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Andacollo Gold

Chile

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The Andacollo low sulphidation epithermal manto-type gold deposit is located some 480 km to the north of Santiago in Chile. Gold has been worked from gravels in the district since Inca times.
(#Location: 30° 14' 48"S, 71° 7' 8"W).

  The Andacollo district comprises a variety of gold, copper and mercury deposits, and is hosted by the lower Cretaceous volcanic-intrusive arc of the Coastal Cordillera, to the west of the main mid to upper Tertiary porphyry copper systems of the main Andean Cordillera. The Andacollo gold deposits cover a north-south elongated area of ~7 x 3 km, located on the NW and western periphery of the Carmen de Andacollo porphyry copper system which is central to the district and represents the root zones of a strato-volcano that was intruded by a high level 98 Ma mineralising stock. See the separate Carmen de Andacollo record for details of the stratigraphy and distribution of mineralisation.

  Placers accumulations, mostly derived from erosion of the veins and mantos are located at distances of tens of metres to as much as 1 km from the apparent bedrock gold sources in gullies crossing the porphyry copper and epithermal deposits.

The stratigraphy within the Cretaceous host sequence to the Andacollo gold deposits is as follows, from the base (after Reyes, 1991):
Arqueros Formation, which is ~500 m thick, and is the oldest stratigraphic unit, outcropping in the western part of the district along a north-trending monocline that dips gently to the east. It is of Barremian age, and consists of marine sedimentary and volcanic rocks, which in the Andacollo area are dominated by volcanic breccia, composed of angular blocks in a clay-rich matrix, and limestone (Muller, 1986).
Quebrada Marquesa Formation, which is Aptian to Albian in age, and is widely distributed, conformably overlying the Arqueros Formation, and outcropping in the central and eastern parts of the district. It is composed of a rhythmic flow sequence of andesitic, dacitic and rhyolitic composition which totals 2000 to 2300 m in thickness, and is capped by a heterogeneous unit of limestone, breccia, conglomerate and andesitic flows (Muller, 1986). In the Andacollo district, it strikes north-south and dips 30°E in the Toro and Socorro sectors in the west, and 10°E in the Tres PerIas area in the NW. Llaumett (1983) and Muller (1986) subdivided the Quebrada Marquesa Formation into six mappable members, two of which, the Cerro Toro and the Andacollo members, host the principal gold and copper mineralisation:
• Cerro Toro Member, ~680 m thick, composed of four rythmic volcanic units, each starting with dark andesites, followed by dacitic volcanic flows, andesite flow breccia, or thin layers of rhyolite, which host the Toro, Socorro, Floridor and Chisperos vein/manto systems.
• Cerro Negro Member, ~300 m thick, composed of andesite.
• Andacollo Member, ~800 m thick, a sequence of alternating andesite and dacite, with an andesite flow breccia towards the top. This member hosts the Churrumata mineralised system towards the base of the pile, and the Tres Perlas system in the breccia in the upper section.
• Pichilingo Member, ~200 m thick, composed of andesite.
• Veintiuna Member, ~350 m of ignimbrite.
• Carbonica Member, ~240 m thick, composed of a section that commences with conglomerate, overlain by limestone and then brecciated andesite.

The intrusive rocks of the district are part of a major a north-south elongated, 50 km long batholith, which varies in composition from diorite through granodiorite to tonalite. Most of the rocks of the district have not been altered or mineralised with the exception of the tonalitic Andacollo porphyry, which outcrops over an area of 2 km2 in the central part of the district (Llaumett, 1975) and is believed to be related genetically to the copper and gold mineralisation.

Veining

  More than 100 veins of various dimensions have been recognized across the district, emplaced in volcanic breccias, along faults, shear zones and the contacts between dykes and host volcanic rocks. In the central and southern parts of the district, they generally strike at 315° but are predominantly east-west in the north, parallel to the main structural trends. The individual veins dip from 70° to vertical and have strike lengths that vary from a few tens of metres to several kilometres. The width of individual veins is controlled primarily by the host rock lithology, and range from centimetres in aphanitic andesite, to as much as 5 to 6 m in more permeable felsic volcanic rocks, such as dacite and andesite flow breccia (Reyes, 1991).
  The distribution of economic mineralization in the veins is discontinuous. Generally sinuous, lenticular ore shoots extend locally for 100 m along strike. Vein contacts are sharp in andesite and diffuse in dacite. The veins are associated with adularia-hematite or chloritic alteration halos, which vary from a few centimeters in width in andesite to 5 to 8 m in dacite (Reyes, 1991).
  The main host to gold mineralisation in the Andacollo mantos is a suite of shallowly dipping dacitic and andesitic flows, flow breccias and pyroclastic units. The location of the mantos is influenced by the NW trending fault set that is dominant in the district. While transgressive in detail however, the mantos are grossly concordant, being stratabound, confined to specific andesite breccias and dacites within this sequence of alternating flows and breccias. They are characterised by potassic (adularia) alteration with associated pyrite and magnetite. The overlying andesite is rarely mineralised in manto form, but contains mineralisation in vertical structural breccias zones. Gold is associated with pyrite as disseminations and as veinlets, although where oxidised, it is liberated and amenable to heap leaching.

  The gold content in the vein selvages is generally ~0.2 to 5 g/t in andesite and 5 to 20 g/t in dacite. Pyrite is the predominant sulphide in the veins, commonly occurring as two generations i). barren, pyrite-I (Muller, 1986), and ii). gold bearing pyrite-II, accompanied by chalcopyrite, sphalerite, galena and cinnabar (Llaumett, 1983). The principal gangue mineral is quartz, which is sometimes amethystine.
  Gold is located in pyrite-II, chalcopyrite and aikinite-bismuthinite intergrowths, along galena and sphalerite crystal margins with quartz, and in the interstices of hematite-quartz assemblages. Grains range from a few microns to >100µm (Reyes, 1991; Oyarzun et al., 1996).
  All of the mineralised veins have been oxidised to depths of 40 to 60 m, with average grades of 5 to 6 g/t Au, and local highs of 100 to 300 g/t. The underlying hypogene sulphide zone within these veins extends to depths of ~250 m, with grades that zone range from 1 to 5 g/t Au (Llaumett, 1983).

Three separate zones peripheral to the porphyry copper deposit have been distinguished (Llaumett, 1983), as follows:
• Internal zone, located adjacent to the porphyry copper deposit, with mineralisation characterised by gold-bearing pyrite, galena, chalcopyrite, sphalerite and specular hematite and gangue minerals are quartz, and locally, calcite. Pyrite varies from massive accumulations to small individual crystals. Sphalerite is either dark and iron rich or pale yellow, whilst chalcopyrite occurs as inclusions within, or rims surrounding sphalerite. Gold grades range from 5 to 56 g/t (Llaumett, 1983). The key veins present in this zone are Socorro, Toro, Fragua and Bahamondes.
• Intermediate zone, located in the northern and southeastern sections of the district, between 1 and 3 km distal to the porphyry copper deposit. It is characterised by gold and copper mineralisation carrying chalcopyrite and small amounts of sphalerite and galena in quartz veins. Some of these veins have been mined for copper. Gold grades vary from from 2 to 25 g/t (Muller, 1986). Significant veins include Arenilla and Altamira.
• External zone, is located in the southern and southeastern parts of the district, >3 km from the porphyry copper deposit. Veins are characterised by an association of mercury and copper with small amounts of gold and silver. Mercury values range from 10 to 4000 ppm and gold averages 0.04 g/t. The mineralization includes cinnabar, pyrite, chalcopyrite, bornite, galena and tennantite. Gangue minerals are mainly quartz and barite (Muller, 1986). Key veins are Mercedes and Dichosa (Reyes, 1991).

Mantos

  The stratabound manto bodies contain disseminated gold, and are only developed in favourable felsic volcanic rocks, i.e., dacite and andesite flow breccia of the Cerro Toro and Andacollo Members. The dimensions of the mineralised mantos in each of these units are strongly dependent upon both the number of veins that cut them, and on the host-rock permeability. The lateral dimension of mantos distal to intersecting veins is proportional to the degree of fracturing, whilst the average gold content of the manto is lower than that in the veins, varying from 1 to 6 g/t Au. Silver ranges 0.5 to 1.5 g/, arsenic is <100 ppm, antimony is <10 ppm, and copper is between 0.02 and 0.1% (Reyes, 1991).
  The mineralisation associated with the mantos is similar to that of the veins, other than the gangue in the latter is essentially composed of quartz. The same two pyrite generations, a barren pyrite-I and mineralised pyrite-II are recognised. Pyrite-I occurs as either irregular shaped, isolated grains or as inclusions in pyrite-II, which is easily distinguishable, occurring as perfect cubes of up to 10 mm. Pyrite-II is accompanied by sphalerite with exsolutions of chalcopyrite, chalcopyrite and galena. A further, late pyrite-III is recognised in quartz veinlets with sericite-kaolinite selvages in the Tres Perlas system. The principal gangue minerals are quartz and calcite, deposited in four phases with differing characteristics. Other gangue minerals include chlorite and hematite (Oyarzun et al., 1996).
  Mineralised mantos hosted by andesite flow breccia have good lateral and vertical continuity, whilst those in dacitic rocks only have lateral continuity in the vesicular parts of the flows and vertical continuity along crosscutting structures. The stratabound bodies are only found to the west of the porphyry copper deposit, comprising an ~7 x 3 km semi-circular zone (Reyes, 1991).
  Five main mineralised manto systems have been recognised from, west to east: Toro-Colorado-Las Vacas-Cabaiias, Socorro, Floridor-Chisperos, Churrumata-Mercedes, and Tres Perlas (Reyes, 1991).
  On the basis of the dominant rock type, the manto mineralisation can be classified into three types (after Reyes, 1991):
• Socorro Norte type, comprising a group of stratabound orebodies developed in andesite flow breccia of the Cerro Negro Member, underlain by aphanitic andesite and overlain by volcaniclastic sandstone. Deposits in this unit form the most continuously mineralised bodies, with lengths that vary from 1 to 1.5 km and thicknesses typically from 10 to 20 m (varying from 1to 3 up to 40 to 50 m). The Toro-Colorado-Las Vacas-Cabaiias, and Floridor-Chisperos zones in the western part of the district are the principal orebodies of this group. Mineralisation occurs as gold-bearing pyrite, sphalerite and traces of chalcopyrite, with two populations of gold concentrations, namely 2 to 3 and 5 to 8 g/t respectively, burt locally as much as 20 to 30 g/t. The richest grades are found in the central part of each manto, close to crosscutting veins and fractures. The NW trending Toro and Socorro faults cut these mantos creating a set of parallel fractures which provided channelways for the mineralising fluids.
• Churrumata type, are which developed in vesicular dacite in the lower part of the Andacollo Member, a unit of strongly magnetic, amygdaloidal, porphyritic dacite overlain and underlain by porphyritic andesite. This type is characterised by subvertical mineralised bodies controlled by faults and fractures in the dacite and by subhorizontal bodies located in the upper, vesicular parts of dacite flows. The subvertical bodies vary from a few to ~30 m wide, 100 to 130 m long, and depths from 40 to 100 m. Subhorizontal bodies are up to 12 m thick, with lateral extents that depend upon the proximity of faults and veins and the intensity of associated fracturing. Zones that are 1 to 2 m wide containing subparallel veinlets are developed parallel to, and between the subvertical bodies. These veinlets contain calcite, quartz and visible gold, and have a sinuous strike. They are 5 to 30 m long, and may be up to a few centimetres thick. They are closely spaced near the subvertical bodies, with a frequency of 5 to 10 veinlets per metre, decreasing to 1 to 5 per metre in zones midway between veins (Llaumett, 1983). The gold mineralisation at Churrumata, which typifies this manto type, is only present in the subvertical and subhorizontal bodies, with the remainder of the dacite being barren. Mineralisation comprises gold-bearing pyrite and lesser amounts of chalcopyrite and sphalerite. Gold distribution is erratic and varies between 3 and 20 g/t in the mineralized bodies and 0.1 and 0.3 g/t in unaltered dacite between the bodies.
• Tres Perlas type, emplaced within dacite and andesite flow breccia in the upper part of the Andacollo Member, which is overlain by sandstone lenses and andesite and underlain by dark aphanitic andesite. The dacite has flow textures defined by elongated vesicles, whilst the volcanic breccia contains smaller fragments than those in the similar lithology of the Cerro Toro Member. These two lithologies host the Tres Perlas Norte and Sur orebodies, which comprise the disseminated gold zones closest to the porphyry copper deposit. The orebodies have lateral continuities comparable to that of the Socorro Norte-type deposits, with the highest gold grades having a strong structural control. Structures also control the subvertical, high-grade lenses, similar to those at Churrumata, although in contrast, the rock between the subvertical lenses is also mineralised. The mineralisation occurs as pyrite, specular hematite, magnetite and traces of chalcopyrite, with an average gold content that is lower than the Churrumata and Socorro orebodies, ranging from 1.3 to 1.5 g/t in the mantos, and 5 to 10 g/t in the subvertical lenses.

Alteration

  The gold mineralisation is associated with propylitic, potassic and argillic alteration.
  The propylitic alteration comprises chlorite, epidote and calcite, and is regional. In thin section, the feldspars can be seen to be partially kaolinised whilst the groundmass is altered to chlorite and calcite (Reyes, 1991). The introduction of pyrite-I, which is very early, probably accompanied the early regional propylitic phase (Oyarzun et al., 1996).
  Potassic alteration has a close spatial association with gold mineralisation, and is the result of the introduction of K
20, which constitutes up to 12 to 13 wt.% in some rocks. This alteration is manifested as both open-space filling, but predominantly, replacement of feldspar by adularia, hematite and chlorite, with lesser amounts of quartz, imparting a pale pink colour to the rock. Under the microscope plagioclase in dacite is seen to be replaced by quartz-adularia, which also fills in spaces between fragments in the flow breccia. Adularia occurs as small idiomorphic crystals in quartz. There is a strong lithologic control to this potassic alteration, which is pervasively developed within the felsic volcanic units. It is only weakly developed in andesite, occurring as narrow halos along fractures. In highly altered rocks, the primary textures are completely obliterated, whilst in moderately altered rocks, there is partial preservation of textures, with alteration minerals confined to veinlets or reaction rims around fragments in the flow breccia. The degree of fracturing and proximity to principal structures and veins dictates the intensity of alteration (Reyes, 1991).
  The vertical boundaries to the potassic alteration vary from abrupt to gradational, controlled by the composition and permeability of the host lithology, whilst lateral limits were controlled by rock permeability, and possibly, by the pressure, temperature and salinity of the mineralising fluids (Reyes, 1991).
  The potassic alteration also produced metasomatic pseudo-breccias by fluids following and permeating outwards from multidirectional intersecting fractures within flow breccia fragments that are >2 to 3 m in diameter, and along fracturing in dacite. The breccia is characterised by 'unaltered' clasts with reaction rims in a 'matrix' of alteration often occurring as parallel alteration bands along fractures (Reyes, 1991).
  Two intensities of gold mineralisation associated potassic alteration have been recognised (Miller, 1986), i). moderate alteration, where the rock is pale grey, contains relics of the original texture, and has small pyrite crystals, ii). a more intense alteration, where the rock is pale, has ghost textures, and cubic pyrite crystals with dimensions of >1 cm (Reyes, 1991).
  The potassic phase is followed by carbonate-I alteration, occurring as large calcite crystals within veins and veinlets (Oyarzun et al., 1996).
  Argillic alteration is generally superimposed on the potassic assemblage, is characterised by kaolinite and sericite, and mainly alters phenocrysts and groundmass. In the Tres PerIas area, late pyrite-quartz veinlets with sericite-kaolinite halos, similar to D veins in porphyry copper systems (Gustafson and Hunt, 1975), cut potassic-altered rock. Remnants of potassic alteration are evident in kaolinite-sericite altered rock in the Socorro and Toro areas (Reyes, 1991).
Carbonate-II alteration, which followed the argyllic phase, occurs as very fine grained and dispersed calcite within the rock matrix, representing a late intense carbonate deposition (Oyarzun et al., 1996).

Gold mineralised volcanic rocks have given ages of 91±6 Ma, suggesting they are marginally younger than the 98±2 and 104±3 Ma from gold bearing rocks in the porphyry (K-Ar; SERNAGEOMIN, reported in Reyes, 1991).

Reserves, Resources and Production

  The Andacollo Gold mine was a 20 000 tpd open pit, heap leach operation owned by Compañía Minera Dayton Limitada in 2000. Life of mine cash costs were $US220 per oz. The mine commenced operation in late 1995. 'Reserves' in 1990 were estimated to be ~15 Mt @ 1.5 g/t Au (Reyes, 1991) or 29 Mt @ 1.2 g/t Au (Oyarzun et al., 1996, quoting Bernstein, 1990).

  Proved + probable ore reserves in 1995 were estimated to be as follows (Lincoln and Tellez, 1995):
      Tres Perlas - 18.79 Mt @ 1.02 g/t Au     at a 0.4 g/t Au cut-off and 1.2:1 strip ratio open pit, and 76% metallurgical recovery;
      Tres Perlas West - 3.85 Mt @ 1.07 g/t Au     at a 0.4 g/t Au cut-off and 1.5:1 strip ratio open pit, and 76% recovery;
      Churrumata - 4.441 Mt @ 1.46 g/t Au     at a 0.4 g/t Au cut-off and 2.4:1 strip ratio open pit, and 65% recovery;
      Socorro Norte - 1.692 Mt @ 2.49 g/t Au     at a 1.0 g/t Au cut-off, underground, and 72% recovery;
      Natalia - 0.809 Mt @ 2.35 g/t Au     at a 1.0 g/t Au cut-off, underground, and 65% recovery;
      TOTAL reserve - 29.583 Mt @ 1.21 g/t Au     at a 1.0 g/t Au cut-off, 1.5:1 strip ratio, and 76% recovery;
            comprising 36 t of contained gold.
      TOTAL drill inferred resource - 54.016 Mt @ 1.10 g/t Au for 59.6 t of Au.
      TOTAL reserve + resource - 85.60 Mt @ 1.13 g/t Au for 95.6 t of contained Au.

Note: The five deposits in this resource are as follows (after Lincoln and Tellez, 1995):
Tres Perlas has dimensions of 600 x 300 m and was 450 m east of the Compañía Minera Dayton Limitada crusher. It is principally a manto that is hosted in a strongly brecciated and sometimes fractured dacitic unit that is 100 to 150 m thick and is potassic and/or sodic altered with a distinctive pink colouration. The manto contains disseminated pyrite or hematite, depending on the depth relative to the base of oxidation. It occurs between upper rhyolitic and footwall andesite units.
Tres Perlas West which is 600 m SW of Tres Perlas and was 500 m south of the primary crusher. It has dimensions of 500 x 100 m and is composed of 4 or 5 mantos that dip at 25 to 30°E, and is in a near identical stratigraphic position to Tres Perlas. The host dacite breccia is up to 45 m thick, and is intruded by numerous andesite dykes and a major quartz monzonite dyke. The form of mineralisation and alteration is similar to that at Tres Perlas.
Churrumata is 1 km WSW of Tres Perlas West and was 600 m SW of the primary crusher. It is hosted by the Churrumata subunit of the Andacollo Member, a sequence of 4 to 5 dacite flow breccias interbedded with andesite flows, overlain and underlain by andesitic sequences. Mineralisation occurs as 4 to 5 mantos that dip at 25 to 30°E, similar in style and alteration to Tres Perlas and Tres Perlas West.
Socorro Norte was 2 km SW of the primary crusher, on the western side of cluster of deposits. It is 400 m long, and ore grade continued down dip for 250 m with a consistent thickness of ~12 m. Mineralisation occurs as a dacite breccia manto, overlain by a thin epiclastic sandstone and then a dacitic volcanic unit. It is underlain by aphanitic impermeable dacite units, and is cut by the major, NW striking Socorro Vein fault, which is relatively high grade and dips SW. The manto is strongly altered, has a distinct pink colouration and insignificant pyrite, sometimes as large cubes.
Natalia was 500 m NE of the primary crusher. It consists of a vein breccia which has pod shaped ore zones that anastomose along strike and down dip. The vein breccia trends WNW, with dips of 45° to near vertical, cutting across stratigraphy. The ore zone has a strike length of 900 m, although the resource was only defined over a 300 m interval. Ore pockets within the vein system are controlled by the stratigraphy intersected, and are locally up to 30 m thick where cutting dacite breccias, and thin in andesites.

The most recent source geological information used to prepare this summary was dated: 1996.     Record last updated: 28/6/2016
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.


Andacollo

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Araya, V., Maldonado, A. and Astudillo, J.,  2012 - Geology of Carmen de Andacollo Deposit: in    Congreso Geológico Chileno, 13, Antofagasta, Chile, 05-09 Agosto,   Articulos de congresos, pp. 25-27.
Lincoln J B, Tellez C  1995 - The Andacollo gold project, IV Region, Chile: in Green S M, Struhsacker E, (Eds), 1995 Geology & Ore Deposits of the American Cordillera, Field Trip Guidebook compendium Geol. Soc., Nevada    pp 492-495
Oyarzun R, Ortega L, Sierra J, Lunar R  1996 - The manto-type gold deposits of Andacollo (Chile) revisited: a model based on fluid inclusion and geologic evidence: in    Econ. Geol.   v91 pp 1298-1309
Reyes M  1991 - The Andacollo strata-bound gold deposit, Chile, and its position in a porphyry copper-gold system: in    Econ. Geol.   v86 pp 1301-1316


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