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Fruta del Norte |
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Ecuador |
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Au Ag
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Super Porphyry Cu and Au
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The Fruta del Norte intermediate sulphidation epithermal gold-silver deposit is located in the Cordillera del Cóndor region of Zamora-Chinchipe Province, in southeastern Ecuador, ~400 km south of Quito, 225 km SE of Guayaquil, ~80 km ENE of the city of Loja and 20 km SSW of the Mirador porphyry Cu-Au deposit. The deposit is also ~10 km west of the locally ENE-WSW border with Peru.
(#Location: 3° 45' 39"S, 78° 30' 22"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-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.
Geology
Fruta del Norte is located in the mineralised corridor of the Cordillera del Condor, which is contiguous with the Corriente Copper Belt that hosts the Mirador porphyry Cu-Au-Ag deposit, and consists of numerous and sometimes spatially juxtaposed porphyry copper, copper-gold skarn and epithermal gold-silver deposits related to metallogenesis within Jurassic arc-related plutonic and attendant volcanic host units, later subjected to Andean tectonics. The terrane is of serrated medium to high relief between elevations of 1200 and 2400 m asl, flanked by the high Andes to the west and merging with the Amazon Basin to the east.
The geology of the area comprises the following, from oldest to youngest (Hennessey et al., 2008):
Palaeozoic
• Metamorphic rocks of the Cordillera Real, found to the west of the Zamora Batholith, ~35 km west of Fruta del Norte.
Triassic to Jurassic
• Pucara Group - a flat-lying sequence found on the eastern margin of the Zamora Batholith in Peru, ~15 km SE of Fruta del Norte, dominantly composed of shallow-water platform carbonates, with an intermediate unit of deeper-water bituminous calcareous shales.
Jurassic
• Zamora Batholith - an elongate, largely undeformed, composite I-type batholith which parallels the Andean cordillera of Ecuador for over 200 km, with an outcropping width of ~42 km. The batholith extends into northern Peru, and is composed of phases of hornblende diorite, monzonite and syenite with localised porphyritic and aplitic dykes and breccia zones. The main magmatic event however is considered to have occurred between 170 to 190 Ma, with lesser younger phases to 140 Ma that overlap with the inliers of Suarez and Misahualli formations and Fruta Andesite which host the Fruta del Norte deposit (Fontboté et al., 2004). In the vicinity of the Fruta del Norte deposit, several poorly defined mineralised phases of the Zamora batholith have been identified. These include the Tranca-Loma, Camp, Sandia and El Tigre porphyry systems, which exhibit anomalous Cu, Au and Mo mineralisation, including an intersection of 75 m @ 0.30% Cu (Tranca Loma; Leary 2005).
• Misahuallí Formation - which is the host to the Fruta del Norte deposit, occurs as north-south aligned inliers within the Zamora batholith. At Fruta del Norte, it comprises hornblende-plagioclase phyric andesites and basaltic andesites, feldspar porphyritic andesitic intrusives, locally voluminous phreatic breccia zones and lesser planar intrusions, and appears to represent a lava-dominated volcanic pile with associated andesitic domes into which numerous sub-volcanic intrusive dykes have been emplaced. At Fruta del Norte, the sequence has been overprinted by a strong to intense chlorite-epidote-carbonate-pyrite propylitic alteration assemblage and/or strong to intense silicification, that destroys some of the original volcanic textures, resulting in a prevailing rock type that is often aphanitic or texturally nondescript. The main lithotypes are:
Andesitic Lavas - is the dominant rock type in the Fruta del Norte area, occurring as an aphanitic to fine-grained hornblende-porphyritic andesite. Tuffaceous volcanic rocks and inter-bedded sediments are locally found in the north and west of the deposit area, with textures suggestive of a primary volcanic breccia/agglomerate formed in a subaerial environment proximal to a volcanic source. The fine-grained volcanic rocks are cut by two main types of intrusions, namely the coherent intrusive feldspar porphyry andesite described below, and much more limited massive, coarse-grained feldspar-hornblende porphyritic diorite dykes that range from less than one metre to several tens of metres thickness. These dykes appear to constitute a swarm that is best developed in the southern part of the deposit. They are often spatially associated with centre line quartz±pyrite±chalcopyrite±magnetite veins that crosscut the dyke and the volcanics into which the dyke was intruded. Veining is not always present in these dykes, suggesting dyke emplacement over time, with only some of the intrusive phases associated with mineralisation.
Phreatic Tuff Breccia - which is the most prevalent of the different types of brecciation encountered within the Fruta del Norte deposit area. It is a phreatic tuff or possibly phreato-magmatic breccia in the central and southern parts of the mineralised system, and comprises pale grey to white sub-rounded to sub-angular and often heavily illite altered fragments of both feldspar porphyry and hornblende-phyric andesite, supported by a fine-grained matrix of silica-illite-pyrite±carbonate altered rock-flour. The dominant clast type reflects the host rock in which the breccia developed. The breccia often has a well-developed cm- to tens of cm scale fluid flow banding. The overall zone of brecciation occurs as a general tabular or elongate series of splaying breccia zones up to 30 m wide, separated by shatter brecciated blocks of wall rock, and is most intense in the central part of the deposit. Morrison (2007) suggested the phreatic breccia formed in the early stages of epithermal mineralisation and overprints the earlier porphyry system and host rocks, and is overprinted by strong, typically illite-pyrite±carbonate or silica-pyrite epithermal alteration. Epithermal veins are best developed along or adjacent to the breccia-wall rock contacts and can be very poorly developed within the rock flour matrix dominant breccia itself. Breccia zones are best developed on the east side of the deposit near the intrusive/volcanic contact where it attains a stratigraphic height of some hundreds of metres and continues to depth.
Lamprophyre - small aphanitic lamprophyre dykes ranging from 0.3 to 3 m thickness represent the youngest intrusive phase in the Fruta del Norte area. The dykes are post-mineral, commonly intruded into gouge zones in faults that cut mineralisation.
Feldspar Porphyry - a distinct, medium-grained, dark to light grey feldspar porphyry, which contains 30 to 60% phenocrysts, mainly plagioclase with subordinate amphibole and biotite, which is often illite-pyrite to silica-pyrite altered, and is best developed in the northern half of the deposit area. This porphyry outcrops east of the East Fault, and underlies the Suarez Formation in the downthrown block to the west of the same fault. The contact with the Misahualli formation andesites is a locally sharp and commonly chilled, dipping at 65 to 70°W where it is not heavily fault-disrupted. The intrusion appears to have an oval shape, elongated north-south, that formed in the footwall to the volcanic rocks, with multiple planar intrusions cutting the volcanics at the contact. It is widest through the central section of Fruta del Norte deposit, but thins and is faulted out to the north, while to the south it swings away from the mineralisation, and interfingers with the volcanics to no longer represent a coherent body. The intrusion may have either originated as an andesite dome emplaced through an actively accumulating volcanic pile, or alternatively, represent a contemporaneous sub-volcanic intrusion. The porphyry/volcanic contact is almost entirely masked by intense veining and mineralisation in the central to northern part of the deposit, where high grade crustiform-colloform veining is commonly best developed at and above the intrusive contact. The rheological contrast between intrusive and finer-grained volcanic units to the west appears to have resulted in enhanced dilation and hydrothermal fluid flow.
• Suarez Formation - which overlies and flanks the Fruta del Norte deposit to the west, is characterised by three distinct stratigraphic units: i). a polymictic conglomerate; ii). an interbedded felsic tuff to volcaniclastic sandstone; and iii). an uppermost sequence of fine- to medium-grained clastic and calcareous beds and subordinate polymictic conglomerate. The Suarez Formation lies within a 12.5 x 2.2 km pull-apart basin, covering an area of ~16.5 km2. The sequence is locally up to 400 m thick west of the West Fault, thinning to the east and tapering out by the East Fault. The north-south trending vertical East and West faults are 300 m apart, and define the margins of the Fruta del Norte mineralisation towards the northern tapering end of the pull-apart basin where the Suarez Formation is only preserved over a width of ~500 m. Over the course of basin filling, sediments accumulated from reworking of an eroding calc-alkaline volcanic terrane (indicated by the predominance of andesitic cobbles in the conglomerate), with lesser clastic sediments derived from units in Peru (e.g. black mudstones and siltstones of the Triassic Pucara Formation). Sedimentary rocks include debris avalanches and flows, ignimbrite like sheets of tuffaceous sandstone, lahars and other sediment flows, and local coal, diagenetic pyrite wads and dark organic silts which suggest transient anoxic/stagnant conditions representing hiatuses between the main depositional events.
The Suarez Formation forms the immediate hanging wall to the Fruta del Norte deposit, and the lower sections of the unit are characterised by localised strong to intense silicification (±marcasite/pyrite) representing a late epithermal pulse. The lower part of the unit has also been subjected to pervasive chlorite-epidote-pyrite propylitisation which has affected the clasts, but not the matrix. Iron-rich smectite (celadonite) is often abundant in the Suarez above the Fruta del Norte deposit. Intense silica or silica-marcasite alteration is preferentially focused in the conglomerate member as a function of its higher porosity and permeability. Pronounced silicification west of the West Fault, and as discrete zones with increasing stratigraphic height, attests to fluid flow away from the main body of the epithermal system.
• Fruta Andesite - an up to 390 m thick massive and locally columnar jointed lava flow of light blue to green, hornblende/plagioclase-phyric trachyandesite, representing a late (Jurassic) pulse of volcanic activity. The flow overlies parts of the Suarez Formation to the west of the West Fault. Ar-Ar dating of amphiboles from the Fruta Andesite yielded an age of 157 to 155 Ma (Stewart, 2007), which correlate with regional calc-alkaline volcanics and intrusions in the Cordillera del Condor. Although described as an andesite, whole rock analyses suggest a trachyandesite composition, which may be influenced by the high alkalis of hydrothermal alteration and not represent the original magma composition (Hennessey et al., 2008).
Unconformity
Cretaceous
• Hollín Formaton - mid Aptian to late Albian, predominantly stacked cross-bedded sandstones, with thinner intervals of inter-bedded mudstone and sandstone with subordinate shales and associated hydrocarbons (typically 2 to 5 cm seams of high vitrininte coal). The sequence is typically flat lying, with dips of up to 7° to the north of Fruta del Norte due to local uplift.
Recent
• Alluvial cover - including alluvial gold workings.
Structure
The Fruta del Norte deposit is located at the intersection of the north-south trending Peñas Fault Zone (which splits into the West and East faults in the deposit area), the NE-SW Rio Blanco Fault and east-west structures, illustrates the structural context of the epithermal system. Collectively the faults are inferred to have undergone complex histories of normal, reverse and strike-slip motion. The sub-vertical (east-dipping) West fault has a post-mineralisation component that truncates the ore over its whole 1.3 km extent (Hennessey and Stewart, 2006). Fault zones comprise three main types, often a complex hybrid of two or more of: i). breccia; ii). granular gouge and
iii). clay gouge. The cataclastites range from non-cohesive rubble, to cohesive non-foliated, to well foliated. Fault zones range from 1 to 2 m in width, to broad mesh-like zones of anastomosing faults and shears that frequently encompass metre-scale panels of deformed wall rock with granular gouge zones often incorporating rounded survivor clasts, some of which are mineralised, as in the East Fault (Hennessey et al., 2008).
Mineralisation and Alteration
The Fruta del Norte deposit is classified as an intermediate sulphidation epithermal gold-silver system (Sillitoe, 2006), with multi-phase quartz-carbonate-sulphide stockwork veining and hydrothermal brecciation that varies from around 80 to 150 m in width at the central part, increasing to over 300 m at the southern end. The epithermal system has been defined over a strike length of at least 1300 m (Hennessey et al., 2008).
A second zone of mineralisation, Bonza-Las Peñas, is located ~1 km to the south of the southern limit of Fruta del Norte, which is also ~1km long, oriented NNW along the contact between the Misahuallí and Suarez formations.
At shallow levels, the stockwork at Fruta del Norte is composed of closely spaced, multi-directional veinlets, while at depth, more discrete, larger (0.5 m to >5 m in width), banded, west-dipping epithermal veins are found, which progressively increase, as the overall stockwork intensity decreases. Hydrothermal brecciation textures vary from fine, millimetre-scale crackle brecciation to matrix-supported brecciated "veins".
The intensity of veining varies along strike and with depth, dropping to less than 5% at the southern end of the system but up to 100% over broad intervals (tens of metres) at the northern end. Similarly, the sulphide content varies systematically, with the upper central part of the system often exceeding 20% sulphide, as alteration and in veins and brecciation, while decreasing to less than 1% in the quartz veins at the northern end of the system.
Veins typically have classic space-filling epithermal textures throughout the system, with crustiform/colloform banding, cockade and bladed calcite (usually pseudomorphed by quartz) evident. The colloform/crustiform banding varies from very fine (sub-mm) to semi-massive. There is also evidence for recrystallisation of opaline silica and chalcedony.
The mineralogy comprises chalcedonic to crystalline quartz, manganese-carbonates (manganoan calcite with lesser kutnahorite and rhodochrosite), calcite, adularia, barite, marcasite and pyrite, as well as subordinate sphalerite, galena, chalcopyrite with trace tetrahedrite and other silver sulphosalts, with rare accessory minerals that include cinnabar, meta-cinnabar (both restricted to sinter), rhodonite, alabandite (only at depth), stibnite and arsenopyrite (both restricted to the basal Suarez Formation), pyrrhotite, hematite, proustite/pyrargyrite, acanthite, native silver, freibergite, boulangerite and jamesonite and their oxidised products, valentinite or senarmontite.
The majority of the gold is microscopic and associated with quartz, carbonates and sulphides. Much of the gold is "free milling", with coarse visible gold common, usually associated with higher grade zones, although ~40% is locked in sulphides. Individual gold grains range from discrete specks <0.1 mm to "broccoli-like" arborescent crystals >10 mm across. Visible gold occurs in all mineralised zones, in quartz or carbonate as well as within pyrite or silver sulphosalt clusters.
Gold fineness is lower at ~750 in the northern part of the deposit, increasing to >900 in the central part of the deposit, where silver sulphosalts contain a percentage of the silver, enhancing the silver:gold ratios to approximately 1:1 in the upper part of the system. At depth and to the south, the system becomes increasingly silver-rich relative to gold, with silver:gold ratios climbing to 10:1. The increasing silver correlates with increasing zinc and lead assays (Hennessey et al., 2008).
Two upflow zones or feeder structures are interpreted within the Fruta del Norte epithermal system, a West upflow zone that represents the bulk of the gold mineralisation at Fruta del Norte, and an East upflow zone that includes the the Bonza-Las Peñas and its northern extension into Fruta del Norte. The West upflow zone runs the entire length of the Fruta del Norte deposit, strikes roughly north-south and dips moderately to steeply west until it is truncated by the West Fault. The East upflow zone only exists on, and south of, the centre of the Fruta del Norte deposit and dips steeply east with a NNW-SSE strike of about 340°. It is interpreted to be the northern continuation of the Bonza-Las Peñas epithermal system (Hennessey et al., 2008).
The two upflow zones/structures converge near the centre of the Fruta del Norte, where the highest grade part of the system is found. The intersection also marks a mineralogical change with the system transitioning from manganese-carbonate-rich to manganese-carbonate-barren and quartz dominated styles (see the mineralogical zones below). To the south, where the two systems diverge, slight differences in geochemistry between the two become evident, with the West upflow zone generally higher in As and Sb and the East upflow zone containing higher Ag (or Ag:Au ratios), Mn, Pb and Zn. The West upflow zone apparently overprints the East upflow zone (Hennessey et al., 2008).
The mineralised envelope at Fruta del Norte encloses four geochemically, texturally and mineralogically distinct zones, as follows:
i). Lower Manganese Stockwork Zone - the largest and most significant zone within the epithermal system, containing the bulk of the tonnes and gold resources. It is characterised by sheeted veins and wispy stockworks dominated by finely crustiform-colloform banded vein textures. In the western half of the deposit the sheeted veins dip moderately west, rolling over to subvertical or steep east dipping in the eastern half. Alteration is dominated by illite-pyrite with patchy carbonate and silica alteration.
ii). Upper Silicified (High Sulphide) Zone - which overlies the Lower Manganese Stockwork Zone with an abrupt contact, and is noted for its absence of manganese carbonates with manganese assays typically <300 ppm. Mineralisation is typically characterised by intense, chaotic stockwork and hydrothermal brecciation composed of chalcedony-marcasite with associated alteration comprising very intense silicification and finely disseminated marcasite, often so pervasive the rock is black. The dominant host is andesite.
iii). Upper Silicified (Low Sulphide) Zone - which is of limited extent, in the central part of the epithermal system, immediately below the Suarez Formation. It is characterised by intense silicification but is depleted in sulphide, grading downwards into the sulphide-rich zone with a transitional contact suggestive of oxidation/alteration along fractures. Stockwork veining and hydrothermal brecciation are evident, both as sulphide-poor veins and as an overprinting sulphide-rich vein phase, interpreted to be related to late epithermal events. This zone is interpreted to represent a palaeo-surface weathering horizon prior to burial of the deposit by Suarez sediments in the Jurassic (Sillitoe, 2007).
iv). Northern Quartz Vein Zone - located at the northern end of the system, characterised by intense quartz veining, usually with >50% quartz, and up to 100% quartz vein material over broad intervals. The quartz is typically white, massive, recrystallised to poorly-banded chalcedony with minor wispy sulphides and rare quartz-replaced bladed calcite textures. This zone also lacks manganese carbonates and is relatively low in sulphide compared to the first two zones described above.
The alteration assemblages within the host andesite comprises proximal intense silicification with fine disseminated sulphide (primarily pyrite and marcasite) within a broad illite-pyrite+silica halo, grading outward into widespread propylitic (chlorite-epidote-calcite) alteration. Smectite (often iron-rich celadonite) and rare kaolinite occurs high in the system, passing downwards into illite, with sericite locally at depth. Carbonate alteration is usually present within the manganese-carbonate stockwork zone.
Alteration is most intense in the upper part of the system, extending to depth along the West Fault and progressively weakening to the east and at depth. The upper part of the system often exhibits such intense silicification and abundant fine sulphide that the host lithology is sometimes difficult to identify (Hennessey et al., 2008).
The epithermal alteration assemblages overprint earlier porphyry-style alteration, evident as rare remnants of patchy potassic (biotite) alteration, while at depth, epithermal veins cut early centre-line porphyry style veinlets. Blotchy propylitic (epidote/chlorite) alteration, interpreted as being porphyry related,is also evident at depth and distal to the epithermal system.
Resources
Published ore reserves and mineral resources (Kinross Gold Corp., 2012) at December 31, 2011, were:
Proven + probable reserves - 25.440 Mt @ 8.21 g/t Au, 11.0 g/t Ag; (not included in resources)
Measured + indicated resources - 25.440 Mt @ 8.21 g/t Au, 10.3 g/t Ag;
Inferred resources - 22.093 Mt @ 5.13 g/t Au, 10.4 g/t Ag.
Published mineral resources (Lundin Mining., 2015) at October, 2014, were:
Indicated resources - 23.5 Mt @ 9.59 g/t Au, 12.9 g/t Ag (225 t of contained Au);
Inferred resources - 14.5 Mt @ 5.46 g/t Au, 11.3 g/t Ag (79 t of contained Au).
This summary is largely drawn from "Hennessey, B.T., Puritch, E.J., Gowans, R.M., Leary, S.F., 2008 - A mineral resource estimate for the Fruta Del Norte deposit, Cordillera del Condor Project, Zamora-Chinchipe Province, Ecuador; an NI 43-101 Technical Report prepared for Kinross Gold Corporation by Micon International Limited, 232p."
The most recent source geological information used to prepare this decription was dated: 2008.
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.
Fruta del Norte
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Selected References:
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Leary, S., Sillitoe, R.H., Stewart, P.W., Roa, K.J. and Nicolson, B.E., 2016 - Discovery, Geology, and Origin of the Fruta del Norte Epithermal Gold-Silver Deposit, Southeastern Ecuador: in Econ. Geol. v.111, pp. 1043-1072.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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