Source component mixing controls the variability in Cu and Au endowment along the strike of the Eastern Andean Cordillera in Peru

Thomas Angerer, Anthony I.S. Kemp, Steffen G. Hagemann, Walter K. Witt, João O. Santos, Christian Schindler, Carlos Villanes

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    Abstract

    Mississippian arc magmatic suites of the Au-rich Pataz and Cu-dominated Montañitas regions in Peru reveal distinct modes of magmatic-hydrothermal petro- and metallogenesis. The distinction is remarkable due to their broad contemporaneity (336–322 Ma), arc-parallel position, and close distance (<50 km) to each other. In both arc regions, petrography, geochemistry, and the tectonic setting of magmatic suites suggest a rapid switch from syn-collisional/compressional to post-collisional/extensional (with ‘A2-type’ signature) emplacement regime. Rocks of the Au-rich Pataz region originate from mixed sources with a contribution from the mantle (εHf > 0 and δ18O of ~ 5.3‰) and assimilated old crust (variously low εHf and δ18O > 5.3‰). The ultimate source of Au in the mineralised Pataz batholith was oxidised (fO2 at FMQ buffer; based on zircon trace chemistry) and alkali-, LILE- and HFSE-enriched, most likely represented by the metasomatised mantle. The syn-extensional emplacement of the relatively reduced (ΔFMQ <0), but unmineralised, A2-type suite involved assimilation of reduced crust. Associated, reduced, magmatic-hydrothermal fluids infiltrated the Au-bearing batholith suite and effectively mobilised and transported and concentrated Au. In the Montañitas region, rocks are oxidised (ΔFMQ > 0) and dominantly mantle derived without significant incorporation of crustal material. Samples from the Cu-mineralised suites indicate the additional contribution of a δ18O <5.3‰ source, potentially melted layer-2 gabbro. In addition, the elevated whole-rock La/Yb and Sr/Y ratios are compatible with minor addition of slab-derived material, which may have enhanced Cu endowment in this region. Late-magmatic, oxidised fluids derived from the younger A2-type suite controlled Cu mobilisation and concentration, while Au behaved largely refractory. In general terms, it is postulated that source mixing in continental arcs is a first-order control of contrasting Cu and Au endowment and that sequential intrusion processes facilitate late-magmatic-hydrothermal mobilisation and concentration of specific metal assemblages.

    Original languageEnglish
    Article number36
    JournalContributions to Mineralogy and Petrology
    Volume173
    Issue number5
    DOIs
    Publication statusPublished - 1 May 2018

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