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Location: 50 km SW from Calama, Chile
Cerro El Plomo 6000, Las CondesSantiagoChile7560623
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The Spence porphyry copper deposit is related to three stocks of dacitic porphyries and tourmaline-bearing hydrothermal breccias, which were emplaced in andesitic volcanic rocks of the Upper Cretaceous–Lower Tertiary. Laser 40Ar/39Ar plateau ages in magmatic biotites of pre- and post-mineralization dacitic stocks show evidence that hypogene mineralization (chalcopyrite, pyrite, molybdenite, and lesser amounts of bornite and tennantite) occurred between 56 and 57 Ma. Unconsolidated gravel deposits and a leach cap cover an oxidation blanket characterized by atacamite, chrysocolla, and minor brochantite, in which the Cu-hydroxychloride is by far the dominating mineral. Underlying the oxide zone is a well-developed chalcocite- and lesser covellite-bearing supergene enrichment blanket. Spence is enriched and oxidised porphyry copper deposit containing in situ copper oxide mineralisation that overlies a near-horizontal sequence of supergene sulphides, transitional sulphides, and finally primary (hypogene) sulphide mineralisation. Representative samples from pit walls and drill cores were selected in order to cover most of the hypogene and supergene zones, including chalcopyrite from the hypogene-enriched sulfides and hypogene zones, chalcocite from the supergene-enriched sulfides and hypogene-enriched blankets, chrysocolla from the oxide blankets, and atacamite from the oxide zone within the host rocks and gravels and leach cap.
