Petrogenesis of gabbroic intrusions in the Valerianov-Beltau-Kurama magmatic arc, Uzbekistan: the role of arc maturity controlling the generation of giant porphyry Cu–Au deposits

Abstract

Arc maturity is considered to play an important role in the mineralization of porphyry Cu–Au deposits. However, the early stage of arc is rarely preserved leading to controversies over arc evolution. Here we investigate gabbroic intrusions from the Akcha, Beleuti and Kalmakyr arc in relation to the Almalyk porphyry Cu–Au orefield in Uzbekistan. The Akcha biotite gabbro shows medium to fine grained texture, and is composed of plagioclase and biotite with minor clinopyroxene, orthopyroxene, quartz and magnetite. The Beleuti and Kalmakyr gabbros display coarse-grained texture, and are dominated by plagioclase, amphibole and clinopyroxene. Zircon LA-ICP-MS U–Pb dating yields ages of 339.9 ± 2.0 Ma for Akcha biotite gabbro, 339.0 ± 3.3 Ma for Beleuti gabbro and 335.0 ± 2.4 Ma for Kalmakyr gabbro. The emplacement ages of these intrusions are older than those of ore-bearing porphyries in the Almalyk orefield, indicating that the gabbros were emplaced during early stage of the arc evolution. The gabbroic rocks are characterized by high Mg# [100 × molar Mg²⁺/(Mg²⁺+Fe²⁺), 41–61], low SiO₂ contents (41.6–50.9 wt%), enrichment in large-ion lithophile elements and negative anomalies for high field strength elements, together with relatively depleted Sr-Nd-Hf isotopic compositions (0.7044–0.7057, —1.59-1.96, 3.38–9.96, respectively). The geochemical features suggest that these rocks were mostly derived from the partial melting of mantle wedge modified by subduction-related fluids. Estimates on water contents based on the composition of amphibole show 4.9–5.3 wt% for the Akcha biotite gabbro, and 5.6–6.8 wt% for the Beleuti gabbro. The oxygen fugacity estimated by zircon Ce/Nd, Ce⁴⁺/Ce³⁺ ratios are lower than those of porphyries from the giant porphyry deposits in the Central Asian Orogenic Belt. Our results, together with previously reported data on porphyries in the Almalyk orefield suggest that the gabbro was formed in an immature arc setting associated with the northward subduction of the Paleo-Turkestan oceanic plate, whereas the ore-bearing porphyries were formed in a mature arc.