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Mocoa

Colombia

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Mocoa is a molybdenum-rich porphyry copper deposit in south-western Colombia, located near Putumayo in the Amazon headwaters, ~450 km SSW of Bogota, ~250 km east of the Pacific coast and 100 km north of the Ecuadorian border (#Location: 1° 14' 35"N, 76° 40' 25"W).

Regional Setting

For detail of the regional setting and geology, see the separate record for the North Andes and Panama copper-gold province.

The Mocoa deposit is located in the area where the Eastern (Oriental) and Central Cordilleras of north and Central Colombia, separated by the Magdalena Intermontaine Depression to the north, converge to become a single morphotectonic entity, the Eastern Cordillera (or Cordillera Real) to the south in Ecuador. To the west, the Central Cordillera is separated from the Western Cordillera (Cordillera Occidental) by the Patia Intermontaine Depression (the Inter-Andean Depression in Ecuador). The Cordillera Occidental is underlain by 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 was accreted during a late Lower Cretaceous (Albian) tecto-metamorphic event (Toussaint, 1999).

To the east of the oceanic terranes, metamorphic rocks, possibly of Proterozoic age, occur as an elongate wedge bounded by strands of a major fault system. These rocks comprise well banded quartz-feldspar gneiss and subordinate amphibolite, correlated with the 1180 Ma Mesoproterozoic Garzon Massif granulite facies metasedimentary and metavolcanic rocks to the north. These rocks are in fault contact with the Late Triassic Payandé Formation in the Mocoa area, which comprise relatively pure limestones and black siltstones, with local more sandy horizons and thin lenses of black chert. To the west of Macoa, these rocks have undergone contact metamorphism by the Mocoa batholith, with limestones silicified, transformed to marble, and locally, to skarn and siltstones changed to hornfels. These are overlain, without obvious evidence of an angular unconformity, by the Late Triassic to earliest Jurassic Saidañia Formation, dominantly composed of interbedded fine grained and tuffaceous (with lesser lavas) andesitic and subordinate dacitic (largely tuffs) volcanic rocks.

The Proterozoic and Late Triassic to Early Jurassic units were intruded by porphyritic stocks and dykes of intermediate composition and, in the SW, the Payandé Formation is cut by the Mocoa batholith.

To the south and SE, all these units are in fault contact with continental red beds of the late Oligocene to early Miocene Orteguaza Formation, a well-bedded succession of mottled reddish-brown and grey, grey, and greenish-grey mudstones and claystones with minor intercalations of sandstone and coal. Tertiary rocks are extensively concealed by late Pliocene to Quaternary conglomerates/terrace gravel with thin (~1 m thick) dacitic tuffs on ridge crests.

Geology

The Mocoa deposit is associated with a partly unroofed stock of biotite dacite porphyry emplaced into a broadly comagmatic, late Early to Middle Jurassic, shallowly dipping and completely bleached andesite and dacite volcanic pile of the Saidañia Formation. It is located 1.7 km NE of the Mocoa batholith, a composite body with surface dimensions of ~40 x 10 km and dominant components of medium grained hornblende and biotite granodiorite and coarse-grained biotite adamellite. Both facies are pyritic and are cut by narrow quartz veins carrying pyrite, chalcopyrite and molybdenite, which are regarded to be related to the batholith formation rather than a fringe to the Mocoa deposit. The batholith has been dated at 210±4 and 198±4 Ma - Late Triassic.

Several relatively small porphyritic bodies (mainly dacitic) outcrop within and to the south of the batholith. Most have only undergone propylitic alteration, although the largest, to which the Mocoa porphyry system is directly related, is pervasively sericitised and, in depth, K-silicate altered, with associated hydrothermal breccias. Samples from these porphyritic bodies and associated alteration have been dated at 166±4, 170±2, 172±2 and 183±3 Ma - Early to Middle Jurassic.

The Mocoa porphyry system is spatially, temporally and genetically related to a partially unroofed dacite porphyry stock, which in outcrop is markedly eccentric with respect to alteration geometry. The stock is broadly tear-drop shaped at surface, with a northern, NE elongated, oval-shaped, 1200 x 400 m section, whereas the southern dyke-like section trends generally north-south and is up to 600 m long but only 200 m wide. The overall stock expands downwards, with the contacts dipping westward to at least 900 m depth. Numerous other smaller dacitic bodies, mainly dyke-like, but including a 75 m thick sill occur in the same area.

The enclosing Saidañia Formation in the deposit area, is subdivided into an upper dacitic member, at least 170 m thick, and a lower 750 m thick andesitic member. The dacitic member, which is entirely within the barren lithocap, overlies the copper-molybdenum deposit along the main central ridge.

The dacite porphyry intrusion contains quartz and plagioclase phenocrysts of up to 6 and 8 mm respectively, although biotite does not exceed 8 mm. Locally, in zones of K-silicate alteration has irregularly distributed poikilitic K-feldspar megacrysts up to 2 cm long, in the dacite porphyry. At least two closely related phases of weakly altered and mineralised late-mineral dacite porphyry occur as irregular bodies, up to 40 m thick within the deposit area, and outcrop as an ~800 m long dyke-like body. These late mineral intrusions post-date the hydrothermal brecciation at Mocoa.

Mineralisation and Alteration

The dacite porphyry stock is intimately related to a complex series of hydrothermal breccias, some of which contain high grade Cu-Mo mineralisation. Most of the breccias occur in the roof zone and along the western flank of the sub-surface southern dyke-like part of the stock. In contrast, breccias are virtually absent from the oval northern part of the stock, from which the roof zone has been removed by erosion, except for two small outlying bodies on its northwestern side. The breccias occur within the outer parts of the stock and in the adjacent volcanic wall rocks, commonly contain fragments of both lithologies, although one is generally more abundant. Most breccias contain angular to subrounded, <3 cm clasts set in a matrix of various combinations of rock flour, sulphides and gangue minerals. The presence of dacite porphyry fragments above the roof of the stock and of isolated fragments rich in hydrothermal phlogopite or K feldspar above the top of the K-silicate zone demonstrates at least 100 m of upward fragment displacement during some phases of brecciation. The bulk of the breccias were emplaced during K-silicate or sericite alteration. A significant number of the breccias are intra-mineral, as demonstrated by the restriction of one or more phases of pre-breccia mineralisation to the clasts. The presence of pre-breccia mineralisation significantly augments the tenor of mineralisation in some breccias, particularly with respect to Mo content.

The orebody has a cylindrical shape and contains chalcopyrite, molybdenite and local traces of bornite.   The sulphides are associated with K feldspar-chlorite-pyrite (especially in the andesite volcanics in the south) and quartz-sericite-pyrite (mainly in dacite porphyry in the north) alteration.   The orebody is largely overlain by a barren quartz-sericite-pyrite lithocap several hundred metres thick.   The high grade Cu-Mo mineralisation is hosted by multiple hydrothermal breccias along the southern roof and southwestern flank of the stock.

A low-grade central core to the deposit, in which grades are seldom >0.15% Cu or >0.015% Mo, occurs as an early, partly preserved K feldspar-phlogopite-actinolite-magnetite alteration assemblage at depths of >400 m. A zone of sphalerite-hedenbergite-garnet skarn within in Late Triassic limestone is tentatively attributed to the Mocoa batholith rather than being an outer halo of the porphyry system.

The outer limit of sericite alteration and 0.5% vol.% pyrite have a diameters of ~1000 to 1500 m and ~1800 m, respectively.

The Mocoa porphyry system is overlain by an up to 260 m thick jarositic zone of partial leaching. However, the rapid erosion during the late Cenozoic precluded development of a significant zone of supergene chalcocite enrichment. The high precipitation caused early complete removal of abundant hypogene anhydrite and supergene gypsum to depths of at least 900 m.

By 1983 drilling had indicated >260 Mt @ a little over 1% Cu equivalent hypogene ore (Sillitoe et al., 1984).
A resource of 588.74 Mt @ 0.27% Cu, 0.061% Mo is quoted by the USGS MRDS database (after Arias and Jaramillo, 1987).

For detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1984.     Record last updated: 2/12/2012
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.


Mocoa

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Sillitoe R H, Jaramillo L, Castro H  1984 - Geologic exploration of a Molybenum-rich porphyry copper deposit at Mocoa, Colombia: in    Econ. Geol.   v79 pp 106-123


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