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Quebradona, Nuevo Chaquiro |
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Colombia |
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Cu Au Ag Mo
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Super Porphyry Cu and Au
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The Nuevo Chaquiro porphyry copper-gold deposit is one of five known porphyry centres at Quebradona, and is located in the Middle Cáuca region of Colombia, in the Department of Antioquia, 60 km SSW of Medellin (#Location: 5° 44' 39"N, 75° 44' 19"W).
Regional Setting
The Quebradona district occurs within the Northern Andean Block, which occupies the northwestern margin of South America in Ecuador, Colombia and Venzuela, and extends into Panama to the NW. It lies within the Cauca-Patia Belt, a broad linear valley that defines the eastern flank of the Western Cordillera of Colombia. This belt is bounded by the parallel Mistrato and Romeral fault systems that are ~35 km apart, to the west and east respectively, separating the oceanic crust domain of the Western Cordillera from the continental Central Cordillera to the east.
For details of the setting, see the separate Northern Andean and Panama copper-gold province record.
Within the Quebradona district, the concealed Nuevo Chaquiroa deposit and intrusive complex, is flanked within a 1 to 2 km radius to the east, south and west, by scattered small, outcropping dioritic to quartz dioritic and granodioritic plugs and dykes with associated alteration and sulphide mineralisation, including the sub-economic La Aurora, La Isabela, La Sola, and El Tenedor occurrences (e.g., B2Gold, 2007).
Geology, Alteration and Mineralisation
The Nuevo Chaquiro deposit is related to an 8.5 to 6 Ma Miocene complex made up of two overlapping dyke/intrusive swarm centres at depth. These centres comprise a concentration of overlapping diorite and quartz diorite dykes and vertical stocks developed over a width of ~1000 m, intruding a thick section of basically flat-lying andesitic tuffs and volcaniclastics of the Miocene to Pliocene aged (11 to 6 Ma) Combia Formation. The latter comprises a succession of volcanic and sedimentary rocks, divided into a Lower Member of basalt and andesite lava flows, agglomeratic breccias and tuffs, and an Upper Member of poorly consolidated conglomerates, gravels, sandstones, tuffs and muds (Durán et al., 2005). This formation fills a 35 km wide pull-apart basin, that defines the Cauca-Patia Belt (or Central Andean Depression) of western Colombia.
The Nuevo Chaquiro intrusive complex includes an early quartz diorite, which is characterised by the highest Au-Cu grades, occurring as chalcopyrite blebs, fractures, stringers and disseminations, followed by three generations of intermineral diorites and quartz diorites containing medium grade ore with chalcopyrite disseminations and fractures, and more magnetite than the early intrusion. These medium grades persist into enclosing tuff wall rocks (Winer, 2016).
The deposit is accompanied by multiple cycles of calcic-potassic and potassic alteration, with later overprinting by sericite-chlorite alteration, each associated with the individual intrusion of either an early or intermineral porphyry. The degree of sericite-chlorite overprinting varies with each intrusion. These multiple cycles are all overprinted by a single, later sercitic/phyllic ± intermediate argillic alteration event superimposed as a broad cap upon the entire system. There is an apparent consistent paragenesis for the alteration/mineralisation associated with each intrusion, taken to suggest a shallow, relatively stable underlying parental magma chamber that fluxed geochemically similar pulses of magmatic/hydrothermal fluid with each porphyry phase (Bartos, 2016).
The depth to the top of the mineralisation is ~250 to 400 m below the surface. Despite the multiple phases of intrusion and alteration, the deposit and intrusive complex exhibits typical overall Cu porphyry alteration zonation, with an inner and deeper, composite core of calcic-potassic alteration featuring biotite, actinolite, epidote and anhydrite with weak Cu, Au and Mo. This zone passes upward and outward into a high temperature, K-silicate cap-like shell, characterised by biotite flooding ± magnetite, accompanied by chalcopyrite and molybdenite, which, in turn, is laterally surrounded by a more distal propylitic zone, comprising an assemblage of chlorite, epidote, illite and carbonate. The mineralisation within these Ca-K and K alteration envelopes is characterised by fine stockworks of randomly oriented veinlets, disseminations and veinlets of quartz, magnetite, pyrite, chalcopyrite and molybdenite, with traces of bornite. The potassic shell grades upward into a cap of chlorite-sericite, which passes laterally into propylitic alteration and upwards into a basin like development of sericitic alteration, comprising an assemblage of muscovite, chlorite, quartz, pyrite ± tourmaline. The sericite zone is 250 to 400 m thick, and encloses limited zones of argillic alteration (Winer, 2016; Bartos, 2016).
At surface, the mineralisation is reflected by alteration that comprises an elliptical 1400 x 800 m, ~1 km2, core of sericite, directly above the concealed deposit, surrounded by extensive chlorite-sericite, and enclosed local potassic remnants. Quartz-Fe oxide stockworks are locally developed within the sericitic core. Within the broader chlorite-sericite zone, there are a number of dykes and small stocks with widths/diameters of ≤100 m within 1 km to the west of the sericite core (the La Isabela prospect), and a larger stock cluster ~2 km to the east (the La Aurora prospect). These two stock clusters have associated zones of potassic alteration within the overall chlorite-sericite zone, whilst the main sericite core above the Nuevo Chaquiro deposit is partially surrounded by a disconnected halo of propylitic alteration and small patches of potassic and argillic assemblages (Winer, 2016).
The Nuevo Chaquiroa intrusive complex can be divided into an eastern early intrusive centre, which contains >0.6% Cu and Au mineralisation, and a contiguous central area immediately to the west, comprising abundant intermineral diorite and quartz diorite, over which a classic ore shell of lower grade mineralisation (>0.3% Cu) appears to have been draped. The high grades associated with the early quartz diorite intrusion are related to a sheeted vein complex that occurs at the cupola or immediately adjacent upper contacts of the intrusive, and is responsible for a >2% Cu cap to this part of the deposit. Quartz veinlet density in this area reaches up to 80 vol.% quartz, accompanied by up to 5 vol.% chalcopyrite. The marginally younger intermineral porphyries created a second contiguous Cu-Au-Mo ore body with an inverted bowl shape, with associated potassic alteration that manifests itself as secondary flooding of the rock matrix by hydrothermal biotite, accompanied by disseminations and hairline veinlets of chalcopyrite, magnetite, pyrite and molybdenite. Molybdenum and silver levels are such that they constitute important by-products (Winer, 2016; Bartos, 2016; Bartos et al., 2014).
On the eastern, higher grade, early porphyry flank, the porphyry-style mineralisation occurs at 350 m depth, whereas in the west it is first encountered at somewhat deeper at 400 to 500 m below surface (Bartos et al., 2014).
The porphyry system is overprinted by an epithermal intermediate sulphidation style of mineralisation, characterised by pronounced north-south oriented carbonate-quartz 'D veins' carrying pyrite and chalcopyrite with occasional sphalerite, galena and gold, and sericitic alteration selvages. These occur on the upper western flank of the deposit in a 600 x 200 m area that was traced from the surface to 300 m depth (Bartos, 2016).
Radiometric dating has returned ages for the porphyry mineralisation of 7.51 Ma (Re-Os) and 7.6 Ma (U-Pb), with 7.53 Ma (Re-Os) for associated epithermal veins, and 6.10 Ma (Re-Os) for a late-mineral quartz diorite, suggesting mineralisation was staged over a period of ~1.4 Ma (Winer, 2016).
The mineralisation has been drill tested (to 2015) over a surface area measuring 1200 x 200 m and to a vertical depth of 1550 m, with the >0.3% Cu grade shell situated at the top of the main set of intrusives, with a concave downward shell shape. Possibilities exist to extend the high grade zone along strike to the north and south and to depth.
Typical sections show the 0.6% Cu shell encompassing the early porphyry has a width of up to ~250 to 500 m, gradually tapering downwards, with rapid closures in the upper and lower ~100 m vertically, and a vertical extent of ~1200 m. This shell encloses a large cap of >1.2% Cu, and in turn, is enclosed within 0.45 and 0.3% Cu shells with more or less common eastern margins, but progressively expanding to the west with maximum widths of ~1200 and 1500 m respectively, and vertical extents of >1700 m. The deepest section of each grade shell corresponds to the early porphyry, shallowing in the core or the complex, before deepening to the west, conforming to an inverted bowl shape.
JORC compliant inferred resources at November 3, 2014 (AngloGold Ashanti website, 2015) within a 0.45% Cu shell, were:
604.5 Mt @ 0.65% Cu, 0.32 g/t Au, 4.38 g/t Ag, 115.9 ppm Mo (171.2 t of contained Au)
This summary been compiled by combining information drawn from the AngloGold Ashanti website and Mineral Resource and Ore Reserve Report 2015, presentations to Simexmin 2016 by Winer, N., and to the Denver Region Exploration Geologists' Society by Bartos, P., in April 2016, as well as an SEG 2014 Conference abstract by Bartos, P. et al..
The most recent source geological information used to prepare this decription was dated: 2016.
Record last updated: 1/7/2016
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.
Nuevo Chaquiro
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Bartos, P.J., Garcia, C. and Gil, J., 2017 - The Nuevo Chaquiro Cu-Au-(Mo) Porphyry Deposit, Middle Cauca Belt, Colombia: Geology, Alteration, Mineralization: in Econ. Geol. v.112, pp. 275-294.
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