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 +/- 2.8Ma, and those for the ore-bearing granodiorite porphyries and monzonite dike from the Sarycheku deposit are 337.8 +/- 3.1 and 313.2 +/- 2.5Ma, respectively. Together with the previous ages, they confine multi-phase intrusions from 337 to 306Ma 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 (Sr-87/Sr-86)(i) ratios of 0.7059-0.7068 for Kalmakyr and 0.7067-0.7072 for Sarycheku and low epsilon(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.