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Gold, Porphyry, Central Andes, Back-arc, Zircon U/Pb dating
Abstract:
The Lindero gold deposit is located in the Southern Puna plateau, northwest Argentina. The deposit is centered in a cluster of six subvolcanic intrusions emplaced at the margin of the Arizaro Basin. Three alteration types were recognized: (i) Ca–Na silicate (clinopyroxene + magnetite + K-feldspar + quartz + calcite ± plagioclase), (ii) K-silicate (K-feldspar + quartz + magnetite ± biotite ± anhydrite) and (iii) chlorite-calcite alteration. The highest ore grades are linked to the K-silicate alteration. The proven plus probable reserves of Lindero are 84,226 t with average grades of 0.63 g/t Au and 0.11% Cu. A previous study assigned Lindero to the iron oxide copper-gold (IOCG) deposit type but many features of Lindero suggest that it is a porphyry gold deposit, including (i) the temporal and spatial link between alteration and the intrusive bodies, (ii) the alteration distribution pattern, particularly the small volume of rock affected by Ca–Na silicate alteration, (iii) the Au-rich and Cu-poor mineralization style. The magmatic complex at Lindero comprises an early-mineral unit (FPD), four inter-mineral units (CPD1, PBFD, CPD2 and DDP) and one post-mineral unit (PMI). In-situ U/Pb SIMS dating of the oldest (FPD), an intermediate (PBFD) and the youngest (DDP) intrusive units, confirms a middle Miocene age. The weighted mean ages of the oldest and youngest units are indistinguishable, with 15.36 ± 0.13 Ma (n = 21) and 15.47 ± 0.11 Ma (n = 16), respectively. Individual ages from each unit range by ~1 m.y. and the overall spread of zircon ages is 15.92 ± 0.23 to 14.44 ± 0.33 Ma. We suggest that emplacement of the subvolcanic stocks took place within this span time, likely at the lower end of this range (15.0–14.4 Ma). Two 40Ar/39Ar ages of hydrothermal biotites from the K-silicate alteration (14.99 ± 0.16 Ma and 14.93 ± 0.12 Ma) indicate that hydrothermal alteration began practically simultaneously with the emplacement of the porphyry units.
All of the intrusive units are similar compositionally. They show a fine- to medium-grained porphyric texture (1–4 mm) comprising plagioclase, amphibole, clinopyroxene and scarce quartz phenocrysts (40–55 vol % of phenocrysts) in a K-feldspar ± quartz microcrystalline (0.02–0.07 mm) groundmass, except in the post-mineral unit which has a cryptocrystalline groundmass. Whole-rock analyses reveal a narrow range of dioritic composition (58.6–61.9 wt % SiO2) and high-K calc-alkaline character for all units. Trace element features (low Ba/Nb ratios, high Nb) of the Lindero magmas indicate a back-arc affinity, similar to those from the Southern Puna and distinct from the Central Volcanic Zone (CVZ) frontal arc. The Sr and Nd isotope ratios (87Sr/86Sr = 0.706042 to 0.706607; 143Nd/144Nd = 0.512501 to 0.512582) from Lindero intrusives are also similar to Southern Puna back-arc volcanic rocks. The Pb isotope ratios of Lindero (206Pb/204Pb = 18.79 to 18.83; 207Pb/204Pb = 15.60 to 15.63; 208Pb/204Pb = 38.66 to 38.74) overlap with both back-arc and arc magmas in the CVZ. The narrow age range, spatial association and uniform chemical and isotopic composition of Lindero porphyry units suggest that were derived from a common magma source, which underwent fractionation and/or crustal assimilation before emplacement as suggest by the low concentrations of Mg, Cr, Ni and Sr. The Lindero porphyry units show chemical and isotopic similarities with those from porphyry gold deposits in the Maricunga belt, Chile, and with the porphyry copper deposits of Argentina located in a back-arc setting; however, they differ from porphyry copper deposits in the frontal-arc setting of Chile, notably by the lack of an adakite-like signature (high Sr/Y ratio).
