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The Panantza and San Carlos porphyry copper-gold deposits are located ~40 km north of the Mirador deposit, 190 km SE of Guayaquil and ~340 km south of Quito in the Corriente Copper-Gold Belt of southeastern Ecuador. San Carlos is ~5 km SE of Panantza.
(#Location: Panantza - 3° 9' 40"S, 78° 27' 1"W; San Carlos - 3° 11' 25"S, 78° 25' 3"W)
The bulk of Ecuador's porphyry Cu±Mo±Au±Ag and porphyry-related epithermal Au±Ag±Cu deposits are of Jurassic and Tertiary (mostly Miocene) age, that define two distinct metallogenic belts (PRODEMINCA 2000; Sillitoe and Perelló 2005; Chiaradia et al., 2009).
The Jurassic deposits form the 150 km long, NNE-SSW trending Corriente Copper-Gold Belt, a narrow eastern, sub-Andean metallogenic belt in the Cordillera Real and Sub-Andean Cordillera del Condor of southeastern Ecuador. These deposits are all associated with Upper Jurassic late porphyry intrusive phases of the Zamora Batholith, and include the Mirador, Mirador Norte, Panantza and San Carlos porphyry Cu, the Fruta del Norte epithermal Au-Ag and the Au-mineralised Nambija skarn field.
A broader Miocene metallogenic belt follows the entire western Andean range or Cordillera Occidental, and has a continuity with the Miocene metallogenic belt of southern Colombia and northern Peru (Sillitoe 1988; PRODEMINCA 2000; Sillitoe and Perelló 2005).
For details of the regional setting and geology, see the separate records for North Andes copper-gold province in Ecuador and the broader North Andes and Panama copper-gold province.
The porphyry copper deposits of the Corriente Copper-Gold Belt are associated with late porphyritic intrusive phases of the Late Jurassic calcalkaline batholiths of the Cordillera Real and sub-Andean Cordillera del Condor. These batholiths, were emplaced into both basement and Mesozoic cover rocks, and lie along and above the mid Palaeozoic suture on the eastern margin of the Lower Palaeozoic arc represented by the Cajamarca-Valdivia-Loja Terrane. To the north, in Colombia, this suture is with the Precambrian Chicamocha Terrane. To the south in Ecuador, where the Chicamocha Terrane is absent, the same suture separates the Palaeozoic arc from the Guiana Shield (Drobe et al., 2007).
One of these Jurassic intrusions, the granitic Zamora Batholith, hosts all the known porphyry-style deposits of the belt. Intrusion of this north-south elongated batholith commenced as early as 190 Ma, although its younger Upper Jurassic porphyry intrusive phases, which are commonly associated with mineralisation, yield ages of 157 to 152 Ma (Gendall et. al., 2000; Coder, 2001).
In the Panantza and San Carlos area, these Jurassic intrusions were emplaced within a suite of marine sedimentary and volcanic rocks of the mid to late Jurassic Misahualli Formation, and possibly also older rocks of the Triassic Piuntza Formation which are mostly exposed along the east side of the batholith, although a 2 x 1 km window is mapped on the western margin of the batholith ~1 km SW of Panantza. Along its western contact, the intrusion is unconformably overlain by sandstones of the Cretaceous Hollin Formation, which are, in turn, conformably followed by more calcareous shales, limestone and sandstone of the Napo Group (Drobe et al., 2007).
Both the overlying sedimentary rocks and the Zamora Batholith granite are intruded by Tertiary dioritic to rhyolitic dykes, sills and plugs along the western edge of the batholith to within <1 km west of the Panantza deposit. These post-mineral dykes generally trend NE in the deposit areas, in contrast to the earlier pre-mineral dykes which tend to trend NW (Drobe et al., 2007).
Drobe et al. (2007) note that a regionally important structure in the area also strikes NE along the base of the main Cordillera Occidental, juxtaposing Cretaceous rocks against older basement to the west, and that Cretaceous units are intensely folded and sheared along this fault.
Mineralisation at Panantza is hosted entirely within the Zamora batholith and related late phase porphyritic intrusions. Where mineralised, these intrusions are predominantly coarse graphic-textured leucogranite and fine-grained aplite, characterised by the absence, or sparse presence of mafic minerals. The margins of the mineralisation are characterised by medium-grained, equigranular granite and quartz monzonite containing 20 to 30% biotite and hornblende, which is more typical of the regional Zamora batholith (Drobe et al., 2007).
The granitic rocks are intruded by hornblende-orthoclase porphyry dykes and tabular plugs, which appear to have been emplaced in two generations, the first slightly predating mineralisation, while the second is late in the mineralisation sequence (Drobe et al., 2007).
The first typically has a more seriate texture, has less quartz 'eyes', occurs as larger dykes than the latter, and is more strongly mineralised. They appear to form a continuous series of intrusions that evolved from pre- to syn-mineral, and were emplaced throughout the mineralising event. An alternatively possibility is that they represent a single intrusive episode, with differences in the degree of mineralisation being a function of the amount of fracturing to which they have been subjected, thus influencing the ingress of mineralising fluids (Drobe et al., 2007).
A hydrothermal breccia interpreted to cut a nearby the late mineral dyke contains clasts of unmineralised porphyry and of mineralised granite, and has lower copper grades (Drobe et al., 2007).
The main ore deposit closely corresponds to the main area of Zamora aplite and leucogranite, over a NW-SE elongated area of ~1500 x 900 m, and intrusion of syn- and late-mineral dykes.
The youngest intrusions, which are of Tertiary age, cut Cretaceous Hollin Formation sandstones, and comprise a single NE-striking, NW-dipping rhyolite dyke in the SE section of the deposit, and several 1 to 2 m thick dolerite dykes, which generally parallel, but may also obliquely cut the former.
Alluvium blankets the floor of the Rio Panantza valley, with local slide-related colluvial fans (Drobe et al., 2007).
Potassic alteration dominates in the mineralised granite and leucogranite host rocks, and the pre- and syn-mineral porphyries, whilst the late-mineral dykes have only undergone moderate chlorite-epidote alteration. The potassic zone occurs within a broader 2 x 1.5 km propylitic zone, which has a narrower, 500 to 1000 m wide arm that extends for a further 2 km to the north (Drobe et al., 2007).
Quartz-sericite-pyrite alteration is confined to structures within the potassic zone, but is pervasively developed marginal to the potassic alteration, extending well to the east into the Panantza Este prospect, and for at least one kilometre west, where it is well exposed in road cuts on either side of the Rio Panantza (Drobe et al., 2007).
Argillic alteration occurs within the supergene zone and extends downward, overprinting the potassic alteration along structures (Drobe et al., 2007).
Mineralization is largely typical porphyry-style hypogene disseminated chalcopyrite, with molybdenite mostly occurring as selvages and within quartz veins, and is developed over an area of ~900 x 600 m, open at depth. Within the potassic alteration envelope, the primary sulphides are chalcopyrite, molybdenite and pyrite, with local anhydrite and gypsum. Higher grade hypogene copper, averaging ~0.8%, is restricted to zones containing more concentrated veinlet-controlled chalcopyrite-pyrite±magnetite. An intense, texture-destroying quartz stockwork extends through the centre of the deposit, from southeast to northwest (Drobe et al., 2007).
Supergene mineralisation comprises both oxide and sulphide enrichment blankets, best developed just north of the Rio Panantza over an area of ~600 x 200 m. Oxide copper minerals are preserved in the leached, saprolite zone, mostly occurring as disseminated and fracture-controlled malachite and chrysocolla, with minor cuprite, pitch limonite and neotocite. The oxide is underlain by blankets of chalcocite coating chalcopyrite and pyrite in strongly argillic-altered host rock (Drobe et al., 2007).
Mineralisation decreases relatively rapidly, over a 50 to 100 m interval, across a NW-striking lineament that defines the east side of the deposit, and more gradually on the south, west, and north. A 200 to 250 m wide zone of low-grade, hypogene copper mineralisation, averaging ~0.4% Cu, extends for at least 400 m NW from the main deposit (Drobe et al., 2007).
Inferred mineral resources at Panantza in 2007, at a 0.4% Cu cut-off, were ~463 Mt @ 0.66% Cu, (Corriente Resources inc. website, 2016)
The San Carlos deposit is also hosted within the Zamora batholith and related porphyritic intrusions. The batholith is predominantly composed of a medium to grained, equigranular granite and quartz monzonite with 20 to 30% biotite and hornblende, and is intruded by minor, slightly younger aplite and leucogranite phases.
Kirk (1999) identified three mineralised porphyries, differentiated mostly on degree of mineralisation and alteration, rather than lithology, namely i). early-mineral monzodiorite porphyry (Unit Jefp), which is only mapped in the extreme southern part of the mineralised system; ii). intra-mineral granodiorite porphyry, and iii). late-mineral granodiorite porphyry (Unit Jhbp), which includes the volumetrically most significant of the porphyries, the 200 to 800 m wide, NNW-SSE trending 'late-mineral' Central Dyke which bisects the higher-grade hypogene mineralisation and is only weakly mineralised in the hypogene zone, although it contains significant exotic copper oxides at shallow depths, within the lateritic profile.
As defined, these three porphyries have a progressive decrease in hypogene copper grades, from as much as 1% to less than 0.2% Cu, and a progressive decrease in the intensity of hypogene alteration in successive intrusions. However, Drobe et al. (2007) suggests that these porphyritic dykes form a continuum that cannot be unequivocally differentiated, and my represent a single intrusive event, with differences in the intensity of mineralisation and alteration related to the degree of fracturing and consequent ingress of mineralising fluids, relative to the host granite. All are essentially hornblende-feldspar porphyries with minor quartz phenocrysts. The less intensely mineralised and altered tend to be more quartz phyric.
A volumetrically minor phase of clearly post-mineral unaltered and unmineralised, weakly porphyritic microdiorite dykes are evident, and may be related to Tertiary diorite intrusions farther west.
Potassic alteration occurs as abundant secondary biotite replacing primary mafic minerals, and predominates in the granite and leucogranite host rocks and the mineralised porphyry dykes. This central potassic zone lies within a broader, ~1800 x 1500 m propylitic zone. Phyllic and intermediate argillic alteration assemblages overprint the mineralised porphyries but not the post-mineral microdiorite porphyry dykes. The late-mineral dykes have undergone moderate chlorite-epidote alteration. Argillic alteration extends down from the supergene zone and overprints the potassic alteration along structures (Drobe et al., 2007).
The simplified alteration paragenesis is as follows (Drobe et al., 2007): potassic (biotite) → chlorite → potassic (pale pink K feldspar) → chlorite+epidote+orange-red K feldspar → intermediate argillic (illite+carbonate+chlorite±smectite±hematite) → phyllic (quartz+sericite +pyrite) → smectite+carbonate (associated with faulting).
Hypogene mineralisation is mostly typical porphyry-style disseminated chalcopyrite and pyrite, with molybdenite occurring as selvages and within quartz veins, and is associated with the potassic alteration event. Anhydrite and gypsum are found locally. Minor subsequent copper mineralisation is vein controlled rather than disseminated (Kirk, 1999). The main zone of mineralisation is found over an area of ~1500 x 900 m and is open at depth.
An oxide copper assemblage was formed in situ over low-pyrite, hypogene-sulphide mineralisation, and as exotic-oxide copper mineralisation transported laterally and precipitated where neutralised by mafic minerals in late mineral or post mineral porphyries. It is preserved in the leached, saprolite zone, and mainly comprises disseminated and fracture-controlled malachite and chrysocolla, with minor cuprite, pitch limonite and neotocite. This oxide zone is best developed to the northeast, extending into the Central Dyke (Drobe et al., 2007).
Supergene sulphide enrichment 'blankets' are best developed under a ridge in the southwest, and consist of chalcocite and minor covellite replacing chalcopyrite and to a lesser degree pyrite. Relict pyrite and chalcopyrite are common, reflecting the immaturity of the secondary enrichment. The upgrading of the supergene copper grade over the hypogene grade is variable from zero to a maximum of about three times the primary grade depending, on the pyrite content. The 1% Cu sulphide blanket covers an area of ~1500 x 750 m mostly to the SW of the Central Dyke (Drobe et al., 2007).
Inferred mineral resources at Panantza in 2007, at a 0.4% Cu cut-off, were ~600 Mt @ 0.59% Cu, (Corriente Resources inc. website, 2016).
The information in this summary is drawn from "Drobe, J., Hoffert, J., Fong, R., Haile, J.P. and Rokosh, J., 2007 - Preliminary assessment report, Panantza and San Carlos copper project, Morona-Santiago, Ecuador; A NI 43-101 Technical Report prepared for Corriente Resources Inc., 247p."
The most recent source geological information used to prepare this summary was dated: 2007.
This description is a summary from published sources, the chief of which are listed below.
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