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Buritica

Colombia

Main commodities: Au Ag
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The Buriticá gold deposits are located ~200 km NW of Bogota, ~72 km NW of the city of Medellín in the Antioquia Department of northwestern Colombia, and ~2 km south of the town of Buriticá (#Location: 6° 42' 19"N, 75° 54' 20"W).

The high grade veins and colluvial deposits of the Buriticá district have been mined since before the arrival of the Spanish Conquistadors, with small high grade veins worked at surface and underground to shallow depths by artisanal miners for gold and silver. The district had been subject to minimal exploration prior to the 1990s, with a significant drilling program commencing in 2009 culminating in significant resources being outlined by 2014.

Regional Setting

  The Buriticá gold deposits lie within the Northern Andean Block, which occupies the northwestern margin of South America in Ecuador, Colombia and Venzuela, and extends into the Panama to the west.
  For details of the setting, see the separate Northern Andean and Panama copper-gold province record.
  Buriticá is the northernmost significant precious metal deposit known to date (2016) in the upper Miocene Middle Cauca belt, which follows the broad Cauca-Romeral fault complex and Central Magmatic Arc of central-north Colombia. This belt contains numerous gold systems, including gold-rich porphyry copper-gold deposits (the largest of which is La Colosa, with a resource of >600 t of gold) and also vein-style precious metal mineralisation in settings described as mesothermal to epithermal, similar to Buriticá. Many of these occurrences and deposits appear to be spatially related to relatively small, high level intrusions of intermediate composition.
  The central magmatic arc is an 80 to 120 km-wide belt of Pliocence to Recent volcanoes extending from south-central Ecuador to central Colombia, the result of the subduction of Miocene-aged Nazca oceanic crust beneath north-western South America along the Ecuador-Colombia trench. Volcanism is dominated by lavas and pyroclastic rocks of andesitic, dacitic and lesser basaltic composition.
  Buriticá is on the western margin of this belt, where it overlaps onto the eastern margin of the allochthonous Chocó Arc Cañas Gordas Terrane. As a consequence, the basement geology of the Buriticá district is dominated by Cretaceous basalts and gabbroic to ultramafic bodies, and stratigraphically overlying Cretaceous turbiditic metasedimentary rocks.

Geology

  The basement geology of the Buriticá district is dominated by Cretaceous oceanic plateau basalts and gabbroic to ultramafic bodies, and stratigraphically overlying Cretaceous turbiditic metasedimentary rocks that belong to the Cañas Gordas Terrane.
  The sedimentary rocks in the district include carbonaceous and variably calcareous pelitic and psammo-pelitic lithologies that commonly strike NNW and are moderately to steeply dipping. These rocks are deformed into shallowly plunging broad open folds, locally with a weak axial surface cleavage, and are mainly metamorphosed to lower greenschist facies grade. To the east and west of the deposit area, Late Cretaceous tonalitic plutonic suites intrude the Cretaceous basement. The western of these, which are closer to the deposit, extend over a north-south elongated area of ~2.5 x 10 km.
  The basement rocks are cut by Miocene hypabyssal intrusions of generally intermediate compositions, varying from basaltic-andesite to relatively mafic dacites. To the west, the dioritic intrusions form relatively large bodies, whilst in the deposit area, there are several clusters of Miocene intrusions that include fine to medium grained and variably porphyritic, steep walled stocks and dyke-like bodies, commonly with intrusive breccia margins. The largest of these intrusion complexes, in the main deposit area, outcrops over an area of ~6 km2. Andesitic dykes are found throughout the deposit area, interpreted to represent offshoots of larger Miocene plutons at depth. Lithogeochemical data from the main intrusion in the deposit area is consistent with a fractionated and hybridized continental-arc, calc-alkaline suite. Radiometric dating of the Buriticá intrusive complexes by Lesage (2011) yielded an age of 7.4±0.1Ma, consistent with the 8 to 6 Ma ages of other intrusive complexes with which porphyry copper-gold and epithermal mineralisation is associated in the Middle Cauca Belt (Bennett et al., 2014).
  The deposit area is cut, and geologically partitioned by a set of regionally extensive north-south to NNW trending faults, which are broadly geometrically similar to the Cauca and Romeral fault systems to the east. To the east of the Yaraguá and Veta Sur vein systems, the steeply-dipping Tonusco Fault system truncates the intrusive complex and related hydrothermal alteration envelopes, with a geometry consistent with significant dip and strike slip movements on this fault postdating both intrusion and alteration. A set of east-dipping faults to the west of, and possibly related to the Tonusco Fault, transect both the Yaraguá and Veta Sur vein system, but appear to only result in minor, dominantly dip-slip dislocation. Other broad-scale structures in the area surrounding the deposit are broadly spaced and trend ENE and WNW, and do not appear to have involved large displacements, although the geometries and distribution of vein systems and alteration patterns are compatible with these structures being active during formation of mineralisation.

Mineralisation and Alteration

  Alteration and mineralisation at Buriticá is interpreted to represent a porphyry-related, carbonate base metal gold vein/breccia system, and is spatially associated with the Miocene intrusion complexes. Three generations of alteration have been mapped surrounding these intrusives (Bennett et al., 2014) as follows:
• Hornfelsing and some silicification around the margins of the intrusions,
• Potassic- (biotite, ±K feldspar, associated with quartz magnetite-rich stockworking) and propylitic alteration mainly but variably affecting the intrusions. The alteration is associated with weak base metal anomalism in the main vein deposit area, but with porphyry style copper-gold mineralisation in the nearby Guarco area,
• A widespread, fracture-controlled phyllic alteration (sericite-pyrite-quartz) which overprints and may obliterate the other alteration assemblages, but is of similar age to the intrusions (Lesage, 2011). In the main deposit area, the extent of the strong phyllic alteration corresponds to the broad Au, Ag, Zn and Pb soil geochemical anomalies.
  Precious metal mineralisation in the two principal vein systems of the main deposit, Yaraguá and Veta Sur (to the east and west respectively), apparently occurred in two main depositional stages:
• Stage I is represented by banded base-metal (Fe, Zn and Pb) sulphide-rich mineralisation with variable amounts of quartz-carbonate gangue and bands, forming a series of sub-parallel narrow vein arrays. In addition, mineralisation of this stage also occurs in dilational veined breccias, which, in places, occupies substantial volumes of both the Yaraguá and Veta Sur systems, although grades typically lower than in the high grade veins. Experience at the Yaraguá mine indicates Stage I mineralisation is non-refractory and recoverable by simple gravity and flotation circuits, with flotation concentrates being cyanided, and gold and silver recovered by the Merill-Crowe process. Wall-rock alteration surrounding Stage I veins comprises narrow selvedges of phyllic assemblages ±K feldspar. This mineralisation and vein margin alteration overprints earlier potassic, phyllic and propylitic assemblages related to the associated intrusions.
• Stage II mineralisation, which is a high-grade gold phase, that is distinctive, both texturally and chemically. It locally cross-cuts and overprints the previous stage as veins and breccia veins, and is characterised by abundant free (and commonly visible) gold in a siliceous and carbonate gangue, with associated arsenical pyrite and with low zinc and lead contents, but relatively high arsenic and antimony with low bismuth. This style is, predominantly found in the Veta Sur system, where it contributes to bonanza grade precious metal subzones, but has also been encountered in the Yaraguá system (Bennett et al., 2014).
  The steeply dipping Yaraguá vein system has been traced over a strike length of 1120 m and 1300 m vertically. Most vein domains in this system strike at ~10°, although one vein set (including the Centena vein), strikes WNW. These vein domains occur in several packages, each occupying a width of over ~150 m across-strike, whilst interleaved wallrocks (mostly andesitic intrusions and intrusion breccias) containing numerous minor veins and zones of breccia-style mineralisation. The Yaraguá vein system is developed over a width of ~400 m, and involves >35 modelled veins, which are most densely developed over the central 140 m width, where >20 of these veins are found. Individual modelled veins vary from <200 to >800 m in length.
  The similarly steeply dipping Veta Sur system has been drilled over 1100 m along strike and 1400 vertically. However, this system strikes around 55° and in its north eastern section, overlaps with and intersects the Yaraguá system. Host rocks include andesitic intrusions and breccias as well as basement metasedimentary and meta-basic rocks. The Veta Sur vein system has a denser core covering an area of ~150 x 400 m involving >22 modelled veins. Additional parallel but shorter veins with lengths of generally 100 to 400 m are found to the NW and SE, becoming sparser and shorter away from the core. To the SW a number of the veins in the denser core, persist for another ~100 to 500 m before terminating. To the NE, the vein swarm overlaps the Yaraguá system but does not persist far past the core area.
  Both vein systems are characterised by multiple, steeply-dipping veins and broader, more disseminated (breccia-style) mineralisation. Both are known to have more extensive high grade mineralisation beyond these dimensions, laterally and at depth (Bennett et al., 2014).
  The mineralised veins can vary from a few centimetres to several metres in width with underground development indicating that mineralised intervals average about 1.6 m true width. Examples of individual vein widths and assays averaged over a number of sampling points, are: San Antonio Vein - 2.01 m @ 13.2 g/t Au, 73.1 g/t Ag, 1.32% Zn; Centena Vein - 1.43 m @ 18.24 g/t Au, 148 g/t Ag, 2.3% Zn; Sophia Vein - 1.97 m @ 113.26 g/t Au, 60.2 g/t Ag, 0.48% Zn; Hanging Wall Vein - 1.55 m @ 15.96 g/t Au, 23.1 g/t Ag, 0.55% Zn (Bennett et al., 2014).
  Images in Bennett et al. (2014) show individual modelled veins have the appearance of massive to semi-massive banded sulphides with zones of brecciation and of quartz and carbonate veining that occur either parallel to the vein margins, or anastomosing within the vein.
  Precious metal-bearing vein and breccia mineralization has been located elsewhere in the Buriticá district, principally in the Guarco, Parjarito, San Augustin, La Estera, La Mano and Pinguro areas. Porphyry copper-gold mineralization has been observed in the Guarco area (Bennett et al., 2014).

Resources

NI 43-101 compliant mineral resources at December 31, 2013 were (Bennett et al., 2014):
      Measured + indicated resource - 8.39 Mt @ 10.4 g/t Au, 31 g/t Ag, 0.5% Zn (87.25 t Au, 261 t Ag),
      Inferred resource - 16.7 Mt @ 7.8 g/t Au, 24 g/t Ag, 0.3% Zn (130.25 t Au, 401 t Ag).

This information in this summary is largely drawn from "Bennett, A., Tahija, L., Vigar, A.J., Recklies, M., Guzmán, Jarra, A.P. and Rykaart, M.E., December 2014 - Buriticá Gold Project, Antioquia, Colombia, Preliminary Economic Assessment; an NI 43-101 Technical Report prepared for Continental Gold Limited by M3 Engineering and Technology Corporation, 272p."

The most recent source geological information used to prepare this summary was dated: 2014.     Record last updated: 11/3/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Buritica

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