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Santo Domingo - Santo Domingo Sur, Iris, Iris Norte, Estrellita
Chile
Main commodities: Cu Fe Au Co S


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The Santo Domingo copper-iron-cobalt (IOCG) project is located ~5 km southeast of the town Diego de Almagro in the Atacama region of Chile, ~35 km northeast of the Mantoverde mine and 130 km NNE of the city of Copiapó. It includes four main deposits Santo Domingo Sur, Iris, Iris Norte and Estrellita and a number of prospects not fully tested (#Location: 26° 26' 26"S, 70° 1' 3"W).

  Artisanal mining, chiefly of gold and iron, has been undertaken in the deposit area from the early 19th century. Modern exploration began in 2002 when Far West Mining Ltd undertook regional airborne geophysics, geological mapping, stream sediment and soil sampling, an induced polarisation survey, core and reverse circulation drilling and resource estimation, culminating in a preliminary economic assessment in 2008. Capstone Mining acquired Santo Domingo from Far West Mining in 2011 and completed a prefeasibility study in the same year. A second feasibility study was commissioned in 2012 and completed in 2014. In July 2015, the Environmental Impact Assessment, including the mine, infrastructure, process facilities, development of a greenfields port and iron concentrate and water supply pipelines, was approved by the Chilean authorities on the basis of the 2014 study. Capstone drilled 14 twinned diamond holes for a total of 3206 m during 2014 and early 2015, to both confirm previous drilling and collect metallurgical samples. However, in late 2015 Capstone announced that it would discontinue work on Santo Domingo due to low copper and iron prices, but continued to maintain the titles and community relations activities. Updates to the 2014 feasibility study were completed in 2018. In January 2019, Capstone drilled 13 twinned diamond drill holes for a total of 3747 m of core, to collect additional material for metallurgical sampling. A new prefeasibility was undertaken in 2019–2020 for an alternative development option that includes a cobalt recovery circuit. An exploration program completed in 2021 totaling 8500 m of drilling confirmed the extension of the mineralised sequence between the Santo Domingo and Iris Norte pits.

Regional Setting

  The Santo Domingo Project deposit cluster is located within the Chilean Iron Belt of the Atacama Fault Zone, a >1200 km long ductile/brittle sinistral strike-slip and dip-slip crustal scale structure that parallels the coast of Chile. The Chilean Iron Belt occupies an area of ~630 km by 40 km that straddles the Atacama fault zone and hosts numerous iron oxide-copper-gold (IOCG) and Iron Oxide Apatite (IOA) style deposits. The Santo Domingo deposits lie on the eastern side of the Atacama Fault Complex, which, in this area, comprises numerous clusters of generally north–south structural breaks in a belt that is ~30 km wide.

  Between ~132 Ma and 106 Ma, a number of tabular mafic to felsic plutonic complexes were emplaced along the Atacama Fault Zone, accompanied by both ductile strike-slip and brittle dip-slip deformation regimes. A number of volcanic- or intrusive-hosted breccia zones were developed in association with the strike-slip and dip-slip faulting, which became sites for the formation of a metasomatic iron oxide and iron-oxide-copper-gold (IOCG) mineralisation.

  For detail of the regional setting of the Chilean Coastal Cordillera and the Atacama Fault Zone in the Central Andes, see the Central Andes and Bolivian Orocline and the Central Andean Coastal IOCG Belt records.

Geology

  The base of the host stratigraphic sequence encountered in drill core in the project area are sedimentary rocks interpreted to be correlatives of the Punta del Cobre Formation which hosts mineralisation at the La Candelaria deposit, ~140 km to the south. The only known outcrop of this formation is a poorly-exposed sequence of sedimentary and volcanic rocks outcropping in the extreme southeastern part of the deposit area. This sequence includes intercalated calcareous sedimentary rocks, crystal tuff, lapilli tuff, hornfels and andesite porphyry. An exposure of thinly laminated, moderately west-dipping, red hematitic siltstone is considered to possibly be a correlative of the hematitic terrigenous basal conglomerate of the Algarrobo Member of the Punta del Cobre Formation as exposed in the Copiapó area (Marschik and Fontboté, 2001). If this correlation is correct, the lithology in this area is in the same stratigraphic position as the host rocks of the Candelaria deposit.
  The presumed Punta del Cobre Formation passes upward into an interdigitated succession of limestone and calcareous marine sediments of the Chañarcillo Group and the predominantly volcanic sequence of andesite flows and volcaniclastic rocks of the Bandurrias Group. The contact between the upper Punta del Cobre Formation and the overlying Bandurrias-Chañarcillo Group sequences is the stratigraphic host location of the deposit at La Candelaria. Limestone units of the Chañarcillo Group vary from a few metres to >100 m in thickness, and can be the dominant lithology over several hundred metres of stratigraphy. They are generally fine grained, massive to thick bedded, and dark to light grey, predominantly occurring in the upper parts of many prominent hills in the area.
  Most clastic rocks in this part of the sequence are tuffaceous sediments or crystal tuffs, with few true 'sediments'. These tuffs and tuffaceous sediments are generally fine to medium grained, massive to poorly bedded and frequently difficult to differentiate from fine-grained, massive flows. Individual units are up to 50 m thick, but can exceed 300 m and comprise the bulk of the stratigraphy, with minor intervals of limestone and andesite lavas. Locally, andesitic volcaniclastic rocks are intercalated with significant volumes of a light grey to cream, aphanitic and, rarely, thinly-laminated rocks, which petrographic investigations suggest are carbonate-potassic feldspar altered tuffaceous andesitic sediments (Ross, 2005). This tuffaceous sequence contains several relatively narrow hematite and magnetite ±copper oxide or sulphide mantos that are up to 12 m thick, and occur sporadically over a 200 m stratigraphic interval, with associated weak to strong actinolite-potassic feldspar alteration. This stratigraphy and related iron oxide-copper mantos have been tentatively identified throughout the deposit and probably underlies most or all of the area.
  Andesitic flows within this sequence range from near aphanitic to coarse-grained feldspar-phyric, but are more often medium grained, with 20 to 30% euhedral, white, prismatic plagioclase ±minor hornblende as phenocrysts set in a grey to brownish aphanitic groundmass. Some flows are massive, whilst others contain abundant amygdales that may be up to 1 cm in diameter, but averaging 1 to 2 mm. These amygdales are filled with varying proportions of quartz, calcite, epidote, chlorite, potassic feldspar, limonite after pyrite and 'almagre' (minute grains of distinct copper minerals admixed with the red hematite or other copper oxides).
  The Bandurrias and Chañarcillo Groups, despite their lithological differences, are interpreted to be contemporaneous, deposited in different parts of the same basin and structurally juxtaposed. However, the andesite-tuff succession that hosts the mantos are considered to be part of the underlying upper Punta del Cobre sequence, suggesting faulted contacts with the structurally-juxtaposed limestone that are correlated with the Chañarcillo Group.
  At least nine intrusive events have been recognised within the district, generally younging eastward and ranging in age from 145 to 90 Ma. These have been grouped into three intrusive phases in the Santo Domingo Project area. The first is a medium-grained equigranular diorite, which best crops out in the southern part of the district, particularly the structural block to the south of the Santo Domingo Sur deposit. In addition, the andesitic tuff sequence has been intruded by fine-grained diorite sills that are present in almost all drill holes cutting the Santo Domingo Sur deposit, varying in drilled thickness from a few metres to >60 m. These sills outcrop to the south of Santo Domingo Sur. Similar sills, that are also encountered, but to a lesser degree in the Iris deposit, are also described as diorite-andesitic porphyritic sills with 30 to 35% fine groundmass and subhedral plagioclase phenocrysts. The diorites are typically altered and in rare cases contain copper mineralisation. These observations have been taken to suggest diorite intrusion is more or less contemporaneous with the mineralising event. Finally, a set of feldspar-hornblende porphyry dykes cut all other rocks types (Daroch and Barton, 2011; Maycock, et al., 2020).
  The Project area is structurally divided by a series of complex, variably north-south, NW, NE and east-west high angle faults with both dip-slip and strike-slip movement, which seem to have been active repeatedly through time. Some low angle, probable reverse faults are also recognised. The dominant structure, the east-west trending, dextral, steeply north dipping, north-side-down block fault, known as the Santo Domingo Fault, is the focus of mineralisation in the east-west aligned Estrellita deposit and Estefánia prospect areas, and is the locus of most of the historic copper production from the project area. The most prominent fault set, as interpreted from magnetic data, trends NW and has fault separations of ~1 km.
  The high-angle faulting has produced a series of fault blocks. Many of the faults separating these blocks appear to mark pronounced lithological changes and structurally juxtapose blocks with different lithological characteristics, suggesting they represent different stratigraphic levels. This has influenced the distribution of manto- and fault-related iron oxide-copper mineralisation at Santo Domino, having, for example, uplifted the central section of the Santo Domingo Sur deposit, bringing the shallow dipping mantos closer to surface. These fault blocks will be further discussed in the individual deposit descriptions below. The project area has only been gently folded along NNE-trending axes.

Alteration

  All of the rocks in the Santo Domingo Project area have been subjected to hydrothermal alteration and mineralisation that is both temporally and spatially zoned on both the deposit and district scales. Alteration styles reflect hydrothermal zoning assemblages from proximal to distal at a deposit scale (e.g., Santo Domingo Sur) and apparently to depth and towards a diorite intrusive complex at a district scale. These features are complex, with multiple overprinting components. According to Daroch and Barton (2011), distal zones are characterised by an assemblage that includes specular-hematite + mushketovite with chalcopyrite ±bornite ±chalcocite, where iron oxides were formed in a complex cycling between hematite and magnetite. Early elongate specularite crystals have been transformed into mushketovite following a zoned pattern, from an inner specularite crystal towards an outer zone with magnetite. The principal Cu sulphide is chalcopyrite which replaces early magnetite-pyrite associations, and occurs interstitial to specularite ±mushketovite, and replacing fractures in early pyrite. The distal alteration assemblages comprise early actinolite ±albite ±epidote ±titanite and later chlorite + actinolite + carbonate ±quartz consistent with low to moderate-temperature Ca(-Na) alteration. The actinolite crystals are fine grained with radial patterns, with shapes preserved after carbonate replacement. K silicate alteration is sparse, occurring as K feldspar in the distal mineral associations.
  Proximal zone mineral associations are predominantly magnetite ±mushketovite with pyrite ±chalcopyrite, reflecting early moderate-temperature Ca(-Na) assemblages, accompanied by epidote ±scapolite ±quartz, followed by actinolite ±albite ±epidote ±titanite and later quartz ±carbonate ±chlorite. Early assemblages are overprinted by later actinolite crystals, which are seen invading and fracturing epidote crystals. Some mineralised veins contain specularite + chalcopyrite ±pyrite and actinolite ±feldspar ±quartz ±epidote with a narrow halo of carbonate. Later quartz ±carbonate and carbonate veins and have a variety of crosscutting relationships (Daroch and Barton, 2011).

  Maycock, et al., 2020) describe a temporal zonation, from early to late as follows:
Sodic (-calcic) - the principal alteration minerals of which are albite, actinolite, chlorite, epidote and titanite, mainly replacing volcanic and intrusive rocks. Scapolite-actinolite-pyroxene veins occur at the southern section of the project area, proximal to and within the diorite stocks and dykes. At surface, actinolite, chlorite and carbonate typically occur infilling amygdales and open spaces. Pink albite replaces plagioclase in the more porphyritic rocks.
Potassic - potassium silicate alteration is less common but is found as K feldspar-chlorite-carbonate-quartz mineral assemblages. Patchy K feldspar mainly replaces plagioclase (albite) and is also found in veins with carbonate and quartz. This alteration is mainly restricted to within the copper-iron mantos.
Carbonate - carbonate rich assemblages are widespread and overprint the previous mineral associations. These include calcite, ankerite, siderite with associated chlorite, quartz veins and stockworks that are commonly found cutting all rock types of the area.
Calc-silicate 'skarn' minerals are found south of Santo Domingo Sur where carbonate rich, and lesser volcanic rocks are in contact with diorite intrusive units. The principal alteration minerals are garnet, mainly andradite, epidote, pyroxene, actinolite and carbonate.

Deposits

  As of February 2020, four deposits had been outlined and three of these were included in the published Ore Reserves, whilst a number of less defined prospects had also been outlined. Of the main deposits, Santo Domingo Sur, Iris and Iris Norte are distributed over an interval of ~4.25 km, from south to north, along a near north-south trending fault zone. Santo Domingo Sur and Iris have been found to be continuous. The fourth, Estrellita, follows the east-west Santo Domingo Fault over a length of ~1.7 km, ~2 km west of the boundary between Iris and Iris Norte (Daroch and Barton, 2011).

Santo Domingo Sur
  The structural block that encloses the Santo Domingo Sur deposit comprises a thick package of andesitic flows, underlain by a sequence of tuffaceous rocks of similar composition that have been intruded by fine-grained diorite sills. The entire package has, in turn, been cut by later feldspar-hornblende porphyry dykes that do not host any mineralisation. In contrast, the structural blocks to the west and east comprise a bedded sequence of gently dipping limestone and intercalated tuffaceous andesitic rocks; and a thick sequence of limestone, respectively.
  The primary hosts to mineralisation at Santo Domingo Sur are the andesitic flows and tuffs which dip gently at ~15° to the NNW below gravel cover. The tuff sequence has been intruded by fine-grained diorite sills that are typically altered, and in rare cases contain copper mineralisation. Mineralisation within the deposit comprises a thick sequence of stacked chalcopyrite-bearing semi-massive to massive specularite-magnetite mantos that have replaced the tuffaceous rocks, with minor associated veins, breccias and skarn bodies. Drilling has outlined a 150 to 500 m thick sequence of these mantos, covering a north-south to NNE-SSW aligned area of ~1300 x 800 m, and has been traced to a depth of >525 m below surface.
  The individual mantos are ~4 to 20 m thick with strike lengths of up to 700 m, and comprise conformable layers of semi-massive to massive specularite and magnetite containing clots and stringers of chalcopyrite. The mantos are radially zoned, from an outer periphery of specular hematite, toward a magnetite-rich core. The Cu-Fe sulphides are more abundant in the specular-hematite domain, with chalcopyrite + pyrite ±bornite. In addition, several steep veins, mainly composed of specular-hematite ±mushketovite and chalcopyrite cut the volcaniclastic rocks, the mantos, and the andesitic flows. The upper limits of the manto sequence are directly below the overlying andesite flows, and are frequently oxidised, contain varying amounts of copper oxides and chalcocite. Mineralisation is best developed in the upper levels in the southern part of the deposit, with the Cu grade and intensity of the mineralisation weakening towards the north and with depth. The high-grade core of the deposit lies along it's southern margin and close to surface, where the bounding fault appears to be the locus of the strongest alteration and mineralisation.
  A zone of hydrothermal brecciation has been outlined in the centre of the deposit, composed of andesite and andesitic tuff fragments set in a fine-grained matrix of iron oxides. It forms a narrow body at depth, which widens toward the surface. The upper part of the breccia is oxidised with both limonite, the dominant iron oxide and copper oxides, and locally native copper. The lower part of the breccia contains chalcopyrite mineralisation and differs from the surrounding rock only in texture.

Iris
  The Iris deposit is essentially blind, masked by Quaternary gravel. It is composed of iron oxide mantos and breccias formed along a NNW striking fault zone to the north of, and semi-continuous with, the Santo Domingo Sur deposit. Along most of it western margin, it is bounded by a west-dipping fault. Mineralisation is developed over a strike length of 1800 m and has been traced from surface to a depth of ~670 m. As projected to surface, it has a width of ~500 m, although when the dip and plunge are taken into account, it's actual average width is of the order of 200 m. Mineralisation occurs close to surface on its southern end and plunges gently towards the north. Where close to surface in the south, old workings over an area of ~100 x 60 m contain copper-oxides such as brochantite and chrysocolla, hosted by a specularite manto and cut by steeply-dipping structures. Overall, the dominant iron oxide in the deposit is hematite and the main copper mineral is chalcopyrite. The distribution of copper is more erratic that at Santo Domingo Sur, reflecting the greater structural control of mineralisation, in contrast to the more continuous stratabound replacement style at the latter.
  The Iris and Santo Domingo Sur deposits are separated by the Iris Mag Zone, which is a separate structural block. Mineralisation comprises strong, up to 40% and more massive magnetite, with typically low grades, on average, of ~0.1% Cu. The host rocks are andesitic flows and andesite breccias with a much smaller tuff component than the other zones. It appears that this part of the deposit has been subjected to an initial high temperature magnetite event but shows little evidence of a later oxidising overprint that has accompanied the higher-grade copper and gold values seen elsewhere.

Iris Norte
  The Iris Norte deposit is ~600 m to the north of the Iris deposit and is also blind, being entirely masked by a gravel sequence. It occurs on the eastern margin of a pronounced gravity high. The deposit is ~500 m wide by 1600 m long, and has been traced to a depth of 320 m below the surface. Unlike Iris and Santo Domingo Sur, the principal host to mineralisation are andesitic flows, with a strong structural control. It has a north-easterly strike, which represents an ~55° rotation compared to Iris. The deposit has been intruded by numerous diorite dykes and sills that have separated it into two lenses. Mineralisation comprises mixed magnetite and specular hematite mantos, a higher proportion of which is magnetite compared to Iris. The main sulphides are pyrite and chalcopyrite.

Estrellita
  The structural block that encloses the Estrellita deposit is composed of an up to 200 m thick package of andesitic porphyries and flows that is underlain by a sequence of volcaniclastic rocks with minor intercalations and interbeds of andesite porphyry, limestone and altered tuff. This structural block has been faulted into a series of four sub-blocks, with each displaced progressively downwards to the north by as much as 75 m.
  The deposit strikes east-west and is a flat-lying to shallow north-dipping tabular body located ~3.5 km NW of Santo Domingo Sur. It covers a plan area of ~900 x 450 m, and is up to 100 m thick. It is thickest in the middle, thinning towards the peripheries.
  Mineralisation is interpreted to occur at a higher stratigraphic level than at the Santo Domingo Sur, Iris and Iris Norte deposits. It occurs as a mixture of manto-style, iron oxide and structurally-controlled, vein-style mineralisation. The core of the deposit comprises a more or less horizontal tabular iron oxide manto that appears to have formed at the intersection of a flat-lying and a steeply-dipping set of specularite bearing structures. Copper mineralisation typically consists of an oxide supergene cap near surface composed of copper oxides such as brochantite, chrysocolla, almagre, cuprite and chalcocite, gradually becoming less oxidised with depth, though a mixed zone, to a sulphide zone where the principal copper mineral is chalcopyite.

Reserves and Resources

  Ore Reserve and Mineral Resources as at 31 December, 2022 (Capstone Copper Reserve and Resources online, viewed March 2024) were:
      Proved + Probable Reserve - 392.326 Mt @ 0.30% Cu, 0.04 g/t Au, 28.2% Fe;
      Measured + indicated Resource - 536.548 Mt @ 0.30% Cu, 229 ppm Co, 0.04 g/t Au, 2% S, 25.7% Fe (Inclusive of Ore Reserves)
      Inferred Resource - 47.903 Mt @ 0.19% Cu, 197 ppm Co, 0.02 g/t Au, 2.2% S, 23.6% Fe.

NOTE: Cobalt has a close association with pyrite within the project deposits. A study has found that 83% of the cobalt in samples analysed was present in solid solution with pyrite. The remainder is likely in locked pyrite particles, in chalcopyrite and, in rare instances, in discrete cobalt sulphide minerals. It can be recovered by producing and treating a pure pyrite concentrate, which can also be used for sulphur/sulphuric acid.

The information in this summary was drawn from both Maycock, J., Luraschi, A., Mendoza, M., Bianchin, M., Rennie, D., Guzman, C., Amelunxen, R., Gingles, M., Kerr, T., Betinol, R., Jones, L. and Bush, G., 2020 - Santo Domingo Project, Region III, Chile; an NI 43-101 Technical Report prepared for Capstone Mining Corp., 516p. and Daroch and Barton (2011), as cited below.

The most recent source geological information used to prepare this decription was dated: 2020.    
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.


Santo Domingo

  References & Additional Information
   Selected References:
Daroch, G.A. and Barton, M.D.,  2011 - Hydrothermal alteration and mineralization in Santo Domingo Sur Iron Oxide (-Cu-Au) (IOCG) deposit, Atacama Region, Chile: in   Lets Talk Ore Deposits 11 th SGA Biennial Meeting, 26-29th September 2011 Antofagasta, Chile,   Proceedings, pp. 448-490.


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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