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Cerro Colorado |
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Panama |
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Cu Mo Ag Au
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Super Porphyry Cu and Au
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The Cerro Colorado porphyry copper deposit is located in northeastern Chiriqul Province, in the Cordillera Central of western Panama, 50 km west of the city of Santiago, 20 km NE of the Pacific Ocean coast and 235 km WSW of Panama City (#Location: 8° 00'N, 81° 47'W).
Tectonic Setting
Cerro Colorado lies within the Chorotega crustal block in the western half of the east-west elongated, ~900 x 300 km Panama microplate, which is a tectonically active area bounded by four separate lithospheric plates. To the east, the Panama Plate adjoins the South American continental plate along a north-south suture, just over the border in western Colombia. To the north, the Caribbean oceanic plate has been subducted below the Panama Plate, with associated north vergent thrust faulting accommodating a component of the convergence. This zone of thrust imbrication and subduction swings SW in the west to become a high-angle strike-slip fault that crosses the isthmus to the Pacific coast. To the south, the Panama plate is bounded by the two contiguous Cocos and Nazca oceanic plates to the west and east respectively. These two plates are separated by a north-south transform that forms a triple junction with the NW-SE boundary between the Cocos and Panama plates, and the east-west boundary between the Nazca and Panama plates. The north-south oceanic transform accommodates the dextral differential movement between the two oceanic plates. The Cocos Plate in the west, has been obliquely subducted northeastward below the Chorotega crustal block in the western half of the Panama Plate. The Nazca plate is separated from the Panama Plate by an east-west sinistral transform faulted South Panama Deformed Zone as it moves eastward parallel to this fault, to be subducted below the South American Plate to the east (Kellogg and Vega, 1995; Kents, 1975; Clark et al., 1977).
The northwestern Caribbean plate includes the Chortis Block of Precambrian continental basement, in what is now Nicaragua and northwestern Costa Rica, which was displaced from the southwestern margin of Mexico by Eocene to Miocene, NW-concave, sinistral strike-slip faulting to be emplaced on the southern to southeastern tip of the Mexican peninsula which is on the southern margin of the North American plate (Mann et al., 2007). This block was bounded to the southwest, south and east by oceanic crust of the Caribbean plate, flooring the Caribbean sea. To the south and west, the oceanic crust comprised the large, thick, oceanic plateau basalts of the Caribbean Large Igneous Province (CLIP), which was formed in the Pacific Ocean between 139 and 69 (mainly from 95 to 88) Ma (Coates et al., 2004; Hoernle et al., 2004; Hradecky, 2011).
By 20 Ma, the Middle America Trench along the south-western margin of the North American Peninsula (Mexico) had extended southeast to subduct the Cocos Plate beneath Mexico, the Chortis Block and the southwestern margin of the CLIP to form a continuous, NW-SE oriented volcanic arc over the southwestern margin of the three terranes, which may have commenced as early as 65 Ma. During this period, Mexico and the contiguous Chortis Block were emergent, but the arc segment developed on the CLIP was at depth, comprising a bathyal submarine volcanic arc developed in a Pacific setting, distant from the continental margin of Mexico. The oldest rocks of this arc now exposed on the northern side of the current isthmus are quartz diorites, granodiorites and basaltic andesites, through dacites to rhyolites, while to the south, the equivalent basement is an accreted suite of dolerite, pillow basalt and radiolarian chert deposited at abyssal depths. These are overlain by precollision Eocene to lower Miocene arc-related rocks, that consist of 4000 m of pillow basalts and volcaniclastic rocks, and biogenic calcareous and siliceous deep-water sedimentary rocks. During this period, as the Caribbean plate and the arc on its western margin rotated eastward, North America and the contiguous Chortis Block on the Caribbean Plate, were separated from South America by ocean, with the Caribbean Plate subducting eastward below the western margin of the South American Plate (Coates et al., 2004).
The precollision Late Cretaceous to middle Miocene open marine units are separated from the overlying syn-collisional shallower neritic sequence of middle to late Miocene age by a regional unconformity at 14.8 to 12.8 Ma. From ~12 Ma, the leading edge of the submarine volcanic arc and the southern margin of the CLIP had collided with the South America Plate, and sections of the arc had become emergent as a string of islands, although there was still a seaway between the Pacific Ocean and Caribbean sea. Another regional unconformity at ~7.1 Ma marks the final stages of collision and emergence, and there are no marine sediments younger than 4.8 Ma, indicating closure of the seaway and complete emergence of the isthmus. By this stage, the current Panama microplate and its tectonic architecture described above was established, continuing to the present (Coates et al., 2004; Kents, 1975). The Chorotega crustal block is the section of the isthmus composed of late Cretaceous to recent volcanic arc rocks, constructed on a basement of late Cretaceous to Palaeocene oceanic crust and marine sedimentary and volcanic rocks of the Caribbean Large Igneous Province, that is located to the east of, and contiguous with, the Chortis Block (Coates et al., 2004).
The ocean plateau basalts and related rocks of the Caribbean Large Igneous Province that form the basement to the Chorotega crustal block in the Panama microplate, were accreted to northwestern South America to the east and SE where they are basement to the Cordillera Occidental which stretches from western Ecuador, through western Colombia and Venezuela, bounded to the east by the Romeral Fault suture (Hauff et al., 2000).
The island arc sequence in the Chorotega block consists of several distinct pulses of volcanism, including Palaeocene to Eocene, mid-Oligocene, late Oligocene to early Miocene, and Pliocene-Pleistocene, most likely separated by times of plate reorganisation (de Boer et al., 1995). The older magmatic suite (66 to 42 Ma) comprise highly deformed basalts, and basaltic andesite flow sheets with rare pillow lavas and volcaniclastic sequences which are tholeiitic, with no recognised porphyry mineralisation related to the associated intrusions (Kesler et al., 1977). The younger andesites and dacites (36 to 29 Ma) are calc-alkaline, as are the more recent rocks associated with a series of stratovolcanoes dated at 21 to 5 Ma (de Boer et al., 1998, 1991), and contain porphyry mineralisation ranging from Oligocene (e.g., the Mina de Cobre Panama deposits 32 Ma) to Pliocene (Cerro Colorado 5 Ma). The Middle Oligocene rocks of the Chorotega block include the 400 km2 Petaquilla batholith, which ranges from gabbros to hornblende granites. The more northerly location of the batholith relative to the axis of the older arc suggests a flattening of subduction (de Boer et al., 1995). Miocene and younger plutonic rocks become progressively more felsic and calcalkaline, with an increase in K2O, corresponding to the evolution of the volcanic arc with time (Kesler et al., 1977). Normal arc magmatism terminated at ~5 Ma due to the collision of a series of aseismic ridges on the Cocos Plate with the developing and emergent Panama microplate (Worner et al., 2009).
Cerro Colorado deposit
The Cerro Colorado deposit is hosted by the 5.9 Ma (Clark et al.,1977) composite high-level Rio Escopeta Granodiorite Pluton, which comprises an older equigranular and a number of younger porphyritic phases, intruding section of a thick, ~30 Ma sequence, largely composed of andesite flows and fossiliferous volcaniclastic sediments (Clark et al.,1977). The Rio Escopeta pluton occurs as a series of isolated exposures over a limited area of ~10 to 20 km across within the largely volcanic sequence that dominates this east-west trending part of the isthmus, and may comprise the upper extremities of a more extensive batholith represented by limited outcrops distributed across the Caribbean side of the isthmus. The surrounding andesites are upper Oligocene in age, black to greenish black, uniformly aphanitic, with some porphyritic sections, and have been subjected to propylitic (epidote-calcite-chlorite-pyrite) alteration (Kents, 1975).
In the Cerro Colorado deposit area, these andesites are cut by the ore-related intrusions, the contacts of which are very irregular, with a number of 40 to 200 m andesite xenoliths occurring within the intrusive masses. Kents (1975) recognised a number of phases of intrusive rocks, confined to a 2 x 3 km area in the immediate deposit area, largely only exposed in creek beds. They fall into two main categories: rhyolites and quartz-monzonites. The rhyolites are characteristically lighter coloured with larger amounts of free-quartz and potash-feldspar, while the principal components of quartz-monzonites are sodium and lime feldspars (Kents, 1975).
The main intrusive phases recognised by Kents (1975), in order of emplacement, are:
Ore-porphyries, which are rhyolitic and the earliest mineral-bearing intrusions. They contain ~46% free-quartz and ~28% K feldspar, and wherever present, are uniformly mineralised with barely-visible, minute sulphides, mainly bornite. The ore-porphyries occupy an ellipsoidal mass with an upper lobate surface, such that only three relatively minor protrusions (the largest 600 x 100 m in an incised valley) outcrop, although in the southwestern section of the ore body, a few hundred metres below the surface, the main mass has dimensions of 600 x 450 m. The andesitic country rock was mineralised within 100 m of the ore porphyry contact.
Quartz-monzonites, which have been dated as Pliocene, at 7.2 Ma (K/Ar), and host the bulk of the mineralisation. At surface, they occur as bodies with dimensions of 200 to 1500 m across, distributed around and within the periphery of the mineralised zone, but at depth coalesces to form the main intrusive mass with dimensions of ~3500 x 1500 m, which surrounds the smaller ore porphyry masses (block diagram in Kents, 1975). This group of intrusions comprises a range of rock types that have a wide distribution, but abrupt and irregular compositional and textural variations that make correlation over short distances (~10 to 50 m) difficult. Kents (1975) suggests a possible interpretation that rocks of slightly different composition and texture were drawn concurrently through separate conduits into the same chamber, to be mingled by magmatic turbulence, but not intimately blended. They have been divided into three overlapping series, based on their Na feldspar content, namely,
i). Granodiorite and felsite in which the Na feldspar content is relatively higher, dominantly occurring as a large pluton to the northeast of the cluster of smaller mineralised intrusions of the deposit area. In the deposit area, these rocks may be locally represented by conspicuous microgranular felsites, that are massive, tight and tough rocks which resemble jasper in appearance. A feldspar-porphyry variety of these felsites resemble the ore-porphyries, such that a visual distinction between the two is not always possible, although there is a difference in chemical composition and habits. While the ore-porphyries occur a single coherent body, the feldspar porphyries are heterogeneously intertongued with the latites. Kents (1975) suggests that, although compositionaly similar, the feldspar porphyries are older than the main granodiorite pluton to the east.
ii). Quartz-monzonite with intermediate amounts of Na feldspars, ranging from a normal medium-grained phase, through porphyritic to latitic quartz-monzonite porphyries, all of which are recognised by their various shades of greenish colour, due to the mostly chloritised pyroxene (up to 20%) they contain. They are also characterised by feldspar phenocrysts that vary from 6 mm, down to a minute fraction of a mm. The quartz-monzonites grade into light grey to whitish cream quartz-feldspar porphyries, containing up to 5% well-developed bipyramidal quartz phenocrysts in a matrix that is normally uniform, in part granulitic, or finely flaky; and
iii). Latites - feldspar porphyries with relatively low Na feldspars. Kents (1975) and Clark et al., (1977) suggest that while these latites are intermingled with the main quartz-monzonite series, there are younger phases up to 800 m across, mainly in the northern margin of the same peripheral annulus intruding quartz-monzonite series rocks.
Quartz-porphyries, that are exposed as an 800 x 1000 m east-west elongated ovoid in the centre of the deposit, that does not persist to a depth of more than a few hundred metres below the general surface, before passing into the main quartz-monzonite intrusion. These rocks have a rhyolitic composition and texture, and a light buff to cream colour. Characteristic components are up to 5 vol.% glassy quartz phenocrysts, many of which have a well-developed bipyramidal habit, and small (~1 mm) feldspars that blend into the rock matrix. The various varieties of quartz porphyry have irregular outlines, and to the west have invaded and displaced the ore-porphyries.
Rhyodacites, which have a light whitish to cream colour, and have a composition very similar to the quartz-monzonites. They have a very erratic distribution, occurring as post-ore dykes cutting the mineralised phases in the central part of the deposit, where they comprise ~12% of the drilled volume, and represent un-mineralised waste. Their centre of distribution broadly coincides with that of the quartz-porphyries, within which they average as much as 30 vol.%, gradually decreasing outwards, with only a few extending into the wall-rock andesites.
Trachyandesites are the youngest consolidated magmatic rocks at Cerro Colorado, occurring as light coloured, porous, extrusive rocks, rich in sodium-feldspar and low in free-quartz. Originally they covered the deposit area, but now only overlie it to the southwest and south.
Interpretation of hydrothermal alteration is divided in the available literature. Kents (1975) argues that, well-defined shells of hydrothermal alteration assemblages cannot be recognised at Cerro Colorado, but that the alteration pattern is spotty and essentially reflects the distribution of different rock types and phases of those rocks. An initial carbonate-chlorite assemblage that resulted in 0.4 to 0.7% Cu in the ore porphyries and ~0.05% in the quartz porphyries and felsites, is interpreted to be overprinted by a second phase of copper sulphide emplacement as further disseminations and veinlets, accompanied by fracture-controlled silicification, and occurs throughout the quartz monzonite series. This alteration is weaker on the margins of the main intrusion. Fracture controlled alteration of the surrounding andesite wall-rocks occurs as thin sericite-chlorite films that may extend for up to 100 m from the intrusive contact, while pervasive alteration of the same andesite is only seen for up to 30 m from the same contact. The most intense phyllic alteration (sericite-carbonate) is exhibited by the post-ore rhyodacite dykes (Kents, 1975).
However, Issigonis et al., (1974), suggest a potassic core, represented by K feldspar in the mineralised porphyries and K feldspar-phlogopite in the wall-rock andesites, occurs at depths of >700 m, surrounded by a sericite-calcite zone, which is in turn enveloped by an extensive sericite zone. Locally an illite zone surrounds the sericite, while an extensive propylitic halo is evident in the surrounding andesites.
According to Nelson (1995), propylitic alteration (epidote-calcite-chlorite-pyrite) affects the andesites and to a lesser extent the granodiorite, while phyllic alteration (quartz-sericite-pyrite) in the immediate vicinity of the deposit is locally referred to as 'latite porphyry' (representing weak phyllic alteration) and feldspar porphyry (strong phyllic alteration with remnant feldspar phenocrysts replaced by sericite). Raynolds (1983) has also suggested that early feldspathic alteration has been destroyed by the phyllic assemblage. Anhydrite is present as a minor component of the phyllic assemblage at depth.
This preceding implies that some of the intrusive phases of Kents (1975) reflect alteration assemblages overprinting and modifying intrusive protoliths. Most likely there was an early potassic phase accompanying the main copper mineralisation reflected by the 'ore-porphyry' remnants, passing out and upwards into propylitic alteration within the outer quartz-monzonite intrusion and andesite wall-rocks, although the latter may in part be a regional pre-ore diagenetic alteration of the andesites. Subsequent overprinting by a destructive phyllic phase in the upper parts of the deposit, produced a core of silica-sericite-pyrite that affected most of the deposit and obliterated much of the initial alteration and mineralisation fabric, except for deeper in the system where the potassic assemblage remained. The phyllic phase altered the quartz-monzonite series to a core of 'felsite/feldspar porphyry', an outer, more weakly altered 'latite' and an upper 'quartz-porphyry', with the more intense alteration zones producing the rhyodacite 'dykes'. This phyllic alteration would have introduced pyrite and possibly further copper sulphides, while redistributing mineralisation from the initial potassic stage. Late stage illite would be seen in the upper parts of the phyllic alteration halo. Anhydrite in the upper parts of the phyllic zone would have been leached, but remained in fractures at depth.
The principal hypogene metallic minerals consists of chalcopyrite, molybdenite and pyrite, occurring as small disseminated grains in the weakly phyllic altered 'latite porphyry' and as larger grains in the strongly phyllic feldspar porphyry assemblage. Fine, 10 µm bornite is uniformly disseminated throughout the ore porphyry, and has resulted in an unusually uniform distribution of grade within that rock type. Raynolds (1983) distinguished five episodes of veining associated with hypogene mineralisation: i). barren quartz veins, which begin the paragenesis, followed by ii). quartz-chalcopyrite-pyrite, iii). quartz-sericite, iv). quartz-sericite-sulphide, and v). massive sulphide veins. Homogenisation temperatures range from 250 to 450°C. Some of the best mineralised portions of the deposit occur within hair line cracks cutting the andesitic country rock, within 100 m of the contact with the porphyry, resulting in the best mineralised portions at Cerro Colorado. Two episodes of hypogene sulphate and carbonate veining postdate this mineralisation, which is in turn cut by post-mineralisation dykes of rhyolite and rhyodacite.
Apparently un-altered quartz monzonite from deep in the system has been dated at 7.2±0.5 Ma (Geochron Labs.). Biotite from the post-mineralisation dykes has been dated at 4.2 Ma (Raynolds, 1983). Unaltered trachyandesite flows dated at 2.5 Ma overlie the deposit (Clark et al., 1977).
Supergene leaching and enrichment, produced an immature chalcocite blanket in which conversion of primary sulphides is incomplete, with ~65% of the copper in the secondary blanket present as supergene sulphides (Galay, 1980), producing a secondary copper grade of 0.57% Cu. The balance of the copper remains as un-replaced primary chalcopyrite (0.31% Cu). The secondary sulphide minerals in the enrichment blanket include roughly equal proportions of chalcocite and covellite, plus minor digenite. The preserved leached and oxidised cap is as thick as 150 m in less eroded areas. Within the supergene blanket, there is significant variability in the thickness (3 to 111 m, averaging 35 m) and grade (0.17 to 2.35% Cu, averaging 0.88%) of the blanket. Enrichment factors are reported to be 1.52x for Copper, 1.02x for molybdenum and 1.23x for silver (Nelson, 1995). The supergene enrichment probably occurred during the interval 4.21 to 2.98 Ma, during the Pliocene erosion of the intrusive complex and of any originally overlying volcanic edifice, prior to the deposition of the trachyandesites at 2.5 Ma.
Estimates of the total resource figures vary, as follows:
USGS, 2008 - 3.73 Gt @ 0.39% Cu, 0.015% Mo, 0.075 g/t Au, 5.2 g/t Ag (although none of the sources cited quote this tonnage);
Nelson, 1995 - 1.3 Gt @ 0.76% Cu, 0.010% Mo, 5.1 g/t Ag, 0.08 g/t Au, at a cut-off grade of 0.4% Cu;
Hargreaves, 1974 and Kents, 1975 - >3 Gt @ 0.6% Cu.
Estimates of the supergene resource include:
70 Mt @ 1.11% Cu, (TexasGulf, 1974),
54 Mt @ 0.85% Cu, (Galay, 1980), and
60 Mt @ 0.88% Cu, (Burgos, 1992).
Much of the information in the description of the Cerro Colorado deposit is sourced from Kents (1975), AIME Reprint 75-S-2, available from the AIME and related websites only.
The most recent source geological information used to prepare this decription was dated: 1995.
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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Clark A H, Farrar E and Kents P, 1977 - Potassium-Argon age of the Cerro Colorado porphyry copper deposit, Panama: in Econ. Geol. v.72 pp. 1154-1158
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Kesler S E, Sutter J F, Issigonis MJ, Jones L M and Walker R L, 1977 - Evolution of porphyry copper mineralization in an oceanic island arc; Panama : in Econ. Geol. v.72 p. 1142-1153
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Nelson, C E, 1995 - Porphyry copper deposits of southern central America: in Pierce F W and Bolm J G (eds.), Porphyry Copper Deposits of the American Cordillera, Arizona Geol. Soc., Digest 20, pp. 553-565
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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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