Marmato - Zona Alta, Zona Baja, Echandia
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The Marmato gold district is located in the Municipality of Marmato, Department of Caldas, Republic of Colombia and is ~80 km due south of the city of Medellin (#Location: 5° 28' 45"N, 75° 36' 0"W).
Marmato is one of the most important historical gold producing districts in Colombia and lies in the heart of the main historical gold producing region where workings date back to 500 BC. The first mention of Marmato followed soon after the colonisation of Colombia in 1525. The first larger scale workings began in 1634. Since then there has been sporadic exploitation and exploration of the district for both gold and silver. No relaible historic production figures are available.
The mining district has been divided into three zones of exploitation, namely:
• Zona Alta, which covers an area of ~1700 x 1100 m in the south, and is above a road level that varies from 1207 to 1298 masl (metres above sea level), to highest topographic point which is at 1705 masl. Prior to rationalisation as Minera de Caldas in 2005, there were 262 small mines and 36 mills operating in this zone. Between 2004 and 2011, total production averaged 1.52 t Au and 1.93 t Ag per annum, not including that from illegal mining;
• Zona Baja, which is everything below Zona Alta over a wider area that extends east to the Cauca River and includes an area of ~3.6 x 2.8 km. Annual production from the main Mineros Nacionales mine in this zone between 2004 and 2011 was ~0.25 Mt @ 3.2 g/t Au, 11.32 g/t Ag for ~0.77 t Au and 1.6 t Ag; and
• Echandia, which lies to the north of the previous two zones and incorporates all levels from the surface downward. Production from this zone has totalled <2 t Au at grades of 3 to 5 g/t Au from a series of veins.
The Marmato gold district is located in the Cauca-Patia (or Inter-Andean) Graben/Depression (or Central Magmatic Belt), on the eastern margin of the Western Cordillera of the Colombian Andes, which is separated from the Central Cordillera to the east by the broad Romeral Fault zone.
This section of the Western Cordillera is underlain by the Romeral Terrane, an allochthonous oceanic plateau basalt domain comprising metamorphosed mafic to ultramafic complexes, ophiolite sequences and oceanic sediments of probable Late Jurassic to Early Cretaceous age (Cediel and Cáceres, 2000; Cediel et al., 2003). The Romeral Terrane was accreted to the continental margin along the Romeral Fault, which lies east of the River Cauca, in the Aptian (125 to 110 Ma). The Central Cordillera, to the east of the Romeral Fault, is largely composed of continental crust of Proterozoic and Palaeozoic-age, comprising metasediments, amphibolites and gneisses, but also includes accreted Lower Palaeozoic island arc rocks on its western margin.
The Romeral Fault zone, which is marked by dismembered ophiolitic rocks, including glaucophane schist, in a tectonic mélange, forms the eastern margin of the Romeral Terrane, and is characterised by dextral displacement, suggesting oblique terrane accretion from the southwest.
Rocks of the Romeral Terrane form the country rock into which the Marmato porphyry stock was intruded, which locally include metabasalts, amphibolites, serpentinites, graphitic schist, biotite schist, sericite schist and chlorite schist that are known as the Arquía Complex (Calle et al., 1984; Durán et al., 2005).
The Romeral Terrane is bounded to the west by the Cauca Fault zone. To the west of this structure, oceanic and island arc terranes of the Dagua-Piñón and Gorgona terranes were successively accreted to the Western Cordillera in the Paleogene and Neogene, culminating in the on-going collision of the outermost Choco (or Panama) arc since 12 Ma in the late Miocene. This reactivated the Cauca and Romeral faults with lateral and reverse movements (Cediel and Cáceres, 2000; Cediel et al., 2003).
The Romeral terrane is partially covered in the Inter-Andean Graben/Depression by continental sedimentary rocks of the Oligocene to Lower Miocene Amagá Formation, comprising grey to green coloured conglomerates, sandstones, shales and coal seams (Durán et al., 2005). These are, in turn, overlain by thick volcanic and sedimentary rocks of the Late Miocene Combia Formation, divided into a Lower Member of basalt and andesite lava flows, agglomerates and tuffs, and an Upper Member of conglomerates, sandstones and tuffs (Durán et al., 2005).
The entire sequence was intruded by a chain of largely Neogene porphyry stocks. Gold mineralisation at Marmato is related to the emplacement of one of these, the Marmato porphyry stock, which intrudes the Arquía Complex metamorphic rocks, the Amagá Formation sedimentary rocks in the Cauca Valley, and the Combia Formation volcanic rocks which outcrop on the western side of the valley for a total width of 25 km, as far as the Mistrato Fault, a segment of the Cauca Fault.
The ages of intrusion, alteration and probably mineralisation, is Late Miocene. The Marmato stock has been dated at 6.3±0.7 Ma (K-Ar on dacite porphyry, probably of the P1 porphyry; Sillitoe et al., 1982), 7.1±0.2 Ma (K-Ar for porphyritic andesite, probably of the P4 porphyry; Rossetti and Colombo, 1999), 6.7±0.06 Ma (Ar-Ar on biotite from andesite porphyry, probably of the P4 porphyry; Vinasco, 2001), and 5.6±0.6 Ma (K-Ar on sericitised plagioclase in dacite, probably of the P1 porphyry from Zona Baja; Pinzón and Tassinari, 2003; Tassinari et al., 2008). The latter date is interpreted to be the time of ore deposition and is slightly younger than the intrusion ages.
The ~11 Ma calc-alkaline and tholeitic magmatism of the Combia Formation is interpreted to be related to transpressional pull-apart basins of the Inter-Andean Graben/Depression over the broad Romeral-Cauca fault complex. The ~6.3 Ma intrusion of the Marmato stock is interpreted to be associated with Combia Formation magmatism and the start of hydrothermal alteration related to reactivation of the Romeral-Cauca fault complex and fracturing of the Marmato stock. This was followed by further reactivation of the Romeral-Cauca fault complex, the final magmatic activity of Marmato stock at 5.6 Ma, and the main mineralisation episode. Combia Formation magmatic activity was terminated at ~5.0 Ma.
For more detail of the regional setting and geology, see the separate record for the North Andes and Panama copper-gold province.
Mineralisation at Marmato is hosted by dacite and andesite porphyries, members of the Marmato stock which is mapped regionally as an 18 x 3 to 6 km with a north-south elongated intrusion (Calle et al., 1984). At Marmato, the intrusion is 4.6 to 5.8 km wide and extends east to the Cauca River, where it is in contact with schist. A roof pendant of Amagá Formation sedimentary rocks occurs ~1 km east of Marmato, forming the more subdued terrain around the town of El Llano (Calle et al., 1984; Durán et al., 2005). Apart from these pendants, the district is almost entirely within the Marmato stock.
Five separate porphyries have been identified at Marmato, distinguished by distinctive textures and crosscutting relationships, although the full extent of individual intrusions has not been fully delimited beyond the limits of the deposit areas of Zona Alta and Echandia. The two main intrusions are dacite (P1) and andesite (P4), while P2, P3 and P5 only occur as dykes. The dominant lithology in the Zona Baja is dacite (P1). The dominant mineralised host rock is a dacite porphyry stock (P1), which has a NE-trending intrusive contact with an andesite porphyry (P4) stock on the NW side of the deposit area, but the extent on the south and east sides is not known. Available mapping suggest the dacite porphyry (P1) covers an area of at least 1600 to 2500 m east-west, by >3000 m north-south in Zona Alta and Zona Baja, and extends north through the Echandia license where there is a large schist roof pendant, and it is cut by broad dykes and sills of andesite porphyry (P4). In the northern part of the Echandia area, P1 is contact with andesite porphyry (P4) to the south and schist to the west.
• P1 Dacite porphyry is composed of crowded phenocrysts of coarse (~5 mm) bipyramidal quartz plus plagioclase, biotite and hornblende. This porphyry forms the bulk of the Marmato stock.
• P2 Dacite porphyry crosscuts P1, and is distinguished by the lower abundant phenocrysts. It is composed of small, elongate quartz, plagioclase, large euhedral biotite and hornblende, and has not been mapped in outcrop but is observed to cross cut P1 in drill core, and appears to occur as small stocks.
• P3 Andesite porphyry, which is characterised by plagioclase-megacrysts, occurs as dykes which trend NW (135°) and east-west (105°), and are up to 400 m long, and cross-cut P1. It has megacrysts of plagioclase up to 15 mm across, with small phenocrysts of plagioclase, biotite, hornblende and minor quartz and magnetite.
• P4 Andesite porphyry, which forms a NE elongated, 1600 x 750 m stock, that is exposed in both the Zona Alta and Echandia sections of the district. It has abundant phenocrysts of coarse plagioclase with biotite, hornblende, magnetite and minor small quartz. It has an intrusive contact with P1 to the SE, whilst numerous dykes of P4 with a NW (120°) to east-west (100°) trend, that are up to 550 m long, extend into the P1 dacite stock. To the NW, P4 is in contact with Arquía Complex schists and Amagá Formation sedimentary rocks. The NE, in Echandia, section of the stock narrows and forms wide dykes and sills. It also appears to occur as a stock in the east, around El Llano, adjacent to a large roof pendant of Amagá Formation sedimentary rocks.
• P5 Dacite, occurs in a small number of dykes that cross cut P1 in a Creek exposure and a few drill cores. It has both coarse and smaller euhedral quartz phenocrysts, with large elongated plagioclase phenocrysts, biotite and hornblende.
Arquía Complex graphite and sericite-chlorite schists occur as roof pendants, the largest of which has dimensions of at least 820 x 300 x 100 m, whilst xenoliths up to 20 m across are found within within in the porphyries. These roof pendants of schist form a string that trends NE within P1, near the P4 contact, whilst a large roof pendant or block of country rock, occurs at the NW contact of P4 at the NW limit of the Zona Alta. A second NE-trending line of roof pendants (or country rock remnants), but of Amagá Formation sedimentary rocks, up to 250 x 100 m across, are found within in P4, parallel to the NW contact.
Hydrothermal phreatic breccias, which are found within the P1 stocks, are often structurally localised along P3 dyke margins and schist contacts. These breccias are narrow (up to ~10 m true width), containing clasts of altered porphyry, schists or sedimentary rocks in a matrix of ground rock which is commonly black, derived from graphite schist, or grey coloured from porphyry.
Veining within the Marmato district is interpreted to be the result of regional tectonic stresses, filling tension fractures related to compression and lateral strike-slip movement on the north-south trending Cauca and Romeral Faults which lie on either side of the deposit. The Marmato deposit occurs as a series of NNW to east-west trending, steep to moderately dipping, gold-bearing, sulphide-rich veins, hosted in a north-south trending late Miocene porphyry complex. This intrusion was emplaced in folded and thrusted Palaeozoic and Mesozoic metamorphic and sedimentary sequences adjacent to the eastern margin of the broadly north-south trending Cauca-Romeral terrane, accompanied by ENE to NW-SE compression. This resulted in north-south trending thrust and transpressional structures along with steep NW and NE conjugate fault zones.
Structural analysis (Telluris Consulting Ltd, 2010) has recognised two principal deformation stages within the intrusive host rocks of the Marmato deposit:
• syn-mineralisation WNW-ESE compression that reactivated some of the basement structures as well as generating a range of second order shear and extensional structures along NNW to E-W trends as well as NNE-trending thrusts,
• continued post-mineralisation compression into the late-Pliocene, to ~2 Ma, resulting in uplift due to renewed thrusting along the main terrane boundaries forming thrust-bounded intermontane basins such as the Cauca-Patia Depression.
Four principal trends of mineralised structures are recognised in the Marmato area:
i). NW trending steep to sub-vertical faults/fractures (140 to 150°),
ii). WNW trending steep to moderately inclined structures (110 to 120°),
iii). East-west trending structures (100 to 90°) that tend to have moderate to relatively low-angle dips, and
iv). ENE to NE trending structures (65 to 80°) with a range of dips.
In addition to these mineralised trends, there is also a set of NNE-trending structures of varying dips, which appear to represent further components of a reverse/thrust fault system. Both the WNW and east-west mineralised veins tend to splay from the main NW trending structures, consistent with extensional and Riedel components of a sinistral shear system. The kinematic indicators imply mineralisation accompanied a phase of WNW-ESE orientated compression, with the NNE-trending reverse faults and conjugate fractures reflecting this same compression.
Post-mineral faulting is reflected by alteration to soft, white clay gouge with ground pyrite on the margins of some veins and veinlets. In some places there is later, coarse euhedral pyrite in the clay gouge. Brittle fault breccias with no clay gouge are also observed.
Two alteration stages are recognised:
• Propylitic alteration which is the earlier, and varies in intensity from veinlet-halo to pervasive. It is characterised by epidote replacement of plagioclase cores, albite replacement of plagioclase rims and matrix and chlorite replacement of mafics, with disseminated pyrite and pyrrhotite. Calcite partially replaces plagioclase where this alteration is only weakly developed. Cross-cutting relationships suggest multiple propylitic alteration events related to repeated intrusions.
• Intermediate argillic alteration which replaces epidote, chlorite and albite, and overprints the propylitic assemblage. It varies in intensity from vein/veinlet selvages to pervasive. The vein and veinlet selvages comprise a strong but generally narrow halo of white to green illite or sericite alteration which grades outwards to pervasive illite and distal smectite. The main disseminated sulphide is pyrite, although pyrrhotite and iron-rich sphalerite are also observed.
The main gold and silver mineralisation historically exploited at Marmato occurs in a steeply dipping sheeted and anastomosing system of sulphide veins and veinlets, with trends as detailed in the Structure section above, although the NW to west-northwest sets are dominant. Although veinlet mineralisation occurs throughout the host porphyry, it varies in intensity within the intrusion.
Two main stages of mineralisation have been recognised:
• Stage 1, which comprises low grade veinlets (but not veins) related to the early propylitic alteration.
• Stage 2, comprising sulphide veins and veinlets, which cross-cut all of the porphyry phases and the phreatic breccias, and is related to the late stage Intermediate argillic sericite-illite alteration. These veins contain sulphides mainly pyrite and a black iron-rich sphalerite (marmatite), with minor amounts of galena, chalcopyrite, arsenopyrite and tennantite-tetrahedrite. At depth some of the veins appear to become more pyrrhotite-rich, with increased chalcopyrite but less sphalerite.
Individual sulphide veinlets and veins vary in width from <1 mm, up to 2 m, and occurs as swarms that define structural zones of from 10 to 80 m true width, separated by zones with weak alteration, isolated veinlets and grades generally less than 0.3 g/t Au. The individual wider sulphide veins are mined underground in a medium scale operation in the Zona Baja, and by artisanal miners in the Zona Alta and Echandia.
In addition to the vein and veinlet mineralisation, there are also stockwork zones of 1 to 5 cm thick veinlets over mineralised widths of >200 m. High grade gold (>10 g/t) intersections may comprise both wider sulphide veins, and 1 to 2 m wide developments of stockwork veinlets.
The known vertical extent of mineralisation in Marmato Zona Alta and Zona Baja is >850 m, extending from the top of Cerro El Burro (1734 masl) to the deepest drill hole which ended in mineralisation at 560 m altitude.
The mineralised vein system occupies a north-south elongated zone of ~1600 x 900 m, of which ~900 m is in Zona Alta and 700 m in Echandia. The dominant vein trend within this zone is NW. Individual veins have good geological continuity and typically vary from 0.5 to 5 m in thickness and extend for 100 to 1000 m along strike, and 100 to 750 m down dip.
Broad grade shells including vein, veinlet and stockwork mineralisation typically vary from 10 to 230 m wide, reaching as much as 340 m in areas of significant veinlet development, whilst extending with good geological continuity for between 200 and ~950 m along strike, and between 100 and 900 m down dip.
At depth, within the central portion of the deposit, a zone of elevated grades define a higher grade, >2.0 g/t "deep zone", which appears to be continuous along strike for ~350 m, with a confirmed down dip extent of up to 500 m, and a thicknesses that varies from 35 to 150 m.
Published mineral resources at Marmato as at June 2012, using a 0.3 g/t Au open pit and 1.5 g/t Au underground cut-off were (SRK Consulting, 2012):
Open pit vein
Measured resource - 8.1 Mt @ 2.29 g/t Au, 8.93 g/t Ag,
Indicated resource - 44.7 Mt @ 2.06 g/t Au, 12.62 g/t Ag,
Measured + indicated resource - 52.8 Mt @ 2.10 g/t Au, 12.05 g/t Ag,
Inferred resource - 7.4 Mt @ 1.76 g/t Au, 8.70 g/t Ag,
Open pit porphyry
Measured resource - 43.1 Mt @ 0.81 g/t Au, 4.11 g/t Ag,
Indicated resource - 313.8 Mt @ 0.70 g/t Au, 5.37 g/t Ag,
Measured + indicated resource - 356.9 Mt @ 0.72 g/t Au, 5.22 g/t Ag,
Inferred resource - 71.8 Mt @ 0.94 g/t Au, 3.19 g/t Ag,
TOTAL open pit combined
Measured + indicated resource - 409.7 Mt @ 0.90 g/t Au, 6.10 g/t Ag,
Inferred resource - 79.1 Mt @ 1.02 g/t Au, 3.71 g/t Ag.
Indicated resource - 0.2 Mt @ 1.96 g/t Au, 15.42 g/t Ag,
Inferred resource - 1.1 Mt @ 2.26 g/t Au, 13.42 g/t Ag,
Indicated resource - <0.1 Mt @ 2.86 g/t Au, 4.31 g/t Ag,
Inferred resource - 5.6 Mt @ 2.69 g/t Au, 2.58 g/t Ag,
TOTAL underground combined
Measured + indicated resource - 0.3 Mt @ 2.05 g/t Au, 14.26 g/t Ag,
Inferred resource - 6.7 Mt @ 2.62 g/t Au, 4.41 g/t Ag.
This summary is drawn from: "SRK Consulting, 2012 - An NI 43-101 mineral resource estimate on the Marmato Project, Colombia, 21 June 2012; an NI 43-101 Technical Report prepared by SRK Consulting (UK) Limited for Gran Colombia Gold Corp. 187p."
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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