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El Cobre |
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Cuba |
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Cu Au Zn Pb Ag
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Super Porphyry Cu and Au
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The El Cobre copper deposit is located 13 km NW of Santiago de Cuba, and ~100 km west of Guantánamo Bay on the southern coast of SE Cuba.
The deposit is located in the Sierra Maestra Paleogene volcanic arc of the southeastern part of Cuba, and is the oldest copper mine in the Americas. The deposit was discovered in 1530 and mined until 1544 (Ansted 1856). During five centuries of activity, >1 Mt of ore @ >14% Cu, and > 2 Mt @ >3% Cu have been extracted. During colonial times (up to 1898), vein-type mineralisation was exploited by underground methods to a depth of up to 300 m. From the 1970s, open pit mining has operated at the Mina Grande and Mina Blanca sectors, although the Cu vein mineralisation continues more than 1 km eastwards (Gitanilla sector). In the 1960s, a series of deep drill holes tested the flanks of the deposit (Barita Occidental, Barita Oriental, Mina Nueva and Melgarejo sectors) leading to the discovery of stratabound polymetallic mineralisation (Cazañas et al., 1998), with gold-bearing polymetallic ores carrying up to 25 g/t Au. in 1998, the copper mine was closed, despite known reserves of >2 Mt @ >1% Cu.
The El Cobre deposit is located in eastern Cuba, within the volcanosedimentary sequence of the Sierra Maestra Paleogene arc. This arc formed as a result of the collision of the eastern Cuban microplate with the North American plate, coinciding with the development and activation of the North Caribbean transform fault, during the Oligocene, which was reactivated during the Miocene, resulting in the modification of the overall structure of Sierra Maestra by wrenching and producing the present southern slope of the range (Rojas-Agramonte et al., 2005; 2006).
The basement to the Sierra Maestra comprises Cretaceous arc-related rocks, unconformably overlain by widespread Paleogene volcanic arc rocks, that are overlain in turn by Middle to Upper Eocene sedimentary rocks of the Puerto Boniato and San Luis Formation (Iturrale-Vinent 1996; Cazañas et al., 1998; Kysar 2001; Rojas-Agramonte et al., 2004; 2005; 2006). The Paleogene volcanic island arc, which developed between the Paleocene and the Early Middle Eocene, is represented by the sequence of >4000 m of volcanic rocks that is the El Cobre Group (Iturralde-Vinent 1996, 1998; Cazañas et al., 1998; Rojas-Agramonte et al., 2006).
The El Cobre Group comprises of volcanic (lavas, pyroclastic flows, subvolcanic sills and their feeder dykes) and volcaniclastic rocks with intercalations of epiclastic material and fossiliferous limestones (Iturralde-Vinent 1996; Cazañas et al., 1998; Kysar 2001; Rojas-Agramonte et al., 2006). The lower and middle sequences of the El Cobre Group, are low-K island arc tholeiites (IAT) (Cazañas et al., 1998; Kysar 2001; Rojas-Agramonte et al. 2006). The arc successions were intruded by calc-alkaline, low- to medium-K tonalites and trondhjemites, typical of intra-oceanic arcs, during the final stages of subduction (Kysar 2001; Rojas-Agramonte et al., 2004). The El Cobre Group is interpreted to have formed in the axial zone of the volcanic arc (Iturralde-Vinent, 1996; 1998), and passes laterally into the Pilón and Caney formations.
The El Cobre Group is divided into three sequences (Méndez, 1997):
The Lower Sequence - mainly thick andesite-basalt flows, interbedded with several hundred metres of thick intermediate to basic tuffs (Cazañas et al., 1998).
The Middle Sequence - which begins with andesite-dacite and conglomerates, passing up into several overlying cycles of explosive volcanism, each consisting of pyroclastic deposits of coarse to very coarse grain size, indicating a proximal source. These cycles are followed by limestone olistostromes, which may be coeval with the emplacement of intermediate to acid lavas. The most important volcanic cycle is close to the base of the sequence, and consists of 300 m of very coarse-grained pyroclastic rocks with >15 mm diameter fragments. This cycle is followed by another, also 300 m thick, composed of finer-grained pyroclastic rocks,but with similar sized with clasts. This unit is capped by limestone olistostromes, and locally by a stratabound zone of anhydrite, barite and Mn oxides mineralisation. Most of the El Cobre deposit is hosted by rocks of the second cycle (Cazañas et al., 1998).
The Upper Sequence - which unconformably overlies the middle sequences, commencing with thick conglomerate beds, which pass vertically into other distal explosive volcanic cycles. Explosive cycles in this upper sequence comprise several tens of metres-thick beds of fine-grained breccias, followed by similarly thick beds of tuffs, and by ash tuffs at the top. Epiclastic components and olistostromic limestones increase towards the top of the series, suggesting the progressive fading of Paleogene volcanism and the subsequent erosion of the volcanic structures (Cazañas et al., 1998).
The El Cobre mineralisation is hosted by volcaniclastic rocks in the upper sections of the of the Middle Sequence, close to the unconformity with the Upper Sequence. Within the vicinity of the deposit, the Middle Sequence comprises coarse-grained pyroclastic rocks, where metre-sized clasts are common at the base of the sequence, while the upper part consists of an interbedding of several tens of centimetre-thick beds of ash tuffs, lapilli tuffs and fine- to medium-grained epiclastic rocks. The pyroclastics vary from andesite-dacitic at the base, to rhyolitic at the top, while two discontinuous, olistostromic reefal limestones, each as much as several tens of metres in thickness, are found interbedded with the pyroclastic rocks at the top of the sequence, possibly representing tectonic instability in the Eocene basin. All of the previous units are also cut by dykes of volcanic rocks, also ranging from andesite-basalt to rhyolite (Cazañas et al., 1998).
The ore bodies are distributed along the >40 km long El Cobre fault that is characterised by cataclastic textures within the wall rocks, and is linked to an east-west fault system that generated two sets of associated joints trending SW and lesser NW. Quartz and copper veining, and hydrothermal alteration are associated with the El Cobre fault, along which a late andesite-basalt dyke has also been intruded, cutting the vein mineralisation. Middle Eocene limestones from Sierra de Boniato in the east post-date this fault, implying the structure and the accompanying fault systems were initiated during the early stages of the volcanic arc (Pérez and García 1997) and influenced the development of later fissure volcanoes. A late Eocene north-south oriented strike-slip fault system, with displacement of several tens of metres, produced a tilted block structure (Pérez and García 1997; Rojas-Agramonte et al., 2005) and influenced the thickness and concentration of ore bodies. The central sections of the deposit, Mina Blanca and Mina Grande, are located on a raised block, and represent the thickest (up to 24 m) development of ore (Cazañas et al., 1998).
Three main styles of mineralisation are distinguished in the El Cobre deposit:
i). Stratiform lenses of manganese oxides, barite and anhydrite, located within an east-west trending belt to the north of the El Cobre fault. Lenses are typically ~1.7 km long and ~100 m wide. The Mn-oxide lenses outcrop over length of ~1 km and are up to 4 m thick, comprising an up to 1.5 m thick basal unit of interbedded jasper and scarce botryoidal cryptomelane and hematite, overlain by a 1 m thick celadonite-hematite bed, followed by a Mn-rich unit of interbedded Mn-silicates (mainly macfallite), chert and pyroclastic rocks cemented by cryptomelane, and capped by 20 cm of massive cryptomelane. Mn-oxides represent the stratigraphically highest level of mineralisation.
An up to 100 m thick unit of bluish, nodular (regular or elongated, several mm to several cm in diameter) anhydrite, interbedded with altered pyroclastic rocks. Near the base of this anhydrite unit, nodules are rimmed by euhedral quartz aggregates with montmorillonite and euhedral pyrite, while a second generation of fine-grained replacement anhydrite is also evident, followed by the replacement of all textural types of anhydrite by several generations of gypsum. Quartz and pyrite veins crosscut this unit, being more frequent lower in the sequence (Cazañas et al., 1998);
Two lenses of barite are found in the Barita and Melgarejo sectors. The first is 4 m thick and nodular, situated at the footwall of the deposit, it is crosscut by quartz-sericite-pyrite veins. The second occurs within the anhydrite-rich unit, near the hanging wall of the deposit and is 3 m thick, with a massive or banded structure and foliated texture, with associated sphalerite, chalcopyrite and minor galena, although euhedral or framboidal pyrite is the most abundant sulphide (Cazañas et al., 1998).
ii). Disseminated, stratabound (polymetallic Zn-Cu-Au mineralisation) - comprising three units (Cazañas et al., 1998);
Lower stratabound mineralisation - where 5 orebodies, each several tens of metres long and 4 to 8 m thick, occur in an area of 700 x 400 m in the west of the Mina Blanca sector where they replace olistostromic limestones. Anhydrite and quartz replace calcite, while anhydrite is, in turn, replaced by hematite, which is pseudomorphed by magnetite and, this, eventually, by pyrite. Close to the El Cobre fault, mineralisation consists of massive micro- to phanerocrystalline quartz aggregates with associated chalcopyrite, pyrite, minor sphalerite and Fe-rich chlorite. Kaolinite commonly occurs as inclusions within quartz. Minor amounts of hessite (Ag2Te), tetradymite (Bi2Te2S) and tellurobismuthite (Bi2Te3) are associated with chalcopyrite (Cazañas et al., 1998).
Intermediate stratabound mineralisation - located several metres above the previous unit, comprising a partly silicified, discontinuous carbonate unit several tens of metres in thick with local Zn-Pb sulphide mineralisation.
Upper stratabound mineralisation - comprising a 1700 m long, 90 m thick, polymetallic Zn-Cu-(Pb) layer (with low Cu/Zn ratios), located to the north of the El Cobre fault, and directly below the anhydrite deposit near the surface. Three zones can be distinguished within the body, from the top down: i). contact zone with the upper anhydrite stratiform deposit, where microcrystalline quartz, montmorillonite, anatase, pyrite and other sulphides replace anhydrite. Pyrite (locally massive) is the most abundant sulphide, and is usually corroded by sphalerite I and chalcopyrite. Sphalerite I is replaced by chalcopyrite and a late sphalerite II. Chalcopyrite locally co-crystallises with bornite and chalcocite; ii). silicification zone which consists of quartz with disseminations of chalcopyrite, sphalerite, pyrite and gold. Quartz, pyrite and sphalerite crystals contain remnants of strongly corroded anhydrite crystals. Gold occurs interstitially among quartz grains or overgrowing early-formed pyrite crystals, and it is sometimes associated with hessite (Ag2Te) and chalcopyrite in small veins that crosscut pyrite (Cazañas et al., 1998); and iii). basal zone which is roughly tabular and several metres thick, consists of strongly kaolinised, sericitised and silicified volcanosedimentary rocks containing sphalerite, pyrite and chalcopyrite (Cazañas et al., 1998).
A mesh of vuggy quartz+chalcopyrite±sphalerite veins, each up to several cm thick, cuts all the above-mentioned units. These veins contain several generations of sphalerite and chalcopyrite, with associated intergrown sericite and kaolinite, and are rich in gold (grains of >100 µm in size, associated with quartz or pyrite). Galena, small native gold grains up to 10 µm in diameter, and calaverite (AuTe2) rimmed by hessite, all occur as inclusions in late sphalerite. A final stage of discrete quartz, calcite and anhydrite veins crosscut all previous associations and infill vuggy porosity (Cazañas et al., 1998); and
iii). Vein mineralisation, which is copper-rich with parallel veins at depth and stockwork development closer to the surface. From west to east, vein mineralisation is continuous along the El Cobre fault system over an area about 1200 x 140 m, from Mina Blanca, which occurs within a SSW-ENE fault, that intersects an east-west structure 400 m to the ENE at Mina Grande, where veining is best developed, and then follows that second structure eastward through Gitanilla. A separate, sub-parallel NE-SW oriented satellite fracture 600 to 800 to the SE of the fault hosting Mina Blanca contains Mina Alta. Minimum known depths of mineralisation are 500 m at Mina Grande and Mina Blanca sectors and 200 m in the Gitanilla sector. At Mina Alta, veins up to 2 m wide are located over an interval of 270 m and to a depth of 130. All have a dip of 80 to 85°S and contain >0.7% Cu ore (Cazañas et al., 1998).
Veining and stockworks are zoned, and may be siliceous near the El Cobre faults progressing to an anhydrite-epidote composition towards the flanks of the deposit. Individual veins are from one to several tens of centimetres in thickness and several hundred metres in length. Deeper, subparallel veins pass upward into siliceous and anhydrite stockworks (adjacent to and further from the fault respectively) containing chalcopyrite and pyrite, with very wide high-grade veins in the transition zone. The stockworks extend over lengths of several hundred metres and comprise veins up to 1 m wide, of variable orientation, although the prevailing direction is the same as the fault (Cazañas et al., 1998).
On the basis of the zonation described above, four vein types are recognised: i). Siliceous subparallel veins - comprising vuggy infill of fine-grained quartz, grading to larger crystals rich in sericite or chlorite inclusions with associated sulphides. The remaining porosity is filled by anhydrite or late-formed calcite. Towards the top of the vein, chalcopyrite is associated with small amounts of hessite (Ag2Te), coloradoite (HgTe), tellurobismuthite (Te2Bi3) and tetradymite (Bi2Te2S); ii). Anhydrite-epidote veins - which comprise cryptocrystalline quartz, followed by radial aggregates of microcrystalline quartz with chlorite, calcite and zoned epidote. Epidote is occasionally associated with pyrite, chalcopyrite and sphalerite. Anhydrite or a late generation of calcite fills the remaining vein porosity. Contacts among these minerals do not show evidence of replacement; iii). Siliceous stockwork - comprising quartz veins accompanied by pyrite and chalcopyrite. Where sulphide-rich, these veins have a symmetric, several centimetres thick, banding of vuggy quartz growths with interstitial pyrite and chalcopyrite. In upper levels, sericite growths on quartz crystals include remnants of anhydrite, calcite, hematite and rutile, while the sericite is corroded by later kaolinite. Chalcopyrite and pyrite occur in the innermost part of the vein, sometimes no vuggy porosity remaining; iv). Anhydrite stockwork - which has only been found in outcrop in the uppermost section of Mina Blanca. It comprises a dense mesh of anhydrite veining, many of which are sub-horizontal. Crystallisation commences with calcite and anhydrite, with low contents of sphalerite and pyrite, infilling all the vein cavity. A second silicification stage ensues, replacing anhydrite and calcite with quartz accompanied by small amounts of chlorite, apatite and pyrite (Cazañas et al., 1998).
A broad zonation of hydrothermal alteration envelopes the deposit, centred on the El Cobre fault system. The uppermost part of the deposit, immediately adjacent to the fault and vein/stockwork mineralisation is strongly kaolinised around the anhydrite stockwork, especially where that stockwork intersects the stratiform anhydrite and barite bodies, producing cryptocrystalline kaolinite and minor smectite. This zone passes outward and downward into a zone characterised by chlorite, silica and pyrite which is extensively developed close to the siliceous veins wherever they occur, and within the lower and intermediate stratabound bodies. Fine-grained Mg-rich chlorite completely replaces the host rock, and is in turn replaced by Fe-rich chlorite, which is dominant in the uppermost part of the vein system. Pyrite development can be intense, with coarse pyrite crystals formed in the replaced rocks. Silicification results in small quartz veins and fine-grained quartz aggregates pseudomorphing pre-existing minerals in the volcanic host rocks. Sericite is only poorly developed, occurring in the uppermost part of the siliceous vein system, accompanied by pyrite and silica. The alteration progression from oldest to youngest comprises - kaolinitisation, Mg-chlorite+pyrite+silica, Fe-chlorite+pyrite+silica, sericitisation, producing an outward and downward pattern from the siliceous and anhydrite stockworks of the fault system of kaolinite, Fe-chloritisation, Mg-chloritisation to an outer halo of propylitised volcanic rocks. In contrast, the volcanic wall rocks of the stratiform Mn mineralisation are characterised by green celadonite alteration (Cazañas et al., 1998).
Cazañas et al. (1998) records details of fluid inclusion studies of sphalerite, quartz, anhydrite and calcite which show salinities of between 2.3 and 5.7 wt.% NaCl equivalent and homogenisation temperatures between 177 and 300°C. Sulphides from stratabound mineralisation yield δ34S values of 0 to +6.0‰, in comparison to those from the feeder zones which fall between -1.4 and +7.3‰. An intra-grain sulphur isotope zonation of ~2‰ is evident in sulphides, with δ34S values usually increasing towards the rims. Sulphate sulphur has δ34S in the range of +17 to +21‰, with the exception of two samples with values of +5.9 and +7.7‰. These authors interpret sulphur isotope data to indicate thermochemical reduction of sulphate from a hydrothermal fluid of seawater origin was the main source of sulphide sulphur, and that most of the sulphates were precipitated by heating of seawater. They conclude the structure of the deposit, mineralogy, fluid inclusion and isotope data all suggest the deposit was sourced from seawater-derived fluids with a probable minor magmatic fluid contribution.
This summary is closely based on Cazañas et al. (1998). For more detail and illustrations, geological maps and sections see that paper.
The most recent source geological information used to prepare this decription was dated: 2008.
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.
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Selected References:
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Cazanas X, Alfonso P, Melgarejo J C, Proenza J A and Fallick A E 2008 - Geology, fluid inclusion and sulphur isotope characteristics of the El Cobre VHMS deposit, Southern Cuba: in Mineralium Deposita v.43 pp. 805-824
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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