The La Colosa porphyry Au deposit is located on the eastern flank of the Central Cordillera of Colombia, within the Middle Cauca metallogenic belt. The deposit contains more than 800 t (22.37 Moz) Au at grades up to 0.8 g/t and is hosted by a composite porphyry stock of dioritic to tonalitic composition, which was emplaced into Triassic-Cretaceous schists of the Cajamarca Complex in the late Miocene (~8 Ma).
The country rocks underwent two ductile deformation events, including development of shear zones, folds, and penetrative foliation, prior to emplacement of the stock. Subsequent brittle deformation reactivated preexisting N- and NNE-trending structures and formed secondary faults due to a change from right- to left-lateral shear sense on regional faults. This switch in stress orientations is attributed to a new plate configuration in the mid-Miocene. The left-lateral movement along regional faults favored emplacement of intrusive centers in dilational pull-apart zones, including the La Colosa stock within the regional Palestina fault zone.
The La Colosa porphyry stock was intruded in three stages, termed early, intermineral, and late, all within a relatively short time interval of ~1.1 m.y. The early and intermineral stages are diorite porphyries and related intrusion breccias, whereas the late stage consists of quartz diorite and tonalite porphyries. The intrusions caused contact metamorphism and hydrothermal alteration that partially obliterated the original texture and composition of the schistose country rocks. The early and intermineral stages are dominated by potassic alteration, with local chloritic alteration in the core of the intermineral stage, sodic-calcic alteration in the deeper parts of the stock, and propylitic alteration confined to the late stage.
Three gold mineralization events are recognized at La Colosa. The first was of porphyry style, during which hypersaline fluids (40–50 wt % NaCl equiv) formed predominantly A- and S-type veinlets and caused multistage wall-rock silicification accompanied by potassic and sodic-calcic alteration. The early-stage intrusions contain the highest gold grades varying from 0.75 to 1 g/t Au, associated with pyrite and minor chalcopyrite, molybdenite, and magnetite in the porphyries and with pyrrhotite-pyrite-melnikovite in the country rocks. In the intermineral-stage intrusions the gold grades drop to 0.5 to 0.75 g/t Au, and pyrrhotite and pyrite are the major sulfides. Gold grades reach low values of <0.3 g/t Au in the late-stage porphyries.
The second gold-precipitating event formed sheeted veinlets of drusy quartz and pyrite with centimeter-wide halos of albite-sericite-pyrite overprinting all other alteration types at the deposit. A ~200°C hydrothermal brine (21–28 wt % NaCl equiv) deposited gold at high grades (>1.5 g/t Au over >10 m drill core intervals) within N-striking normal faults that developed during and after emplacement of the porphyry stock. The third mineralization event was supergene, with Au enrichment confined to late porphyries and characterized by sulfide boxworks, resulting in gold grade increases from 0.3 to 1.2 g/t Au.
Colombia has been a major gold producer since precolonial times. Currently known gold deposits include orogenic gold, porphyry Cu-Au, porphyry Au, transitional porphyry-epithermal, epithermal, and placer gold (Sillitoe et al., 1982; Rodriguez and Warden, 1993; Sillitoe, 2008). The La Colosa deposit represents a major porphyry gold deposit within the metal-rich Middle Cauca metallogenic belt of Colombia.
The La Colosa deposit, 100% owned and explored by Anglo-Gold Ashanti Ltd, is located 30 km west of the city of Ibague, capital of the Department of Tolima, Colombia (Fig. 1). The deposit site is characterized by steep topography on the eastern flank of the Central Cordillera with elevations between 2,300 and 3,500 m above sea level (a.s.l.). The deposit was discovered in 2006 during an extensive greenfields exploration program. Current ore estimates based on 139,000 m of drilling (AngloGold Ashanti, 2016) include an indicated resource of 821 million tonnes (Mt) at 0.85 g/t Au (22.37 Moz) and an inferred resource of 242 Mt at 0.78 g/t Au (6.1 Moz).
Information from surface exposures and drill holes was used to develop the first description of the geology and hydrothermal alteration of the deposit (R. Sillitoe, unpub. report, 2007; Lodder et al., 2010). The present paper integrates new information on rock types, structures, and hydrothermal alteration as well as geochronology and some fluid inclusion analyses. These new data permit constructions of a detailed 3-D model of the deposit, and they provide insights into the structural, magmatic, and hydrothermal events that formed this significant gold deposit in the northern Andes.
In 1984 the Colombian Geological Survey conducted a stream sediment survey in the central part of the Central Cordillera in the Departments of Quindio and Tolima. During this survey, several geochemical anomalies were identified, including an anomalous zone related to the San Antonio gold mine, where minor amounts of gold were exploited from hydrothermal breccias hosted by siliceous schists of the Cajamarca basement complex (Lodder et al., 2010). In 1997, Greystar Resources Ltd. drilled two holes into these hydrothermal breccias, with assays indicating erratic, anomalous concentrations of gold, copper, molybdenum, and arsenic (Lodder et al., 2010).
From 1999 to 2003, AngloGold Ashanti launched a comprehensive greenfields exploration program for porphyry and epithermal gold deposits in the Eastern, Central, and Western Cordillera of Colombia, which included regional database compilation, target generation, and field prospect analysis. The target generation permitted definition of prospective blocks, one of which was the Ibagué-Mariquita block located along the eastern flank of the Central Cordillera, Department of Tolima. Based on the information collected by the Colombian Geological Survey, Greystar Resources, and Conquistador Mines, a visit was made by AngloGold Ashanti to the San Antonio area in 2000 as a part of an agreement with Conquistador Mines. At this time, access to the area was restricted because of unpredictable security conditions. However, during the brief visit, boulders of hydrothermally altered diorite porphyry with porphyry-style veinlets were identified, and the San Antonio area was recommended for follow-up exploration once the security situation allowed it (Lodder et al., 2010).
In 2006, field work started in the Ibagué-Mariquita block with a stream sediment (–200 mesh) sampling program. In this prospective block, several geochemical anomalies were encountered, and the stream sediment sample from La Colosa creek (0.7 g/t Au) was found to be associated with boulders of diorite porphyry containing quartz-sulfide and magnetite veinlets and biotite-rich schist (Lodder et al., 2010). Subsequent mapping and rock sampling in La Colosa Creek identified two areas with potassic alteration and porphyry-style veinlets. The first diamond drilling program began in February 2007. The initial holes, COL-001 and COL-002, intercepted 138 m with 1.51 g/t Au and 220 m with 1.45 g/t Au, respectively. These findings confirmed that the surface geochemical anomaly persisted at depth and indicated the potential for a >150 t (>5 Moz) gold deposit. Resource drilling continued through to 2016. The continuations of the gold resource to the north and at depth are currently still undefined.
Tectonic Setting of the Northern Andes
In their northernmost extension, the Andes divide into three main branches, the Western, Central and Eastern Cordilleras (Fig. 1). The Western and Central Cordilleras are separated by the Cauca Valley, and the Central and the Eastern Cordilleras are separated by the Magdalena Valley, an intermontane basin. The Llanos basin marks the foreland basin between the Andes and the Guyana Shield.
History of terrane accretion and related magmatism
Each branch of the northern Andes is characterized by its own tectonic history, linked to interactions between the Caribbean, Nazca, and South American plates (Burke et al., 1984; Kellogg and Vega, 1995; Meschede and Frisch, 1998; Cediel et al., 2003). These interactions resulted in complex combinations of subduction, magmatism, terrane accretion, thrusting, and uplift as well as local extension and formation of foreland basins (Aspden and McCourt, 1986; Taboada et al., 2000; Cediel et al., 2003; Corredor, 2003; Moreno-Sanchez and Pardo-Trujillo, 2003; Gómez et al., 2005a, 2005b; Sarmiento-Rojas et al., 2006; Bayona et al., 2008; Nie et al., 2010; Spikings and Simpson, 2014; Mora-Bohórquez et al., 2017). Based on the geologic history, it has been postulated that several terranes accreted episodically to the South American continent between the Cretaceous and the Miocene (Cediel et al., 2003; Restrepo et al., 2011).
The Eastern Cordillera (Chicamocha terrane after Cediel et al., 2003, or Chibcha terrane after Restrepo et al., 2011) is an asymmetric, doubly vergent fold-and-thrust belt of predominantly Mesozoic sedimentary sequences overlying Paleozoic and Precambrian basement rocks (Gómez et al., 2005a, 2005b; Moreno et al., 2011; Van der Lelij et al., 2016). The Central Cordillera contains an eastern terrane (Cajamarca-Valdivia terrane after Cediel et al., 2003, or Tahamí terrane after Restrepo et al., 2011), which is composed of autochthonous Paleozoic basement rocks and volcano-sedimentary sequences of the deformed and metamorphosed Cajamarca Complex (Ordoñez-Carmona et al., 2006; Villagómez et al., 2011; Blanco-Quintero et al., 2014; Martens et al., 2014). Plutonic rocks of Jurassic-Cretaceous age, including the Ibagué and Antioquia batholiths, intruded the basement rocks of the Central Cordillera and reflect magmatic activity during subduction of the Farallon plate (Spikings et al., 2015). The boundary between Chibcha terrane (Eastern Cordillera) and Tahamí terrane (eastern part of the Central Cordillera) is interpreted to be localized along the NE-trending Otu-Pericos and NNE- to N-trending Palestina fault systems (Restrepo et al., 2011; Martens et al., 2014; Mora-Bohórquez et al., 2017).
The authochthonous rocks of the Central Cordillera are divided from the oceanic crust of the Western Cordillera by the Romeral fault zone, a tectonic melange several kilometers wide (Moreno-Sánchez and Pardo-Trujillo, 2003; Villagómez et al., 2011; Mora-Bohórquez et al., 2017). The Romeral fault zone cuts the Cretaceous oceanic arc of the Quebradagrande Complex, which was accreted to the Central Cordillera at about 117 to 107 Ma (Moreno-Sánchez et al., 2008; Villagómez et al., 2011; Blanco-Quintero et al., 2014). The allochthonous Western Cordillera consists of oceanic terranes with predominantly mafic to ultramafic igneous and volcano-sedimentary rocks that represent the accreted remnants of the Caribbean large igneous province, which collided with South America at ~75 Ma (Villagómez et al., 2011; Spikings et al., 2015). This collisional event caused the formation of a subduction-related magmatic arc (Bayona et al., 2012), tectonic shortening, the onset of uplift and exhumation of the Central Cordillera, and formation of the foreland basins of the Middle Magdalena basin east of the Central Cordillera during the Paleocene (Nie et al., 2010; Saylor et al., 2010; Borrero et al., 2009; Parra et al., 2012).
The age of major deformation in the Central Cordillera at the latitude of the Middle Magdalena Valley basin is early-to mid-Eocene, as evidenced by the Middle Magdalena Valley unconformity. This regional pediment surface, which developed over approximately 15 m.y., formed during eastward migration of the axis of uplift in the Central Cordillera (Gómez et al., 2005b; Moreno et al., 2011; Parra et al., 2012).
In the Miocene, the Panama-Choco block collided with the Western Cordillera (Barat et al., 2014), producing the present-day northward convexity of Panama. It also induced further uplift of the Central Cordillera, exhumation of deeper crustal levels, and formation of an intermediate-silicic magmatic arc related to eastward subduction of the Nazca plate (Coates et al., 2004; Borrero et al., 2009; Nie et al., 2010; Farris et al., 2011; Barat et al., 2014). The present-day manifestation of ongoing volcanic activity is the Pliocene-Quaternary volcanic belt of the Northern volcanic zone (Fig. 2; Borrero et al., 2009; Londoño, 2016).
Present-day deformation in the Central Cordillera
The Central Colombian block, which is bounded to the south by the Ibagué fault, to the west by the Romeral suture zone, to the east by the Eastern Cordillera, and to the north by the left-lateral Bucaramanga strike-slip fault, is characterized mainly by NE-striking thrust faults and a few SE-trending strike-slip faults (Fig. 1; Corredor, 2003).
Based on analysis of earthquake focal mechanisms and kinematic fault slip data, it has been shown that the ESE-subducting Caribbean plate is still influencing the deformation style to approximately 5° N latitude, inducing left-lateral displacement along major fault systems, such as the Romeral fault system (Ego et al., 1996; Corredor, 2003). According to Vargas and Mann (2013) a W-trending crustal-scale structure, the Caldas tear (Fig. 1), can be inferred by means of earthquake focal analysis. This structure traverses the Cordillera and may be correlated with the northern boundary of the Pliocene-Quaternary Northern volcanic zone.
According to global positioning system (GPS) measurements and geophysical data, eastward subduction of the Nazca plate is continuing with a convergence rate of approximately 6 cm/y relative to stable South America (Fig. 1; Colmenares and Zoback, 2003; Egbue and Kellogg, 2010). The oblique convergence along the western margin of northern South America causes northeastward sliding of the entire North Andean zone (Ecuador and Colombia) along the major fault systems (Veloza et al., 2012; Alvarado et al., 2016).
Structural Geology and Deformation History at La Colosa
The La Colosa deposit is located east of the continental divide on the crest of the Central Cordillera, between the regionally extensive Romeral fault zone (approx. 6 km to the west of La Colosa) and the Palestina fault system (Fig. 2). Based on differing definitions of tectonic terranes in the Central Cordillera, the La Colosa deposit is located within the Tahamí terrane (Restrepo et al., 2011) or the Cajamarca-Valdivia terrane (Cediel et al., 2003).
La Colosa is situated within a belt of alternating green and black schists and metasedimentary rocks of the Cajamarca Complex. The protoliths of this sequence are interpreted to be a volcano-sedimentary sequence (Blanco-Quintero et al., 2014). The entire Cajamarca Complex underwent green-schist- to amphibolite-facies metamorphism. In the northern part of the Cajamarca Complex (north and west of the city of Medellín), U-Pb ages indicate that this metamorphism was of Triassic age (U-Pb SHRIMP dating on zircons; Restrepo et al., 2011). However, rocks of the Cajamarca Complex between the city of Ibagué and La Colosa have yielded 40Ar/39Ar plateau ages that record a Late Jurassic (157.8 ± 0.6–146.5 ± 1.1 Ma) metamorphic event (Blanco-Quintero et al., 2014).
Based on field observations, drill core data, and published information on the tectonic evolution of the northwestern Andes, the deformation history at La Colosa can be divided into two preporphyry ductile deformation events followed by a syn- to postporphyry brittle deformation event (Horner et. al., 2016; Fig. 3). These are described separately in the following paragraphs.
The most obvious feature of ductile deformation D1 is a penetrative foliation, S1, which often lies parallel to the original sedimentary layering, S0. The affected rocks typically display millimeter- to centimeter-wide spacing of individual foliation planes. Crenulation, small-scale folding, and lenses of remobilized quartz are common. The S1 foliation generally strikes north-south (NW-SE to NE-SW) and dips steeply to the east and west. The predominance of E-dipping foliation planes indicates closed, slightly W vergent folding. Fold axes B1 plunge subhorizontally to moderately to the north and south. Ductile shear zones are common. They are often oriented parallel or subparallel to foliation S1; however, their orientation is more variable with strikes to the north, northeast, and northwest. Dips ranges from vertical to shallow.
Locally a second foliation, S2, can be observed, in particular in the westernmost part of the La Colosa area, approaching the Romeral fault zone and the Quebradagrande Complex. This second ductile deformation event, D2, overprints S0 and S1 (Fig. 3). The second foliation S2 forms open folds with shallow to moderately dipping limbs; fold axes B2 show shallow to moderate plunges toward the east and southeast. Shear zones with moderate to shallow dips to the east resemble the axial surface plane of the D2 fold structures. At or close to ductile shear zones, quartz remobilization, crenulation, and folding are more intense.
Brittle deformation (faulting, fracturing, and brecciation) is evident in the metamorphic basement rocks of the Cajamarca Complex as well as in the younger intrusive stocks of the La Colosa area. N-striking faults follow inherited weaknesses at existing ductile shear zones and along fold structures. However, NE- and NW-striking brittle faults are the most prominent brittle structures. The main NE- to NNE-striking structural feature is the regional Palestina fault system, characterized by a series of parallel faults, such as the La Ceja fault or the NE-3 fault (Fig. 4), with minor faults branching off and joining the main fault strands. Based on drilling information, individual faults consist of multiple fault core and damage zones reaching a width of up to 250 m. Kinematic indicators on the NE-striking faults indicate predominantly left lateral and minor right-lateral and normal movement. The N- and NW-striking structures, such as the La Colosa fault, are controlled and limited by the major NE-striking strands of the Palestina fault system. Based on kinematic indicators, these faults display extensional and minor strike-slip characteristics. The geometry of the N- and NW-trending secondary faults and the NE-trending strands of the Palestina fault system and the kinematic relation resemble a sigmoidal-shaped pull-apart structure. At the northern end of the La Colosa area a N-striking structural corridor contains a set of parallel N-striking, E-dipping extensional faults that control high-grade gold mineralization. East- to ESE- and ENE-striking faults and fracture zones are common and represent late-stage extensional deformation, crosscutting the earlier structures.
Characteristics and Sequence of Intrusions and Country Rocks at La Colosa
The La Colosa site contains an intrusive complex with two magmatic centers known as the La Colosa and San Antonio porphyry stocks (Fig. 4), hosted by schistose country rocks of the Cajamarca Complex. The complex is present over a map area of 3.5 km2, and it includes a series of porphyry intrusions with compositions ranging from diorite to tonalite. The San Antonio porphyry stock, which lies some 1.5 km to the southeast of the La Colosa stock, includes a late diorite porphyry and hydrothermal breccias (Fig. 4). Whole-rock analyses confirm a medium- to high-K calc-alkaline composition for the La Colosa porphyry stock (Gil, 2010), exhibiting enrichment of large-ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HSFE) and heavy rare earth elements (HREE), respectively. The 143Nd/144Nd (0.512759–0.512870) and 87Sr/86Sr (0.707265–0.704510) isotope compositions indicate derivation from a mantle source (Gil, 2010). The concentrations of Nb, Ta, and Ti show negative anomalies, which are characteristic of suprasubduction zone magmatism (Betancourt, 2014).
Three main stages of intrusions make up the La Colosa porphyry complex: early (55–62 wt % SiO2), intermineral (59–66 wt % SiO2), and late (61–64 wt % SiO2) (Gil, 2010). These stages are distinguished by their textures, phenocryst mineralogy, and crosscutting relations, as detailed in the following.
La Colosa early porphyry
The early intrusions occupy a surface area of ~0.35 km2 and include five diorite porphyries (E0, E1, E2, EDM, and E3) and three early intrusion breccias (EBX1, EBX2, and EBXDM) (Fig. 4). Early porphyry E0 (Fig. 5A), which is observed only in drill core, is characterized by 20 to 30 vol % of plagioclase and hornblende phenocrysts in a microcrystalline groundmass. Diorite porphyry E1 is equigranular (Fig. 5B), fine to medium grained, with a crowded texture and 60 to 70 vol % of plagioclase, amphibole, and orthoclase phenocrysts. Diorite porphyry E2 is medium to coarse grained with 20 to 30 vol % of subhedral to euhedral plagioclase and hornblende phenocrysts (Fig. 5C). The EDM porphyry is a fine- to medium-grained diorite with 40 to 50 vol % of subhedral to euhedral plagioclase, hornblende, and biotite (Fig. 5D). The last early diorite porphyry, E3, is fine grained, with 30 to 40 vol % of anhedral to subhedral plagioclase phenocrysts (Fig. 5E). E3 intruded the earlier porphyries and generated intrusion breccias (sensuSillitoe, 1985), such as EBX1 (with E1-dominant clasts; Fig. 5F), EBX2 (with E2-dominant clasts; Fig. 5G), and EBXDM (with EDM-dominant clasts) cemented by porphyritic E3 diorite (Fig. 5H).
La Colosa intermineral porphyry
Two intermineral stocks have been distinguished, covering a total map area of over 0.33 km2 (Fig. 4). The earlier of these, I1, is a porphyry with subhedral plagioclase and hornblende phenocrysts (Fig. 5I). The later stock, I2, is a diorite porphyry containing 20 to 30 vol % of subhedral to euhedral plagioclase and hornblende phenocrysts (Fig. 5J). Crosscutting relationships show that I2 intruded I1 and thereby formed intrusion breccia IBX (Fig. 5K). The coarse-grained early-stage and intermineral diorite porphyries are difficult to distinguish due to their compositional and textural similarities. The main distinguishing criteria, following Sillitoe (2000), are the lower alteration intensity, lower veinlet density, and lower gold contents of the intermineral diorites.
La Colosa late porphyry
The late porphyry stage at La Colosa has a map extent of ~2.7 km2, and it is made up of diorite (LD), quartz diorite (QZD), and tonalite (TO) porphyries as well as intrusion breccias (QZDBX), some along the contact with country rock schist (DBX; Fig. 4). The principal rock is the tonalite porphyry (TO) with phenocrysts of plagioclase, quartz, biotite, and hornblende (Fig. 5K), located in the northeast corner of the La Colosa Complex (Fig. 4). This rock also occurs as NW-striking, subvertical to E-dipping dikes cutting the early porphyry intrusions and schistose country rocks and following existing faults, which formed within the pull-apart structure (Fig. 4).
San Antonio porphyry stock
The San Antonio porphyry stock (Fig. 4) is divided into three late diorite porphyry intrusions (LS, IS1, and IS2). Their mutual contacts and their contacts with the schistose country rocks are marked by intrusion breccias (SABX) characterized by subangular clasts of wall rock, IS1, and IS2, cemented by late diorite porphyry LS (Fig. 5S). The last phase of the San Antonio stock consists of hydrothermal breccias (HYBX) containing subangular to subrounded clasts of wall rock and minor diorite in a rock-flour matrix that is locally cemented by sulfides or carbonate minerals (Fig. 5T).
The country rocks at La Colosa are regionally metamorphosed black schists, green schists, and metapelites of greenschist-facies grade. The emplacement of the La Colosa and San Antonio porphyry stocks caused contact metamorphism that transformed the proximal country rocks into hornfels (Lodder et al., 2010), thereby obliterating the original volcano-sedimentary layering of the protoliths.
3-D model of lithologic units
Figure 6 shows the 3-D configuration of all the lithologic units described above, based on surface mapping and core logging. For simplicity, rock types have been grouped as follows: early (early porphyries and early intrusion breccias EBX1, EBX2, and EBXDM), intermineral (intermineral porphyries and intrusion breccia IBX), late (tonalite and quartz diorite porphyries), and wall rock (surrounding schistose country rock and intrusion breccias along its contacts with porphyries SBX).
Distribution of Hydrothermal Alteration and Sulfides in the La Colosa Deposit
Based on surface mapping, detailed core logging, and previous work (Lodder et al., 2010), eight broad mineral assemblages of hydrothermal alteration are distinguished macroscopically (Fig. 7): Sodic-calcic (albite + actinolite ± epidote; Fig 7A); potassic (biotite ± K-feldspar), which occurs mainly as vein infill and as pervasive replacement of wall-rock ferromagnesian minerals, such as biotite and hornblende (Fig. 7B, E, F); quartz-sericitic (white to gray mica + quartz; Fig. 7G), which results in partial destruction of the original rock textures; chloritic (chlorite alone; Fig. 7I), which is a late-stage alteration replacing biotite and amphibole; propylitic (chlorite + epidote ± albite ± calcite ± actinolite; Fig. 7J); intermediate argillic (smectite + illite), which overprints the early alteration types, forming patches and selvages (Fig. 7H); silicification (Fig. 7K); and supergene argillic Fe oxide alteration (kaolinite + iron oxides and oxyhydroxides), which is a weathering product showing boxwork textures after sulfide minerals (Fig. 7L).
The 3-D distribution of these alteration assemblages is shown in Figure 8. The various stages of porphyries and the country rocks show distinctive alteration patterns and sulfide mineral assemblages. At the La Colosa intrusive center the predominant type of hydrothermal alteration in the early porphyries is moderately intense potassic alteration (secondary biotite ± K-feldspar). This alteration type with pyrite as the predominant sulfide is locally restricted to the upper part of the early intrusive stage. The deepest members of the early stage, the EDM porphyry and EBXDM breccia, are dominated by sodic-calcic alteration (as centimeter-scale patches and halos around quartz and magnetite veinlets) with subordinate potassic alteration. The early-stage sulfides are pyrite, chalcopyrite, and molybdenite accompanied by hydrothermal magnetite.
The intermineral stocks and breccias are dominantly altered by potassic (secondary biotite) and sodic-calcic assemblages (again confined to centimeter-scale patches and halos around quartz and magnetite veinlets) with some cores of chloritic alteration. Pyrite is the most abundant sulfide, followed by pyrrhotite, which is commonly found close to the contacts with the country rocks. The dominant hydrothermal alteration types in the late porphyries are propylitic followed by intermediate argillic.
Hydrothermal alteration of the San Antonio intrusions is predominantly propylitic with relicts of secondary biotite and sodic-calcic alteration. In contrast, the hydrothermal breccias are dominated by intermediate argillic alteration. Disseminated sulfides include pyrrhotite and pyrite with sporadic occurrences of arsenopyrite, chalcopyrite, and molybdenite. Melnikovite (colloidal pyrite) has been identified as a breakdown mineral after pyrrhotite.
The metamorphic country rocks are partially overprinted by hydrothermal potassic alteration (Fig. 5M) that extends along foliation planes. Less abundant alteration types are several stages of silicification and localized sodic-calcic alteration (Fig. 5P).
In contrast to the other alteration types, quartz-sericitic alteration is clearly structurally controlled: it is spatially limited to a N-striking corridor of extensional faults, which mainly crosscut the early porphyries, in particular E3. Thus, early potassic alteration in E3 is overprinted by quartz + sericite. The N-striking faults are also locally affected by intermediate argillic alteration.
During visual logging of the drill core the volume fractions of sulfides in the rocks were defined by counting the number of mineral grains visible under the hand lens. The results allowed four 3-D zones of sulfide-oxide enrichment to be distinguished. The first is characterized by 0.2 to 0.5 vol % chalcopyrite, 0.5 to 1 vol % molybdenite, 0.5 to 1 vol % magnetite, and 1 to 2 vol % pyrite associated with early porphyries EDM, EBXDM, and part of E3. The second zone is associated with early porphyries and early intrusive breccias, and it contains 3 to 6 vol % pyrite and 0.2 to 0.5 vol % magnetite. The third zone is characterized by 1 to 3 vol % pyrrhotite and 1 to 2 vol % pyrite in intermineral porphyries and part of the country rock. The fourth zone is characterized by 0.5 to 2 vol % pyrite, 1 to 2 vol % pyrrhotite, and 0.5 to 2 vol % melnikovite in the schistose country rock (Fig. 9).
Nine main veinlet types have been distinguished (Fig. 10) based on their mineralogy, textures, and alteration halos. The typology and nomenclature of the veinlets follows the descriptive criteria for veins and veinlets in porphyry Cu deposits given by Gustafson and Hunt (1975), Sillitoe (2000), and Seedorff et al. (2005). The most common veinlets are A and S types (Fig. 11), as described below.
Early biotite (EB) is the earliest veinlet type recognized at the deposit (Lodder et al., 2010). It consists of fine-grained biotite that fills sinuous veinlets (<2 mm wide) associated with the early porphyries (Fig. 10A). No alteration halos are present beside these veinlets.
A-type veinlets are characterized by vitreous or granular quartz along with sulfides, mainly pyrite, pyrrhotite, molybdenite, and chalcopyrite, as well as magnetite. These veinlets (1–10 mm wide) are also sinuous and occur with or without albite ± actinolite alteration halos (Fig. 10B, C). Studies of fluid inclusions (L. Diamond, unpub. report, 2013) show that the quartz precipitated from hypersaline solutions of 40 to 50 wt % NaCl equiv (the inclusions contain daughter crystals of halite and sylvite and accidentally trapped crystals of chalcopyrite and molybdenite). The minimum temperatures of quartz precipitation are set by the homogenization of the inclusions to the liquid phase between 340° and 450°C.
B-type veinlets (2–5 mm wide) occur in rectilinear networks and are filled with granular quartz with central sutures lined with pyrite (Fig. 10D); they occur in potassic-altered early and intermineral porphyries, but they are scarce.
M-type veinlets (1–5 mm wide) are sinuous or rectilinear and are irregularly filled with magnetite with or without actinolite, K-feldspar, pyrite, molybdenite, and chalcopyrite. Some of these veinlets are bordered by albite + actinolite halos. They occur in all rock types and are spatially associated with the E3 and EDM porphyries in the deep parts of the La Colosa porphyry stock (Fig. 10E).
N-type veinlets (1–5 mm wide) are filled with albite accompanied by actinolite and sulfides (Fig. 10F). These rectilinear veinlets are bordered by sodic-calcic alteration, and they occur mainly in the early and intermineral porphyries.
Actinolite (AC)-type veinlets (<3 mm wide) are rectilinear, contain traces of pyrite, pyrrhotite, molybdenite, and melnikovite, and are spatially associated with potassic and sodic-calcic alteration in the early and intermineral porphyries and in the schistose wall rocks (Fig. 10G). They show no alteration halos.
S-type veinlets (1–5 mm wide) are rectilinear, are dominated by pyrite and pyrrhotite, and occur in all rock types in the La Colosa stock and its country rocks (Fig. 10H).
D-type veinlets (1–5 mm wide) are rectilinear, filled by pyrite, and display fine-grained quartz-sericitic halos (Fig. 10I). They are confined to the N-striking dilational corridor.
Sheeted quartz veinlets (closely spaced and 1–10 mm wide; Fig. 10K) contain patches of albite + pyrite ± visible gold ± illite ± carbonates, and locally they show crustiformcolloform banding (Fig. 9J). The same mineral assemblage occurs as alteration halos around the veinlets. These rectilinear veinlets are confined to the N-striking dilational corridor. Analyses of fluid inclusions (L. Diamond, unpub. report, 2014) show that the drusy and crustiform quartz precipitated at temperatures between 200° and 350°C from a low-salinity (1.7–4 wt % NaCl equiv) aqueous solution containing traces of CO2. A second, CO2-free fluid with high salinity (21–28 wt % NaCl equiv) infiltrated the quartz along microfractures, depositing gold + pyrite + albite + apatite + anatase between 150° and 200°C. Residual cavities in the veinlets were subsequently filled by calcite. Other minor veinlet types distinguished at La Colosa are monomineralic K-feldspar, chlorite, and gypsum.
Mineralization and Metal Zoning
Contours of metal concentrations (Au, Cu, Mo, Ag) were fitted to the drill core database using Leapfrog software. The database contains 65,572 samples analyzed for gold by fire assay and by ICP for trace elements, retrieved from 138,696 m of drilling. Surface samples have been included to constrain the fits of the contours. The dispositions of the resulting 3-D iso-surfaces of average metal contents (hereafter termed “shells”) reveal correlations between ore and host rock types, contacts, hydrothermal alteration, vein types, and brittle structures.
Metallurgical and geometallurgical studies on composited and selective samples indicate that gold occurs predominantly as native gold, as electrum, and in minor quantities as gold tellurides and gold-silver tellurides (Leichliter, 2013). Gold occurs as isolated grains and as inclusions or fracture fillings in pyrite, pyrrhotite, and silicate minerals such as feldspar and quartz (Fig. 12).
Three gold mineralization events have been recognized at La Colosa. The first is associated with the various porphyry phases of the intrusive complex (Fig. 13A, B). The early porphyries and early intrusion breccias have gold ranging in grade from 0.75 to 1.0 Au g/t. Veinlets of A and S type are abundant, and potassic alteration is common. The gold grade of the intermineral porphyries and breccias is lower, from 0.5 to 0.75 Au g/t. In the late-stage porphyries, gold grade drops to <0.3 Au g/t. In this first event, gold was concentrated at the contact zone between the intrusions and their country rocks and also in fracture zones that follow the NNW- to N-trending D1 folds in the country rocks. These zones can be traced through the schistose basement rocks for 1 km north and south of La Colosa porphyry stock.
The second gold event was focused within the N-striking extensional faults that crosscut the early, intermineral, and late-stage porphyries as well as the country rocks. Sheeted quartz veinlets are abundant, and they contain quartz and pyrite with centimeter-wide halos of albite + sericite ± carbonate (Fig. 12D). In the late tonalite porphyry, this second event is represented by sparsely distributed pyrite ± sericite veinlets (<3 per m in drill core) with elevated gold grades (>1 g/t Au; Fig. 12). At La Colosa these high-grade sheeted quartz veinlets occur along the strike of the faults for hundreds of meters into the northern part of the deposit, and their extent remains open for further exploration. The third gold event was a local product of supergene enrichment in the late tonalite and quartz diorite porphyries due to weathering that produced clays and iron oxides (Fig. 13A). Thus, the gold grade is enriched from 0.1 Au g/t in unaltered rocks to 1.5 Au g/t in weathered rocks.
The grade shells for 0.5, 1.5, and 2.0 g/t Au illustrate the distribution of gold within the deposit (Fig. 13A, B; Digital App. 1). The 0.5 g/t Au shell encompasses early and intermineral intrusions and some portions of the schistose country rock. In detail, three separate zones can be recognized within this shell. The first shows grades between 0.5 and 0.7 g/t Au and is associated with porphyries E3 and EDM and the breccia EBXDM. All of these rocks exhibit potassic alteration and contain A-and M-type veinlets with pyrite, chalcopyrite, and molybdenite. This first gold zone partly overlaps with a grade shell that contains the highest values of copper (>0.1%), molybdenum (>0.01%), and silver (>1 g/t) at La Colosa (Fig. 13A).
The second zone within the 0.5 g/t Au shell (Fig. 13A) is hosted by early porphyries (E0, E1, E2, and E3) and breccias (EBX1 and EBX2) and a portion of intermineral porphyries. Disseminated pyrite is the dominant sulfide. A-, S-, and D-type veinlets occur within the zone and contain uncommon magnetite, chalcopyrite, molybdenite, and K-feldspar. The gold ranges in grade from 0.75 to 1 g/t Au. The third zone is marked where the abundance of pyrrhotite exceeds that of pyrite (Fig. 13A). In this zone intermineral porphyries and intermineral intrusion breccias predominate, showing potassic and subordinate chloritic alteration, which is dominant in the central part of the intermineral porphyry where the gold grade drops from 0.7 to 0.5 g/t Au.
Part of the gold in the 0.5 g/t Au shell (Figs. 4, 13A) is hosted by the hornfelsed schistose country rock, which shows silicification, potassic, and sodic-calcic alteration, and which contains A- and S-type veinlets with pyrrhotite, pyrite, and melnikovite. In total, the 0.5 g/t Au shell contains 1,107 Mt of ore with 684 t (22 Moz) of gold, averaging 0.62 g/t.
The grade shells for 1.5 and 2.0 g/t Au (Fig. 13A, B) are hosted by early, intermineral, and late porphyries and intrusion breccias, as well as country rock. The 1.5-g/t Au shell extends over 1.2 km of surface length and contains 196 Mt of ore with 271 t Au (8.7 Moz) at 1.38 g/t Au whereas the 2.0 g/t Au shell covers 0.75 km and contains 40.7 Mt of ore with 84 t Au (2.7 Moz) at 2.04 g/t Au.
Deep copper, molybdenum, and silver zone
Copper, molybdenum, and silver are closely associated in space, and their highest grades are found in the eastern part of the deposit between 2,400 and 2,600 m a.s.l. (Fig. 13A). Average grades are 0.11 wt % Cu, 0.017 wt % Mo, and 2.3 g/t Ag.
Fifty-five samples from La Colosa deposit and adjacent porphyry stocks were selected for radiometric age dating using the U-Pb, Re-Os, and K-Ar methods (Table 1; Fig. 14). Sample preparation and analysis were carried out at VU Geoservices Corporation (Arizona).
U-Pb analyses were used to date the crystallization ages of the La Colosa porphyry stock and its adjacent intrusive centers, as well as to obtain information on the maximum deposition age of the schistose country rocks. Re-Os dating was used to constrain ages of the mineralization, and K-Ar dating was used to determine cooling ages and constrain hydrothermal alteration events.
Heavy mineral concentrates of the <350-µm fraction were separated using conventional techniques at ZirChron LLC. Zircons from the nonmagnetic fraction were mounted in epoxy and slightly ground and polished to expose the surface and keep as much material as possible for laser ablation analyses. Cathodoluminescence images were acquired to guide the analyses. Spots on the zircons were analyzed using a New Wave Nd:YAG UV 213-nm laser (30-µm spot size and 10 Hz) coupled to a ThermoFinnigan Element 2 single collector, double-focusing, magnetic sector inductively coupled plasma-mass spectrometer (ICP-MS). Each analysis consisted of a short blank analysis followed by 250 sweeps through masses 204, 206, 207, 208, 232, 235, and 238, taking approximately 30 s. Time-independent fractionation was corrected by normalizing U/Pb and Pb/Pb ratios of the unknowns to the zircon standards. For this study, two zircon standards were used: Peixe, with an age of 564 Ma, and FC-1, with an age of 1099 Ma. Uranium-lead ages and errors (2σ) were calculated using Isoplot® (Ludwig, 2012), a geochronological toolkit for Microsoft Excel.
La Colosa district intrusive rocks
The U-Pb ages of the La Colosa porphyry stock (Figs. 14, 15A, B) are between 7.4 and 8.5 Ma. These ages indicate the early, intermineral, and late stages at La Colosa were emplaced during a short time span of ~1.1 ± 0.2 m.y. The adjacent porphyry stocks (within a 10-km radius of La Colosa) returned ages between 6.3 and 8.4 Ma (Figs. 14, 15A), defining a magmatic cluster. The stock at the California prospect ~7 km southeast of La Colosa (Figs. 14, 15A) was dated at 3.4 Ma and thus represents a younger magmatic event in the area, which correlates with the Pliocene to Recent volcanic activity of the Northern volcanic zone (Digital App. 2).
Re-Os and K-Ar ages
Molybdenite was dated by the Re-Os method, with all sample preparation and analyses carried out at the University of Arizona. Approximately 0.5 g of Re-Os spikes were loaded and digested using the Carius tube method. The tube was placed in an oven and heated to 240°C for 24 h. Rhenium and Os were separated using the distillation technique, in which the Os is collected into cold HBr, dried down, and redissolved in 0.1 HNO3. Rhenium was extracted using AG1-X8 anion exchange column chemistry. Rhenium and Os were later loaded on Ni and Pt filaments, respectively, with Ba salts to enhance ionization. Measurements were made using the negative thermal ion mass spectrometer (NTIMS).
The Re-Os ages of molybdenite indicate that mineralization occurred during two distinct periods (Table 2). The first was simultaneous with the emplacement of the early, intermineral, and late porphyries of La Colosa stock (8.24 ± 0.08–8.18 ± 0.06 Ma). The second period was dated at 3.24 ± 0.06 Ma at the San Antonio porphyry stock.
Zircon ages in country rocks
Detrital zircons in eight samples of schistose country rock from the Cajamarca Complex were analyzed by the U-Pb method (Table 3; Fig. 14). Several zircon populations were identified, with major peaks at 255 to 99, ~600 to 400, and ~1200 to 1000 Ma. The ages indicate several sources of detrital zircons. In addition, the age populations constrain the maximum age of deposition of the Cajamarca Complex to between ~243 and ~99 Ma (Early Triassic to beginning of Late Cretaceous) (Digital App. 3, 4).
Age and deformation of the country rocks
The new radiometric dating and structural results presented in this study allow the age and deformation history of the country rocks at La Colosa, the Cajamarca Complex, to be better defined. According to Vargas et al. (2005), the basement of the Cajamarca Complex is Devonian gneiss (~393 Ma). Based on 40Ar/39Ar dating of metapelites and amphibolites of the Cajamarca Complex close to La Colosa, a Late Jurassic metamorphic event was suggested by Blanco-Quintero et al. (2014). This Late Jurassic event correlates with crystallization ages between 159.6 and 155.7 Ma for the Ibagué batholith obtained by Villagómez et al. (2011) and Cochrane (2013). U-Pb detrital zircon ages obtained in the present study vary from 243 to ~99 Ma. The oldest deposition ages of the Cajamarca Complex are Triassic and have been identified in its eastern part near the Ibagué batholith. Radiometric ages gradually pass to Jurassic (174–162 Ma) at the latitude of the town of Cajamarca. In the western areas close to the crest of the Central Cordillera (La Línea ridge) the youngest ages have been found from 138 to 99 Ma. Although the Romeral fault system, with the San Jeronimo fault as its easternmost branch, marks the boundary between the Cajamarca Complex to the east and the Quebradagrande Complex to the west, the exact location of this boundary is not well defined. The ages of the Quebradagrande Complex vary from Barremian to Albian (Moreno-Sanchez et al., 2003; Nivia et al., 2006; Blanco-Quintero et al., 2014), similar to the detrital zircon age in the western part of Cajamarca Complex. The gradual decrease in the age of the Cajamarca Complex toward the Quebradagrande Complex to the west indicates that both complexes may have had a similar origin but a different metamorphic history and extent of exhumation.
The deformation event D1 that affects the Cajamarca Complex may be associated with the main regional metamorphism and the onset of the Andean orogeny in the Late Cretaceous to Paleocene (e.g., Nie et al., 2010; Parra et al., 2012). Alternatively, it may be correlated with an earlier, probably Late Jurassic metamorphic event, as suggested by Blanco-Quintero et al. (2014). The D2 ductile deformation may, therefore, reflect the later Andean deformation marked by thrusting of the Cajamarca Complex onto the accreted terranes to the west as a result of the collision of the South American margin with the Caribbean large igneous province during the Late Cretaceous-Paleocene (Villagómez et al., 2011; Spikings et al., 2015).
Structural controls on magmatism and hydrothermal activity
The local field observations made in this study and those made at the regional scales by other workers provide insight into the development of structures that focused magmatic activity at the La Colosa site. The regionally extensive, NNE-striking Palestina fault system, which hosts the deposit, may have originated as a ductile, deep-seated shear zone that evolved to a fault system with brittle characteristics. Some workers have attributed right-lateral displacement to the Palestina system (e.g., Cediel et al., 2003; Mora-Bohórquez et al., 2017). However, global positioning system (GPS) data (Kellogg and Vega, 1995), fault kinematic studies (Ego et al., 1996; Acosta et al., 2004; Cortés et al., 2006), and analyses of seismic data (Taboada et al., 2000; Corredor, 2003; Cortés and Angelier, 2005) have suggested that the shear direction along the Palestina and other regional fault systems, such as the Romeral fault (Figs. 2, 16), has changed from right-lateral to left-lateral since the mid-Miocene. The trigger for this change is thought to have been the interaction of the South American plate with the E-migrating Caribbean plate and the docking of the Panamá-Chocó block. This modified the Miocene stress field north of latitude 4–5° N, inducing WNW- to NW-directed compression and thereby reversing shear along the regional fault systems.
A consequence of this reversal of stress orientation was the formation of secondary faults, such as N-striking synthetic Riedel shear structures (R), some of which seem to have followed inherited anisotropy within the metamorphic basement rocks, such as the D1 ductile shear zones. NW- to W-trending faults were also generated, interpreted here as antithetic Riedel shear faults (R'), as indicated by their slight right-lateral senses of movement. Subsequent movement along the regional faults developed N- to NW-striking extensional faults, indicating extension in east-west to southwest-northeast direction. This promoted linkage of several faults in between the major strike-slip faults, generating local pullapart structures and thus creating extensional centers into which the La Colosa and San Antonio stocks intruded (Fig. 3). The NW-trending contact between the early and the intermineral porphyries and the NW-trending tonalitic dikes (Fig.4) reflects the stress regime with NE-SW–directed extension, which acted during the emplacement of the magmatic stock at La Colosa. The emplacement of the La Colosa and San Antonio intrusive stocks is, consequently, associated with and delimited by pull-apart structures controlled by the regional NE-trending strike-slip fault system.
Whereas the Palestina fault system clearly controls the longitude of the La Colosa and San Antonio stocks, another large-scale tectonic feature may have influenced the latitude of their host pull-apart structures. According to the interpretation of current seismicity data, a W-trending tear zone (Caldas tear) associated with the subducting slab of the Nazca plate has been proposed by Vargas and Mann (2013). This tear zone extends for approximately 240 km in an east-west direction at about 5° N and is colinear with a now-extinct, ~9 to 12 Ma oceanic spreading ridge. Furthermore, this tear structure is the likely boundary between two different subducted slabs having different dips. It may, therefore, have played a role in localizing the La Colosa deposit.
Following the intrusive activity at La Colosa and San Antonio, uplift and progressive deformation along the regional structures continued to develop the N- and NW-striking normal faults within the local pull-apart structures. These extensional faults crosscut the porphyries as well as the metamorphic basement rocks and acted as conduits for the late hydrothermal fluids that precipitated the high-grade, sheeted quartz-pyrite Au veinlets (Figs. 3, 12). Although deformation along the major structures continued during this period of postporphyry emplacement, other fault orientations, including the contact zones of the pull-apart structure, were unfavorable for the deposition of high-grade mineralization.
Hydrothermal alteration and mineralization at La Colosa
Whereas the U-Pb ages at La Colosa show that the composite porphyry stock was emplaced between 8.5 and 7.4 Ma, the Re-Os ages for molybdenite show that mineralization occurred at ~8.2 Ma, simultaneous with emplacement of the early, intermineral, and late porphyry stocks (8.24 ± 0.08–8.18 ± 0.06 Ma). The K-Ar closure ages of secondary biotite indicate that the deposit passed through approximately 300° to 400°C soon thereafter, at 8.0 ± 0.8 Ma. The entire intrusive and hydrothermal mineralization events thus occurred within a relatively short, ~1.1 m.y. time interval. However, Re-Os ages for molybdenite at the San Antonio porphyry stock show that mineralization there occurred significantly later, at ~3.24 Ma. Evidently, the hydrothermal systems at the two magmatic centers are unrelated to each other.
Gold deposition at La Colosa was primarily controlled by the sequence of intrusions. The early porphyries carry the highest gold grades (0.75–1.0 g/t Au), then the intermineral porphyries follow with intermediate grades (0.5–0.75 g/t Au), and finally the late porphyries have the lowest grades (<0.3 g/t Au). Part of the gold mineralization is hosted by the country rock adjacent to the mineralized stocks. Most of the gold is situated in A- and S-type veinlets with pyrite as the dominant sulfide, accompanied by potassic alteration. The best-developed Cu-Mo-Ag mineralization is associated with early porphyries and breccias (E3, EDM, and EBXDM) deep in the deposit, but total tonnages are subeconomic. Although this zone overlaps with some of the gold-rich zones, its geometry and depth suggest that Cu, Mo, and Ag are basically decoupled from gold. Overall, many features of the mineralization at La Colosa, including the porphyritic rock textures, the range of hydrothermal alteration assemblages and veinlet types, and the relatively short time interval (~1.1 m.y.) during which the stocks were emplaced are quite typical of Au porphyry deposits elsewhere (e.g., Sillitoe, 2000).
A more unusual and economically key feature at La Colosa is the occurrence of a significant portion of the total gold resource (~8 Moz) in late extensional faults that crosscut the intrusive complex and strike northward far into the surrounding country rocks. The sheeted quartz-pyrite veinlets that contain the high-grade gold (>1.5 g/t) in this structural corridor are accompanied by sericite + albite alteration rather than by the dominantly potassic alteration characteristic of the earlier porphyry-stage mineralization. Presumably the sheeted quartz-pyrite veinlets formed from fluids that evolved from a parent magma chamber well below the level of the drilled stocks.
Mineralized magmatic belts of Colombia
Several metallogenic belts have been defined as a function of the tectonomagmatic evolution of the northern Andes (Sillitoe et al., 1982; Rodríguez and Warden, 1993; Sillitoe, 2008; Leal-Mejia, 2011; Fig. 16). Among these, three metallogenic zones of Miocene to Pleistocene age are recognized: the Middle Cauca belt, the Cauca-Nariño belt, and the California-Vetas district. Besides the La Colosa porphyry gold deposit, the Middle Cauca belt includes porphyry Au-Cu (e.g., Titiribi, Leal-Mejia, 2011; Quinchia, Bissig et al., 2017), porphyry Cu-Au-(Mo) (Nuevo Chaquiro, Bartos et al., 2017), and epithermal vein-type Au-Ag deposits (e.g., Marmato, Tassinari et al., 2008; Mendoza and Ordoñez, 2014; Buriticá, Lesage et al., 2013). The California-Vetas district hosts an isolated cluster of porphyry Cu-Mo, porphyry Au-Cu, and epithermal Au-Ag vein systems (Felder et al., 2005; Horner et al., 2010; Mantilla Figueroa et al., 2013; Bissig et al., 2014; Rodríguez Madrid et al., 2017). The Cauca-Nariño belt includes numerous known gold occurrences associated with As-Ag-Sb-Pb-Zn metal signatures spatially associated with middle to late Miocene porphyry stocks such as Berruecos, Dominical, Cerro Gordo, Berruecos, El Vergel, (Leal-Mejia, 2011), and Piedra Sentada (Gomez-Gutierrez and Molano-Mendoza, 2009).
Mineralization in the Middle Cauca belt as well as in the California-Vetas district shows strong structural control, which is linked to the tectonic reorganization in the North Andean region during the mid-Miocene, when the Panamá-Chocó block docked against the South American continent. As is the case for La Colosa within the Palestina fault system, other known ore deposits in the Middle Cauca belt are associated with regional N- to NNE-striking fault systems, such as the Romeral and Cauca faults (e.g., Nuevo Chaquiro, Marmato, and Buriticá). As mentioned above, in several of these deposits, reversal of the general shear sense from right to left lateral has been proposed or confirmed (e.g., Rossetti and Colombo, 1999; Sillitoe, 2008).
It is thus intriguing that the onset of mid-Miocene magmatic activity coincided with slip reversal along these major regional structures in the northern part of the northern Andes. This correlation may have important implications for further exploration of mineralized systems in the Middle Cauca belt.
Summary and Conclusions
The following conclusions can be drawn from this study:
The La Colosa deposit is located within the Tahamí terrane of the Central Cordillera of Colombia. Metamorphosed and deformed Mesozoic rocks of volcano-sedimentary origin (Cajamarca Complex) constitute the country rocks. Two ductile deformation events have been identified within this Complex. The first is characterized by tight, W-vergent folding and development of a penetrative foliation and shallow N- and S-plunging fold axes. The second event, overprinting the first in the western part of the La Colosa area, shows open folding with E- to SE-plunging fold axes and shallow E-dipping shear zones.
The regional NNE-striking Palestina fault system crosses the entire deposit area. The fault system is composed of several fault strands and intermediate splays. Like the adjacent Romeral fault system, the Palestina system reversed its slip direction from right lateral to left lateral due to a major change in the stress regime during the docking of the Panamá-Chocó block to South America in the mid-Miocene. The slip reversal reactivated older structures and formed new brittle, extensional structures.
Within the regional Palestina fault system, the La Colosa deposit is located in a pull-apart structure that formed upon brittle reactivation of old ductile features and the generation of secondary Riedel shears.
This extensional zone favored emplacement of the composite magmatic stocks of the La Colosa intrusive complex. The complex is composed of three main intrusive pulses denoted early, intermineral, and late stage.
The porphyry stock was emplaced during a short time span of approximately 1.1 m.y. between ~8.5 and ~7.4 Ma. Re-Os dating at La Colosa suggests that hydrothermal molybdenite mineralization occurred simultaneously with emplacement of the early, intermineral, and late porphyry stocks (8.24 ± 0.08–8.18 ± 0.06 Ma). Consistent with this, K-Ar closure ages of secondary biotite indicate that the La Colosa deposit cooled through 300° to 400°C at 8.0 ± 0.8 and 7.9 ± 0.8 Ma. In contrast, at the San Antonio porphyry stock the Re-Os ages indicate a later mineralization event at 3.24 Ma.
Potassic alteration is the dominant alteration type in the La Colosa porphyry stock. The schistose country rock also shows weak to strong potassic alteration accompanied by multiple stages of silicification. Pyrite is the dominant sulfide in the gold-rich (1.5–2.0 g/t Au) zones of the deposit. Pyrrhotite forms an aureole in the country rock. Close to the contact, pyrite and pyrrhotite coexist. Late-stage quartz-sericite alteration is associated with N-trending normal faults.
The early porphyry intrusions have the highest gold contents inside the 0.5 g/t Au shell. This gold-rich zone lies above and overlaps a zone of magnetite, chalcopyrite, and molybdenite that is present in the deepest explored portion of the deposit.
Following emplacement of the stocks and the porphyry-type Au-Cu-Mo mineralization, continued extensional deformation during uplift of the Central Cordillera formed a set of N-trending normal faults. These faults were sites for renewed hydrothermal fluid flow and deposition of the second, high-grade Au mineralization within sheeted, drusy quartz veinlets bordered by sericite-albite halos. These veinlets crosscut the early, intermineral, and late porphyry intrusions as well the country rocks in the northern area of the La Colosa deposit.
Most of the known gold deposits in the Central Cordillera, including La Colosa, belong to the Middle Cauca metallogenic belt. These deposits are all associated with late Miocene (10–6 Ma) intrusive stocks (Leal-Mejia, 2011; Mantilla Figueroa et al., 2013; Bissig et al., 2014; Bartos et al., 2017).
Mineral discovery and prefeasibility projects are always the products of successful teamwork. We thank AngloGold Ashanti for giving us the opportunity to present this study, which summarizes the work of numerous geologists involved in the exploration program and prefeasibility study: Timoleon Garzon, Edwin Palacio, Anwuar Urquiza, William Pulido, German Gonzalez, Cesar Garcia, Javier Gil, Andrés Giraldo, Geronimo Valencia, Daniel Saenz, Andres Rodriguez, Juan Pablo Valencia, Luis Arteaga, Andres Cepeda, Felipe Rodriguez, Mayeli Gomez, Paula Montoya, and Jennifer Betancourt. The initial mineralization model was constructed by Richard H. Sillitoe and Ruben Padilla in 2007. Their training over years produced a generation of porphyry gold geologists. AngloGold Ashanti greatly supported the training in 3-D modeling and geostatistical analysis. The authors are thankful for the discussions and validations with Vaughan Chamberlain and Tim Thompson, in particular, on improving the quality of 3-D models and the exploration benefits during the drilling phases. We gratefully acknowledge Richard Sillitoe, David John, and Stuart Simmons for their critical and constructive comments that helped to improve the manuscript.
Andrés Naranjo is chief of exploration at Continental Gold, based out of Medellin, Colombia. He received a bachelor’s degree in geology and is currently studying for a master’s degree in earth science from Universidad de Caldas, Colombia. Andrés was an exploration geologist with Anglo-Gold Ashanti from 2008 to 2017 and worked 11 years in greenfield exploration, target generation, and drilling advanced projects. He is a member of the Society of Economic Geologists, the Society for Geology Applied to Mineral Deposits, and the Australasian Institute of Mining and Metallurgy.