone of strong quartz-K feldspar alteration corresponds to the main bornite-digenite core of the sulphide zoning pattern. To the east, away from the West Fault, the mineralogy changes first to bornite-chalcopyrite to chalcopyrite-pyrite and then to barren country rock in the propylitic altered margin of the system. Westward from the core, adjacent to the main West Fault, this zoning is interrupted and over printed by the distinctly younger pyrite-chalcopyrite-bornite-digenite main stage veins, accompanied by pervasive quartz-sericite alteration.
Supergene modification resulted in a partly preserved leached cap and extensive oxide ore that replaces an upper chalcocite blanket overlying a high grade supergene blanket that persists to nearly 800 m below surface in the zone of fault brecciation and pervasive pyritic main stage quartz-sericite alteration. Some leached copper was moved laterally to form the Mina Sur (Exotica) secondary deposit in younger channel gravels.
The combined production + reserve/resource for the district, including the main Chuquicamata mine, Radomiro Tomic, MM and Mina Sur is approximately 11.4 Gt @ 0.76% Cu.
Return to top More detailed description
Spence ...................... Saturday 18 October, 2003.
A major new, un-developed, relatively un-disturbed deposit to provide a perspective on how such deposits are expressed at surface and in exploration data, and how they may be found.
Spence is a major porphyry copper deposit in northern Chile, located 150 km northeast of Antofagasta, adjacent to the road to Calama and Chuquicamata (#Location: 22° 47' 50"S, 69° 15' 10"W).
Return to top
This deposit, with the Cerro Colorado mine to the north (east of Inquique) is managed as part of the Pampa Norte Division. It was discovered by RioChilex in 1996, and is located along the prominent Antofagasta-Calama Lineament. It contains both supergene enriched and hypogene ore.
Mineralisation is associated with a complex of granodioritic to quartz-monzonite stocks with accompanying Paleocene dykes dated at 57 Ma. The long axis of porphyry intrusion and mineralisation is orientated NNE, sub-parallel to the Antofagasta-Calama Lineament, with the mineralised system having plan dimensions of ~3700 x 900 m. The emplacement of these intrusives into an upper Jurassic to lower Cretaceous suite of largely andesitic volcanics and volcaniclastics was controlled by a normal, NE-SW trending structural system which resulted in at least three distinct intrusive events and hydrothermal breccias.
The first intrusive pulse is represented by a quartz-feldspar porphyry (QFP1), accompanied by a late magmatic potassic alteration core of biotite and K feldspar with associated A and B type veins, fringed by a propylitic halo of chlorite-pyrite alteration. This pulse produced the main hypogene chalcopyrite rich mineralisation which is also the focus of the development of supergene enrichment. The second intrusive pulse (QFP2) of very similar composition was concentrated in the southern part of the deposit, forming a more compact centre with little fracturing, surrounded by intrusive breccias with clasts of QFP1 produced by explosive volatile components of the system. Subsequent activity, still mainly in the southern part of the deposit produced further hydrothermal breccias which are classified on the composition of their matrix into molybdenum breccias (with a sulphide rich matrix of molybdenite, pyrite and chalcopyrite) and a tourmaline breccia with a tourmaline-quartz matrix.
A major episode of phyllic (quartz-sericite-pyrite) D-type vein alteration with strong associated pyrite commenced from before the introduction of the QFP2 porphyry and continued through the breccia phase. This additional sulphide was important to the subsequent development of supergene leaching and enrichment. A third, limited intrusive phase of feldspar porphyry, subsequent to the breccias, is also in the southern part of the deposit and is characterised by the absence of copper and of quartz.
The deposit is entirely masked by 50 to >100 m of gravel which, in general, overlies a leached cap that passes down into a zone of supergene oxides, an enriched chalcocite blanket and then to hypogene sulphides. Several episodes of supergene blanket development and leaching are indicated.
Copper minerals within the oxide zone are atacamite and brochantite, underlain by a blanket of chalcocite, covellite and pyrite at the interface between the oxide and hypogene sulphide zones. Hypogene sulphides are chalcopyrite, bornite, molybdenite, tennantite and pyrite (Cameron et al., 2006).
On the basis of laser 40Ar-39Ar measurements of supergene alunite, Rowland and Clark (2001) identified a long period of supergene activity from 44.44±0.54 to 27.74±5.42 Ma, which was extended by a K-Ar date of 20.9±2.2 Ma. The deposit is covered by piedmont gravels of Miocene age, which are indurated, and for the most part, are poorly sorted with a fine-grained matrix that makes them impermeable, except where fractured, or in better sorted layers, as near their base (Cameron et al., 2006).
The atacamite-brochantite assemblage is locally continuous through the leached cap into the basal gravels, indicating oxide deposition continued after supergene oxidation and gravel deposition. Mostly the gravels rest on a leached cap. The preserved leached cap is ~20 to 100 m thick, while the atacamite-brochantite oxide layer is generally ~45 m thick. The supergene sulphide blanket is 50 to 100 m thick, thinning at the margins. The supergene sulphide zone contains irrregular patches of mixed supergene and hypogene sulphides at the transition to the hypogene zone (Cameron et al., 2006).
Published JORC compliant ore reserves and minerals resources at 30 June 2012 (BHP Billiton, 2012) include:
Total measured + indicated + inferred resources, for,
Oxide - 62 Mt @ 0.92% Cu;
Low-grade Oxide - 143 Mt @ 0.20% Cu;
Supergene Sulphides - 219 Mt @ 0.86% Cu;
Transitional Sulphides - 33 Mt @ 0.65% Cu;
Hypogene Sulphides - 2.351 Gt @ 0.43% Cu.
Total proved + probable reserves, (included in resources) for,
Oxide - 36 Mt @ 0.85% Cu;
Low-solubility Oxide - 143 Mt @ 0.20% Cu;
Supergene Sulphides - 23 Mt @ 1.04% Cu;
Sulphide - 165 Mt @ 0.94% Cu.
This summary was based in part on Tapia (2003), Cameron et al. (2006) and the BHP Billiton 2012 Annual Report.
El Salvador ...................... (Sunday 19 - travelling Antofagasta to El Salvador) Monday 20 October, 2003.
The El Salvador supergene enriched and hypogene porphyry deposits and the related Damiana exotic copper accumulations are located in the Indio Muerto district of Northern Chile, some 140 km east of the Pacific coast port of Cha–aral, 1100 km north of Santiago and 200 km south of the Escondida deposit. They contain in excess of 12 Mt of copper metal and are operated by the Codelco Salvador Division.
Return to top More detailed description
The Indio Muerto district is a 6 km long, NNE oriented, mineralised trend within a 200x50 km Tertiary volcanic field that also embraces the Potrerillos porphyry copper ores 25 km to the north. This field represents andesitic to rhyolitic activity in two distinct superposed magmatic events, older Paleocene (63-58 Ma) and younger two stage Eocene activity comprising an initial 44 Ma phase and a later 42 to 41 Ma pulse, both with porphyry copper mineralisation, the younger being far more important.
At the main El Salvador/Turquoise Gulch deposit the thick Cretaceous andesitic volcano-sediments, unconformably overlain by Tertiary andesites and rhyolites, are cut by a generally vertical, composite granodioritic porphyry plug with a diameter of around 2 km representing a four phase multiple intrusive and hydrothermal event responsible for the mineralisation.
Some 75% of the 5 Mt of copper at Turquoise Gulch was contained in early mineralisation, emplaced with the K Porphyry (the second intrusive phase, following the barren X Porphyry) predominantly associated with distinctive, discontinuous quartz veins - A veins - containing a disseminated K-silicate assemblage of alkali feldspar-biotite-anhydrite with bornite-chalcopyrite or chalcopyrite-pyrite. Bornite decreases outwards from the central zone of K-silicate alteration, through chalcopyrite and pyrite dominant shells, to the outer propylitic zone where minor chalcopyrite-magnetite veining is found, giving way further out to specular hematite. A transitional phase of quartz veining - B veins - post-dated the third intrusive phase, the L Porphyry, which removed part of the early stage alteration/mineralisation. B veins lack K feldspar, but comprise quartz with anhydrite, tourmaline and sulphides (molybdenite and chalcopyrite) bands. Late, irregular and ÔlacingŐ D veins, characterised by abundant pyrite and fracture controlled sericite and sericite-chlorite alteration have associated disseminated mineralisation. They contain lesser, but upward increasing bornite, chalcopyrite, enargite, tennantite, sphalerite or galena, accompanied by an anhydrite gangue and minor quartz.
Supergene enrichment as a 1.5 km diameter, 200 m thick chalcocite blanket was responsible for the original commercial orebody at Turquoise Gulch comprising an approximate pre-mining (1957) reserve of 300 Mt @ 1.6% Cu as secondary Cu sulphides replacing chalcopyrite and bornite and coating pyrite, with kaolinite and alunite alteration. Supergene processes have also moved leached copper laterally to form the Damiana and Quebrada M exotic copper deposits. The total resource in the Indio Muerto district is estimated to be 866 Mt @ 1.41% Cu, 0.01% Mo, 0.115 g/t Au.
Collahuasi ...................... Tuesday 21 October, 2003.
The Collahuasi porphyry copper project is based on two large deposits 7 km apart in northern Chile, Ujina and Rosario, located some 180 km south-east of the Pacific coast port of Iquique. Both are developed within the West Fissure Zone which passes through Chuquicamata 200 km to the south and is a continuation of the Domeyko fault system that straddles Escondida even further south. The operation produced around 0.185 Mt of Cu in 2002 and is managed by the Compania Minera Dona Ines de Collahuasi owned by Falconbridge (44%), Anglo American (44%) and a Japanese consortium.
Return to top More detailed description
Further porphyry mineralisation occurs in the Profunda deposit directly to the east of Ujina, below 400 m of gravel cover, and at the Quebrada Blanca deposit 10 km to the west of Rosario. High grade massive sulphide veins at La Grande, related to the Rosario porphyry have been surrounded by secondary enrichment halos, while a small exotic copper oxide deposit, Huinquintipa is hosted by palaeo-channel gravels draining Rosario. The four porphyry deposits are developed on three separate intrusive/mineralised centres defining a 25 km long, broadly east-west belt of mineralisation. All lie within a block of upthrown Permo-Triassic volcano-sedimentary rocks between the two major north-south faults of the West Fissure system, the West Fault and the Rio Loa Fault to the east. Thick, folded Jurassic sediments lie to the west of the West fault, while Tertiary volcanics and Quaternary sediments are found to the east.
At Ujina, strongly fractured and leached Permo-Triassic rhyolites and andesites are intruded by the mineralised granodioritic Ujina porphyry. Hypogene mineralisation occurs as a low grade core of chalcopyrite-bornite grading outwards into the main chalcopyrite-pyrite ore zone and then to low grade pyritic mineralisation with lesser chalcopyrite. The mineralisation typically occurs as veins, on fracture planes and as disseminations in the matrix of the host. Primary alteration comprises a core of potassic feldspar alteration, grading out into a potassic biotite ring, then to a quartz-sericite phase containing the higher grade primary ore, then to propylitic alteration with reduced grades. Moderate to intense argillic alteration (sericite, kaolinite and montmorillonite) accompanies supergene chalcocite-covellite enrichment of hypogene sulphides below leached upper levels (promoted by fracturing and faulting and breccias) and a lower zone of low grade oxides.
Rosario is a 2 x 1.5 km dome shaped mass of copper mineralisation within a variety of hosts, including andesites, rhyolites, volcanic sandstones and calcareous sediments of the Permo-Triassic host sequence. These have been intruded by a three phase igneous complex of porphyritic medium grained potassic granodiorite, a grey, mineralised porphyritic dacitic sill and a medium grained porphyritic granodiorite, the Rosario Porphyry. Mineralisation occurs as a series of concentric shells of disseminations and veins and is strongly influenced by a set of NW trending, SW dipping faults controlling the emplacement of the porphyries, primary mineralisation and associated vein systems. Bornite predominates in the high grade potassic altered core at depth, adjacent to the Rosario porphyry and vein system, grading outwards and upwards into zones of bornite-chalcopyrite to chalcopyrite-pyrite to pyrite with decreasing quantities of chalcopyrite on the periphery. Potassic alteration accompanied the first phase on mineralisation, which was succeeded by the largely fault controlled main phase veins with associated quartz-sericite, obliterating the potassic assemblage. The main phase vein system commenced with pyrite, minor chalcopyrite, bornite and tennantite, followed by a second pulse that produced abundant bornite with minor chalcopyrite, enargite, "grey copper" and sulphosalts and secondary chalcocite in the upper parts of the deposit. Supergene enrichment is irregular and less significant than at Ujina.
Reserve and resource figures for the Collahuasi project as estimated in 2002 amounted to proven+probable reserves of 1.85 Gt @ 0.92% Cu and resources of 2.31 Gt @ 0.70% Cu. The Aur Resources controlled Quebrada Blanca had 339 Mt @ 0.84% Cu leach ore.
Cuajone ...................... (Wednesday 22 - travelling Iquique to Moquega) Thursday 23 October, 2003.
The Cuajone porphyry copper-molybdenum deposit is one of a trio of major porphyry centres (the others being Toquepala and Quellaveco) spread over a north-west trending interval of 30 km, some 100 km south-east of Arequipa, and 42 km north-east of Moquegua in southern Peru. The three deposits together contain more than 25 Mt of copper metal and are operated by Southern Peru Copper Corporation, owned principally by Grupo Mexico 54.6% and Phelps Dodge 14%.
Return to top More detailed description
All three are members of a belt of Paleocene to early Eocene porphyry deposits in the Main Arc Domain of Andean magmatism and were emplaced between 56 and 52 Ma. This mineralisation is the culmination of a major continental volcano-plutonic arc, represented by the calc-alkaline to shoshonitic subaerial volcanics of the Cretaceous to Paleogene Toquepala Group. This group comprises andesite, dacite and rhyolite flows, breccia flows, agglomerates and ash flows, underlain by Upper Triassic to Jurassic marine volcanic and sedimentary suites and by Precambrian metamorphics of the Arequipa Massif accreted block which are cut by Palaeozoic granitoids and the major polyphase Cretaceous to Palaeogene Andean Batholith.
The mineralisation and alteration at Cuajone is directly related to a 2.5 x 0.7 km, NW-SE elongated, multistage latite porphyry intruding basaltic andesites and the overlying rhyolite porphyries of the locally 80 to 65 Ma Toquepala Group. The first, pre-mineral intrusive in the mine area was a north-south elongated, 66.7 ±1.7 Ma porphyritic diorite stock, approximately 1 to 2 km to the west of the orebody. This was followed by the three phase 56 to 52 Ma latite porphyry stock. The first pulse was responsible for the bulk of the hypogene copper and molybdenum mineralisation at Cuajone and the associated intense alteration of both the latite and surrounding Toquepala Group andesites and rhyolites. This pulse, where not heavily altered, is a porphyry with phenocrysts of quartz and feldspar in a cryptocrystalline matrix. Alteration takes the form of a potassic core, characterised by biotite-magnetite-K feldspar-silica, grading upwards and outwards to biotite-magnetite-silica and laterally into an extensive outer pyrite-propylitic alteration envelope with a radial extent of up to 4 km. Mineralisation is in both the latite and andesite wall rocks. The second magmatic pulse produced the Barren Latite Porphyry, while the third is only weakly altered and has no copper or molybdenum mineralisation.
At a late stage of the Latite Porphyry complex, during the of the first erosive period, the interaction of downward percolating meteoric waters with the rising hypogene hydrothermal fluids produced an intense phyllic quartz-sericite-pyrite zone superimposed on the upper parts of the first pulse Latite Porphyry mineralised system to produce a higher grade zone of copper-molybdenum ore as chalcopyrite and molybdenite within the Latite Porphyry and the Toquepala Group rhyolites.
The 900 m wide hypogene ore zone at Cuajone was overlain by a gently west dipping, average 20 m thick, blanket of supergene sulphide enrichment carrying more than 1.5% Cu. The main chalcocite layer is overlain by a 15 to 40 m thick partially oxidised upper zone averaging 1.3% Cu. These are in turn overlain by an up to 120 m thick hematite bearing leached capping with 0.1 to 0.2% Cu. Argillic alteration associated with the supergene ores include kaolinite, montmorillonite, illite and dickite.
To 1999, 425 Mt @ 0.97% Cu had been treated, while a reserve of 1.4 Gt @ 0.64% Cu, 0.033% MoS2 remained at a 0.4% Cu cutoff.
Antamina ...................... (Friday 24 - travelling Lima to Hauraz) Saturday 25 October, 2003.
Antamina, which is located in the western Cordillera of northern Peru, some 285 km north of Lima, is the world's largest known porphyry/skarn deposit of copper and zinc with a geological resource of 990 Mt @ 1.2% Cu, 1.0% Zn, 0.03% Mo, 13 g/t Ag, within a global resource of 1.5 Gt which is open at depth on the limbs of the deposit. The mine is operated by the Compañia Minera Antamina owned by BHP Billiton (33.75%), Noranda (33.75%), Teck Cominco (22.5%) and Mitsubishi (10%).
Return to top More detailed description
Antamina is an oxidised, garnet rich calcic skarn developed in the carbonate wall rocks of a sub-economic, calc-alkaline porphyry copper-molybdenum occurrence and has a garnet-pyroxene chemistry typical of copper skarns, but with an anomalously high grade outer zinc rich shell. The host carbonates are ensialic back-arc basin sediments of Cretaceous age and are located within an Eocene thrust and fold belt in the eastern part of the Western Cordillera. Late Tertiary sub-aerial volcanics covered this part of the Western Cordillera and a series of small high level Miocene stocks were emplaced, followed by the late Miocene Cordillera Blanca Batholith to the west of Antamina. The ore deposits of the Western Cordillera Poly-metallic Belt are related to this Miocene magmatism and include the skarns at Antamina, carbonate replacement at Cerro de Pasco, porphyry copper as at Michiquillay, porphyry copper-gold at Cerro Corona and epithermal gold-silver at Yanacocha and Pierina.
The mineralisation at Antamina is associated with a 9.8 Ma composite core intrusion of quartz-monzonite porphyry divided into early, middle and late phases. The early (or main) phase formed a stock which was responsible for the porphyry and skarn mineralisation. The middle and late phases occur as dykes and sills. The early and middle phases have associated biotite and k-feldspar alteration and quartz vein stockworks with associated Cu-Mo porphyry type mineralisation averaging 0.2% Cu and only minor phyllic and argillic alteration.
Approximately half of the Cu mineralisation is found within phreatic breccias and associated veined endoskarn host, while most of the skarn mineralisation is exo-skarn dominated by massive garnet. The skarn is concentrically zoned from brown, out to green garnet which has minor diopside, then to marbleised limestone, with massive sulphides and a localised wollastonite-diopside zone on the outer margin of the skarn. The outer marbleised limestone contains mantos of skarn and veins. Over 90% of the skarn is mineralised containing significant amounts of Cu with subordinate Zn, Ag & Mo. Higher grade Cu is concentrated within the red-brown garnet zone and in endo-skarns. Zinc averages around 0.2% in the brown garnet zone but is generally over 2% in the outer green garnet zone which is 100 to 200 m wide at the surface, narrowing with depth. Both brown and green garnet zones are andradite rich, with individual garnets having grossular-andradite cores and andradite rims. The skarn has only been subjected to minor, fracture controlled retrograde alteration characterised by chlorite, epidote, actinolite, quartz, calcite, sericite and clay.
The sulphides are coarse, low total iron and interstitial to the garnet crystals. The principal ore minerals are chalcopyrite and sphalerite, with ubiquitous pyrite and magnetite. Current proven+probable reserves are 530 Mt @ 1.22% Cu, 1.01% Zn.
The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. Many of these sources are listed with the "More detailed descriptions" of these deposits linked to each summary above.