California-Angostura district - Angostura, La Bodega, La Mascota
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The Angostura, La Bodega and La Mascota gold-silver deposits are distributed over a ~4 km interval of an 11 km long, NE trending high to intermediate sulphidation epithermal system that forms the core of the California-Angostura gold district. The deposits occur in that order, from NE to SW, and located in the in the western branch of the Eastern Cordillera of northeastern Colombia near the border with Venezuela, some 400 km NNE of Bogotá, and ~67 km NE of the city of Bucaramanga in the Department of Santander. The deposit is situated at elevations of from 2400 to 3500 metres above sea level (masl).
(#Location; La Bodega - 7° 22' 50"N, 72° 53' 44"W).
Gold is reported to have been discovered in the California district in 1549 during a Spanish military action, although it had already been the site of much earlier artisanal activity by the indigenous Sura people. Spanish colonials exploited two open-pit operations in the district at San Antonio in the La Baja portion and at La Perezosa, immediately SW of La Mascota and NE of Angostura respectively. Production continued on a small scale through the next two and a half centuries. In the early 19th and 20th centuries the British company Colombian Mining Association and French company Francia Gold and Silver undertook operations that included a mill and smelter just outside the town of California.
In 1947, the Anaconda Copper Mining Company took an option on a property at La Baja that encompassed the present La Bodega deposit and conducted exploration via tunneling and 746 m of drilling. Core recoveries were reportedly so poor that insufficient information was available to justify a large option payment and Anaconda withdrew. Nippon Mining Company undertook drilling in the La Baja area in 1967, whilst exploration was undertaken by Placer Development and Ingeominas (Instituto Colombiano de Geología y Minería) in the 1970s and 1980s respectively.
Exploration by Greystar, later to become Eco Oro Minerals Corp., commenced in 1994, culminating in an NI 43-101 resource at the Angostura deposit and prefeasibility study in 2010. The deposit has been defined over a strike extent of 2 km and a width of 1 km.
After initiating work on the deposits commencing in 2006, Ventana Gold Corp. acquired the La Bodega and adjacent La Baja property in June 2010, covering section of the California-Angostura trend and encompassing the La Bodega and La Mascota deposits. NI 43-101 resources were published for these deposits in 2010.
In addition to the works described above, small-scale artisanal operations over the years have driven a large number of adits and tunnels on various parts of the deposits exploiting high-grade veins and shoots.
The California-Angostura epithermal system is situated in the northern Andes ranges, within the western branch of the Eastern Cordillera. In this region, the Eastern Cordillera splits into two mountain ranges, one that swings north-easterly into Venezuela, forming the Sierra de Mérida Massif, and a western branch which trends north south and exposes the Santander Massif. The two ranges are separated by the Cretaceous to Holocene Maracaibo Basin. These three elements together form the Maracaibo Sub-plate, a composite of numerous lithotectonic provinces, representing the tectonically displaced northwestern-most portion of the Guyana Shield, which was transported NW from the Late Cretaceous. The California-Angostura epithermal system is located on the junction of these two ranges
The Santander Massif and Maracaibo Sub-plate are bounded to the west by the NNW to north-south trending, composite Bucaramanga-El Carmen Fault System. Some 50 to 75 km to the east, the parallel to sub-parallel Santander Fault cuts the massif, and in the north forms its eastern margin with the Maracaibo Basin.
The oldest rocks in the Santander Massif are the Precambrian Bucaramanga Formation gneisses, schists, quartzites and migmatites which were regionally metamorphosed to upper amphibolite facies during the Palaeozoic. These metamorphic rocks were intruded by the composite Triassic to Jurassic and Cretaceous Santander Plutonic Batholith, including diorite, monzonite, quartz monzonite, granodiorite and tonalite, accompanied by felsic to andesitic volcanism. During subsequent back-arc development, a number of basins formed, including the Maracaibo basin, which were filled with transgressive marine sediments. Immediately to the west of the California-Angostura, the Bucaramanga Formation gneisses and Santander Plutonic Batholith are overlain by a Cretaceous sequence of siltstone, chert, limestone and conglomerate, underlain further to the west by a pre-Devonian supracrustal sequence.
During the Late Cretaceous to Paleocene/Eocene, folding and thrusting of the Eastern Cordillera resulted in basin inversion and uplift, and intrusion of Middle Miocene porphyritic bodies of rhyodacitic and dacitic composition. This uplift and accompanying erosion had commenced during the Late Eocene to Early Oligocene, complemented by the reactivation of older structures, and continued during the Middle to Late Miocene. Subsequent renewed uplift has persisted to the present, accompanied by rapid basin inversion.
Gold mineralisation in the California-Angostura gold district was developed in association with Middle Miocene stocks along the NE-SW trending Rio La Baja Fault, a splay of the sub-parallel, regional scale, Rio Cucutilla Fault, which is 2.5 km to the SE, but converges to the NW. These faults are members of a regional set of similarly NE-SW oriented structures, and a lesser NW-SE set, developed between the generally north-south to NNW trending, sinistral Bucaramanga-El Carmen and Santander fault systems. The California-Angostura gold district is ~25 km east of the main Bucaramanga-El Carmen Fault System.
District Geology and Structure
In the California-Angostura gold district, amphibolite facies gneisses, schists, quartzites and migmatites of the Proterozoic Bucaramanga Formation are intruded by the composite Santander Plutonic Batholith.
According to Godoy et al. (2012), the California-Angostura gold district lies close to the northwestern margin of a generally north-south elongated ~5 x 11 km exposure of the regional Triassic to Jurassic suite of monzonite, granodiorite, tonalite of the Santander Plutonic Batholith. This author also shows a younger Mesozoic, apparently early Cretaceous, NE-SW elongated, ~7.5 x 2.5 km diorite to quartz-monzonite to granite intruded along this northwestern margin of the larger intrusion. The mineralised structure of the California-Angostura system passes along the northwestern tip of this latter intrusion, partly in the intrusive and partly within the Bucaramanga Formation gneisses. According to Stubens (2015), this section of the intrusion is predominantly composed of quartz-monzonite and granite, with increasing proportions of diorite, granodiorite and tonalite to the south. The same author shows small isolated intrusions of leucocratic tonalite to the NE of this intrusion in the Angostura deposit, whilst small bodies of Middle Miocene porphyry, from <100 m, up to 1 km in diameter, are mapped within the confines of the batholith and in the gneisses along and to the SE of the Rio La Baja and Angostura faults.
Altman et al. (2010), note that in the La Bodega and La Mascota section of the mineralised trend, the dominant lithology is well foliated, comprising mafic facies biotite-amphibole and felsic facies quartzo-feldspathic bands of the lower amphibolite facies Bucaramanga Formation gneiss package. The quartzo-feldspathic bands frequently have an equigranular texture, making them difficult to differentiate from the leucocratic intrusive rocks. Altman et al. (2010) also record that equigranular to porphyritic, generally leucocratic, dykes and stocks of quartz-diorite to quartz-monzonite composition have been emplaced along NE and NW trending structures. These are taken to belong to the Santander Plutonic Batholith, although the same authors suggest some porphyritic phases may be of Tertiary age.
To the west, (at the California end of the mineralised trend), the Bucaramanga Formation gneisses are overlain by the Cretaceous Tambour and Rosablanca formations, which are juxtaposed with the batholith across the Rio La Baja/Angostura faults.
All of these rocks have been intersected by a swarm of parallel to subparallel, generally ENE to NE trending, steeply north-dipping structures which contain >200 individual veins and composite veins in the Angostura deposit and a similar number in the La Bodega-La Mascota deposit area. This fault zone, the the Rio La Baja and Angostura fault complex, is intruded by small Middle Miocene porphyritic bodies of rhyodacitic and dacitic composition.
NE-SW trending dextral strike-slip faults are the principal structures of the district, and define a dilation zone. Five vein/fault stages have been identified, from oldest to youngest (Godoy et al., 2012):
• NE-SW striking faults that have steep to moderate NW and SE dips.
• NW and SW striking structures that have dips from sub-vertical to ~60° NE and SW.
• East-west to ENE-WSW striking structures, with dips ranging from 65 to 85° to the north and NE, and south and SW.
• North-south and NE-SW striking, low-angle, 20 to 50° west, north and south dipping structures.
• NW-SE, north-south, and NE-SW trending structures with dominant steep to moderate west dips, and east-west striking structures with steep north and south dips.
Mineralisation and Alteration
Hydrothermal alteration overprints regional metamorphic assemblages with varying intensities, varying from partial alteration to a total replacement of the original mineral assemblage. The most intense hydrothermal alteration surrounds quartz-alunite veins, where the original host rock is completely replaced. Distal to these veins, sericite, illite, smectite, kaolinite and finally chlorite may occur. Illite is the most common alteration mineral, followed by sericite, although alunite, kaolinite, chlorite and an interlayer illite-smectite are also frequently developed mineral phases. Some supergene alunite is also observed.
The quartz veins are initially formed by partial silicification of the original altered rocks, producing patches of micro-granular quartz. As silicification persists, it culminates in complete silicification, assimilating the primary quartz remnants, to produce a granular, sometimes vuggy, crustified quartz. A late event of high-temperature white quartz occurring in veinlets is observed in the lower levels of the deposit.
Mineralisation, alteration and veining in the various sections of the mineralised complex is as follows:
The Angostura deposit has a strike extent of ~2 km, a width of ~1 km, and extends from 2400 m.a.s.l. to 3470 m.a.s.l. It is limited to the NW by the Angostura Fault and to the southeast by the sub-parallel Móngora Fault. Mineralisation trends to the SW to and partially across the WNW-ESE Páez Fault into the narrower La Bodega deposit. To the north, mineralisation appears to terminate fairly abruptly against the east-west Mortino Fault in the Cristo Rey area, beyond which only narrow, isolated veins have been encountered.
Within these confines, the deposit has been sub-divided geographically into four principal zones: Veta de Barroc in the north, Central, the main core section of the deposit, with the smaller Los Laches to the east, and El Silencio in the south.
Angostura has been described as a structurally controlled, high-sulphidation, epithermal gold deposit. Mineralisation occurs in a swarm of structures with the trends listed above, which generally have steep dips. Whilst the deposit as a whole has a NE-SW trend, the boundaries between the main Veta de Barroc, Central and El Silencio zones trend ENE-WSW, and the dominant strike of the vein swarms is ENE-WSW to east-west. The Los Laches zone strikes NE-SW, separated from the Centtral Zone by a gap of ~200 m.
The intensity of the resultant fracturing is proportional to the degree of secondary porosity and permeability of the host rocks, and hence the density and grade of mineralisation. These mineralised structures vary in width from <2 m for individual veins to >40 m thick composite structures, with strike lengths that range from <50 m to >1 km. Over 200 individual veins and composite veins have been identified.
Within these structures, mineralisation is present as bands, veinlets, stringers and silicified hydrothermal breccias. Alteration and mineralisation are stronger in the intrusive host rocks in the upper parts of the mineralised system, whilst the meta-sediments appear to be a poorer host to the gold-silver.
The gold and silver mineralisation principally occurs within pyrite that occurs in bands, veinlets, stringers, hydrothermal breccias or stockworks that are composed predominantly of quartz, pyrite and alunite which entirely replacing the primary host rock. Sulphide associations include pyrite-digenite-tetrahedrite and pyrite-chalcopyrite ±digenite-tetrahedrite. Locally pyrite is the sole sulphide.
Very fine-grained electrum and gold-silver tellurides occluded in pyrite have been identified as gold-telluride (probably calaverite - AuTe2) and as a gold-silver telluride (probably petzite - Ag3AuTe2). The fine grained character and the frequent occurrence of gold in tellurides together result in the primary mineralisation being partly refractory.
Oxidation is irregular, influenced by structure. It has penetrated to depths of up to and sometimes exceeding 170 m along specific structures, but is more generally ~10 to 30 m on the margins of the deposit, and generally from 40 to 100 m in its central parts. Where oxidation is incomplete, sulphides are mantled by limonite in a transition zone typically with sulphide sulphur concentrations of 1 to 2% (as compared to ~0.5% in the actual oxidation zone).
• La Bodega and La Mascota
The gold-silver-copper mineralisation delineated at La Bodega is part of the same, large, NE trending, fault controlled mineralising system that also hosts the adjacent Angostura deposit. The first drill hole just to the SW Angostura intercepted 106.45 m of 7.81 g/t Au utilising a 0.5 g/t lower cut-off grade.
A number of mineralised zones have been defined or indicated. The La Bodega zone, immediately to the SW of the Central zone at Angostura; La Mascota zone, which is basically continuous to the SW of La Bodega; La Mascota SW Extension an irregular zone on the extension of the La Mascota and a second short parallel trend ~100 m to the SE; Las Mercedes and Aserradero zones, which are on two parallel trends ~200 and ~400 m to the SE of La Mascota respectively. The three main zones, La Bodega, La Mascota zone and La Mascota SW Extension occur over a strike length of ~3 km and widths of 60 to 150 m.
Mineralisation at La Bodega, has been defined to inferred resource status over a strike length of 350 m and >250 m down dip, which is 55 to 75°N, whilst at La Mascota, the mineralised strike length is ~800 m, and >275 m down dip, which is generally 65 to 80°N. La Mascota appears to thin to the NE from the thickest portions in the SW. Scout drilling suggests the resource continues to greater depths in both zones, persisting to at least 500 m at La Bodega. The two zones are separated by a gap in which mineralisation persists, but over a narrower interval indicated to be ~10 to 15 m.
Mineralisation is present as a series of sheeted, to anastamosing, NE, NW and east-west striking, generally steeply north-dipping faults containing discrete and sheeted composite veins. Individual veins are often separated by more weakly altered and mineralised host rock. Mineralised structures vary from <1 m thick discrete veins, to composite structures that are >50 m wide.
The altered and mineralised veins in the La Bodega zone are characterised by a quartz-alunite assemblage separated by pervasively phyllic (quartz-sericite-pyrite) altered host rock, often with network quartz-sulphide veinlets in the wall rock and with local propylitic alteration. As such, the La Bodega zone comprises a mass of pervasive phyllic alteration enveloping discrete and multiple, sheeted composite vein sets across a width up to 150 m.
Gold principally occurs as 5 to 35 µm particles along fractures and crystal planes of fine-grained pyrite, and also in gold-silver tellurides which occur as inclusions within pyrite.
In contrast, in the La Mascota zone, wall rock alteration associated with the mineralised vein/breccia structures is more restricted comprising a thin (1 to 5 m) selvage of illite-pyrite-titanite and propylitic alteration (chlorite-epidote). Mineralised structures vary from discrete silica-alunite veins, up to composite, sheeted sets as much as 40 m thick.
Breccias, of both hydrothermal and tectonic origin, are common in La Mascota and have a variety of matrix textures and represent multiple, crosscutting mineralising events. In addition, the La Mascota zone contains substantially more copper and silver, and is distinctly different from the La Bodega zone, in which pyrite is dominant. Gold largely occurs as 5 to 40 µm particles along sulphide crystal boundaries associated with fine-grained pyrite, chalcocite, enargite, minor covellite and bornite as well as chalcopyrite with sphalerite.
The style of mineralisation is variable over the 3 km strike length from the La Bodega to the La Mascota SW Extension zone. Base metals increase from NE to SW, from pyrite dominant in La Bodega, with zinc, copper and silver increasing significantly from La Mascota to La Mascota SW Extension.
Host rock appears to exert a control on mineralisation, which appears to be best developed in the felsic intrusives and in gneisses that are dominantly of quartzo-feldspathic composition. In contrast, where the host is tightly banded mafic gneiss or amphibolite, the mineralised structures tend to be more restricted, narrower and well defined. Altman et al. (2010) suggests the isotropic, homogenous, quartzo-feldspathic lithologies may have deformed in a more brittle manner than the anisotropic, banded, mafic gneisses, thus creating increased permeability.
Gold and silver mineralisation in excess of a 0.5 g/t Au cut-off is generally associated with the presence of disseminated, fine-grained pyrite and variable amounts of accompanying silica-alunite, whilst higher-grade shoots with 4.5 to several hundred g/t Au, are characterized by dense silicification comprising chalcedonic or microcrystalline quartz, 5 to >15% fine-grained, bluish-grey pyrite and frequently one or more phases of vuggy, hydrothermal brecciation. A sooty, dark grey, very fine-grained pyrite or marcasite is also observed to be associated with high-grade intersections.
The individual vein structures are enveloped by disseminated mineralisation, with a gold content ranging from 0.2 to 1.0 g/t Au and little associated veining. This style of mineralisation is most extensively developed in the northeastern section of the La Bodega zone, where intersections such as 154 m of 1.65 g/t Au and 63 m of 0.80 g/t Au are principally due to disseminated envelopes that have minor associated veinlet arrays, but are mainly characterised by disseminated, fine-grained pyrite and variable, moderate silicification. These mineralised zones comprise the following, in order of volume: i). silica-illite altered wall rocks; ii). hydrothermal breccias; iii). relatively narrow quartz-alunite-sulphide veins; iv). tectonic breccias; and v). silicified faults.
Veins and stockwork developments frequently split, change direction and display an en echelon character. Most of the high-grade gold mineralisation is confined to narrow silicified zones with associated hydrothermal breccias and quartz-sulphide veins. Copper sulphides and gold were introduced in multiple pulses, whilst visible gold is generally associated with heubnerite (MnWO4).
Illite with quartz and pyrite are common wall rock alteration product, and occurs in close proximity to advanced argillic minerals, although in the La Bodega zone, sericite dominates. Altman et al. (2010) observes, that unlike most high sulphidation gold deposits, where the wall rocks are extensively leached by acid fluids, advanced argillic minerals at La Bodega are mostly restricted to relatively narrow mineralised structures, and true vuggy texture is uncommon, occurring mostly in clasts within hydrothermal breccias.
In general, the depth of oxidation varies from 10 to 30 m, principally controlled by fracture density and consequent permeability, but persists to as deep as 100 m down those structures where permeability is best developed. The degree of oxidation is variable, with fine sulphides protected by silica remaining intact even at the surface.
Published NI 43-101 compliant mineral resource estimates are as follows:
As at July, 2010, (cut-off not quoted) based on USD850/oz gold, USD12/oz silver (Greystar Resources press release, 15 July, 2010):
Open pit mineral resources
Oxides - Measured + indicated resource - 105.8 Mt @ 0.41 g/t Au, 3 g/t Ag (=43 t Au, 352 t Ag);
- Inferred resource - 6.3 Mt @ 0.44 g/t Au, 3 g/t Ag (=2.7 t Au, 18 t Ag);
Transition - Measured + indicated resource - 124.8 Mt @ 0.66 g/t Au, 6 g/t Ag (=82 t Au, 748 t Ag);
- Inferred resource - 5.5 Mt @ 0.84 g/t Au, 6 g/t Ag (=4.6 t Au, 34 t Ag);
Sulphides - Measured + indicated resource - 128.0 Mt @ 1.20 g/t Au, 6 g/t Ag (=153 t Au, 769 t Ag);
- Inferred resource - 14.5 Mt @ 1.43 g/t Au, 6 g/t Ag (=21 t Au, 87 t Ag);
TOTAL - Measured + indicated resource - 358.6 Mt @ 0.78 g/t Au, 5 g/t Ag (=279 t Au, 1850 t Ag);
- Inferred resource - 26.3 Mt @ 1.07 g/t Au, 6 g/t Ag (=28 t Au, 150 t Ag);
Underground mineral resources
Sulphides - Measured + indicated resource - 1.283 Mt @ 3.95 g/t Au, 17 g/t Ag (=5 t Au, 21 t Ag);
- Inferred resource - 3.76 Mt @ 3.61 g/t Au, 16 g/t Ag (=14 t Au, 62 t Ag);
As at July, 2015, using a 2.0 g/t Au cut-off (Eco Oro website, 2016):
All ore types mineral resources
Measured + indicated resource - 20.67 Mt @ 3.93 g/t Au, 17.3 g/t Ag (=81 t Au, 358 t Ag);
Inferred resource - 9.17 Mt @ 4.08 g/t Au, 16.7 g/t Ag (=37 t Au, 153 t Ag).
La Bodega - La Mascota as at November, 2010, using a 2.0 g/t Au cut-off (Altman et al., 2010):
La Bodega Zone
Inferred resource - 4.67 Mt @ 4.1 g/t Au, 10.6 g/t Ag, 0.1% Cu (=19 t Au, 50 t Ag).
La Mascota Zone
Inferred resource - 18.67 Mt @ 4.0 g/t Au, 21.6 g/t Ag, 0.14% Cu (=75 t Au, 403 t Ag).
La Mascota SW Extension Zone
Inferred resource - 3.81 Mt @ 3.2 g/t Au, 36.2 g/t Ag, 0.17% Cu (=12 t Au, 138 t Ag).
Las Mercedes and Aserradero Zones
Inferred resource - 0.65 Mt @ 3.1 g/t Au, 10.3 g/t Ag, 0.05% Cu (=2 t Au, 6 t Ag).
TOTAL @ 2 g/t Au cut-off
Inferred resource - 27.7 Mt @ 3.9 g/t Au, 21.5 g/t Ag, 0.14% Cu (=109 t Au, 595 t Ag).
TOTAL @ 1 g/t Au cut-off
Inferred resource - 64.13 Mt @ 2.46 g/t Au, 15.7 g/t Ag, 0.09% Cu (=158 t Au, 1005 t Ag).
This summary has been drawn from:
• O'Prey, M., 2008 - California-Vetas Property, California-Vetas Mining District, Department of Santander, Colombia: an NI 43-101 Technical Report prepared for Ventana Gold Corp., 37p.;
• Altman, K.A, Sim, R., Davis, B.M., Prenn, N.B., Elfen, S.C. and Fisher, B.R., 2010 - Preliminary assessment, La Bodega Project, Department of Santander, Colombia: an NI 43-101 Technical Report prepared for Ventana Gold Corp., by Samuel Engineering, 242p.;
• Godoy, M., Farr, G., McKittrick, R. and Engels, J., 2012 - Updated Preliminary Economic Assessment on the Angostura Gold-Silver Underground Project, Santander Department, Colombia; an NI 43-101 Technical Report prepared for Eco Oro Minerals Corp. by Golder Associates Perú S.A., 294p.;
• Stubens, T.C., 2015 - Technical Report on the Updated Mineral Resource Estimate for the Angostura Gold-Silver Deposit, Santander Department, Colombia: an NI 43-101 Technical Report prepared for Eco Oro Minerals Corp., by Micon International Limited, 628p.
• Burton, P., (Ed.), 2011 - The Colombian Mining Industry; World Gold Analyst Special Report, GFMS World Gold Ltd., London, 92p.
The most recent source geological information used to prepare this summary was dated: 2015.
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.
Bissig, T., Clark, A.H., Rainbow, A. and Montgomery, A., 2015 - Physiographic and tectonic settings of high-sulfidation epithermal gold-silver deposits of the Andes and their controls on mineralizing processes: in Ore Geology Reviews v.65, pp. 327-364.|
Rodriguez Madrid, A.L. Bissig, T., Hart, C.J.R. and Mantilla, F.L.C., 2017 - Late Pliocene High-Sulfidation Epithermal Gold Mineralization at the La Bodega and La Mascota Deposits, Northeastern Cordillera of Colombia: in Econ. Geol. v.112, pp. 347-374.|
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