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 petrogenesis.

位于中亚造山带（Central Asian Orogenic Belt）西段的阿尔马雷克斑岩矿集区，是亚洲第二大斑岩成矿区域，因此长期受到地质学界的广泛关注。本文报道了萨尔切库（Sarycheku）与卡尔马克尔（Kalmakyr）矿床的成矿相关斑岩的锆石U-Pb年龄、主量与微量元素地球化学数据，以及Sr-Nd同位素组成。其中，卡尔马克尔矿床的含矿石英二长斑岩与花岗闪长斑岩的锆石U-Pb年龄（经激光剥蚀电感耦合等离子体质谱法（LA-ICP-MS）测试）分别为326.1±3.4 Ma和315.2±2.8 Ma；萨尔切库矿床的含矿花岗闪长斑岩与二长岩脉的锆石U-Pb年龄则分别为337.8±3.1 Ma和313.2±2.5 Ma。结合已有年代学数据，限定阿尔马雷克矿集区的多期岩浆侵入活动发生于337~306 Ma之间。
地球化学特征显示，所有样品均属于钾玄岩系列，相对高场强元素（high field strength elements）而言显著富集大离子亲石元素（large-ion lithophile elements），且Nb/U重量比值极低（0.83~2.56）。卡尔马克尔矿床样品的初始锶同位素比值(⁸⁷Sr/⁸⁶Sr)i为0.7059~0.7068，萨尔切库矿床样品为0.7067~0.7072；两者的εNd(t)值均较低，卡尔马克尔为-1.0~-0.1，萨尔切库为-2.3~0.2，指示其岩浆主要起源于受板片流体改造的交代地幔楔，并伴随大陆地壳物质通过同化-分离结晶作用（assimilation-fractional-crystallization）过程的加入。
与典型斑岩型铜矿床相比，阿尔马雷克矿集区的含矿斑岩属于钾玄岩系列，而非钙碱性系列。此外，尽管该区域的岩浆事件形成于大陆弧构造环境，但萨尔切库与卡尔马克尔矿床的含矿斑岩并未呈现环太平洋巨型斑岩矿床所具有的典型埃达克岩（adakites）地球化学特征，表明其成岩过程并未涉及板片熔融（slab-melting）作用。