Andean Cu-Au-base metals province - Northern Andes in Ecuador
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The Northern Andes and Panama copper-gold province occupies an interval of ~2500 km from far northwestern Peru, through Ecuador, Colombia and Panama to eastern Costa Rica in Central America, and largely corresponds to the area in which: i). an allochthonous, largely intra-oceanic arc was accreted to the western margin of the complex Proterozoic and Archaean shield of South America during the Lower Palaeozoic; and ii). a complex of allochthonous Cretaceous oceanic plateau terranes accreted onto the northwest margin of South America, connecting it to the southern tip of North America, during the Oligocene to Miocene.
Tectonic and Geological Setting
According to Cediel et al. (2003) the Northern Andes differs substantially from the Peruvian and Central Andes in a number of aspects, including the nature and age of underlying basement and continental margin, the nature and evolution of stress field regimes during uplift, the nature and age of subducting oceanic
crust, and the timing and style of deformation and magmatism.
For detailed description of the regional setting and geology of the whole Northern Andes and Panama copper-gold province, and context to the description of the Ecuadorian section outlined below, read the separate North Andes and Caribbean record.
Specific details of the Ecuadorian sector of the province may be summarised as follows:
In northern and central Ecuador five major physiographic divisions are recognised (Litherland et al., 1994), from east to west, namely:
i). Oriente foreland basin, including the Sub-Andean Zone on its western margin, in eastern Ecuador, which is covered by flat-lying Palaeozoic and Late Triassic to Tertiary sedimentary rocks that largely overlie Archaean and Proterozoic basement of the Guiana Shield. The Sub-Andean Zone comprises two en echelon, NNE trending structural highs, the Cutucú and Napo uplifts, to the SE and NW respectively. These two large antiformal culminations are limited by steeply dipping ESE vergent thrust faults, and are separated by the narrow Pastaza depression. The core of the Napo uplift in the Cordillera del Condor is composed of generally subhorizontal Late Triassic to Cretaceous sedimentary and volcanic rocks, with various NNE elongated Jurassic granitoids in the west (including the ~162 Ma Abitagua and 190 to 150 Ma Zamora batholiths to the north and south respectively). The Abitagua batholith intrudes similarly aged, undeformed, porphyritic, silicic lavas, associated flow breccias and pyroclastic rocks of the late Jurassic Misahualli volcanic sequence. The Zamora batholith, further south, cuts feldspar micro-porphyritic andesites, hornblende andesites, and dacites with a series of small associated high-level, subvolcanic intrusions, also of the Misahualli volcanic sequence. The less topographically pronounced Cutucú uplift is predominantly composed of a continental-type sequence of tuffaceous grey siltstones and sandstones with basaltic (at least in part tholeiitic) lavas, and overlying to lateral continental red-beds (Baby et al., 2013; Christophoul et al., 2002; Apsden and Litherland, 1992).
ii). NNE-trending Eastern Cordillera (Cordillera Oriental/Cordillera Real), (the 'Central Continental Sub-plate' of Cediel et al., 2003) that comprises a para-autochthonous terrane, mostly composed of Lower Palaeozoic meta-sedimentary and (meta-)igneous basement units representing an accreted intra-oceanic to continental margin arc (Litherland et al., 1994; Pratt et al., 2005). In the north the sequence comprises garnet-biotite schists and paragneisses with minor amphibolites. In the south however, low-grade phyllites, quartzites and semi-pelitic schists predominate, but towards the east these are replaced by a narrow elongate belt of medium- to high-grade schists and gneisses. Towards the west they are intruded by synkinematic granitoids, which are garnet-bearing, two-mica intrusives with peraluminous (approximately S-type) petrochemistry, which are ubiquitously deformed and metamorphosed, ranging from weakly to intensely (migmatite, augen gneiss and mylonite). Attempted dating of these granitoids have returned broadly Triassic K-Ar ages, which may represent resetting, and it is suggested they were probably emplaced during the Palaeozoic (Cediel et al., 2003; Litherland et al., 1994). These rocks have all been deformed by a subvertical to steeply west-dipping, Andean trending, second schistosity, and towards the east are deformed into an eastward vergent nappe complex (Apsden and Litherland, 1992).
iii). The Inter-Andean Depression/Graben, (also known as the Valle Interandino or Chaucha Terrane) is a 30 to 50 km wide transpressional pull-apart structure, above which a number of overlapping elongated basins are filled with volcanic, volcaniclastic and sedimentary rocks. These volcano sedimentary rocks, and associated Tertiary intrusions, vary from Paleocene to mid-Miocene in age in far northern Peru and southern Ecuador, underlying and areally overlapping similar Late Miocene to Recent sequences in Ecuador, which become dominant in Colombia. These rocks are distributed over a width of ~150 km, lapping well onto the Eastern and Western Cordillera, and within the depression, conceal a broad, NNE-trending suture zone that separates the eastern para-autochthonous terrane, and the western allochthonous province. This suture zone is also the core of an 80 to 120 km wide corridor of Pliocene to Recent Andean-type volcanism, part of a ~1000 km long chain of stratovolcanic cones that extends north into Colombia. The Inter-Andean Depression is separated from the Eastern Cordillera by the broad Peltetec Fault, a continuation of the Romeral Fault in Colombia. The Tertiary magmatism closely parallels the Chile-Peru Trench that is to the west. The major Calacalí-Pujilí-Pallatanga fault zone (an extension of the Cauca Fault in Colombia), marks the eastern margin of the Western Cordillera, and most likely corresponds to the western limit of the suture zone (Vallejo et al., 2009; Spikings et al., 2010). This broad suture zone corresponds to an upwardly expanding flower structure fault zone and comprises a complex structural melánge of rocks from both the bounding terranes (Cediel et al., 2003).
Rocks within this structurally complex zone are taken to include the southern extension of the Romeral Terrane in Colombia, composed of interleaved blueschists and eclogites, mafic oceanic volcanic and Palaeozoic continental schist protoliths.
iv). NNE-trending Western Cordillera (Cordillera Occidental), also known as the Piñón Terrane - the basement to which comprises an allochthonous, late Cretaceous oceanic plateau sequence, composed of dislocated segments of the Caribbean Large Igneous Province ocean plateau, accreted between ~75 and 70 Ma. Transcurrent fault displacement along generally north-south trending faults has structurally juxtaposed volcano-sedimentary successions of different ages, but similar lithologies within the terrane. These exotic oceanic plateau basement rocks are overlain by middle Eocene to Oligocene volcanic and volcaniclastic island arc sequences (dominated by the Macuchi Unit). This island arc sequence, which represents the dominant outcrop in the Western Cordillera and its western foothills in northern-central Ecuador, are only locally covered by minor younger volcanic and sedimentary deposits. Several Tertiary intrusions are exposed in this terrane, and tend to be aligned with broadly orogen-parallel fault zones that have strike lengths of several hundred kilometres (e.g., the Chimbo-Toachi shear zone), and persist to mid- and deep-crustal levels where they are defined by 35° east-dipping faults (Guillier et al., 2001). This suite of Tertiary intrusions includes Oligocene to Miocene intrusive centres of batholithic dimensions (PRODEMINCA 2000; Schütte et al., 2012).
v). Coastal forearc region, a coastal plain covered by Paleogene to Neogene sedimentary rocks composed of material shed from the Andes, which is also underlain by the same basement sequence of late Cretaceous oceanic plateau rocks as the Western Cordillera, and is also part of the Piñón Terrane (Litherland et al., 1994).
In southern Ecuador, the main NNE structural trend/corridor that prevails in northern-central Ecuador (as described above) is disrupted where the Dolores-Guyaquil Megashear, swings SW into the Gulf of Guayaquil. A little to the south, the major Bulabula fault also swings to the west to cross the Pacific coast where it appears to defines the southern limit of the Piñón Terrane. Both the Dolores-Guyaquil Megashear and Bulabula fault appear to be splays of the NNE trending regional structural corridor, which is in part cut to the south of the Gulf of Guayaquil by an WNW-ESE arc-transverse structure, the Piñas-Portovelo fault zone. To the east, however, the main NNE-SSW structural corridor continues south into Peru, swinging to a NNW-SSE trend across the Huancabamba Deflection zone, an east-west continental scale basement feature. The Piñas-Portovelo fault, which appears to have been influenced by the Huancabamba Deflection zone, defines the northern margin of the WNW-ESE trending Amotape range (or Amotape arch) to the south, which is composed of (Proterozoic-)Palaeozoic and Mesozoic metamorphic rocks, and includes blueschist facies rocks and an associated eclogitic harzburgite complex, indicating the fault represents a Jurassic to early Cretaceous suture zone. This suture zone, has influenced the later tectonic fabric. A string of 26 to 16 Ma porphyry intrusions of dioritic to granodioritic composition, the Cangrejos-Zaruma Intrusive Belt, have intruded the hinge of the WNW-ESE trending Cangrejos antiform, that parallels the Piñas-Portovelo Fault, which is ~10 to 15 km to the south (Spencer et al., 2002).
Oligocene to early Miocene volcanic and volcaniclastic rocks of the Saraguro Group constitute the major Tertiary arc assemblage in southern Ecuador, overlain by mid- to late Miocene volcanic units, and by volcaniclastic and sedimentary intramontane basin deposits. However, extensive late Miocene to recent volcanic cover sequences are absent to the south of the arc-transverse Piñas-Portovelo fault, which marks the beginning of a break before their re-occurrence further south in northern Peru (Hungerbühler et al., 2002).
A Cretaceous back-arc basin, the Lancones trough, is developed to the south of the Amotape arch and west of similar basement exposed as the Olmos arch within the main Andean trending structural corridor. This back-arc basin contains a thick succession of Cretaceous turbiditic rocks, and a volcanic complex that is characterised by an ~10 km thick, mantle derived sequence of basaltic volcanic rocks (Benavides-Caceres, 1999). This back-arc basin is predominantly in far northwestern Peru, and apparently predates accretion of the Piñón terrane.
Throughout the Tertiary, oblique plate convergence between the Farallon/Nazca and South American plates along the Ecuadorian margin to the north of Guayaquil, has been accommodated by oblique subduction slip, and by trench-parallel, dextral forearc sliver displacement along major orogen-parallel fault zones. Currently, the dominant component is oblique subduction slip (Daly 1989; Ego et al., 1996; Hungerbühler et al., 2002). Compressive to transpressive pulses during the Miocene, at 19 and 10 to 9 Ma, deformed intramontane basin deposits of the main arc region (Hungerbühler 1997; Hungerbühler et al., 2002). Opening of the Inter-Andean Depression as a ramp to half ramp basin has been in a transpressional setting since ~6 Ma (Winkler et al., 2005). These events are broadly temporal correlates of the compressive pulses recorded south of the 'Huancabamba Deflection' which marks the change in trend of the Andean trend to the south of Guayaquil, from NNE-SSW in Ecuador, to NNW-SSE in Peru (Benavides-Cáceres 1999; Noble and McKee 1999). Since the late Miocene, Miocene and younger oceanic crust of the Nazca Plate has been subducted eastward below the Colombia-Ecuador section of the Chile-Peru Trench. The ~200 km wide, east-west elongated Carnegie Ridge seamount chain in the Nazca Plate, has contributed to minor slab shallowing, reflected by concomitant eastward frontal arc migration and arc broadening in onshore north-central Ecuador (Guillier et al., 2001; Schütte et al., 2012).
Distribution of Mineralisation
Copper and gold mineralisation in Ecuador is concentrated in two main temporal belts, as follows:
Jurassic deposits, which 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 zone of the Cordillera del Condor of southeastern Ecuador. These deposits form a limited belt that is not apparently continuous to the north or south. All are associated with Upper Jurassic 160 to 150 Ma late porphyry intrusive phases of the 190 to 150 Ma Zamora Batholith, and include the,
• Mirador, Mirador Norte, Panantza and San Carlos porphyry Cu-Au deposits and the Kutucus Cu skarn (5 km ENE of Panantza) associated with late phase (152 to 156 Ma) dykes of porphyry within the body of the Zamora Batholith (163.8±1.9 Ma);
• Fruta del Norte, an intermediate sulphidation epithermal Au-Ag deposit, hosted within an inlier of andesitic Jurassic Misahuallí Formation rocks within the Zamora batholith, and
• Nambija Au-mineralised skarn field, comprising a number of deposits, hosted within the shallowly dipping continental and marine volcano-sedimentary rocks of the Triassic Piuntza unit, which include minor limestone and calcareous shale underlain by a flat roofed section of the Zamora Batholith.
• Other prospects and deposits include the narrow, more structurally controlled zones of Trinidad, San Miguel, La Florida and San Luis which are located to the North of Panantza, prior to the Zamora Batholith passing below the Mid- to Late-Jurassic Misahuallí basaltic to andesitic volcanic country rocks. West of Mirador, the Chancho group (Chancho Norte, Chancho and Chancho Sur) is similarly narrow and structurally controlled, with the south end opening to a 'horse tailing' of diffuse mineralisation before disappearing beneath Cretaceous Hollin Formation quartzite cover rocks. The southern section of the belt includes the El Hito and Santa Barbara porphyry Cu-Mo and Cu-Au deposits, 80 km south of Mirador (Drobe et al., 2013).
For detail, click on the links above to the separate records.
Porphyry and epithermal deposits are distributed along the ~150 km wide corridor of volcano-sedimentary rocks that parallel the coast, and the current Chile-Peru Trench, some ~260 km to the west. The volcano-sedimentary sequences vary from Paleocene to mid-Miocene in age in far northern Peru and southern Ecuador, overlapping similar Late Miocene to Recent sequences in central to northern Ecuador, which become dominant in Colombia. These rocks are accompanied by Tertiary intrusions and are centred on the Inter-Andean Depression, extending onto the Eastern and Western Cordillera. The mineralised corridor is continuous into Peru to the south and SE and into Colombia to the NNE. Within this belt, significant copper and/or gold mineralisation is concentrated in two main districts,
• Imbaoeste district, 75 km north of Quito, in northwestern Ecuador, where the Oligocene to early Miocene Saraguro Group and overlying Late Miocene to Recent arc sequences are both developed and overlap, with the older succession predominantly to the west. Both sequences are cut by Tertiary intrusions that host both Eocene and Miocene porphyry and epithermal mineral deposits. The principal of these include the Miocene Llurimagua/Junin Cu-Mo-Au deposit and the Eocene Alpala Cu-Au deposit (Rohrlach et al., 2015), 60 km to the NE.
• Azuay-El Oro district, an ~100 x 100 km area that is ~440 km to the south of Junin, and 100 to 200 km SSE of Guayaquil in southern Ecuador, dominantly composed of Miocene deposits. It includes the Chaucha porphyry Cu-Au, the Gaby-Papa Grande, porphyry Cu-Au, Quimsacocha high-sulphidation, Portovelo-Zaruma intermediate-sulphidation and Rio Blanco low-sulphidation epithermal Au deposits. For detail click on the links. Other lesser deposits include the:
Cangrejos porphyry Au-Cu deposit is located ~10 km north of the Piñas-Portovelo fault. It occurs within multiple, nested late Oligocene to early Miocene, quartz-dioritic to granodioritic plutons, intruded into pelitic phyllites and schists of the Palenque unit of the Jurassic El Oro Metamorphic complex to the north, west and south, and by the Oligocene to early Miocene Saraguro Group volcanic rocks in the SE. All of these rocks are intruded by an ~1 km diameter plagioclase-hornblende porphyry stock, as well as a number of variably oriented porphyry dykes, and crackle breccia bodies that are associated with several porphyry intrusions (Potter 2004), accompanied by pervasive hydrothermal alteration (particularly silica flooding) and weak sulphide mineralization (mainly chalcopyrite). Gold-copper mineralisation occurs as chalcopyrite ±bornite disseminations associated with Au, as well as chalcopyrite- and molybdenite-bearing quartz veinlets hosted by the intrusions. Local, high-grade Au mineralisation (up to 10 to 30 g/t) is found within multi-directional quartz (-tourmaline) veinlets (Potter 2004). The mineralization is accompanied by sodic-calcic alteration and silicification, partly affecting all intrusive lithologies, as well as some breccia bodies. A quartz-molybdenite veinlet cutting sodic-calcic altered quartz-diorite yielded an Re-Os age of 23.51±0.09 Ma (Schütte et al., 2012). Non-NI 43-101 historic resource estimates of several hundred million tonnes of 0.5 to 1.5 g/t Au, <0.2% Cu have been quoted (e.g., Schütte et al., 2012). An NI 43-101 technical report for Odin Mining and Exploration dated 2010 concluded there was insufficient drilling to estimate a resource.
El Mozo, a mid-Miocene high-sulphidation epithermal Au deposit hosted by a sequence of early to mid-Miocene volcanic and volcaniclastic rocks. Gold mineralisation is predominantly associated with pyrite in vuggy silica bodies (with associated advanced argillic alteration) controlled by multi-directional, high-angle faults. Spatially associated intrusive rocks comprise partly argillic altered granodiorite porphyry dykes and seriate diorite and granodiorite plugs. Hypogene alunite from associated advanced argillic alteration gave a 40Ar/39Ar age of 16.9±0.2 Ma (Schütte et al., 2012). An inferred resource of 3.5 Mt at 2.3 g/t Au (0.5 g/t Au cutoff) is quoted by Schütte et al. (2012).
Tres Chorreras porphyry-related breccias, polymetallic veins/replacement and epithermal Au. This mid-Miocene mineralisation is hosted by volcanic rocks of the Oligocene Saraguro Group. The deposit area includes a pipe-shaped, 600 m diameter diatreme breccia body, and a number of post-diatreme, irregularly shaped diorite and granodiorite plugs and dykes. The breccia is composed of subrounded to subangular juvenile clasts within a partly friable, variably mineralised rock flour matrix. Mineralisation occurs as i). possibly porphyry-related polymetallic veins, massive sulphide replacement bodies, and disseminations within the diatreme breccia body and along the faulted contact between the diatreme breccia structure and a diorite intrusion; and ii). as a set of NE-trending, subvertically dipping, epithermal Au bearing quartz-base metal veinlets, also hosted by the diatreme breccia and a diorite intrusion. Extensive silicification and argillic alteration affects both the intrusions and the diatreme breccia (Melling et al., 2007; Schütte et al., 2012). Re-Os analysis of molybdenite from the breccia matrix and from a polymetallic vein gave ages of 12.93±0.05 and 12.75±0.06 Ma (Schütte et al., 2012).
• Bolívar district, which is defined by two sub-economic (in 2016) porphyry Cu (±Mo±Au) systems, Telimbela and Balzapamba. Mineralisation at these two systems is spatially associated with sections of the Oligocene to mid-Miocene Telimbela and Balzapamba batholiths, which are dominantly hornblende±biotite-bearing granodiorite, tonalite and quartz-diorite, emplaced within gently east-dipping, basic volcanic and volcaniclastic rocks of the middle Eocene to Oligocene Macuchi island arc sequence (MMAJ/ JICA 1991).
At Telimbela, porphyry Cu (±Mo±Au) mineralisation occurs as several broad NE trending zones, largely centred on several NE-trending, early to mid-Miocene, partly porphyritic, hornblende- and biotite-bearing quartz-diorite and granodiorite dykes (each a few to a few tens of metres thick) and stocks (up to 800 m in diameter) in the southern portion of the Telimbela batholith. Mineralisation includes chalcopyrite-, pyrite- and molybdenite-bearing quartz veinlets and disseminations with associated potassic, sodic–calcic and overprinting phyllic alteration, hosted by all intrusive lithologies (including the batholith), and locally by the Macuchi volcaniclastic country rocks also. Hydrothermal brecciation, with chalcopyrite-pyrite±molybdenite disseminated in the biotite- and chlorite-bearing breccia matrix, occurs around the contact zones of some intrusions. Re-Os geochronology of batholith-hosted molybdenite-quartz veinlets associated with potassic alteration yielded ages of 19.17±0.07 Ma (Schütte et al., 2012).
The Balzapamba porphyry Cu (±Mo±Au) system is located in the northern part of the Oligocene to early Miocene Balzapamba batholith. Mineralisation occurs as chalcopyrite, pyrite and molybdenite disseminations within batholith, and disseminated within the silicate-sulphide matrix of elongated hydrothermal breccia zones within the batholith. The matrix is predominantly composed of biotite, quartz and chlorite. The copper sulphides are erratically distributed and associated with potassic and, locally, sodic–calcic alteration, overprinted by phyllic assemblages. A number of thin (a few metres thick) NE-trending quartz-diorite porphyry dykes are also potassic altered and mineralised. The system appears to represent the eroded root zone of a porphyry system. Molybdenite associated with a potassic alteration assemblage in the batholith returned Re-Os ages of 21.50±0.08 Ma (Schütte et al., 2012).
The most recent source geological information used to prepare this summary was dated: 2014.
This description is a summary from published sources, the chief of which are listed below.
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Schutte P, Chiaradia M, Barra F, Villagomez D and Beate B, 2012 - Metallogenic features of Miocene porphyry Cu and porphyry-related mineral deposits in Ecuador revealed by Re-Os, 40Ar/39Ar, and U-Pb geochronology: in Mineralium Deposita v.47 pp. 383-410|
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