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Location: 107 km NW from Puerto San Julian, Argentina
National Route N3. Km 2146 Magallanes, Santa CruzArgentina
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GeologyThe Cerro Vanguardia district is located within the southern Deseado Massif in the Santa Cruz province of Patagonia, Argentina. The Deseado Massif is an extensive rhyolite province of Middle to Upper Jurassic age. The most important geological feature in the Deseado Massif is an extended plateau formed by pyroclastic, epiclastic and extrusive rocks that were part of a strong explosive volcanic event. The volcanic event was associated with regional extensional tectonics developed during the Middle to Upper Jurassic and related to the opening of the Atlantic Ocean. The rocks representing this magmatism are termed the Bajo Pobre Formation and Bahia Laura Group. The Bajo Pobre Formation comprises andesites, basalts and mafic volcanic agglomerates. The Bahia Laura Group includes both the Chon Aike Formation (ignimbrites, tuffs, volcanic breccias, agglomerates, lavas, and domes) and the La Matilde Formation (tuffs and epiclastic volcanic interlayered with ignimbrites).Deposit typeThe Middle to Upper Jurassic ignimbrites and volcanic rocks of the Chon Aike Formation host the low-sulphidation epithermal gold and silver deposits. The thickness of the ignimbrite sequence is estimated to have exceeded 1,000m but some lateral variations have been identified across the district. Epithermal gold-silver bearing structures cut across all Jurassic rocks in the stratigraphy. The two main ignimbrite units, the Masiva-Lajosa and Granosa, host most of the mineralised veins.The Masiva-Lajosa ignimbrite occurs at the top of the sequence while the Granosa ignimbrite occurs towards the bottom. These two ignimbrites are separated by two thinner, polymictic ignimbrite units (Brechosa and Brechosa Base) and a sequence of stratified crystal to ash-rich tuffs (Estratificada unit). The base of the sequence is a mixed unit of stratified ignimbrite intercalated with fine-grained tuffs (Estratificada Inferior ignimbrite).Mineralisation styleCerro Vanguardia is in the core of the 60,000 km2 Deseado Massif, one of the most extensive volcanic complexes in southern Patagonia. The Deseado Massif is deposited over Paleozoic low-grade metamorphic basement rocks. These rocks are exposed in erosional windows through overlying Cretaceous sediments and Tertiary to Quaternary basalts. The orebodies comprise a series of low-sulphidation epithermal vein deposits containing gold and large quantities of silver, which is recovered as a by-product. Mineralisation characteristicsThe mineralisation is concentrated in steeply-dipping quartz veins that cut the flat-lying ignimbrites and volcanoclastic rocks. The Cerro Vanguardia district contains more than 100 gold and silver-bearing epithermal veins for a cumulative exposed vein strike extension of more than 240km, of which 55 veins are currently known to contain economic gold and silver mineralisation.The veins at Cerro Vanguardia consist mainly of quartz and adularia and contain minor electrum, native gold, silver sulphides and native silver as fine-grained disseminations. Vein textures are mainly characterised by colloform-crustiform banding, pseudomorphic quartz lattice textures, massive-to-vuggy quartz veins and vein breccias. 40Ar/39Ar dating on adularia from the Osvaldo Diez vein yielded ages of around 153Ma while the age of the thick sequence of ignimbrites hosting the veins has been dated between 166Ma and 150Ma.
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