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La Colosa

Colombia

Main commodities: Au Ag
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The La Colosa porphyry porphyry gold (copper-poor) deposit is located in the Central Cordillera of Colombia, in the department of Tolima, ~8 km NW of Cajamarca, 30 km WNW of Ibagué and 165 km west of Bogota (#Location: 4° 27' 52"N, 75° 29' 28"W).

The Miocene La Colosa deposit is developed over the subduction zone related to the convergence of the Nazca and South American tectonic plates, and is in close proximity to the convergence of the subparallel, regional, north-south to NNE-SSW trending Romeral (to the west), Palestina and Otu-Pericos (to the east) faults, which coalesce to form a single fault zone just to the south of La Colosa.

The Central Cordillera is bound to the east by the Magdalena River valley, and on the west by the Cauca River valley (Cauca-Patía graben; Aspden et al., 1987; Taboada et al., 2000). The Romeral fault system, composed of high-angle NNW- to NNE-trending reverse faults with a dextral strike-slip component, divides the cordillera into two domains, an eastern continental and a western oceanic domain (McCourt et al., 1984; McDonald et al., 1996; Taboada et al., 2000).

The eastern domain of the cordillera belongs to the Cajamarca-Valdivia terrane of the Central Continental Sub-Plate Realm of Cediel and Caceres (2000), or the Tahami terrane of Toussaint (1999). It is composed of pre-Mesozoic polymetamorphic basement intruded by subduction related Mesozoic batholiths and stocks (Taboada et al., 2000). During the Lower Palaeozoic, an allochthonous to predominantly parautochthonous accretionary prism of Ordovician to Silurian age was sutured directly onto the composite Guiana Shield along the trace of the paleo-Palestina fault system, which includes its eastern splay the Otu-Pericos fault, accompanied by Cordilleran-type orogenic deformation and regional metamorphism (McCourt et al., 1984; Cediel et al., 2003; Kennan and Pindell, 2009). This prism is now represented by greenschist to lower amphibolite metamorphic facies pelitic and graphite-bearing schists, amphibolites, intrusive rocks and rocks of ophiolitic origin (Cediel et al., 2003). The margin of the composite Guiana Shield, to the east of the paleo-Palestina - Otu-Pericos fault system is occupied by the exotic metamorphic rocks of the Proterozoic Chicamocha Terrane that was welded to the Amazon craton during ~1300 to 900 Ma, Orinoco orogeny (Cediel et al., 2003).

From the Late Paleozoic to the Early Cretaceous, an intercontinental rift was developed over most of the Central Continental Sub-Plate Realm (Cediel and Caceres, 2000) accompanied by deposition of marine and continental cover sequence and the emplacement of numerous intrusions (Aspden et al., 1987; Cediel et al., 2003). The extensional regime was terminated by the Aptian-Albian, followed by a transpressive regime up to the Recent (Cediel et al., 2003).

The western domain of the cordillera has been included into the Western Tectonic Realm of Cediel et al. (2003), or the Calima terrane of Toussaint (1999), and is part of the Caribbean Large Igneous Province oceanic plateau that extends to the south into Ecuador, and to the north into Panama and Venezuela and underlies the Caribbean Sea. It was formed by the progressive accretion of a series of thick oceanic plateau terranes that were accreted to, rather than subducted below, the South American plate. This followed on from a reorganisation of drift direction and velocity between the oceanic Farallon and South American plates in the early Cretaceous resulting in the oblique collisions and partial subduction or obduction of these oceanic terranes from the SW (e.g., Pindell and Kennan, 2001, 2009; Cediel et al., 2003; Kennan and Pindell, 2009). A number of these episodes of oceanic crustal accretion have occurred since the late Lower Cretaceous (Albian), the first along the Romeral-Cauca fault zone (McCourt et al., 1984; McDonald et al., 1996), and then along a series of major, parallel, dextral faults developed progressively to the west of each new accreted terrane slice (Cediel et al., 2003). These terranes formed the western part of the Central Cordillera and the Western Cordillera and have been affected by periods of uplift, erosion, and subduction-related calc-alkaline magmatism (Cediel et al., 2003). While these exotic terranes were being accreted between the mid Cretaceous and Paleocene, a series of granitic batholiths were emplaced in both the Western Tectonic Realm and the Cajamarca-Valdivia terrane to the east, mainly focused on major longitudinal fault zones.

La Colosa lies within the eastern domain of the Central Cordillera, to the east of the Romeral and Palestina fault zone.

A series of intermediate intrusions were emplaced during the Cenozoic, mainly concentrated along the Palestina and Romeral faults, affecting the rocks on either side of the Central Cordillera. Quaternary calc-alkaline volcanism was also developed within the same region, including the volcanic terranes of the Nevado del Ruiz, Nevado del Tolima and the Cerro Machin volcanoes, which are all part the Central Cordillera volcanic arc that resulted from the subduction of the Nazca plate beneath the South American plate. Regionally, the Cajamarca-Valdivia terrane includes a basement of Palaeozoic metamorphic rocks, mainly schists, intruded by Jurassic to Paleocene quartz diorites and granodiorites, and by Neogene andesitic to dacitic hypabyssal rocks with associated porphyry mineralisation.

The La Colosa deposit is part of a cluster of gold-bearing, late Miocene calc-alkaline porphyritic diorite-granodiorite intrusions that outcrop within an area of ~20 km2 to the north and northwest of the town of Cajamarca and the point of the northward divergence between the major regional Palestina and Romeral fault systems.

The following rocks outcrop in the La Colosa deposit area:
Palaeozoic metamorphic rocks of the Cajamarca Complex, which comprise alternating packages of black graphitic schists, quartz-sericite schists, calcareous green schists and chlorite-actinolite schists, plus some quartzite and thin marbles and are exposed mainly in the SW part of the deposit area. Neogene granitic intrusions (diorites, quartz diorites and dacites) have locally transformed the metamorphic sequence in hornfels. The foliation in the metamorphic rocks predominantly trends of north-south, with sub-vertical dips. Metamorphic rocks and hornfels enclose the bulk of the mineralised porphyries.
Neogene hypabyssal intrusions and intrusive breccias, the mineralised intrusive complex contains three early and two intermineral diorite porphyries that are cut by a late intermineral granodiorite porphyry (Lodder et al., 2010), and include (after Garzon, 2011)
• Early and inter-mineral diorite composite stock - medium crowded diorite; coarse diorite porphyry; intrusion breccia with coarse diorite porphyry clasts; two fine diorite porphyry pulses; a coarse diorite porphyry; and an intrusion breccia. The early diorite porphyries are altered to potassic and sodic-calcic mineral assemblages, and cut by multiphase stockwork veinlets and with disseminated mineralisation, are characterised by grades of >1 g/t Au, and average about 200 ppm Cu and 1 g/t Ag, with traces of anomalous molybdenum.
• Late-mineral diorite composite stock - composed of a medium diorite porphyry and a quartz diorite phase. Potassic and sodic-calcic alteration also affects these porphyries, but grades are, on average, <0.4 g/t Au.
• Late-mineral granodiorite stock and dykes - occurring as an ~5 km
2 mapped body of granodiorite to dacite porphyry occupying the NE section of the deposit area, with accompanying series of dykes that are all less than 40 m in thickness but show continuity over at least 600 vertical metres. This porphyry is mostly barren, with only erratic anomalous gold grades, which are all <0.4 g/t, with only weak to moderate propylitic and intermediate argillic alteration.
Quaternary deposits, that cover the rocks described above, and comprise (Garzon, 2011),
• volcanic accumulations, occurring as layers of interbedded ash and pyroclastics with thicknesses in excess of 5 m, and
• colluvium, made up of metamorphic and intrusive rock float, much of which is mineralised.

The early and intermineral diorite porphyries cover a NNW elongated area with a length of ~1500 m and a width of 500 to 700 m, immediately to the SW of the late-mineral granodiorite stock, and around the southern margin of the latter, is contiguous with and to the west of the late-mineral diorite stock. The latter covers an area of ~1 km
2 to the SSE of the late-mineral granodiorite stock. A belt of contact metamorphic schists trends SSE from the southern margins of the early and intermineral diorite and Late-mineral diorite stocks for ~2 km over a width of 400 to 600 m (Lodder et al., 2010).

The various dioritic rocks are mainly composed of plagioclase and biotite, although the biotite is usually secondary. Hornblende has been observed passing into biotite and chlorite, while some have garnet in the cores of the plagioclase. In general the breccias comprise clasts of the different intrusive diorite pulses, while some have metamorphic and hornfels, all set in a matrix of intrusive diorite. Quartz diorite rocks are medium grained and have a porphyritic texture, with plagioclase and sometimes rounded bi-pyramidal quartz. The dacitic rocks have porphyritic textures, and are composed of plagioclase, hornblende, biotite and bi-pyramidal quartz up to 13 mm across (Garzon, 2011).

Both the intrusive and the metamorphic rocks are influenced by NE and NW structural trends. The NE structures include the La Casucha and Casa Vieja faults to the NE and SE of the deposit area respectively. Recent normal movement is apparent on both, possibly related to the regional Palestina fault system. The NW structures include the Belgica and Colosa faults which are on the SW and NE magins of the mineralised system. The cluster of parallel faults distributed over a width of 100 to 200 m faults that are associated with the Belgica fault, have accommodated reverse movement and in part define the contact between the metamorphic sequence and the intrusive rocks. In addition, the different intrusive bodies that make up the Colosa complex have a preferred NNW orientation, suggesting the NW structural trend influenced their emplacement (Garzon, 2011).

The host rocks to the Colosa deposit were hydrothermally altered by fluids that accompanied both the dioritic and granodiorite/dacite intrusions, and show a close temporal relationship with the different pulses. Both the first dioritic, and the last granodiorite/dacite pulse are hydrothermally altered and mineralised, although the granodiorite/dacite intrusive (the most recent) has a lesser intensity of mineralisation.

The hydrothermal fluids formed zones of alteration characterised by (after Garzon, 2011):
Sodic-calcic assemblages, represented by the presence of chlorite, actinolite and albite. The paragenesis of the main alteration and mineralisation commenced with pervasive sodic-calcic alteration overprinted by the potassic stage (see below), which was in turn, cut by a second sodic-calcic event. The first pulse has a patchy distribution, and was subsequently invaded by secondary biotite, while a second pulse was observed in a breccia with potassic altered dioritic fragments within a matrix containing abundant albite and actinolite. The second sodic-calcic alteration clearly overprints the potassic assemblage and is largely confined to irregular, centimetre-scale patches and well defined veinlets. The patches and veinlets contain epidote, actinolite and chlorite, typically with white, 'albite-rich' haloes.
Potassic assemblages, which principally occur as secondary phlogopitic biotite and subordinate K feldspar, mainly as a pervasive replacement of the porphyries, especially the early phases, with associated veining and mineralisation. Biotite occurs disseminated, as nests and early biotite (EB) veinlets. K feldspar is disseminated and as veinlets. Magnetite accompanies the potassic assemblages as disseminated grains and as veinlets (M-type; c.f. Clark and Arancibia, 1995). Sinuous A-type quartz veinlets and quartz veinlets with sutures of pyrite or chalcopyrite (B-type; c.f. Gustafson and Hunt, 1975), and stockwork quartz veinlets accompany the potassic assemblage. Chlorite and epidote are found as disseminations and veinlets, and actinolite occurs as veinlets. Molybdenite accompanies the potassic alteration phase as veinlets, disseminations and in associated with quartz veinlets, while pyrite is disseminated and as veinlets, and chalcopyrite is disseminated, in veinlets and is associated with quartz veinlets. The potassic alteration and accompanying mineralisation occurs mainly in the central part of the deposit affecting the early intrusions, but is also present in the intermediate and late intrusions. In addition, it affects the hornfels, usually occurring as bands of biotite and actinolite, and veinlets of actinolite. This mineral association contains 1 to 7% pyrite; 1 to 5% magnetite and traces of chalcopyrite and molybdenite, with traces of chalcocite, covellite and bornite in fractured secondary minerals.
Intermediate argillic assemblages are only weakly developed and only form mappable zones in the dacite and in the northern limit of the deposit. They are characterised by the association of chlorite-sericite-clay, with sericite replacing biotite and plagioclase, while smectite and illite clays, together with chlorite and epidote replaced plagioclase. The intermediate argillic alteration is accompanied by disseminations and veinlets of pyrite, with some pyrite veinlets having sericite-illite selvedges. Remnant K feldspar and quartz-pyrite-molybdenite veinlets of the potassic assemblage are found within the intermediate argillic zone. Intermediate argillic chlorite-sericite-clay alteration surrounds and overprints the potassic alteration, occurring as disseminations, in nests and fracture zones, and affects early, intermediate and late intrusions. It contains up to 10% pyrite and trace molybdenite.
Phyllic assemblages, which like the intermediate argillic alteration, is only weakly developed and only forms mappable zones in the dacite and in the northern limit of the deposit. They are characterised by the presence of sericite and pyrite disseminations, pyrite veinlets with sericite halos, quartz veinlets with iron oxides, and locally by chlorite. This alteration generally destroys rock textures and overprints intermediate argillic assemblages. It mainly occurs in the eastern part of the deposit, as a north-south to NNW oriented zone affecting the early, intermediate and late intrusions, the metamorphic rocks and hornfels. In parts remnant quartz veinlet stockworks of potassic alteration are preserved, while in some areas there are nests of pyrrhotite. Phyllic zones contain up to 5% pyrite and as much as 7% pyrrhotite.
Silicification has affected the hornfels bands, occurring as massive zones and veinlets following the foliation, accompanied by pyrite as disseminations, veinlets and in quartz veinlets. These zones are observed in the western and southern part of the Colosa deposit close to intrusive bodies.

The early intrusion porphyries appear to have been altered and mineralised at the same time, since there is scant evidence for veinlet introduction between the three intrusive events. The gold content of the early intrusive porphyry phases is similar. The veinlets at La Colosa appear to span the potassic and sodic-calcic alteration events. The earliest veinlets only contain biotite, although most subsequent early veinlets are composed of quartz, magnetite, pyrite, pyrrhotite plus minor chalcopyrite and molybdenite, and are dominated by either quartz or magnetite. The main determinant of gold grade in the diorite porphyries or dacite intrusive stock is the intrusive phase in which the mineralisation is hosted. The 'Early intrusions' host the highest and most consistent gold grade (average >1.1g/t Au), while the Intermediate intrusion diorites have grades averaging <0.7g/t Au, and the late dacite phase generally only has >0.3g/t Au close to the contact with 'Early intrusions' diorite phases (Garzon, 2011).

The sodic-calcic and potassic alteration, with or without chlorite, have the best gold grades. Areas with intense illite alteration generally average <0.3 g/t Au. The contact breccias and hornfels developed at the contact between porphyritic rock and schist present a mineralised halo of at least 60 m with an average gold grade of >1 g/t Au (Garzon, 2011).

Gold mineralisation within the intrusive bodies is associated with pyrite and occurs in both A- and B-type quartz veinlets. Within schists and hornfels it is also predominantly related to pyrite veinlets along foliation planes associated with hydrothermal quartz. Gold grains vary from almost pure gold to much lesser amounts of gold-silver telluride. The gold grains are generally fine, ~15 µm, although coarse grained gold of ~116 µm has been found in the metamorphic rocks. Gold grains are both liberated and 'locked' in sulphides and silicates. A significant amount of gold is associated with K feldspar and plagioclase. Sulphide minerals associated with gold are dominantly pyrite, with lesser amounts in pyrrhotite and arsenopyrite.

Weathering has produced iron oxides and clays to a depth of no more than 30 m.

The age of the intrusive bodies, mineralisation and alteration is between 8.5 and 7.5 Ma (U-Pb in zircon, K-Ar in biotite and hornblende, and Re-Os in molybdenum; Garzón, 2011). According to Lodder et al. (2010), U-Pb (zircon) dates performed using laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS; Gehrels et al., 2008) for syn-mineral diorite through to intermineral granodiorite porphyry at La Colosa and adjacent intrusions span the range from 8.3 to 7.9 Ma (H. Leal, unpub., 2010).

In plan view, in 2010, the La Colosa resource envelope had a SSE tapering tear-shape, with a long dimension of 1800 m and maximum width of 750 m (Lodder et al., 2010).

Published mineral resources at 31 December, 2011 (AngloGold Ashanti, 2012), at a cut-off grade of 0.5 g/t Au were:
    Inferred resource - 515.98 Mt @ 0.98 g/t Au for 505.99 t of contained gold.

Published JORC compliant mineral resources at 31 December, 2015 (AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2016), at a cut-off grade of 0.3 g/t Au were:
    Indicated resource - 821.67 Mt @ 0.85 g/t Au;
    Inferred resource - 242.51 Mt @ 0.78 g/t Au;
    TOTAL resource - 1064.18 Mt @ 0.83 g/t Au for 885.33 t of contained gold.

The most recent source geological information used to prepare this summary was dated: 2011.     Record last updated: 8/12/2016
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.


La Colosa

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Garzon, T.,  2011 - Discovery of the Colosa gold-rich porphyry deposit: in   NewGenGold 2011 Conference, Case Histories of Discovery, 22-23 November 2011, Perth Western Australia, Louthean Media, Perth,   Proceedings volume,  pp. 229-240.
Leichliter, S ., Hunt, J., Berry, R., Keeney, L., Montoya, P.A., Chamberlain, V., Jahoda, R. and Drews, U.,  2011 - Development of a Predictive Geometallurgical Recovery Model for the La Colosa, Porphyry Gold Deposit, Colombia: in   The First AusIMM International Geometallurgy Conference, Brisbane, Queensland, 5 - 7 September 2011 The Australasian Institute of Mining and Metallurgy: Melbourne,   Proceedings Vol. pp. 85-91.
Lodder, C., Padilla, R., Shaw, R., Garzon, T., Palacio, E. and Jahoda, R.,  2010 - Discovery history of the La Colosa Gold Porphyry Deposit, Cajamarca, Colombia: in    Society of Economic Geologists, Denver,   Special Publication 15, pp. 19-28.
Montoya, P.A., Keeney, L., Jahoda, R. Hunt, J., Berry, R., Drews, U., Chamberlain, V., and Leichliter, S.,  2011 - Geometallurgical Modelling Techniques Applicable to Prefeasibility Projects La Colosa Case Study: in   The First AusIMM International Geometallurgy Conference, Brisbane, Queensland, 5 - 7 September 2011 The Australasian Institute of Mining and Metallurgy: Melbourne,   Proceedings Vol. pp. 103-111.


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