Segovia, Frontino - Providencia, Sandra K, El Silencio, Las Verticales
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The Segovia (previously Frontino) gold mining district is located within the Colombian Central Cordillera, ~130 km NE of Medellin, in the Department of Antioquia, north-western Colombia.
(#Location: 7° 5' 12"N, 74° 41' 28"W)
Early prospecting commenced at the nearby Remedios area in 1560, but was limited due to the location and difficult terrain limiting access mineable areas. By the mid-18th century mining activity had almost been abandoned. A second phase of gold mining began following independence and an influx of investment from Great Britain, through London-registered mining companies. Mining in the district began on a larger scale around the early 1850s at both Segovia and Remedios. The mines of the district have subsequently been in operation for over 150 years and produced an estimated 150 t of gold at an average production grade of 9.7 g/t Au.
The Segovia-Remedios deposits are hosted by the north-south to 30° elongated Segovia Batholith, which covers an area of ~5600 km2, is predominantly composed of granodiorite to diorite. The batholith is 10 km wide at Segovia, and is bounded by two regional steeply dipping, north-south to 350° striking faults, the Otú and Nus faults on the western and eastern sides respectively. Both are considered to be younger than Lower Cretaceous and form part of the regional Palestrina Fault System that straddles the batholith. The Otú Fault post-dates the Nus fault and has a sinistral displacement of some 66 km. It separates the batholith from Palaeozoic rocks of the lower Palaeozoic Cajamarca Group, mostly quartz-sericite, graphitic and metavolcanic schists with felsic gneissic intercalations. Further to the west, further to the west the Cretaceous Antioquia Batholith and Santa Isobel stock intrude the same sequence.
On the east side of the batholith, the Nus fault trends 350° and is interpreted to have a steep dip and dextral displacement of >50 km. It juxtaposes the batholith with a 3 to 5 km wide strip of Cretaceous rocks which include basic volcanic rocks and a package of sedimentary rocks, mainly siltstones, limestones and sandstone. These Cretaceous sequences are bounded to the east by the Bagre fault, which trends 20° in the south and 10° in the north, with a sinsitral displacement interpreted to be >50 km. fault. The sequence to the east of this fault is composed of Proterozoic gneisses and migmatites and further intrusives, similar to those of the Segovia Batholith.
The region also contains localised unconsolidated alluvial material and the prevailing climatic conditions have resulted in the formation of a thick layer of yellow to brownish saprolite which may persist to depths of 30 to >60 m. The saprolite is related to an erosional surface marked by an incised plateau.
The Segovia Batholith has been assigned an Upper Jurassic age based on a single age date of 160±7 Ma by K/Ar dating of hornblende (Feininger et al., 1972). However, some more recent dating in the region suggest it may be younger and mid- to late-Cretaceous in age (~68.4±5.5 to 84.1±5.5 Ma, Echeverry et al., 2009). It predominantly comprises grey-green medium grained diorite to quartz diorite with local rapakivi textures and variations from quartz monzonite to granodiorite and gabbro (González and Londoño 2002). It is intruded by dolerite and andesitic dykes along structures that are considered to comprise one of the controls of the gold mineralisation.
For more detail of the regional setting and geology, see the separate record for the North Andes and Panama copper-gold province.
Local Geology, Structure and Veining
The Segovia deposits are entirely within the Segovia batholith, which locally comprises a coarse grained (~5 mm), equigranular, fairly dark coloured granodiorite with white plagioclase, quartz and dark green hornblende.
Three principal phases of deformation and fracturing are recognised, as follows:
• D1 - an early compression that produced two sets of fractures, one with a 10 to 40° trend and 30°E to SE dip, and a second vertically dipping set striking at 295 to 310°. This event is associated with the emplacement of a series of both steep and shallowly dipping, pre-mineralisation basaltic and diorite-andesite dykes;
• D2 - a broadly north-south to NNE-SSW oriented Paleogene (?) compression stage, that produced a clean set of fractures at 270 to 310° which dip at 25 to 30°N. If of Paleogene age, this deformation accompanied the docking of the Dagua-Piñón Terrane to the west;
• D3 - an east-west to WNW-ESE oriented post-mineralisation late-Miocene compressional event, accompanied by steep to vertically dipping fractures which strike at 325° (and NE ?). Again, if of late-Miocene age, this event would correlate with the docking of the Gorgona Terrane and/or Choco Arc of the Panama Microplate.
Gold mineralisation is hosted by a series of quartz-sulphide veins filling certain of these fractures, which have three main trends:
• a D1 set with a north-south to NE strike, dipping at 30° E;
• an east-west to NW striking D2 set that dip at 30° to the N or NE; and
• those that strike NW, with a dip of 65 to 85° NE (D1 ?), occurring on the west side of the district parallel to a NW trending segment of the Otú Fault.
The low angle veins have formed along fractures and thrust faults which often display thrust duplex structures, resulting in pinching and swelling of the veins. These is no evidence of any systematic change in grade through these pinch and swell structures. The average width of the quartz veins in these structures is 0.95 m, with a maximum of up to 9 m. Locally, an intersection lineation can be observed in these veins, plunging toward 60°, sub-parallel to the plunging high grade shoot mineralisation trend observed in resource modelling, implying an influence of cross structures on grade.
Quartz veins commonly follow 2 to 3 m thick mafic dykes or sills which may occur in either or both the hanging wall and the footwall of, or within, the mineralised vein. These dykes are basaltic and have coarse (7 mm) white plagioclase phenocrysts in a fine grained, dark-coloured matrix. The presence of dykes is taken as a guide to ore in exploration.
The mineralised veins are offset vertically by >50 m by high angle reverse faults (D3 ?) which trend NE with dip of 65 to 85°NW, and NW with dips of 85°W to 65°E.
The geological history has been summarised as follows: i). intrusion of granodiorite; ii). development of low angle fault system during D1; iii). intrusion of basic porphyry dykes along the low angle fractures and faults; iv). formation of quartz-sulphide veins along the low angle structures; v). late stage high angle D3 reverse fault movement causing vertical off-sets of the mineralised quartz veins.
Mineralisation and Alteration
Structural and dating data suggest the gold-rich, base metal mineralisation at Segovia accompanied Paleogene deformation related to oblique accretion of outboard terranes and was subsequently reactivated during late-Miocene post-mineralisation deformation, the event associated with porphyry Au-Cu mineralisation in the Cauca belt to the west.
The Segovia deposit is composed of a series of veins that include the Providencia Vein which generally strikes at 100° dipping at 30° NE, and can be traced for ~2 km along strike; the Las Verticales Vein System which occurs over a strike length of >3 km on a trend of 140° and dips at 75° NE. The Sandra K Vein typically strike at 9°, dips 29°E, and persists over a 1.3 km strike length. The El Silencio Vein strikes at 50°, dips 27°E, and is 2.2 km long. 27 other veins are known at the Segovia Operations.
A three stage vein paragenesis recognised at the Providencia, El Silencio and Sandra K veins: i). quartz veins; ii). quartz-pyrite; and iii). quartz-pyrite-gold/electrum galena-sphalerite infilling fractures.
The mineralised veins are composed of massive white quartz with minor calcite, with massive, ribbon and locally drusy quartz textures. The veins may also exhibit breccia textures with clasts of the enclosing wall-rock.
Sulphides occurring within mineralised veins are coarse grained, and occur as simple bands or as clots, mainly comprising pyrite, with late galena and sphalerite. Minor, fine grained arsenopyrite, chalcopyrite, and rare scheelite, pyrrhotite and marcasite are also observed. Higher grades are apparently related to high proportions of galena and sphalerite. High grade ore shoots containing 150 to 200 g/t Au can contain up to 50% sulphides and have a NE plunge in the principal veins. Gold and electrum are fine grained (<20 µm), with coarser visible gold evident in bonanza grade ore.
Wall-rock alteration occurs as narrow selvages that are a few metres wide enveloping the veins, and affects both the mafic dykes and the granodiorite. Alteration in the mafic dykes takes the form of potassic (biotite) and illite assemblages. The granodiorite has been subjected to propylitic alteration with selective mineral replacement by chlorite, epidote, pyrite and calcite. The wall rocks adjacent to the main veins often also contains quartz-sulphide veinlets.
Reserves and Resources
Published mineral resources at Segovia at a 3 g/t Au cut-off, as at August 2012 (SRK Consulting on Gran Colombia website, 2016) including pillars was as follows:
Un-mined resource - Measured + indicated resource - 0.447 Mt @ 20.2 g/t Au;
Inferred resource - 0.088 Mt @ 20.1 g/t Au;
Pillars in current operation - Inferred resource - 0.392 Mt @ 14.8 g/t Au;
Pillars in historic mined area - Inferred resource - 0.224 Mt @ 26.9 g/t Au.
Un-mined resource - Measured + indicated resource - 0.263 Mt @ 10.9 g/t Au;
Inferred resource - 0.422 Mt @ 8.2 g/t Au;
Pillars in current operation - Measured + indicated resource - 0.077 Mt @ 12.3 g/t Au;
Inferred resource - 0.014 Mt @ 14.4 g/t Au.
Un-mined resource - Inferred resource - 0.681 Mt @ 11.6 g/t Au;
Pillars in current operation - Inferred resource - 1.309 Mt @ 9.7 g/t Au.
Un-mined resource - Inferred resource - 0.771 Mt @ 7.1 g/t Au.
Un-mined resource - Measured + indicated resource - 0.709 Mt @ 16.8 g/t Au (=11.9 tonnes of contained Au);
Inferred resource - 1.962 Mt @ 9.5 g/t Au (=18.6 t of contained Au);
Pillars in current operation - Measured + indicated resource - 0.077 Mt @ 12.3 g/t Au (=0.95 t of contained Au);
Inferred resource - 1.715 Mt @ 10.9 g/t Au (=18.6 t of contained Au);
Pillars in historic mined area - Inferred resource - 0.224 Mt @ 26.9 g/t Au (=6.0 t of contained Au).
TOTAL contained gold - 56 tonnes.
This summary is largely drawn from: Parsons, B. et al., 2014 - Preliminary economic assessment on the Segovia and Carla Operations, Department Of Antioquia, Colombia; an NI 43-101 Report, prepared by SRK Consulting (UK) Limited for Zandor Capital S.A., 242p.
The most recent source geological information used to prepare this summary was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
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