Petrogenesis of Quebrada de la Mina and Altar North porphyries (Cordillera of San Juan, Argentina): Crustal assimilation and metallogenic implications

FRANCHINI, MARTA; DI GIUSEPPE, NOELIA; MAYDAGÁN, LAURA; BOUHIER, VERÓNICA; LUIS DIMIERI; CHIARADIA, MASSIMO; REY, ROGER

Geoscience Frontiers

Universidad de Geosciencias de Beijing

Lugar: Beijing; Año: 2016 p. 1 - 25

1674-9871

We investigate the geology of Altar North (Cu-Au) and Quebradade la Mina (Au) porphyry deposits located in San Juan Province(Argentina), close to the large Altar porphyry copper deposit (995 Mt,0.35% Cu, 0.083 g/t Au), to present constraints on the magmatic processesthat occurred in the parental magma chambers of these magmatichydrothermalsystems.Altar North deposit comprises a plagioclase-amphibole-phyric daciteintrusion (Altar North barren porphyry) and a plagioclase-amphibolebiotite-phyric dacite stock (Altar North mineralized porphyry, 11.98 ±0.19 Ma). In Quebrada de la Mina, a plagioclase-amphibole-biotite-quartzphyricdacite stock (QDM porphyry, 11.91 ± 0.33 Ma) crops out.High Sr/Y ratios (92-142) and amphibole compositions of Altar Northbarren and QDM porphyries reflect high magmatic oxidation states (fO2=NNO +1.1 to +1.6) and high fH2O conditions in their magmas. Zones andrims enriched in anorthite (An37-48), SrO (0.22-0.33 wt. %) and FeO(0.21-0.37 wt. %) in plagioclase phenocrysts are evidences of magmaticrecharge processes in the magma chambers.Altar North and Quebrada de la Mina intrusions have relativelyhomogeneous isotopic compositions (87Sr/86Sr(t) =0.70450-0.70466, εNd(t)= +0.2to +1.2) consistent with mixed mantle and crust contributions intheir magmas. Higher Pb isotopes ratios (207Pb/204Pb = 15.6276-15.6294)of these intrusions compared to other porphyries of the district, reflectan increase in the assimilation of high radiogenic Pb components in themagmas. Ages of zircon xenocrysts (297Ma, 210Ma, 204Ma, 69 Ma) revealedthat the magmas have experienced assimilation of Miocene, Cretaceous,Triassic and Carboniferous crustal rocks. Fluids that precipitated sulfides in the Altar deposit may have remobilized Pb from the host rocks, as indicated by the ore minerals being more radiogenic (207Pb/204Pb = 15.6243-15.6269) than their host intrusions. Au/Cu ratio in Altar porphyries (average Au/Cu ratio of 0.14× 10-4 by weight in Altar Central) is higher than in the giant Mioceneporphyry deposits located to the south: Los Pelambres, Río Blanco and LosBronces (Chile) and Pachón (Argentina). We suggest that the increase inAu content in the porphyries of this region could be linked to theassimilation of high radiogenic Pb components in the magmas within theselong-lived maturation systems.