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The Gramalote gold deposit is located near the town of Providencia in the Department of Antioquia, Colombia, ~230 km NW of Bogota and 110 km NE of Medellin in northern Colombia.
(#Location: 6° 30' 34"N, 74° 54' 44"W).
The Gramalote deposit is located in the Central Cordillera, the central of three mountain ranges which collectively constitute the Colombian Andes. The Central and Eastern of these cordillera are separated by the Magdalena River Valley, whilst the Central and Western are on opposite sides of the Cauca-Patia (or Inter-Andean) Graben/Depression, with the western boundary of the Central Cordillera defined by the Romeral fault system.
The Central Cordillera has a Precambrian and Palaeozoic polymetamorphic basement (McCourt, et aI., 1984) that has been intruded by numerous Mesozoic and Cenozoic batholiths and stocks, the most significant of which is the Antioquia Batholith. Regionally, the western limit of the polymetamorphic basement is formed by the generally north-south striking, dextral transcurrent Lower Cretaceous Romeral-San Jacinto fault system (McCourt, 1984; Aspden and McCourt, 1986). This metamorphic basement coincides with the Cajamarca-Valdivia Terrane of Cediel and Caceres (2000) and Cediel et al. (2003), which dominates the northern portion of the Central Cordillera, and comprises the Valdivia, Cajamarca and Ayura-Montebello Groups. These units include mostly lower to middle greenschist to lower amphibolite grade meta-sedimentary rocks (meta-pelite, muscovite schist, graphitic schist, graphitic quartzite, marble) and oceanic volcanic/intrusive rocks of ophiolitic affinity (amphibolite, meta- olivine gabbro, -dunite, -pyroxenite, -chromatite, serpentinite). They form a parautochthonous accretionary prism of early Palaeozoic age, which underwent regional metamorphism and was sutured to the palaeo-continental margin to the east along the Palestina Fault during the Ordovician-Silurian (Cediel et al., 2003). To the west of the Romeral-San Jacinto Fault complex, the Cajamarca-Valdivia Terrane basement is juxtapose against a complex of allochthonous oceanic terranes emplaced along the northern Andean margin throughout the late Mesozoic and Cenozoic.
For detail of the regional setting and geology, see the separate record for the North Andes and Panama copper-gold province.
The earliest of the Mesozoic magmatism events is represented by large batholiths and widespread coeval volcaniclastic sequences which were emplaced during the Jurassic.
The Gramalote deposit is hosted by the subsequent Antioquia Batholith, one of the most extensive of those intruded during the Late Cretaceous.
This batholith is a trapezoid shaped, multi-phase, calc-alkaline, I-type intrusive complex, predominantly composed of tonalite with local diorite and granodiorite phases, which is exposed over an area exceeding 8500 km2. According to Cediel et al., (2003), the batholith was emplaced during the arrival and accretion of the Dagua oceanic terrane along the Pacific margin of the northern Andes during the interval from the late Cretaceous to Eocene. Intrusion followed reactivation of the Palestina fault zone and initiation of the Romeral Romeral-San Jacinto Fault system from the Aptian-Albian, resulting in a period of uplift and erosion of the Palaeozoic basement and Mesozoic volcano-sedimentary sequences (Cediel and Caceres, 2000). The cooling history of the batholith is syn- to late-syntectonic.
Major, generally WNW to NW trending lineaments occur within the batholith, especially in its eastern sector, consistent with rotation and sinistral shear, possibly related to pene-contemporaneous dextral reactivation of the Palestina fault system and dextral transpression along the Romeral fault system. Subordinate zones of north-south extension and NE-striking shear are related to the regional sinistral shear pattern and appear to control the focusing of late magmatic and hydrothermal activity and gold (±Cu, Mo, Pb, Zn, Ag) mineralisation in sections of the batholith (Gustavson Associates, 2012).
The intrusive complex of the Antioquian batholith has a holocrystalline and medium to coarse grained phaneritic texture, with a mineralogy dominated by plagioclase and quartz ±K feldspar, and up to 20% mafic phases, which vary from coarse grained biotite-rich to hornblende-dominant (depending on locality). Trace minerals include magnetite and titanite. The formation of regional-scale chlorite after biotite and epidote after hornblende are attributed to metasomatic effects (after Gustavson Associates, 2012).
The batholith contains hornblende gabbro-diorite xenolithic blocks that range from centimetres to kilometres across, interpreted to represent an early- or pre-batholithic phase. The main phaneritic tonalitic intrusions are cut by various generations of centimetre to metre scale dykes, including porphyritic diorite and quartz diorite, aplite and simple quartz-K feldspar ±muscovite and biotite pegmatite, whilst small plugs of bi-pyramidal quartz+biotite bearing hypabyssal granodiorite porphyry have been identified at a number of sites (after Gustavson Associates, 2012).
Numerous radiometric age dates from the tonalitic phases of the batholith (Cediel et al., 2003), employing a range of dating techniques, are scattered from ~90 to ~60 Ma, with a strong cluster ~70 Ma.
The following is drawn from Sepulveda, 2003; Coughlin, 2004; and Valencia, 2006, as quoted in Gorham, 2008 and Gustavson Associates, 2012. The Gramalote deposit is entirely hosted within, and underlain by medium to coarse-grained biotite ±hornblende tonalite and granodiorite of the Antioquia Batholith. In the deposit area, the modal composition of the batholith ranges from 75 to 85% plagioclase, 10 to 16% quartz and 5 to 8% biotite.
Throughout the deposit area, the main intrusion is cut by dykes that are a few to several tens of centimetres in thickness, ranging in composition from dioritic to granodioritic and granitic. However, these dykes represent no more than 3 vol.% of the rock mass, and may be grouped according to composition, texture and orientation into those of:
• Hypabyssal porphyritic texture, which are generally of dioritic to granodioritic composition, with variable phenocryst assemblages, including biotite, biotite-plagioclase, biotite-hornblende and plagioclase-biotite-bipyramidal quartz.
• Granitic composition with aplitic to pegmatitic texture which are of either sub-vertical or sub-horizontal orientation. The aplite and pegmatite are commonly transitional. These dykes are composed of K feldspar-quartz±biotite and/or muscovite. They represent at least three pulses of injection and crystallisation, in paragenetic order, i). medium-grained equigranular aplogranites containing K feldspar-quartz-biotite, ii). K feldspar-quartz±mica pegmatite and iii). K feldspar-quartz aplite. All of these granitoid dykes are interpreted to be associated with sub-solidus conditions of emplacement and deformation within the batholith. They commonly exhibit plumose textures, and fill conjugate extensional and tension gash arrays.
The various intrusive phases within the Gramalote district were emplaced in the following order: i). batholithic tonalite; ii). batholithic granodiorite; iii). hypabyssal porphyritic diorite/granodiorite dykes; iv). biotite aplogranites; v). K feldspar-quartz-biotite pegmatite; vi). sinuous glassy quartz segregations; vii). high-angle aplitic dykes; viii). low-angle aplitic dykes; and ix). sheeted magmatic-hydrothermal quartz arrays.
U-Pb zircon analyses of dykes from selected drill core and surface samples from the Gramalote Ridge area have yielded ages of 60 Ma, suggesting the mineralisation at Gramalote is associated with the last phases of crystallisation of the Antioquia Batholith. This is supported by the presence of aplites and pegmatites that are also related to a late crystallisation phase, with a high content of volatiles (AngoGold Ashanti, 2011).
The Antioquia Batholith appears to be free of penetrative plastic or pseudo-plastic deformation, apart from occasional millimetre to centimetre-scale mylonitic C-S planes characterised by ribbon structure in quartz and fish structure in biotite. However, evidence of semi-brittle and brittle reactivation is widespread, especially at Gramalote, where it is reflected in well developed systems of discrete syn-tectonic faults and fractures, with slickenside development and quartz-sericite-calcite-pyrite mineral infillings. Arrays of conjugate rectilinear or stepped tensional meso-fractures are spatially associated with hangingwall or footwall zones along fault fractures, and are commonly infilled with quartz, sericite, pyrite and calcite.
The Gramalote mineral systems is located between two curvi-linear, NNW to NW to east-west-trending (from east to west), macro-scale lineaments which splay off the Palestina fault in the east, and transect the Antioquia Batholith before merging to the west of Gramalote. These are the Nus River and El Socorro lineaments, to the north and south respectively. These lineaments are interpreted to have accommodated sinistral-oblique, ~NE verging shear displacement. Differential movement along the two lineaments generated NNW-striking mega-scale (hundreds of metres) tensional dilation which allowed the formation of hundreds of individual centimetre-scale veinlets with hydrothermal infilling and alteration. These vein arrays are longitudinally limited by conjugate, NE-striking sinistro-lateral planar shears. Detailed mapping in the immediate Gramalote deposit area indicates lithologic homogeneity, with more than 95% of the rock mass being holocrystalline tonalite/granodiorite. However, alteration assemblages related to mineralisation, are variable, and are closely linked to the structural evolution of the area. The structural framework defines the main sectors and satellites to the Gramalote deposit, which include within a 3 km radius: i). Gramalote Ridge, ii). La Trinidad, iii). Monjas, iv). San Antonio, v). La Reina, vi). La Maria, vii). El Limon, viii). La Malasia, ix). Cristales, x). El Retiro, xi). La Cascada/Felipe, xii). Las Torres, xiii). El Mango, xiv) El Barzal and xv). La Concha.
Mineralisation and Alteration
Gramalote is an intrusive-hosted, structurally controlled, stockwork gold-silver deposit, where mineralisation is controlled by NE-SW trending strike-slip shear zones and NNW-SSE trending extensional shear zones and dilational fractures, all of which cut the tonalites and granodiorites of the Cretaceous Antioquia Batholith.
Three distinct deposits have been delineated at Gramalote, namely Central Gramalote, Trinidad and Monjas West. Monjas West is located 2.6 km along the westward strike extension of the Gramalote Ridge from Central Gramalote, whilst Trinidad is ~3 km NW of Gramalote Ridge.
The main zone of mineralisation at Central Gramalote has been traced over a NE-SW (~70°) strike for ~1100 m, vertically down to 450 m below the topographic surface, and occurs within several zones that periodically coalesce both along strike and down-dip. These zones are generally contiguous, structurally-controlled corridors, shear zones, sheeted and conjugate vein arrays and alteration halos, and vary from a few tens to 200 metres in true width, with vertical to sub-vertical dips to the SSE. The style of alteration and mineralisation of both satellite deposits is similar to the main Gramalote Ridge area.
Zones of alteration and sheeted fracturing commonly contain composite gold grades of >1 g/t Au over tens of metres, although these structurally related elements may be separated by metre-scale zones of fresh, barren, unmineralised and unaltered intrusive rock. Alteration occurs as both broad zones and narrow selvedges around veins. The latter range from a few up to >100 mm.
The development of veining, alteration and mineralisation as follows, after Sillitoe (2005, 2006) and Gustavson Associates (2012, 2014):
i). An early set of sparse fine grained quartz-calcite veins and numerous veinlets that cut both the granodiorite and the dykes swarms. Many of the veinlets form sheeted arrays parallel to the granitic and aplitic dykes, possibly forming a temporal continuum with the dykes. These veins/veinlets contain very fine grained pyrite and generally do not host gold.
ii). Quartz-pyrite-chalcopyrite-gold veinlets with associated prominent, ~1 cm wide K feldspar selvages. These veinlets are composed of translucent quartz, and contains a few disseminated grains of pyrite and chalcopyrite but are the most important gold hosts. Gold occurs as fracture fill in pyrite along with chalcopyrite. Some are similar to A-type veinlets in porphyry copper deposits, with non-matching walls, suggestive of emplacement in a semi-ductile environment. Others contain molybdenite along their edges and resemble B-type veins. According to Gustavson Associates (2012), these selvages comprise, from mineralised vein to fresh rock: a). a proximal zone of K feldspar alteration, with muscovite fully replacing chlorite and increased carbonate content, and complete magnetite destruction; b). an intermediate sericitisation zone, accompanied by incipient carbonate development, with pyrite replacing magnetite; and c). a distal incipient sericite zone, involving alteration of plagioclase to fine mica, with partially oxidised magnetite, and biotite converted to chlorite. Biotite phenocrysts adjacent to within and the outer K feldspar selvages are chloritised.
iii). Quartz-calcite-white mica veinlets with up to 2 cm wide muscovite/sericite selvages. These veins typically contain a little more pyrite (which is granular) and chalcopyrite (±sphalerite), along with muscovite and late-stage crystalline calcite. Chalcopyrite occurs as prominent centimetric clots within the veins, where it always accompanies the gold.
The second and third quartz vein types are auriferous and cut all intrusive phases at Gramalote, including the equigranular granodiorite pluton and cross-cutting aplite and porphyry dykes. Tellurides are also found within these last two vein types. Veinlets composed predominantly of either K feldspar or muscovite/sericite accompany the corresponding quartz-veinlet generations, but carry only very minor amounts of sulphides and are deficient in gold. Gold appears to be confined to the quartz veinlets, with no disseminations in the selvages or country rock, and consequently the gold grade is proportional to the vein density. Where present gold occurs as native grains commonly intergrown with chalcopyrite and telluride minerals. Petrographic work indicates the gold occurs as 5 to 20 µm grains associated with fractures and inclusions within pyrite and cavities associated with sulphosalts [aikinite PbCuBiS3, matildite AgBiS2] and tellurides (hessite [Ag2Te] (Leal, 2007 and Cabral, 2007).
The alteration assemblages, particularly the potassic suite, are accompanied by varying degrees of magnetite destruction which is clearly defined in airborne and ground magnetic data over the granodiorite and tonalite host rock which is weak to moderately magnetic.
Oriented core data suggests the veins have random orientations with a weak preference for a 045° strike and subvertical orientation.
Following the last two main veinlet stages, zones up to 30 m wide were subjected to pale-green coloured sericitic or intermediate argillic alteration, containing only very minor pyrite and essentially no chalcopyrite.
The saprolite and 'saprock' portion of the deposit represents a small percentage of the mineralisation. The saprolite thickness varies from 5 to 30 m, averaging ~15 m. Local artisanal miners are active within the saprolite portion of the deposit. In the Gramalote Central area they mine narrow (<50 cm) north-south to NW trending sub-vertical mineralised quartz veins (Gustavson Associates, 2014).
Reserves and Resources
Published ore reserves and mineral resources at 31 December, 2011 (AngloGold Ashanti, 2012) were:
Measured + indicated + inferred resources, Main Zone - 76.13 Mt @ 0.65 g/t Au for 49.65 tonnes of contained gold.
Measured + indicated + inferred resources, Trinidad - 22.20 Mt @ 0.55 g/t Au for 12.21 t of contained gold.
Measured + indicated + inferred resources, TOTAL - 98.33 Mt @ 0.63 g/t Au for 61.86 t of contained gold.
This summary is largely based on the following reports: Gorham, J., 2008 - Updated report on the Gramalote Property, Department of Antioquia, Colombia; an NI 43-101 Technical Report prepared by Dahrouge Geological Consulting Ltd for B2Gold Corp., 347p.; Hulse, D.E., et al., 2012 - NI 43-101 Technical Report on Resources, Gramalote Project, Providencia, Colombia; an NI 43-101 Technical Report prepared by Gustavson Associates for B2Gold Corp., 85p.; Hulse, D.E., Sobering, G., Newton, M.C., Malhotra, D. and Daviess, F., 2014 - NI 43-101 Preliminary Economic Assessment, Gramalote Project, Northwest Colombia; an NI 43-101 Technical Report prepared by Gustavson Associates for B2Gold Corp., 174p.; AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2011, pp. 168-169; AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2014, p. 133.
The most recent source geological information used to prepare this summary was dated: 2014.
This description is a summary from published sources, the chief of which are listed below.
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