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Porphyry Related Copper & Gold Deposits in the Cordillera of the Americas
16 March to 8 April 1998 - In Two Separate Modules
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DESCRIPTIONS of ORE DEPOSITS
Module 1 - South America
Mon 16 to Thurs 26 March 1998
Module 2 - North America
- Disputada-Los Bronces, Chile.
- El Teniente, Chile.
- El Indio, Chile.
- Chuquicamata, Chile.
- El Abra, Chile.
- Escondida, Chile.
- Alumbrera, Argentina
Mon 30 March to Wed 8 April 1998
- Mission, Arizona, USA
- Sierrita, Arizona, USA
- Ray, Arizona, USA
- San Manuel, Arizona, USA
- Silver Bell, Arizona, USA
- Bingham Canyon, Utah, USA
- Highland Valley, BC, Canada
- Iron Mask District/ Afton-Ajax, BC Canada
MODULE 1 - SOUTH AMERICAThis module started with a one day seminar in Santiago, Chile which provided an overview of the tectonics, geology and metallogeny of the Andean porphyry belt, as well as details of the regulatory, social and political environment of exploration and mining in the region. Deposits were visited in the following order.
The Los Bronces and adjacent Rio Blanco mines exploit a single porphyry Cu deposit which is located on the western side of the Andes in Central Chile, some 69 km west of Santiago. Like the El Teniente deposit a 100 km to the south, it was formed in the early Miocene to Pliocene, and is one of the largest of the ore deposits of this age grouping. The Los Bronces open pit mine, which is operated by the Exxon subsidiary Compañia Minera Disputada de Las Condes SA, exploits a tourmaline rich breccia complex superimposed on the west side of an earlier major porphyry copper system. The Rio Blanco underground and open pit mine of the Codelco Andina Division is principally developed in the north-central part of this older porphyry. Mineralisation was emplaced between 7.4 and 4.9 Ma. Production from Disputada in 1993 was 157 400 t Cu, from 12.11 Mt of ore with a head grade of 1.29% Cu. Reserves in 1993 were 1318 Mt @ 0.8% Cu. The Codelco Andina Division operation has a capacity of 200 000 tpa of contained Cu.
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The El Teniente orebody is associated with a 5 Ma quartz-dioritic to dacitic porphyry intruded into a sequence of Cretaceous to Quaternary volcanics, continental sediments and felsic to intermediate intrusives. The immediate hosts are Miocene andesitic lavas and pyroclastics that are cut by the "Braden Formation" which comprises a brecciated inverted circular cone. Three zones of hypogene alteration are recognised, namely an early magmatic potassic K feldspar-biotite stage with an outer propylitic zone of chlorite-epidote-calcite-magnetite-pyrite, and a multi phase hydrothermal phyllic quartz-sericite-anhydrite, with associated tourmaline, epidote and calcite. The latest hydrothermal phase is within the Braden Formation pipe and includes anhydrite, siderite and gypsum. Hypogene mineralisation is zoned with bornite in the central core, surrounded by chalcopyrite, grading outwards to pyrite. Mineralisation is found in veins and as disseminations in the wall rocks. Within the outer rim of the Braden Formation pipe there is a zone of late stage tennantite-tetrahedrite, pyrite gypsum and ankerite. Late stage supergene mineralisation produced chalcocite and lesser covellite, with associated argillic alteration and anhydrite leaching. Production commenced in 1906. In 1993 some 33 Mt of ore with a head grade of 1.10% Cu were milled to produce 305 600 t Cu. Mining is principally by block caving. The mine is operated by Codelco.
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El Indio is a high sulphidation Au-Ag-Cu deposit hosted by welded ash flow tuffs that underlie andesitic volcanics in a north-south striking fault bounded depression. Mineralisation appears to be related to the emplacement of small felsic domes and is of late Miocene age. The ore is present as fault controlled veins covering an area of 500 x 150 m. Mineralisation was emplaced in two well defined phases, namely initial massive enargite-pyrite veins, and second stage quartz-gold veins. The massive sulphide veins are accompanied by kaolinite and lesser alunite alteration and are rich in Cu, but gold poor. The second stage has associated sericite and lesser pyrophyllite, with local bonanza gold grades. The mine is operated by Barrick Gold. Total production plus reserves are 23.2 Mt @ 6.6 g/t Au, 50 g/t Ag, 4% Cu.
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Chuquicamata is probably the greatest porphyry copper deposit of them all. Mineralisation is spread over a strike length along the West Fissure fault for around 13 km. Like Escondida, it represents hypogene mineralisation emplaced in the Eocene to Oligocene, associated with a 34 Ma, highly altered, quartz-monzonite and enclosing granodiorites. It contains an early K-feldspar stable chalcopyrite-bornite-molybdenite suite, an over printing early hydrothermal chalcopyrite-molybdenite-sericite vein set and a late stage hydrothermal pyrite-chalcopyrite-enargite-intense sericite-silica phase. In addition it has been strongly oxidised in the upper levels, and undergone supergene enrichment to form a chalcocite blanket. Adjacent is the 180 Mt @ 1.6 to 1.8% Cu "La Exotica" or Mina Sur orebody comprising chrysocolla, atacamite, Cu-wad etc. in younger channel gravels and the underlying bedrock representing secondary transported Cu in the base and bedrock of a palaeo-alluvial channel. Reserves are of the order of 5 billion tonnes in 1993, containing near 45 Mt of Cu, as quoted in 1994. The mine has been in operation on a large scale since 1915. Until 1952 only oxide ore was mined. Currently a mix of oxide, supergene blanket and hypogene mineralisation is treated. Production at Chuquicamata in 1993 totalled 533 000 t Cu from 83.5 Mt of ore milled, with a head grade of 1.3% Cu. The open pit covers an area of some 3.8 x 2 km, and is near 500 m deep. Chuquicamata is a Codelco mine.
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El Abra is located some 42 km to the north of Chuquicamata and is based on a 400 Mt oxide resource with an average grade of 0.75% Cu. This is to be treated with modern SX/EW technologies. It is said to overlie around 800 Mt at near 1% Cu as hypogene sulphides. Like Chuquicamata, with which it was apparently synchronous, the hypogene orebody at El Abra is developed along the West Fissure fault and is emplaced in Tertiary intermediate intrusives, which in turn are part of an intrusive-volcanic complex the oldest members of which date back to the Jurassic. The hosts are diorite to syenite in composition. Multiple phases of intrusion, mineralisation and alteration are implied. Deep erosion has removed the upper parts of the system. The ore has a low pyrite content, and this with the basic nature of the hosts resulted in little supergene enrichment, but strong oxidation of the ore. The dominant oxide is chrysocolla. El Abra is a joint venture between Codelco and Cyprus Amax.
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Escondida is a large supergene open pit deposit in northern Chile, located some 160 km to the east-south-east of Antofagasta. It has a geological resource of 1760 Mt @ 1.59% Cu and mineable reserves of 662 Mt @ 2.12% Cu. The hypogene mineralisation, which is generally <0.5% Cu, is of Eocene to Oligocene age, associated with diorite-monzonite porphyries, while the supergene enrichment is believed to have taken place during the middle Miocene. Production in 1993 totalled 389 000 t Cu from 17.6 Mt of ore with a head grade of 2.4% Cu. The mine is a joint venture between BHP Copper and Rio Tinto.
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The Baja de la Alumbrera porphyry copper orebody is the most significant deposit in Argentina to date. It is located on the eastern margin of the Miocene to Pliocene belt, some 250 to 300 km to the east of the main belt of Chilean ore deposits. Alumbrera is centred on a two phase dacite-porphyry stock emplaced within a large andesitic strato-volcano. The deposit has a central cylindrical core of K-silicate alteration with high grade Cu-Au zones rich in quartz and magnetite, surrounded by sericite and supergene kaolinite, with an external propylitic zone. The supergene profile is immature, and economic ore is principally hypogene. The measured resource is of the order of 695 Mt @ 0.51% Cu, 0.66 g/t Au, with a high grade core of 118 Mt @ 0.64% Cu, 0.92 g/t Au. Mining is by open pit. Minera Alumbrera, the operator is a joint venture between MIM, North Ltd & Rio Algom.
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MODULE 2 - NORTH AMERICA
This module commenced with a one day workshop in Tucson, Arizona, presented by Professor Spencer Titley, entitled "Geology of the Porphyry Deposits in the Western United States". Its aim was to provide a context to the deposits visited, and an overview of the tectonics, geology and metallogeny of the western US, as well as the styles of mineralisation represented in the region.
The Mission Complex open pit mine exploits skarn ore within a Palaeozoic sequence of dolomite, limestone and quartzite which is intruded by a sill of 56.7 Ma Palaeocene quartz-monzonite porphyry. Limestone is the principal host, with a skarn alteration assemblage of garnet-diopside, wollastonite and epidote-garnet-K feldspar in the limestones, while mineralised veins in the silicates have K silicate and phyllic selvages. Historic production + reserves in 1994 totalled more than 1100 Mt @ 0.67% Cu, 4 g/t Ag. In the same year the mine produced 128 100 t Cu from 21.36 Mt of ore with a head grade of 0.6% Cu. The mine is operated by ASARCO Inc., and is located just to the south of Tucson Arizona.
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The Sierrita mine, which is close to Mission, is a low grade hypogene orebody in porphyries and volcanics. It illustrates both the importance of fracturing and the grades that can be economically treated in an established mining district. The orebody occurs on the outer rim of the late Cretaceous to Tertiary, multi-phase, Ruby Star Granodiorite batholith. Mineralisation is centred on a complex of stocks which are late stage, co-magmatic phases of the batholith. The ore, which is fracture controlled, occurs near the contact between the batholith and both a Jurassic batholith and Triassic volcanics. The Sierrita deposit is a low total sulphide, low pyrite and low grade orebody. Alteration is only poorly developed, although the fracture density is relatively high. All of the different porphyries of the intrusive complex are mineralised and fractured, as are all other rocks within the mine. Production from 1968-78 was 200 Mt @ 0.27% Cu, 0.022% Mo, with reserves in 1978 being 387 Mt @ 0.30% Cu, 0.035% Mo. In 1989 reserves were 510 Mt @ 0.34% Cu, in 1993 477 Mt @ 0.3% Cu. Production in 1993 was 142 000 t of Cu, 5500 t of Mo, 40 t of Ag. The mine is operated as a complex with the nearby Twin Buttes deposit by Cyprus Sierrita Corporation.
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The Ray orebody in Arizona illustrates a number of important characteristics of porphyry mineralisation, particularly the influence of host rock composition on ore style, alteration and supergene modification. There is no clear mineralised, central porphyry body, although numerous porphyry masses are evident, possibly representing a tilted porphyry plug/complex. Ore is hosted by both a major dolerite sill and the quartz-mica schists of the enclosing Middle Proterozoic Pinal Schists. Ore within the dolerite is of economic hypogene grade with no associated supergene enrichment, reflecting the reactivity of the host. In contrast the low reactivity Pinal Schists have only sub-economic hypogene mineralisation, but economic supergene ore. Historic production + reserves are in excess of 1200 Mt @ 0.6 to 0.8% Cu. In 1993 8.15 Mt of ore at 1.6% Cu were treated for 130 300 t of Cu. The mine is operated by ASARCO Inc.
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The San Manuel - Kalamazoo mines exploit a large porphyry style deposit by underground block caving techniques north of Tucson in Arizona. The classic Lowell and Guilbert porphyry model was based on this deposit. It is virtually unique in that it is centred on a 67 Ma Palaeocene quartz- monzonite porphyry plug intruding a larger Middle Proterozoic porphyritic quartz-monzonite body. Approximately half of the orebody is hosted by each. As such this deposit is almost unique in being hosted by a virtually homogenous mass of rock. Historic production + reserves have been near 1000 Mt @ 0.7% Cu. The mine is operated by BHP Copper.
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There are a number of deposits at Silver Bell which have been exploited in the past and others that remain un-mined. This deposit group illustrates the processes and zonation within a leached cap-supergene enriched system. All of the known deposits at Silver Bell are confined to an arcuate, pyritic zone of alteration that is 12 km long and 0.35 to 3 km wide. This zone is superimposed on Palaeozoic clastic and carbonate sediments, Mesozoic alaskite, dacite and andesite porphyry and a series of quartz-monzonite stocks which are each from 500 to 1500 m in diameter and localised within the alteration corridor. Low grade hypogene mineralisation is centred on these stocks, with laterally more extensive economic supergene blanket ore developed from these accumulations. At Silver Bell it is possible to see the full section through the leached cap, supergene blanket to the hypogene mineralisation in the mined out pits, and the original exposure of the leached capping in the un-mined sections of Silver Bell North. Historic production at Silver Bell is of the order of 92 Mt @ 0.71% Cu, 2.2 g/t Ag, with approximately 102 Mt @ 0.47% Cu, or 80 Mt @ 0.6% Cu, remaining in North Silver Bell. The deposit is controlled by ASARCO Inc., and is located to the north-west of Tucson in Arizona.
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Bingham Canyon is located near Salt Lake City in Utah. The mine first operated in 1904 with the supergene cap being exploited with grades of around 1.5% Cu and 0.5 g/t Au. The orebody is centred on the late Eocene quartz-monzonite Bingham Stock, one of a number of intrusive phases in the mine area. The orebody has a well defined series of shells as defined by alteration and sulphide mineralogy. The ore is present as both disseminated porphyry style mineralisation and as two major skarn orebodies within Palaeozoic carbonates. Since the very early years all production has been from hypogene ore. Historic production of the porphyry mineralisation to 1972 was 1240 Mt @ 0.91% Cu, while in 1995 reserves were 1020 Mt @ 0.59% Cu, 0.38 g/t Au. The two skarn orebodies contain 61 Mt @ 1.9% Cu, 0.38 g/t Au and 81 Mt @ 2.8% Cu, 1.6 g/t Au. Ore production is from a large open pit, from which some 52.4 Mt of ore were extracted in 1994, with a head grade of 0.63% Cu, to yield 310 100 t Cu, 15.86 t Au, 8700 t Mo and 135 t Ag. The mine is controlled by the Rio Tinto subsidiary Kennecott.
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On arrival in Canada a one day workshop was held at the Mineral Deposit Research Unit of the University of British Columbia with Professor John Thompson on the "Geology of the Porphyry Copper & Gold Deposits of Western Canada". This was aimed at providing a context and background overview of the porphyry deposits of the region, which has tectonic and geologic differences to the western US.
The Highland Valley complex comprises a series of large low grade hypogene deposits within calc-alkalic intrusives in central British Columbia, Canada. Like most of the porphyry deposits in western Canada, it occurs within one of the Palaeozoic to Mesozoic displaced terranes accreted onto the western margin of the Canadian Shield, in contrast to those of the western US which are within the cratonic terranes. This mine is currently the only significant porphyry operating in the Canadian Cordillera. The total reserve + historic production amounts to around 2000 Mt @ 0,45% Cu, and includes the Lornex, Highmont, Valley Copper, JA, Bethlehem, Krain and Craigmont deposits. The mineralisation is located within the core of the 1000 km2 210 to 200 Ma Jurassic Guichon Creek Batholith, a multiphase body ranging in composition from tonalite to diorite to granodiorite. This batholith cuts a co-magmatic andesite pile and falls within the Quesnellia Displaced Terrane. Mineralisation is associated with swarms of younger dykes within the youngest, most acid phase of the batholith. In 1994 some 44.7 Mt of ore averaging 0.42% Cu was milled for 186 800 t Cu, 30 t Ag and 0.2 t Au. The mine is operated by Highland Valley Copper, a joint venture between Cominco Ltd, Rio Algom Ltd, Teck Corp and Highmont Mining.
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Iron Mask District/ Afton-Ajax
The 30 x 5 km alkalic Iron Mask Batholith hosts one larger and six small orebodies of the Afton-Ajax group of deposits which are distributed over an area of 10 x 4 km. The batholith is a sub-volcanic multiple intrusion emplaced between 205 and 190 Ma, composed predominantly of diorite, but includes phases ranging from gabbro to syenite composition. It intrudes comagmatic andesitic volcanics of the upper Nicola Group. The dominant style of mineralisation is chalcopyrite with calcite infilling fractures and veinlets, and as disseminations and blebs. The Iron Mask Batholith has widespread saussuritic (propylitic) and K-feldspar alteration as well as high magnetite contents (up to 10% in the intrusive) and massive magnetite zones. The area within a 1 km radius of each deposit is marked by an epidote-albite-chlorite-magnetite-carbonate alteration assemblage, increasing towards ore, and by irregular weak pyritic haloes. The mines are now exhausted. Production + reserves in 1984 amounted to 71 Mt @ 0.75% Cu, 0.44 g/t Au, 4 g/t Ag. This visit will comprise a briefing, surface traverse and an inspection of remaining un-rehabilitated pits in the company of the Resident Geologist of the local British Columbia Geological Survey.
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For more information contact: T M (Mike) Porter, of Porter GeoConsultancy (email@example.com)This was another of the International Study Tours designed, developed, organised and escorted by T M (Mike) Porterof Porter GeoConsultancy Pty Ltd (PGC) in joint venture with the Australian Mineral Foundation (AMF). While the reputation and support of the AMF contributed to the establishment of the tours, after it ceased trading at the end of 2001, PGC has continued to develop, organise and manage the tour series.
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