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Mirador, Mirador Norte |
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Ecuador |
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Cu Au Ag
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Super Porphyry Cu and Au
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The Mirador and Mirador Norte (3.5 km to the NW), porphyry copper-gold-silver deposits are located in the Corriente Copper-Gold Belt of southeastern Ecuador, at 800 to 1800 m asl in the province of Zamora-Chinchipe, ~340 km south of Ecuador's capital city of Quito, 70 km ESE of the city of Cuenca, and ~5 km to the west of the locally north-south border with Peru (#Location: Mirador - 3° 34' 41"S, 78° 26' 8"W).
Regional Setting
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 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 a NNE-SSW trending Jurassic magmatic arc that is represented by the composite Zamora Batholith and attendant continental volcanic rocks of the Misahualli Formation, comprising andesitic lavas and phreatic tuff-breccia, and the Suarez Formation sequence of conglomerates, volcaniclastic sandstones and mixed carbonate and clastic rocks. This arc, which extends almost 2000 km northwards into Colombia, was developed subparallel to the continental margin ~150 to 200 km to the WNW, during an extensional regime, prior to the accretion, during the late Cretaceous, of allochthonous oceanic plateau terranes that now constitute much of western Ecuador and Colombia.
The deposits of this metallogenic belt 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.
Mirador
Geology
The Mirador and Mirador Norte deposits are hosted by Late Jurassic granite, leucogranite and porphyries of the 190 to 170 Ma Zamora Batholith. This regionally extensive batholith is one of a number of Jurassic intrusions in the Cordillera Real and sub-Andean regions of Ecuador, and comprises primarily diorite and granodiorite and related calc-alkaline intrusive rocks. The younger Late Jurassic sub-volcanic porphyry intrusive phases of the Zamora Batholith have been dated at 158 to 152 Ma and may be coeval with the regional Misahualli Formation volcanism. To the south of the Mirador deposit, the resistant Cretaceous Hollin Formation quartz sandstones unconformably overlie the Zamora batholith and cover the southern limits of the Mirador mineralised complex (Drobe et al., 2008; 2013).
The wall rocks of the Mirador porphyry copper-gold system are part of the Zamora batholith, mainly medium-grained, equigranular granite/granodiorite, with minor leucogranite phases along the west and southwest margins, and rare late dolerite dykes up to two metres in width. There are also scattered xenoliths of calc-silicate altered shale. Hornblende and biotite within the granite/granodiorite in the deposit area are mostly replaced by brown to black secondary biotite, which is the most obvious indicator of potassic alteration (Drobe et al., 2008; 2013).
In the Mirador deposit area, the Zamora granite is intruded by the suite of younger porphyritic rocks mentioned above, the oldest of which are north and east striking, pre-mineral dacitic feldspar-hornblende porphyry dykes (156.2±1.0 Ma; U-Pb; Drobe et al., 2013) which precede the first pulse of Cu-Au mineralisation and are overprinted by potassic alteration, although, based on the degree of alteration, a dyke in the southern part of the deposit appears to be slightly older than the northern dykes. These porphyritic dykes are distinguished from the Zamora granite in strongly altered zones and in leached surface exposures by their large hornblende phenocrysts and equant feldspar crystals (Drobe et al., 2008; 2013).
A large, ~400 m diameter, syn-mineral, vertical breccia pipe is found, slightly off-centre in the mineralised system, loosely centred on the early dykes. This breccia is mostly clast-supported, composed of angular to sub-angular fragments of the early porphyry dykes, Zamora granite and quartz-vein fragments set in a matrix of rock flour and fine rock and vein fragments. The clasts exhibit an even potassic alteration with no alteration rims, and vary from centimetres to more than a metre in average size. The matrix contains coarse fillings of chalcopyrite-pyrite and anhydrite. The early porphyry dykes can be traced into the breccia as trains of fragments and intact blocks, and where fragments dominate over matrix, the dykes can be mapped through the breccia (Drobe et al., 2008; 2013).
A sparse, NE-striking, NW-dipping suite of 10 to 30 m thick dacitic hornblende-feldspar-quartz porphyry dykes cut both the breccia and the wall rocks of the deposit. Based on degree of alteration and mineralisation, these dykes are post-mineral, with a quartz-rich variety apparently the youngest in the series (Drobe et al., 2008). They have been dated at 153.1±1.3 Ma at Mirador (U-Pb; Drobe et al., 2013). This set of dykes is larger and more numerous along the SE and NW margins of the mineralised system, where they have textures interpreted to be transitional from porphyritic to tuffaceous. Where the dykes resemble a crystal tuff, they are quartz rich, with minor lithic fragments, and are distinguished from the porphyritic portion of the dyke by intense argillic alteration of the matrix giving it lighter, buff colour. These rocks are sparsely fractured relative to the mineralised equivalents, lack any quartz veining, and are fresh to chlorite altered. Outcrops are blocky and resistant and weather to a characteristic bright red clay due to the oxidation of abundant magnetite. Large rhombs of orthoclase are common in the main dykes. Smaller dykes in the northwest portion of the deposit are dark grey with albite phenocrysts dominating the texture. A large central dyke has abundant coarse hornblende phenocrysts, in addition to subhedral albite, orthoclase and quartz phenocrysts (Drobe et al., 2013).
The youngest rocks are 10 to >100 m wide post-mineral hydrothermal breccia dykes and irregular breccia masses, characterised by a polymictic, angular to subrounded clast assemblage of mineralised and unmineralised rock. They occur at the margins of most late dacitic hornblende-feldspar-quartz porphyry dykes (dacite) and as irregular 'diatremes' around the north and northwest margins of the mineralised zone. The dominant clast type is often dependent upon whether the breccia intruded mainly mineralised rocks, or post-mineral intrusions. Common clasts of black shale and fresh Zamora granite, which are not known to occur within several kilometres of the deposit, indicate the fragments have traveled significant distances. The large diatreme north of the deposit, and outside of the mineralised zone, is composed almost exclusively of shale fragments. The clasts are set in a matrix of finely ground rock, which in places also contains milled sulphide minerals where cutting mineralised Zamora granite. These breccia dykes and masses preferentially intrude contacts of the post-mineral dacite dykes and are most common along the southeast margin of the deposit. Intrusive breccia also occurs as irregular plugs around the north and NE margins of the mineralised zone, with the largest plug occurring outside of the mineralised zone to the north of the deposit. Copper grades within the intrusive breccias range from very low to slightly less than the deposit average, depending on the amount of mineralised rock incorporated. Outcrops of this breccia are massive and very sparsely fractured. In drill core, the breccia is the least fractured lithology in the deposit (Drobe et al., 2008; 2013).
This suite of dykes and breccias covers an area of 1 x 1 km, with individual intrusion having a gross NE-SW trend.
All the intrusive rocks are unconformably overlain by the Aptian to Albian Hollin Formation quartz sandstone, which on the southern contact with the deposit is marked by 50 m high cliffs (Dawson and Makepeace, 2003). This indicates the mineralisation and associated subvolcanic units were exposed at surface by the middle Early Cretaceous (~127 Ma; Drobe et al., 2013).
Mineralisation
Where exposed in road cuts and other excavations, the bulk of the mineralisation at Mirador and Mirador Norte is a weathered and leached laterite, occurring as a tan to brown saprolite with residual silica and abundant iron oxides. Deep weathering has remobilised Cu, but left Au and Mo as largely immobile elements to define a geochemical footprint of the deposit, with their anomalies coincident with mineralisation at depth. There is also a well-defined Zn depletion coincident with the Au and Mo anomalies. Cu, which is very mobile in saprolite, forms a patchy, displaced anomaly, tending to concentrate on propylitic-altered and post-mineralisation intrusions which have a higher carbonate content (Drobe et al., 2008; 2013).
Supergene Cu enrichment is found beneath the saprolite, occurring as secondary chalcocite coating the hypogene sulphides. This zone has undergone intense argillic alteration, which with the chalcocite mineralisation, diminishes gradually with depth. The supergene enrichment zone has a relatively flat upper surface, but more uneven lower boundaries with the underlying hypogene mineralisation. The transition from the leached cap to supergene sulphides, or direct to hypogene mineralisation, can be sharp, on a centimetre scale, or a mottled interval that extends over a thickness of several metres in the vicinity of fracture zones where uneven clay alteration persists to greater depths. Argillic alteration also extends to greater depths within the breccia, due to deeper penetration of meteoric waters within the easily dissolved breccia matrix (Drobe et al., 2008; 2013).
The leached capping is preserved and thickest under ridge crests and eroded in any significant drainage valley with perennial streams, where hypogene and supergene mineralisation may be exposed. Although the supergene blanket forms <10% of the total resource, its high Cu grade, softness and shallow depth make it economically important (Drobe et al., 2013). The supergene enrichment blanket averages <13 m in thickness and is parallel to the present topography, beneath a superficial leached cap that averages <22 m (Dawson and Makepeace, 2003).
The Mirador porphyry system has a well-defined alteration halo, with a large (~4 km2), phyllic quartz-sericite alteration zone covering a large part of the wall rock and porphyry dyke set. This alteration has overprinted a potassic alteration zone whose remnants are small secondary biotite nuclei mainly in the granite/granodiorite. An approximately 1.5 x 1.0 km zone of silicification overprints the centre of the system (Dawson and Makepeace, 2003).
The hypogene Cu-Au mineralisation at Mirador forms a roughly circular zone with a diameter of ~400 to 450 m as defined by the 0.4% Cu contour, surrounding the early breccia pipe, but skewed a little to the SW. Zones of >0.6% Cu straddle the outer contract of the breccia pipe. The hypogene mineralisation mostly occurs as disseminations and fine fracture fillings of chalcopyrite and pyrite in potassic-altered Zamora granodiorite and early porphyry dacite, and as coarse blebs of those sulphides accompanied by dominant purple anhydrite filling interstices in the matrix of the early breccia pipe. Potassic alteration is evident as secondary biotite after mafic minerals. Despite the difference in mineralisation style, there is no statistical difference in Cu or Au grades within the breccia compared to other host lithologies. The total sulphide concentration of ~4% is relatively constant across the deposit, with chalcopyrite greater than pyrite within the central potassic core, and weak, sporadic bornite only occurring in the SE quarter of the deposit. Abundant magnetite occurs along the northwestern edge of the deposit but is not associated with Cu-Au mineralisation (Drobe et al., 2008; 2013).
A >300 m deep, ~100 m wide zone of massive, milky quartz flooding occurs near the southwestern edge of the early breccia pipe and has lower copper grades. Drilling to 1000 m below the surface at Mirador has continued to intersect homogeneous hypogene Cu grades of ~0.6% Cu, indicating a vertical geometry to the mineralisation (Drobe et al., 2008; 2013).
Gold is present as fine inclusions divided equally between chalcopyrite and pyrite, as well as in a native form. Molybdenite occurs in a halo of early stage quartz-molybdenite veins with a preferred east-west orientation, outboard of, but partially overlapping, and possibly predating, the main copper mineralisation. The >50 ppm Mo contour defines an ~200 m wide annulus with a diameter of ~900 m (Drobe et al., 2008; 2013).
Mineralisation and alteration at Mirador began as an east-west oriented stockwork of barren, milky, A type quartz veins at ~156 Ma, following emplacement of the early trachytic hornblende-feldspar porphyry dykes at ~158 Ma in a fault and/or fracture zone. Early Cu-Au-Ag-Mo mineralisation, with associated potassic alteration, closely followed the early quartz stockwork, predominantly as disseminations and fine fracture controlled veins of chalcopyrite-pyrite within granite and the early porphyry dykes. The intensity of mineralisation was variable within the early porphyry dykes, which seem to have been variably fractured influencing the ingress of hydrothermal fluids, some being less permeable than the surrounding older granodiorite (Drobe et al., 2008; 2013).
The early breccia pipe appears to have formed after the initial disseminated mineralisation, based on the presence of disseminated chalcopyrite within breccia clasts. Cu-Au mineralisation continued to be formed following emplacement of the breccia, occurring as coarse chalcopyrite, pyrite, anhydrite and rare bornite in open spaces between breccia fragments, mixed with fine comminuted rock matrix, and as disseminations and fine fracture-filling of chalcopyrite peripheral to the breccia. Mo was concentrated in a halo peripheral to the Cu-Au mineralisation. Toward the waning stages of mineralisation, the NE-striking, NW-dipping hornblende-feldspar porphyry dykes intruded all of the mineralised units within the deposit, followed closely by phreatic 'pebble' dykes along reactivated dyke margins as well as the formation of isolated diatremes. The consistent association of dacite dykes with post-mineral breccia dykes suggests the two units are at least in part coeval. The larger, late breccia diatremes on the northwest margin of the deposit area differ in that they are dominated by shale fragments, and while they may be the same age as dacitic breccias, they seem to be rooted in rocks not exposed at surface (Drobe et al., 2008; 2013).
Sparse, thin (<10 cm), subvertical relatively gold rich (>10 g/t Au) veins of massive pyrite, chalcopyrite±galena±sphalerite cut the late hornblende-feldspar porphyry dykes. Although volumetrically insignificant, they present evidence of a minor, very late mineralising event.
In the upper weathered zone, to ~300 m depth, the Zamora granite appears highly fractured. This is a weathering effect, resulting from the hydration of anhydrite to gypsum, which fractures the rock due to the accompanying volume expansion, followed by dissolution of the gypsum from the newly created veinlets. The rock is relatively competent below the level where anhydrite and gypsum are affected by weathering and leaching. Argillic alteration penetrated to depth within the gypsum dissolution fracture system, decreasing from very strong within the supergene zone, to weak at the gypsum-anhydrite front about 300 m below (Drobe et al., 2008; 2013).
Mirador Norte
Geology and Mineralisation
Mirador Norte is located 3 km NNW to NW of Mirador, along the trend of the soil geochemical anomaly. It is relatively recessed and very poorly exposed, although gossanous saprolite after the phyllic alteration halo is exposed in road cuttings.
The geology is less complicated, largely comprising equigranular Zamora granodiorite intruded by primarily NW-SE aligned, but with subsidiary NNE-SSW splays, of dacitic feldspar-hornblende porphyry dykes, and lacks a breccia pipe or any post-mineral intrusions. The mineralisation, dominant alteration, and metal ratios are similar in composition to Mirador but are more structurally controlled, without the coincident circular zoning of metals and alteration. Copper grades are similar in both granodiorite and dacitic porphyry dykes, although on the southern margin of the deposit, the copper grades in porphyry show some variation relative to the granite, with Cu grades changing across dyke contacts. The dike contacts appear to have controlled fluid flow more at the margins of the deposit than at the centre of the deposit, where fracturing may have been more pervasive and less prone to control by lithology.
Hypogene mineralization mainly comprises disseminated and stockwork chalcopyrite, which, as at Mirador, has undergone surficial leaching, preserved as a leached cap up to 40 m thick overlying the secondary enrichment blanket that averages 14 m in thickness. The enrichment zone is immature, with chalcocite coatings on chalcopyrite and pyrite. The enriched zone grades down into primary, disseminated chalcopyrite mineralisation. Higher grade areas are associated with structurally controlled, fine-grained, dark-grey silica flooding that can carry >5% chalcopyrite. Alteration is mostly potassic in the form of black to brown secondary biotite and is almost completely overprinted by a propylitic (chlorite + epidote) assemblage, which, unlike the fringing chloritic alteration at Mirador, is spatially coincident with it. Local coarse anhydrite is preserved at deeper levels below the gypsum front. The potassic alteration assemblage grades into intense quartz-sericite-pyrite alteration along the west side of the deposit, while along the NE side propylitic alteration extends to the north, beyond the potassic alteration. Early and barren quartz veining is only significant in the northwest third of the deposit.
Mineralisation takes the form of overlapping NW-SE aligned, U shaped zones of Mo and Cu that open to the SE, and covers an area of ~750 x 750 m (Drobe et al., 2013).
Resources
Mineral resources at Mirador include (Corriente Resources, 2008) at a 0.4% Cu cut-off were:
Measured + indicated resources - 438 Mt @ 0.61% Cu, 0.19 g/t Au, 1.5 g/t Ag; plus
Inferred resources - 235 Mt @ 0.52% Cu, 0.17 g/t Au, 1.3 g/t Ag.
Mirador Norte which is ~3 km NNW of the main Mirador copper-gold deposit (Corriente Resources, 2008) has at a 0.4% Cu cut-off:
Indicated resources - 171 Mt @ 0.51% Cu, 0.09 g/t Au; plus
Inferred resources - 46 Mt @ 0.51% Cu, 0.07 g/t Au.
Total measured + indicated resources at Mirador and Mirador Norte - 609 Mt @ 0.58% Cu, 0.16 g/t Au
Total inferred resources at Mirador and Mirador Norte - 281 Mt @ 0.52% Cu, 0.15 g/t Au
This summary is largely drawn from: "Drobe, J., Hoffert, J., Fong, R., Haille J.P. and Collins, J., 2008 - Mirador Copper-Gold Project, 30 000 tpd Feasibility Study, Zamora-Chinchipe Province, Ecuador, prepared for Corriente Resources Inc., 160p." and "Makepeace, D.K. and Chapman, P., 2002 - Mirador Project, Corriente Copper Belt Southeast Ecuador, Preliminary Assessment, Technical Report and Cashflow Models, prepared for Corriente Resources Inc., 34p."
The most recent source geological information used to prepare this decription was dated: 2013.
Record last updated: 7/12/2016
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.
Mirador
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Selected References:
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Drobe J, Lindsay D, Stein H and Gabites J, 2013 - Geology, Mineralization, and Geochronological Constraints of the Mirador Cu-Au Porphyry District, Southeast Ecuador : in Econ. Geol. v.108 pp. 11-35
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