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| Chuquicamata Copper Mine |
250km north-east of Antofagasta,
1,200km north of Santiago. (22°17'S 68°54'W)
Classification: open pit copper mine.
OreType : Copper porphyry.
Dimension: Size of pit: L=4,500m, W=3,540m, D=800m.
Production: Gold-Copper 650,000 metric
Tons annually. A=2,800m a.s.l.
Overview
Chuquicamata,
in northern Chile, is the world’s greatest orebody. It was mainly controlled by
initial intrusions (probably at 36 to 33 Ma) through mineralization (last major
hydrothermal event at 31 Ma) to postmineral brecciation and offset by the West
Fault system. The Chuquicamata Porphyry Complex consists of the East Porphyry,
West Porphyry, Banco Porphyry and Fine Texture Porphyry. Potassic
alteration, the early stage of
alteration, affects all porphyries. Veins of quartz molybdenite, up to 5 m wide
and cutting all porphyries, were emplaced between the early and the main
stages. Main-stage veins occupy many of the same structures of the early stage
and may involve massive remobilization of earlier mineralization. The late
stage formed digenite with relatively coarse grained covellite from deep in the
sericitic zone. A leached capping and oxide copper ore, replacing an upper
chalcocite blanket, overlie a high-grade supergene chalcocite body that extends
up to 800 m in depth.
Summary of Geological
Setting
Chuquicamata
is closely related to Eocene, early Oligocene porphyritic intrusions that occur
within the middle to late Cenozoic, north-south striking Domeyko Fault system.
Pre-Oligocene rocks
The
oldest rocks in the Chuquicamata district occur in a north-northeast trending
belt of Paleozoic metasedimentary and metaplutonic rocks. These rocks include gneissic granite, metadiorite, quartz
diorite, and minor tonalite recrystallized in varying degrees to amphibolite.
Eocene-Oligocene intrusions
The
porphyritic rocks in the Chuquicamata pit, with the dominantly barren Fortuna
Complex to the west and the intensely mineralized Chupui Porphyry Complex to
the east, are separated by the major postmineral West Fault. Rocks with
textures essentially identical to those of the Chuqui Porphyry Complex extend
northward at least 9 km through the Radomiro Tomic mine (Cuadra et al., 1997;
Cuadra and Rojas, 2001).
Rock Types
Fortuna
Intrusive Complex
The Fortuna Intrusive Complex borders
to the open pit and contains only low-grade mineralization. It has been structurally
juxtaposed against the intensely mineralized Chuquicamata Porphyry Complex by
large-scale, postmineral movement on the Wets Fault, which is documented by
Dilles et al. (1997), Tomlinson and Blanco (1997), and previous workers. The
Fiesta Granodiorite phase of the Complex is volumetrically dominant and is
intruded by small irregular bodies of San Lorenzo Granodioritic Porphyry and
minor Tetera Aplite Porphyry. Fiesta Granodiorite is weakly mineralized with
copper oxides in the uppermost northwestern benches of the pit. Sulfides occur only
near contacts of the San Lorenzo porphyries.
Pre-Chuqui
porphyry intrusions
The Elena and East Granodiorites are
exposed on the eastern margin of the pit. They intrude metasedimentary rocks
that were originally shale and sandstone with minor limestone. Whereas Elena Granodiorite
is mineralogically and texturally similar to the East Porphyry, the East
Granodiorite is texturally distinctive and clearly older. A radiometric dating
of the Elena Granodiorite indicates a Jurassic (dating of zircon) to Early
Cretaceous age (dating of biotite), published by Ambrus (1979). All of these
rocks at the east edge of the pit are essentially poor of mineralization.
Chuqui Porphyry
Complex
Practically the entire Chuquicamata
orebody is hosted by the Chuqui Porphyry Complex, made up of East, West, Fine
Texture, and Banco porphyries. Their textures vary widely, and most exposures are
affected by some degree of hydrothermal alteration and pervasive cataclastic
deformation. The probably oldest and largest intrusion is the East Porphyry
with hypidiomorphic-granular texture. The West Porphyry is finer grained and
with quartz eyes in an aplitic groundmass. Locally both porphyries are weakly
foliated. Banco Porphyry is more porphyritic and finer grained than East
Porphyry, which it intrudes. From West Porphyry it differs in having an
abundance of small plagioclase crystals in the aplitic mass. The Fine Texture
Porphyry is distinctly finer grained than normal East Porphyry but has also a
hypidiomorphic-granular texture. Contacts with East Porphyry may be abrupt but
usually faulted. Because of the overprinting of most dikes by quartzsericite alteration,
their identification is very difficult. Furthermore is seems, that Banco and
Fine Texture porphyries have been affected by all of the same stage of
alteration and mineralization as the East Porphyry.
A large part of the copper at Chuquicamata occurs in veins and
veinlets filling faults and faultrelated shatter zones. In the main orebody
practically all of these fractures have been opened and mineralized more than
once. Early-stage veinlets of quartz and quartz-K feldspar contain no or only
very minor sulfide. They are cut by more continuous quartz veins, to 5 cm wide,
containing minor molybdenite and traces of chalcopyrite. Large banded quartz veins,
known as blue veins, are typically 1 m or more in width. They contain abundant
molybdenite and truncate the previous veins. Furthermore, they are commonly
surrounded by sericitic alteration, but this is due to superposition of younger
pyritic veins following the same structures. Veins and veinlets of the main stage
contain pyrite, chalcopyrite, bornite, and digenite, decreasing amounts of
quartz and increasingly well-developed sericitic alteration halos. Locally, the
earliest of these veins appear to contain pyrite without Cu sulfide (Lindsay et
al., 1995). Relatively late main stage veins contain enargite ± pyrite and
minor sphalerite. Later on, veinlets and fractures are filled with relatively
coarse grained covellite (to 1 mm) and digenite with and without pyrite.
Hypogene Alteration and
Mineralization
Just like El Salvador and many other
porphyry copper deposits, vein relationships lead to the definition of an early
stage defined by K feldspar stable alteration and early quartz veinlets, a
transitional stage defined by quartz-molybdenite veining, and a main-stage
defined by pyrite-bearing veins with sericitic halos. A more unusual and
controversial late stage is defined by coarse-grainedcovellite-digenite
veinlets without pyrite and possibly hypogene sphalerite rims on other sulfides
(Fréraut, et al., 1997).
Supergene Mineralization and
Alteration
After Taylor (1935) and Jarrell (1944) the
rich oxide copper orebody has been largely mined out, but considerable
resources of lower grade material remain in the north end of the pit and beyond
(North zone, Fig. 1; Cuadra et al., 1997; Ossandón and Zentilli, 1997). Oxide
ores contain a large variety of minerals but in chief antlerite, brochantite,
atacamite, chrysocolla, and copper pitch. Also residuals of chalcocite are
implied. The ore was overlain by leached capping and was an eastward and upward
extension of the chalcocite zone, indicating it was a supergene chalcocite
enrichment blanket oxidized in situ. It is the upper of two chalcocite blankets
with a leached horizon in between. A lower enrichment zone has more reactive
alteration assemblages and contains decreasing chalcocite and/or covellite
proportions downward. In the central zone of intense brecciation, the two
enrichment blankets (copper leaching and chalcocite enrichment) merge and reach
their maximum depth.
Mining and processing
Coldec uses conventional open pit mining methods at
Chuquicamata. A conventional truck-and-shovel operation constitutes the mining
activity. Large quantities of the ore are crushed within the pit. Underground
conveyors transport the crushed ore to the mill bins.
An Outokumpu flash smelter is installed for smelting the
concentrate. The concentrate is then passed through a converter with electric
furnace. After the slag is cleaned, the concentrate passes through four Pierce
Smith converters. Blister copper is then sent to six anode furnaces. The
electrolytic refinery has a capacity of 855,000t per annum. Three anode casting
wheels were installed and are fed by the furnaces.
Underground mining at Chuquicamata
The new underground mine, scheduled begin operations in 2019,
will comprise of four production levels, a 7.5km main access tunnel, five clean
air injection ramps, and two air-extraction shafts.
The tunnels will deepen the mine by nearly 787m by the end of
production in 2060. The underground mine will be developed at an estimated cost
of $4.2bn and will produce an estimated 140,000 tonnes of ore per day. It mine
is expected to produce 366,000t of copper and 18,000t of fine molybdenum per
year.
Sinclair Knight Merz undertook the conceptual engineering of the
mine, including identification, option studying and analysis for ore excavation
and handling. For ore extraction, panel caving and macro blocks were studied.
Three extraction panels were identified at different depths for both options.
Excavation panels identified for panel cavings were at 1,841m, 1,697m and
1,409m above sea level and 1,841m, 1,625m and 1,409m for macro blocks.
The copper ore reserves of the Chuquicamata underground mine are
estimated to be 1,700mt grading 0.7% copper and with an average molybdenum
content of 502ppm.
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