Santa Cruz, Argentina
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The Cerro Vanguardia low sulphidation epithermal gold-silver deposits are located in Santa Cruz Province, southern Patagonia, Argentina, ~110 km NNW of Puerto San Julián on the Atlantic coast and 280 km SSW of Comodoro Rivadavia (#Location: 48° 23' 15"S, 68° 15' 47"W).
Mineralisation is exploited from a series of open pits and multiple underground mines distributed over an area of ~15 x 10 km within a 543 km2 mining lease. Exploration at Cerro Vanguardia commenced in the late 1980s, undertaken by the state owned Fomicruz (Fomento Minero de Santa Cruz Sociedad del Estado) and Minera Mincorp (a JV between Anglo American Argentina Holdings Limited and Perez Companc, a local private company). The initial open pit began operating in 1998, supplemented from 2010 by shallow underground mining to access deeper high-grade mineralisation. Heap-leaching started in 2012. The mine has been operated by AngloGold Ashanti since 1998.
The deposit complex is hosted within the ~60 000 km2 Deseado Massif which also hosts a series of other low sulphidation Au-Ag deposits and deposit clusters. These deposits are spread across the width of Argentina, from the Atlantic coast (Cerro Moro) to San José, 500 km to the WNW. The Cerro Bayo deposit occurs 110 km to the west again, in the Andes of eastern Chile. These deposits are exposed within a number of inliers of the Jurassic Chon Aike Large Igneous Province, surrounded by largely flat lying to gently dipping post-Jurassic sequences and superficial cover. The Cerro Vanguardia deposits comprises the largest vein field within the Massif, with >100 individual veins totalling a composite ~200 linear km of veining. Fifty-seven of these veins were known to contain economic gold and silver mineralisation in 2016.
The regional setting and geology of the Deseado Massif is described in the Southern Andes and Patagonia record.
Within the Deseado Massif, the Middle to Late Jurassic Chon Aike Large Igneous Province is represented by the Bahía Laura Volcanic Complex which is dated at between 187 and 151 Ma (Haller, 1997). This complex is inferred to overlie a basement of sandstone and pelites of the Mid- to Upper-Triassic El Tranquilo Formation, as evidenced by clasts in veins and intrusions within the exposed overlying sequence. It is subdivided into the following formations:
• Roca Blanca Formation, the lowest exposed unit, comprising ~990 m of Lower Jurassic sandstones and sandy tuffs;
• El Piche Formation, composed of amygdaloid basalts and andesites(?) assigned to the Lower Jurassic (Jovic 2010). Previously equated with the Bajo Pobre Formation (e.g., Páez et al., 2010), which Guido and Jovic (2014), interpret to be higher in the sequence (see below), with the El Piche Formation being a separate and older unit.
• Chon Aike Formation, predominantly Middle to Upper Jurassic rhyolites which have calc-alkaline, peraluminous and high potassium signatures, accompanied by minor dacites and trachydacites. These extrusives built an extensive rhyolitic plateau, dominated by large volumes of pyroclastic material (~90% ignimbrites with subordinate intercalated lavas) erupted as high fluidity ash flow tuffs (Pankhurst et al., 1998; Panza and Haller, 2002; Guido, 2004);
• Cerro León Formation, typically porphyritic subvolcanic laccoliths, sills and dykes, which de Barrio et al., (1999) and Jovic et al. (2008) regarded as representing feeders and intrusive equivalents of the Bajo Pobre Formation that Sharpe et al. (2002) interpreted to be equated with the El Piche Formation of Guido and Jovic (2014). However, dactic and rhyolitic dykes attributed to the Cerro León Formation cut through into the highest volcanic unit of the Chon Aike Formation (Guido and Jovic, 2014);
• Bajo Pobre Formation, composed of intermediate to basic calc-alkaline, mostly andesitic, volcanic rocks, dominated by lavas, with subordinated ash flow tuffs and agglomerates which interfinger with the upper Chon Aike Formation and/or the tuffs of the lower La Matilde Formation, representing the extrusive equivalent of the Cerro León Formation (Panza and Haller, 2002);
• La Matilde Formation, a homogeneous Upper Jurassic sequence of ash fall tuffs and reworked volcaniclastic sediments deposited in low energy fluvial and lacustrine settings, with minor ash flow and air fall tuff intercalations. It is intercalated with and partially a lateral equivalent of the upper ignimbrites of the Chon Aike Formation (de Barrio et al., 1999).
The sequence described above is unconformably overlain by sandstone and course clastic sedimentary rocks of the Lower Cretaceous Bajo Grande Formation
Within this complex, the Chon Aike Formation, which hosts the mineralisation, has been divided into six units, as follows, from the base:
• Estratificada Ignimbrite - a mixed unit of ignimbrites interspersed with stratified tuffs;
• Granosa Ignimbrite - a heavily welded crystallographic flow tuff with a thickness of >150 m;
These are followed by three thinner units that together total <100 m in thickness, namely,
• Upper Stratified Ignimbrite - poorly welded pumiceous ignimbrite and tuff layers with a marked stratification;
• Brechosa Base Ignimbrite and
• Brechosa Ignimbrite - both of which comprise poorly welded polymorphic and stratified flow deposits;
• Masiva-Lajosa Ignimbrite - the uppermost unit that is represented by >100 m of massive welded crystal ignimbrite, containing lesser stratified pumice.
The entire Chon Aike Formation sequence within the deposit area is cut by pre- and syn-mineralisation dactic and rhyolitic dykes equated with the Cerro León Formation.
Mineralisation characteristically occurs as a low-sulphidation, low temperature epithermal vein system, developed in faults hosted by the Middle to Upper Jurassic volcanic sequence. These veins are narrow and occur within a vertical range of about 0 to 200 m below surface in a structural pattern related to two major shear directions, trending north-south (Concepcion) and east-west (Vanguardia). This shearing has produced two sets of veins. The first set dominates and strikes at ~320°, generally dipping 65 to 90°NE, while the other set that is mainly on the northern margin of the vein field, strikes on average at 105°, varying from east-west to ENE to NE and dips 60 to 80°SW. Both sets cut the entire volcaniclastic sequence that is dominantly flat lying. The veins are thickest and have the highest gold grade within the more competent Granosa and Masiva-Lajosa ignimbrites. They comprise multiple mineralising pulses of quartz-chalcedony and sericite-adularia with sulphides of base metals and Au-Ag minerals occurring as minor electrum, native gold, silver sulphides and fine-grained native silver disseminations. This assemblage is mainly characterised by dark colloform-crustiform banding, pseudomorphic quartz-lattice textures, massive to vuggy quartz veins and vein breccias, as well as replacement textures. High-grade Au-Ag mineralisation is associated with the dark crustiform to colloform bands which are composed of quartz-adularia with finely disseminated pyrite, now oxidised to limonite. The veins have sharp contacts with the enclosing ignimbrite, which also hosts narrow stock-work zones that are weakly mineralised, and appear to have been cut by a sequence of north-east trending faults that have southerly movement with no important lateral displacement.
The age of the mineralisation was calculated to be between 155 and 153 Ma (40Ar/39Ar dating of adularia), corresponding to the upper Jurassic (Sharpe et al., 2002).
Mineralisation of Cerro Vanguardia is interpreted to be genetically related to the sub-volcanic intrusions of the Cerro León Formation. This intermediate magmatism is the last volcanic event prior to the mineralisation, and both affect all previous units, except for the reworked deposits of La Matilde. The latter are interpreted to be relatively contemporaneous with the mineralisation, based on the presence of intercalated ferruginous silica beds regarded to be the surface expression of fossil geothermal deposits of hydrothermal fluids rich in silica and iron. The linkage with the intrusives is supported by geochemical vectors, defined by Au and Ag distribution patterns indicating highest grade concentrated in the vicinity of intermediate magmatism. Veining was the result of the mixture of magmatic fluids and meteoric waters.
Reserves and Resources
Measured + Indicated + Inferred Mineral Resources at 31 December, 2002 (AngloGold Ashanti, 2003) were:
TOTAL Resource - 12.9 Mt @ 7.68 g/t Au, 53.29 g/t Ag, for 98.7 t Au.
Remaining Measured + Indicated + Inferred Mineral Resources at 31 December, 2006 (AngloGold Ashanti, 2007) were:
TOTAL Resource - 42.399 Mt @ 2.93 g/t Au, for 124.051 t Au.
Remaining AngloGold Ashanti 92.5% share of the Measured + Indicated + Inferred Mineral Resources at 31 December, 2016 (AngloGold Ashanti, 2017) were:
Open pit veins - 12.01 Mt @ 5.31 g/t Au, for 63.79 t Au;
Stockwork ore - 15.55 Mt @ 0.56 g/t Au, for 8.7 t Au;
Underground veins - 2.07 Mt @ 9.66 g/t Au, for 20.3 t Au;
Heap leach stockpile - 4.55 Mt @ 0.58 g/t Au, for 2.64 t Au;
TOTAL AngloGold Ashanti share of Mineral Resource - 34.19 Mt @ 2.78 g/t Au, 67.05 g/t Ag, for 95.16 t Au and 2292 t Ag.
TOTAL Mineral Resource - 36.96 Mt @ 2.78 g/t Au, 67.05 g/t Ag, for 102.87 t Au and 2477 t Ag.
AngloGold Ashanti 92.5% share of the Ore Reserves at 31 December, 2016 (AngloGold Ashanti, 2017), which are included in Mineral Resources, were:
Open pit veins - 2.24 Mt @ 5.67 g/t Au, for 12.68 t Au;
Stockwork ore - 6.57 Mt @ 0.65 g/t Au, for 4.26 t Au;
Underground veins - 1.43 Mt @ 8.76 g/t Au, for 12.49 t Au;
TOTAL AngloGold Ashanti share of Ore Reserve - 10.23 Mt @ 2.88 g/t Au, 55.46 g/t Ag, for 29.43 t Au and 567 t Ag.
Exclusive Mineral Resources at 31 December, 2016 (AngloGold Ashanti, 2017) which are located between the pit design and the Mineral Resource shell and exists due to the difference in the economic parameters used: - 25.55 Mt @ 2.43 g/t Au, for 62.07 t Au.
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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