GEOLOGICAL SETTING & MINERALIZATION
Ecuador’s main physiographic elements broadly coincide with geological domains. The flat-lying western Costa forearc and the eastern Oriente foreland areas bound the Andean mountain chain that splits into two segments, the Western (Occidental) and Eastern (Oriental) cordilleras. These two cordilleras in Ecuador are separated by the Inter-Andean Depression.
Volcanic/magmatic arcs within Ecuador span the Jurassic to Quaternary periods. The Bramaderos property lies near the junction of Cretaceous and Miocene age magmatic arcs.
Some of the oldest exposed rocks in Ecuador are of Palaeozoic to Precambrian age that outcrop as a fault-bound inlier along the western side of the Eastern Cordillera.
A Jurassic volcanic arc broadly coincides with the eastern side of the Eastern Cordillera and extends the full length of Ecuador but terminating in northern Peru. This Jurassic arc hosts several porphyry Cu-Au and epithermal Au deposits, some economic, at Mirador, Mirador Norte, Panantza-San Carlos and Fruta de Norte in southeast Ecuador.
Magmatism retreated ocean-ward during the Cretaceous, with a Cretaceous arc running the length of Peru and encroaching northward and terminating in southernmost Ecuador. This Cretaceous belt is not known to be a major porphyry belt other than for a short segment of the belt in coastal central Peru where it hosts the Los Pinos and Almacen deposits and Ellana, Tingo, Puquio, Anita de Tibilos and Chavez N2 prospects. The northern end of this Cretaceous arc includes the large Tangula batholith which lies just south of the Bramaderos property, and which is a member of the coastal batholiths further south in Peru.
During the Late Cretaceous, several allochthonous (i.e. transported from a distance) oceanic terranes were accreted to the Ecuadorian margin and form the basement to the forearc (i.e. the Costa forearc) of the presently active Quaternary volcanic arc that runs along the Inter-Andean Depression. In southwest Ecuador the allochthonous Amotape terrane, which is a distinct tectonic block that extends into northern Peru, accreted to the Ecuadorian margin during the Palaeozoic to earliest Cretaceous. The El Oro Massif is a microtectonic block at the northern edge of the Amotape terrane which imparts an east-southeast structural trend that is anomalous relative to the main north-northeast trend of Ecuador.
The stratigraphic sequence that dominates the Bramaderos property forms part of the Cretaceous-age Celica-Lancones Basin in southwest Ecuador and northwest Peru. This extensional basin formed over the Amotape Block and its suture with the Andean margin (Jailard et al., 1996). The basin hosts large VMS deposits at Tambo Grande in Peru and was deformed in the Late Cretaceous.
During the subsequent Palaeocene to Eocene epochs, extensive volcanics of the Sacapalca Formation erupted east of the Cretaceous arc and along the southerly extension of the Inter-Andean Depression.
Superimposed on these older volcanic arcs is a zone of Miocene magmatism which runs north-south along the full length of Ecuador, and which hosts numerous porphyry and high-sulphidation deposits/prospects. The Miocene Belt is quite wide in east-west dimension in central-south Ecuador where intrusions of diorite, quartz diorite and granodiorite form the ESE-trending Cangrejos-Zaruma intrusive belt controlled by a deep-seated or crustal scale fracture system along the northern edge of the Amotape terrane.
The Miocene epoch was an exceptionally metallogenically fertile period in southern Ecuador and northern Peru. A belt of magmatic-related Au-Cu and Cu-Au deposits, mostly porphyry-related systems, of Miocene age extend from around 3oS in Ecuador down to 8.5oS in Peru. The Miocene belt is less prolifically mineralized north and south of this latitude range. This interval of high Cu-Au fertility is genetically related to the subduction of the Inca Plateau beneath this sector of the Andean margin during the Miocene. The inland deflection of contours on the Wadati-Benioff Zone (depth to zone of seismicity related to the subducting plate) is a consequence of buoyant and flat subduction of the Inca Plateau segment of the plate, resulting in widespread compressional tectonics in the over-riding plate which, a key factor in porphyry Cu-Au metallogenesis.
The local geology of the Bramaderos region is quite poorly known, with minimal historical mapping effort compared to other parts of the country. Unit and Formation names vary significantly between various published papers as ongoing work by several university-based research groups try and unravel the complex stratigraphy of this part of southwest Ecuador.
The geology of the region is discussed in relation to the Cariamanga 1:100,000 map sheet.
At this local scale, the geology is dominated by the Cretaceous volcaniclastic sediments belonging to the extensional Celica-Lancones basin, and which are intruded by the Cretaceous-age composite Tangula Batholith. The dominantly granodioritic Tangula Batholith has metamorphosed the surrounding Cretaceous sediments.
The Celica Formation in the region inter-tongues with the overlying Alamor Formation elsewhere, and occurs across the Bramaderos property and south of it. It comprises submarine andesitic volcanics that represents the products of a volcanic arc of Albian (lower Cretaceous) age. The best sections of the Celica Formation Lower Cretaceous arc series (over a 1000m thick) and its sedimentary cover are located in the fault-bounded Rio Playas depression west of Catacocha.
In detail the Celica Formation (which hosts the porphyritic and subvolcanic intrusions at Bramaderos) consists of massive lava flows, welded tuffs, pillow breccias, conglomerates and agglomerates of mainly andesitic composition with some intercalated limestone lenses.
The overlying Alamor Formation is poorly defined and poorly understood and may represent the upper part or a lateral equivalent of the Celica Formation.
Higher in the sequence the 150-200m thick Naranjo Formation unconformably overlies the Celica or Alamor volcaniclastic rocks. The lower parts comprise pebbly and fossiliferous calcareous marls and coarsens upward into marls, fossiliferous limestones and greywacke beds.
The Casanga Formation (200-400m thick) overlies the Naranjo Formation comprises turbiditic greywackes with nodular limestone.
Unconformable Continental Deposits overly the Casanga Formation and comprise purple-coloured red-beds and volcanics including continental shales, siltstones and volcaniclastic beds that crop out east of El Naranjo.
The Rio Playas Formation – could be restricted to yellow-coloured sediments (conglomerates, sandstones and shales) that unconformably overly the undated Red Beds, the Casanga or Alamor Formations.
The Unconformable Continental Deposits and Rio Playas Formation could be westward lateral equivalents of the Sacapalca Formation of Palaeocene to Eocene age. The Sacapalca Formation east of Bramaderos comprises thick sub-aerial andesitic tuffs and agglomerates with fluval red-beds in the upper parts. Near Nambacola, Catamayo and Catacocha, capping fluvial red-beds on top of the Palaeocene-Eocene Sacapalca Formation might correlate with the undated Unconformable Continental Deposits and Rio Playas Formation described above.
Finally, a series of porphyritc diorite stocks and plugs intrude the Celica Formation within the Bramaderos property. Similar stocks are shown further east penetrating the Sacapalca Formation, and so these were considered to be of Tertiary age. However, historical dating of a quartz-diorite stock at Curiplaya, obtained a U/Pb age of 92 +/- 1.0 Ma which concluded the Curiplaya porphyry intrusions as being Cretaceous age.
The geology of the Bramaderos property comprises Cretaceous volcanics of the Celica Formation which have been intruded by quartz diorite and hornblende diorite porphyritic intrusions along NNE- to NE-trending structures, and potentially localized by WNW-trending cross-structures.
Mapping by Ecuador Gold defined a minimum 3km by 1km size intrusion of hornblende diorite which is elongate WNW, hosts the main mineralization at the Bramados Main porphyry prospect, and defines a WNW-tending ridge, herein called the “Bramaderos Ridge”. Further north lies a mapped quartz diorite intrusive of similar dimension intruding the Celica Formation. Smaller bodies of younger rhyodacite tuff overlie these intrusions.
Extensive zones of argillic alteration affect the Celica Formation over an area exceeding 4 x 1 km, and were mapped by EcuaGold as phyllic alteration forming a NNE-trending belt that encompasses the Bramaderos Main and Porotillo prospects as well as other centres of porphyry stockwork mineralization further to the SSW. In addition, a >2km by 1km NNW-trending belt of argillic alteration at the Limon porphyry prospect is associated with a strongly leached cap above locally exposed porphyry quartz vein stockworks. Further afield the extensive areas of argillic alteration are transitional through intermediate argillic alteration to peripheral propylitic alteration.
Several centres of potassic alteration are associated with the main areas of outcropping quartz stockwork mineralization at the porphyry prospects.
Mineralization on the Bramaderos property is dominated by a cluster of broadly north-northeast aligned centres of porphyry-related quartz stockwork mineralization associated potassic alteration centered within and above centers of hornblende diorite intrusions. These intrusive centers are overprinted and surrounded by extensive zones of phyllic and argillic alteration that collectively overlap and merge into an altered corridor approximately 5 kilometres long by up to 1.5 kilometres wide.
The Limon porphyry system lies slightly off trend, northeast of the linear north-northeast trend of quartz stockwork systems that define the prospects at Bramaderos Main, Melonal, Porotillo, Ghangue Stockwork and Yeso Bramaderous.
The potassic cores and associated quartz stockworks tend to form distinct conical shaped hills on the property and are surrounded at lower elevations by zones of phyllic and argillic alteration that inturn are transitional outward to propylitic alteration.
Lying in peripheral areas west of these centres of porphyry mineralization are two main epithermal prospects – West Zone Breccia and Bramaderos Vein.
Each of the main mineral prospects are discussed sequentially below.
The Limon prospect, located 2.5km northeast of the Bramaderos Main prospect, is an undrilled porphyry Au-Cu system that lies northeast of the main NNE Bramaderos trend. Access to the area is gained by walking track (approximately 1-hour) east from the Pan American highway. The area is dominated by a an elongate zone of highly ferruginous and leached phyllic/argillic alteration that represents a classic leached cap over (and/or lateral to?) the exposed quartz stockwork mineralization. The area is marked by a natural vegetation anomaly that is 1.2km long by around 400 metres wide which is centred on the southeast-draining “Limon Creek”, a tributrary that flows into the Rio Playas. The vegetation anomaly, which may be exacerbated by livestock grazing, reflects the nutrient-poor rocks caused by severe acid-leaching as a result of oxidizing sulphide (dominantly pyrite) which generates acid in the weathering profile.
The upper northwest bank of the Limon Creek alteration exposures is associated with outcropping and intense quartz stockwork veining in fine- to medium-grained hornblende diorite, in the vicinity of 634,400mE, 9551030mN. Quartz vein stockworks vary from moderately developed sets of bi-directional quartz B-veins in weakly magnetic intrusive protolith to intense multi-directional quartz stockworks and sheeted quartz B-veins. Locally these areas of strong quartz stockwork veining are associated with fracture-controlled smears of malachite generated by surface oxidation. The presence of elevated Cu values and the intense leaching observed at surface both suggest that significant copper grades can be expected at depth below the level of near-surface oxidative leaching.
To the southeast of the main stockwork exposures relict quartz B-veins are locally observed within the lower parts of the argillized and leach cap, suggesting that the extent of stockwork veining in the sub-surface will significantly exceed the areas of their most obvious surface exposure.
The Bramaderos Main prospect is a porphyry Au-Cu system located near the centre of the Bramaderos property. It has received the most drilling of all prospects on the property and with the most encouraging and consistent Au intersections.
A broad and likely structurally-controlled northwest-trending ridge (“Bramaderos Ridge”) extends southeast from the conical Bramaderos Hill and hosts stockwork and sheeted quartz veining over an area of approximately 840m in the northwest dimension by 440m in the northeast dimension. Veins comprise both quartz and quartz-magnetite B-veins as well as fine magnetite micro-veins. The stockwork and potassic alteration is focused on quartz hornblende hornblende diorite intrusions that have intruded volcaniclastic breccias, tuffs and volcanic sediments of the Celica Formation.
The main alteration minerals in the potassic alteration zones are silica, magnetite, chlorite +/- relic biotite +/- epidote +/- actinolite +/- pyrite +/- chalcopyrite. The zones of potassic alteration at Bramaderos Main have been deeply overprinted by retrograde phyllic and/or argillic and intermediate argillic alteration, such that biotite is largely converted to chlorite, magnetite is partially converted to hematite, illitic clays are widespread and the main indicator of the potassic precursor alteration is the high abundance of disseminated and veinlet magnetite.
The mineralized quartz veins at Bramaderos Main are typically B generation veins that typically form thin, spidery and curving networks as well as zones of strong sheeted veining suggesting local structural controls on the stockworks.
The wall rocks to the stockwork veined intrusive rocks tend to weather recessively due to the widespread phyllic/argillic alteration that is most intensely developed in the more permeable strata of the host Celica Formation. A number of gullies incise deeply into the sides of Bramaderos Ridge and Bramaderos Hill, and the surrounding hills. These gullies expose thick sequences of late-stage argillic alteration that is associated with typically red-orange hued clays due to oxidation of sulphide, predominatly pyrite from the interpreted ‘pyrite shell’ that forms a halo around the Bramaderos Main porphyry system.
The lower flanks of Bramaderos Ridge and Bramaderos Hill are characterized by an abundance of late-stage gypsum veining which is interpreted to reflect oxidation of sulphides during weathering, and subsequent downward transport and reprecipitation of sulphate as gypsum in late fractures.
The Melonal prospect area comprises silica-sericite-altered quartz diorite with up to 5% disseminated and veinlet-controlled iron oxides produced by oxidation of pyrite. Moderately to strongly silicified porphyry outcrops occur on the summit of the Melonal hill and highly fractured, ferruginous and gold-bearing andesitic tuffs of the Celica Formation occur along the northern margin of the prospect. Argillic-altered andesitic lapilli tuff has also been noted from Melonal along with propylitic- and sericite-altered quartz diorite float. Quartz stockwork veining has been mapped historically at Melonal over an area of 650m north-south by around 200m east-west. The abundance of pyrite described at surface may mean that Melonal lies within the western pyrite halo to the Bramaderos Main porphyry system, although this concept will be tested with ongoing exploration.
Porotillo is a NW-elongate and conical-shaped hill located midway between Melonal and Ghangue Stockwork. It is cored by an quartz hornblende diorite intrusive with widespread, generally sparse, but locally more intense quartz stockwork veining. A shallow 2-3m thick east-dipping bed of beige-coloured and medium-grained granular gypsum has been mined on a small-scale near the southeast end of the Porotillo Prospect.
A quarry located along the access road on the north side of Porotillo exposes a highly fractured and magnetic quartz diorite intrusive with abundant disseminated magnetite and magnetite micro-veinlets with rare porphyry-related quartz B-veins. Minor malachite spotting and smears occur in places associated with oxidized chalcopyrite-bearing quartz veins.
West Zone Breccia
The West Zone Breccia is currently understood to be an epithermal mineralized breccia body located 2.2 kilometres northwest of the Bramaderos Main prospect, although it has also been described by historical workers as a shear zone.
Historical work depicts a strongly Au-mineralized breccia approximately 140m long by approximately 75m wide with a low-grade marginal breccia facies which inturn lies within a hornblende diorite porphyry intrusion.
The central part of West Zone is characterized by dacite porphyry that is intruded by a fractured, light to dark grey, angular clast-supported intrusive/hydrothermal breccia 200m by 70m in dimension and located in a fault striking 30o and dipping 65o to the southeast. The breccia matrix is described as being siliceous. The area contains argillic alteration on surface and sericitic alteration in fresh rock. Hematite and jarosite are observed in fractures, with primary pyrite content estimated to be less than 10%.
The presence of elevated As and Sb at the West Zone Breccia in conjunction with local vuggy textures leaves open, though no means confirms, the possibility that West Zone is a high-sulfidation breccia body. Further trenching and mapping is required to clarify the genesis of the West Zone mineralization.
The best trench was WZTR-2 which yielded 41.7m @ 3.70 g/t Au within oxidized quartz-sericite-pyrite altered hydrothermal breccia. Trench WZTR-5 yielded 22m @ 4.74 g/t Au within strongly silicified hydrothermal breccia with disseminated pyrite.