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

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.Zhiguo Cheng, Zhaochong Zhang, A. Turesebekov, B.S. Nurtaev, Lijuan Xu, M. Santos

Carboniferous porphyry Cu-Au deposits in the Almalyk orefield, Uzbekistan: the Sarycheku and Kalmakyr examples

The Almalyk porphyry cluster in the western part of the Central Asian Orogenic Belt is the second largest porphyry region in Asia and hence has attracted considerable attention of the geologists. In this contribution, we report the zircon U–Pb ages, major and trace element geochemistry as well as Sr–Nd isotopic data for the ore-related porphyries of the Sarycheku and Kalmakyr deposits. The zircon U–Pb ages (Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)) of ore-bearing quartz monzonite and granodiorite porphyries from the Kalmakyr deposit are 326.1 ± 3.4 and 315.2 ± 2.8 Ma, and those for the ore-bearing granodiorite porphyries and monzonite dike from the Sarycheku deposit are 337.8 ± 3.1 and 313.2 ± 2.5 Ma, respectively. Together with the previous ages, they confine multi-phase intrusions from 337 to 306 Ma for the Almalyk ore cluster. Geochemically, all samples belong to shoshonitic series and are enriched in large-ion lithophile elements relative to high field strength elements with very low Nb/U weight ratios (0.83–2.56). They show initial (⁸⁷Sr/⁸⁶Sr)i ratios of 0.7059–0.7068 for Kalmakyr and 0.7067–0.7072 for Sarycheku and low εNd(t) values of −1.0 to −0.1 for Kalmakyr and −2.3 to 0.2 for Sarycheku, suggesting that the magmas were dominantly derived from a metasomatized mantle wedge modified by slab-derived fluids with the contribution of the continental crust by assimilation-fractional-crystallization process. Compared to the typical porphyry Cu deposits, the ore-bearing porphyries in the Almalyk cluster are shoshonitic instead of the calc-alkaline. Moreover, although the magmatic events were genetically related to a continental arc environment, the ore-bearing porphyries at Sarycheku and Kalmakyr do not show geochemical signatures of typical adakites as reflected in some giant porphyry deposits in the Circum-Pacific Ocean, indicating that slab-melting may not have been involved in their petrogenesisZhiguo Cheng, Zhaochong Zhang, Fengmei Chai, Tong Hou, M. Santosh, A. Turesebekov and B.S. Nurtae

Carboniferous porphyry Cu–Au deposits in the Almalyk orefield, Uzbekistan: the Sarycheku and Kalmakyr examples

<p>The Almalyk porphyry cluster in the western part of the Central Asian Orogenic Belt is the second largest porphyry region in Asia and hence has attracted considerable attention of the geologists. In this contribution, we report the zircon U–Pb ages, major and trace element geochemistry as well as Sr–Nd isotopic data for the ore-related porphyries of the Sarycheku and Kalmakyr deposits. The zircon U–Pb ages (Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)) of ore-bearing quartz monzonite and granodiorite porphyries from the Kalmakyr deposit are 326.1 ± 3.4 and 315.2 ± 2.8 Ma, and those for the ore-bearing granodiorite porphyries and monzonite dike from the Sarycheku deposit are 337.8 ± 3.1 and 313.2 ± 2.5 Ma, respectively. Together with the previous ages, they confine multi-phase intrusions from 337 to 306 Ma for the Almalyk ore cluster. Geochemically, all samples belong to shoshonitic series and are enriched in large-ion lithophile elements relative to high field strength elements with very low Nb/U weight ratios (0.83–2.56). They show initial (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7059–0.7068 for Kalmakyr and 0.7067–0.7072 for Sarycheku and low ε_Nd(t) values of −1.0 to −0.1 for Kalmakyr and −2.3 to 0.2 for Sarycheku, suggesting that the magmas were dominantly derived from a metasomatized mantle wedge modified by slab-derived fluids with the contribution of the continental crust by assimilation-fractional-crystallization process. Compared to the typical porphyry Cu deposits, the ore-bearing porphyries in the Almalyk cluster are shoshonitic instead of the calc-alkaline. Moreover, although the magmatic events were genetically related to a continental arc environment, the ore-bearing porphyries at Sarycheku and Kalmakyr do not show geochemical signatures of typical adakites as reflected in some giant porphyry deposits in the Circum-Pacific Ocean, indicating that slab-melting may not have been involved in their petrogenesis.</p