Centinela - Esperanza
Super Porphyry Cu and Au|
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The Esperanza porphyry copper-gold deposit is located ~60 km south of Calama (near Chuquicamata), in northern Chile. It is 3 km SE of the El Tesora exotic copper deposit. Both Esperanza and El Tesora belong to the Centinela cluster of deposits and have been merged into the same operation. Other deposits include Telégrafo (2.4 km south of Esperanza), Shererezade, Caracoles (3.2 km SSW of Esperanza), Centinela and Polo Sur (#Location: Esperanza - 22° 58' 20"S, 69° 3' 35"W).
See the separate El Tesora record also.
Regional and District Setting
Much of the region is masked by the middle Eocene to middle Miocene age Calama and Tambores Formations composed of moderately consolidated gravels.
The Centinela cluster of >10 middle Eocene deposits and occurrences are distributed over an NNE trending interval of ~35 km, within a 25 km wide, fault-bounded belt of i). Late Cretaceous andesitic, pyroclastic and volcano-sedimentary rocks of the Quebrada Mala Formation and ii). early Eocene andesitic, dacitic and rhyodacitic flow domes of the Estratos de Cerro Casado. This fault bounded belt is located between the Palaeozoic basement exposures of the Cordillera de Domeyko to the east, and an early Cretaceous volcanic sequence in the Coastal Range to the west.
Magmatic activity within the district persisted over an interval of almost 80 m.y., from the Early Cretaceous to the Eocene. The oldest plutonic rocks intruded in the vicinity of the Centinela cluster comprise Early Cretaceous olivine-pyroxene gabbros and hornblende bearing quartz diorites dated at between 124 and 100 Ma (U-Pb zircon and K-Ar ages; Mpodozis et al., 1993b; Marinovic and García, 1999). The intruded country rocks were Jurassic marine limestones of the northern Chile back-arc basin, ~100 km east of the Early Cretaceous magmatic front, which was located in the Coastal Range (Boric et al., 1990).
Subsequent volcanic and intrusive events took place in the Late Cretaceous, when the Quebrada Mala Formation volcanic sequence and a group of coeval 70 to 66 Ma pyroxene diorites to rhyolite porphyries and 70 and 66 Ma (U-Pb zircon) flow domes, were generated after the Andean arc front migrated eastward into the Centinela region. Volcanism persisted after the Cretaceous-Tertiary deformation event, with eruptions from stratovolcanoes and small collapse calderas that were active from the early Paleocene (64 Ma) to the early Eocene (53 Ma).
A diverse group of epizonal intrusions, mainly ~60 Ma pyroxene-biotite quartz diorite to monzodiorite and 58 to 57 Ma hornblende-biotite granodiorite, plus other andesitic to dioritic porphyritic intrusions, were emplaced into the Mesozoic units and Paleogene volcanic edifices over this period.
Incaic magmatism and mineralisation in the district took place between 45 and 39 Ma (Mpodozis et al., 2009; Perelló et al., 2010) and began ~12 to 10 m.y. (based on U-Pb zircon ages from intrusive rocks and Re-Os ages in molybdenite) after the termination of early Eocene volcanism. Mpodozis and Cornejo (2012) note that this event coincides with the mostly copper-barren early phase of Incaic intrusions at Chuquicamata-El Abra (45 to 42 Ma) and Escondida (44 to 41 Ma). The oldest of these intrusive rocks at Centinela include minor 45 Ma pyroxene-biotite and quartz diorites. However, in contrast to Chuquicamata-El Abra and Escondida, numerous mineralized porphyry centres were emplaced at Centinela between 44 and 39 Ma, which together with some barren stocks, form a 40 km long, north to NE trending belt, including at least 10 discrete intrusive complexes. A syntectonic sequence of conglomerates and volcaniclastic sandstones, accumulated contemporaneously with porphyry copper emplacement, occurs as interbedded layers of dacitic block and ash deposits and tuffs with 42 to 39 Ma U-Pb zircon ages.
The oldest porphyry systems in the cluster, dated at 45 to 43 Ma, are found in the SW section of the belt. The age generally progressively decreases to the NE, to 39 Ma at the extremity of the trend. The porphyry belt is cut obliquely by a 3 to 5 km wide, north-south trending fault zone of intense deformation that was active both during and after porphyry emplacement, and controls the geometry of the porphyry complexes. It is the northern termination of the Sierra de Varas fault, which stretches for >250 km along the western border of the Cordillera de Domeyko (Mpodozis et al., 1993; Sotoet al., 2005). Those deposits to the west and east of the fault zone are largely undeformed. Those to the west are associated with subvertical hornblende-biotite dacite dyke swarms intruded into Paleocene volcanic/subvolcanic rocks (e.g., Centinela) or early Eocene rhyolitic dome complexes (Polo Sur, Perelló et al., 2010).
The oldest deposits include Centinela (45 to 44 Ma) and Shererezade (44 to 43 Ma), followed by several barren 43 Ma pyroxene-hornblende dioritic stocks and lacoliths, although the similar aged 43 Ma Pilar is a mineralised porphyry copper system. A new pulse of copper-bearing intrusions was emplaced at Polo Sur between 42 and 41 Ma, whilst apparently barren 42 Ma dacitic porphyries have been documented at Penacho Blanco. The 41 to 39 Ma Mirador deposit, towards the NE end of the trend, is the youngest porphyry deposit recognised to date and occurs east of the fault zone, where it is structurally undisturbed. Copper mineralisation hosted within Jurassic marine limestones and evaporites in this same area (Mora et al., 2009) is associated with a group of subvertical multiphase, west to NW-trending intrusions.
Deposits within the fault zone are of intermediate 42 to 40 Ma ages and are all associated with tilted swarms of porphyry dykes (e.g., Caracoles, 42 to 41 Ma; Telégrafo, 42 to 40 Ma; Esperanza, 42 to 40 Ma; Llano, 41 Ma; Perelló et al., 2004, 2010; Bisso et al., 2009; Mü̈nchmeyer and Valenzuela, 2009; Swaneck et al., 2009), emplaced into moderately to steeply dipping units that were disrupted by major postmineral reverse, normal and strike-slip faults. The Esperanza and Telégrafo deposits occur within a long-wavelength, asymmetric, basement-cored anticline, bounded in the footwall to the west by a moderately E-dipping thrust fault (the Telégrafo fault). The core of the anticline is sliced by the subvertical Coronado and Llano faults and mineralisation is associated with easterly inclined porphyry dykes intruding ~40 to 50°W dipping Triassic to Upper Cretaceous rocks.
The mineralised and altered host rocks of the Esperanza and Telégrafo porphyry deposits, are thrust to the west, over the Telégrafo fault, to overlie barren, unaltered mid-Eocene (42 to 39 Ma) sedimentary and volcanic rocks that were deposited whilst the porphyry intrusions were being emplaced at depth. The subvertical Esperanza fault bounds the mineralised limb of the anticline to the east and places Jurassic limestones over Late Upper Cretaceous rocks, although it also includes an important component of strike-slip movement.
The younger Llano and Coronado faults, which have both sinistral and large, down-to-the-east components of displacement, part of which is late Miocene or younger (<10 Ma), are superimposed over the Telégrafo-Esperanza system.
The 42 to 40 Ma (Swaneck et al., 2009) Caracoles deposit, 10 km SSW of Esperanza, is hosted in 64 to 60 Ma early Paleocene volcaniclastic rocks, associated with a steeply SE-dipping swarm of thin dacite porphyry dykes. These dykes follow the Las Lomas duplex, defined by two north-south trending subvertical structures, the Las Lomas and Centinela faults, that have undergone both early dextral and later sinistral displacement. The control of these structures on intrusion is consistent with emplacement of the porphyry along a zone of pre or synmineral strike-slip deformation.
Deposit Geology and Mineralisation
A series of structurally controlled, medium-grained granodiorite porphyry dykes intrude Quebrada Mala Formation massive andesite flows and interbedded pyroclastic and calcareous volcano-sedimentary horizons at Esperanza.
Hydrothermal alteration comprises of a core of potassic alteration (biotite dated at 41.3 ±0.3 Ma) partly overprinted, and surrounded by a halo of intermediate argillic, phyllic, and propylitic assemblages.
Hypogene Cu-Au mineralisation predominantly occurs in multiple stockworks of pyrite poor, A- and B-type veinlets carrying chalcopyrite and bornite with quartz, K feldspar, biotite, magnetite, apatite and anhydrite. These veins are observed to be directly associated with the potassic alteration assemblages. Minor molybdenite which accompanies these veinlets has been dated at 41.80 ±0.13 Ma. Anhydrite becomes increasingly abundant with depth in the potassic core, and locally, forms a large structurally controlled massive body with interbedded proximal skarn which is rich in garnet and diopside.
The overprinting intermediate argillic alteration assemblage includes chlorite, illite, smectite and greenish sericite, with accompanying chalcopyrite and pyrite. However, the phyllic quartz-sericitic alteration is not accompanied by copper mineralisation, being dominated by disseminated and veinlet pyrite in D-type veinlets.
Within the upper 150 m of the deposit, supergene-oxide copper mineralisation is developed, predominantly comprising atacamite and chrysocolla with lesser brochantite, copper wad and copper-rich clays. Minor chalcocite, covellite, native copper and cuprite are found near the redox interface.
Reserves and Resources
Resource figures in 2001 were as follows (Perello et al., 2004):
71 Mt @ 0.42% Cu - oxide zone,
443 Mt @ 0.63% Cu, 0.26 g/t Au - hypogene sulphide zone, 0.3% Cu cut-off
128 Mt @ 1% Cu, 0.48 g/t Au - hypogene sulphide zone, open pit resource.
Published reserve and resource estimates at 31 December, 2010 were as follows (Antofagasta PLC website, 2012):
Measured + indicated resource - 890.7 Mt @ 0.47% Cu, 0.011% Mo, 0.17 g/t Au; plus
Inferred resource - 1031.9 Mt @ 0.31% Cu, 0.008% Mo, 0.06 g/t Au; including
Proved + probable reserve - 587.0 Mt @ 0.55% Cu, 0.010% Mo, 0.22 g/t Au.
Measured + indicated + inferred - 121.3 Mt @ 0.33% Cu; including
Proved + probable reserve - 102.4 Mt @ 0.35% Cu.
Note the oxide ore is treated seprately at the El Tesora operation .
Published ore reserve and mineral resource estimates at 31 December, 2015, were as follows (Antofagasta PLC Annual report, 2015):
Measured + indicated resource - 2249.9 Mt @ 0.41% Cu, 0.012% Mo, 0.14 g/t Au; plus
Inferred resource - 965.7 Mt @ 0.32% Cu, 0.011% Mo, 0.09 g/t Au;
TOTAL resource - 3215.7 Mt @ 0.38% Cu, 0.012% Mo, 0.14 g/t Au; including
Proved + probable reserve - 1840.4 Mt @ 0.44% Cu, 0.012% Mo, 0.15 g/t Au.
Measured + indicated + inferred resource - 337.7 Mt @ 0.41% Cu; including
Proved + probable reserve - 188.5 Mt @ 0.44% Cu.
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 3/5/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Mpodozis, C. and Cornejo, P., 2012 - Cenozoic Tectonics and Porphyry Copper Systems of the Chilean Andes: in Hedenquist J W, Harris M and Camus F, 2012 Geology and Genesis of Major Copper Deposits and Districts of the World - A tribute to Richard H Sillitoe, Society of Economic Geologists, Denver, Special Publication 16, pp. 329-360|
Perello, J, Brockway H and Martini R, 2004 - Discovery and geology of the Esperanza porphyry copper-gold deposit, Antofagasta region, northern Chile: in Sillitoe R H, Perello J and Vidal C E (Eds.), 2004 Andean Metallogeny: New Discoveries, Concepts and Updates Society of Economic Geologists, Special Publication 11, pp 167-186|
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