Metallogeny and geodynamics of the Aktiuz–Boordu Mining District, Northern Tien Shan, Kyrgyzstan

Abstract

The Aktiuz–Boordu Mining District is located in the Northern Tien Shan in the eastern part of Kyrgyzstan. The region is characterized by nappe-folding structures and comprises strongly deformed Precambrian and Lower Paleozoic sedimentary, volcano-sedimentary, and metamorphic sequences. Metamorphic rocks are represented by crystalline schists, para- and orthogneisses, marble, migmatite, amphibolite and eclogite lenses. These rocks are thought to be the oldest in the Northern Tien Shan. The 2780-Ma Aktiuz complex has a total thickness of 2800–2900 m. Available U–Pb zircon age data for the Kemin Group migmatites yield ages of 2200 ± 50 Ma. Based upon their geological setting, multistage metamorphism and isotopic ages of retrograde metamorphism (1.1–1.9 Ga), these rocks reach a few kilometers in thickness and are subdivided into Archaean and Paleoproterozoic. The Archaean and Paleoproterozoic basement metamorphic rocks contain mineralization of various ages and types, including porphyry Cu, Au-sulphide, Au–Bi, barite, epithermal base metal and Au–Ag, REE and rare-metals. Two ore fields have been identified within the Aktiuz–Boordu Mining District, they are: (1) Taldybulak–Boordu, with Au, base metal, and porphyry Cu systems; (2) Aktiuz, with REE, rare- and base-metal deposits. Within the Paleozoic Taldybulak–Boordu volcanic structure, deposits and occurrences of Au are present at Taldybulak Levoberezhny, Chimbulak Zapadny, Karamoko and Kuranjailyau; of Pb at Boordu, Taldybulak Stary, Chimbulak Vostochny and Chimbulak Zapadny; of Mo at Karabulak, and of Cu at Berkut-Kashka. Almost all are found along the periphery of deeply eroded volcanoes. Rock types in the Taldybulak–Boordu ore field mainly comprise chlorite–amphibole and amphibole schists, amphibolites, and migmatites of the Paleoproterozoic Kuperlisay suite. The younger Paleoproterozoic Kokbulak and Kapchigay suites are represented by mica schists and granite gneisses, which are separated from the Kuperlisay suite by a fault. The basement rocks are cut by a complex network of Paleozoic sub-1 volcanic intrusions, accompanied by eruptive rocks and breccia pipes. These represent the root system of Middle and Late Paleozoic volcanoes. At the Taldybulak Levoberezhny Au deposit, the principal ore-controlling structures are inter- and intraformational shear, thrust–fault zones and tectonic mélange injected by intrusions that underwent metasomatic alteration. Gold-sulphide disseminated mineralization is associated with the upper part of Taldybulak shear zone and is associated with monzodiorites and quartz–tourmaline veinlets. The Aktiuz ore field is located in an uplifted block of the basement rocks, including gneisses and crystalline schists (the Archean Aktiuz complex and Paleoproterozoic Kemin Group). They are intruded by Neoproterozoic mafic and ultramafics rocks, Late Ordovician–Silurian collision granites and granodiorites, Devonian monzodiorites, alaskites, alkalic and subalkalic two-feldspar granites, and Permian–Triassic syenites, subalkalic leucogranites, granophyres and granite-porphyries. The Kuperlisay leucogranite intrusion is an elongated body with several ridge-like projections of granophyres. For the Kuperlisay leucogranite a K–Ar age of 260 Ma was obtained, whereas the K–Ar age of granophyre is 225–230 Ma. Analyses of the Aktiuz granite samples taken from the Kutessay open pit yielded an age of 416 Ma (U–Pb zircon SHRIMP method). The Aktiuz ore field contains deposits of Th at Kuperlisay and Kutessay II, and of REE and base metals at Aktiuz and at Kalesay. All are hosted by leucocratic alkali granites, granophyres, granite-porphyries and albitites. Significant reserves and resources of REE and base metals ores for the Aktiuz and Kutessay II deposits make them economically important.