Quartz textures, trace elements, fluid inclusions, and in situ oxygen isotopes from Aktogai porphyry Cu deposit, Kazakhstan
Li C. Shen P. Seltmann R. Zhang D. Pan H. Seitmuratova E.
1 February 2025Walter de Gruyter GmbH
American Mineralogist
2025#110Issue 2272 - 292 pp.
The Paleozoic Aktogai Group in Kazakhstan ranks among the 30 largest porphyry Cu deposits globally. The Aktogai deposit is the largest one in the Aktogai Group and is characterized by intensive potassic alteration where the dominant orebody occurred. However, its mineralization processes still need to be clarified. Our investigation focused on the texture, trace elements, fluid inclusions, and in situ oxygen isotopes of the quartz from the ore-related tonalite porphyry and associated potassic alteration at Aktogai to trace the deposits mineralization processes. Ti-in-quartz thermobarometry, fluid inclusion microthermometry, and geological characteristics indicate that the ore-related magma at Aktogai originated from a shallow magma chamber at∼1.9 ± 0.5 kbar (∼7.2 ± 1.9 km) and intruded as the tonalite porphyry stock at ∼1.7-2.4 km. The potassic alteration and associated Cu mineralization comprise five types of veins (A1, A2, B1, B2, and C) and two types of altered rocks (biotite and K-feldspar). Among them, nine types of hydrothermal quartz were identified from early to late: (1) VQA1 in A1 veins and RQbt in biotite-altered rocks; (2)VQA2 in A2 veins and RQkfs in K-feldspar altered rocks; (3) VQB1 in B1 veins and VQB2E in B2 veins; and (4) quartz associated with Cu-Fe sulfides (VQB2L, VQBC, and VQC) in B and C veins. Titanium contents of the quartz decreased, while Al/Ti ratios increased from early to late. Fluid inclusion micro-thermometry and mineral thermometers reveal that VQ A1, RQbt, and hydrothermal biotite formed under high-temperature (∼470-560 °C) and ductile conditions. VQA2, RQkfs, VQB1, and hydrothermal K-feldspar formed during the transition stage from ductile to brittle, with temperatures of ∼350-540 °C. The rapid decrease in pressure from lithostatic to hydrostatic pressure led to fluid boiling and minor involvement ofmeteoric water (∼11-14%) in the mineralizing fluid. Extensive recrystallization in VQA1 to VQB1 was associated with repeated cleavage and healing of the intrusion. With cooling, K-feldspar decomposition and hydrolysis increased. Fluid cooling and water-rock reactions resulted in the co-precipitation of Cu-Fe sulfides, white mica, chlorite, VQBC, and VQC at temperatures of ∼275-370 °C and brittle conditions. The Paleozoic Aktogai deposit exhibits formation depths and fluid evolution processes similar to Mesozoic and Cenozoic porphyry Cu deposits worldwide. The close association between Cu-Fe sulfides and later quartz formed under intermediate-temperature conditions at Aktogai implies that Cu-Fe sulfides are not precipitated under early high-temperature conditions in porphyry Cu deposits.
Central Asian Orogenic Belt , emplacement depth , Fluid evolution , fluid source , Ti-in-quartz thermobarometry
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Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
Institutions of Earth Science, Chinese Academy of Sciences, Beijing, 100029, China
University of Chinese Academy of Sciences, Beijing, 100049, China
Center for Russian and Central EurAsian Mineral Studies, Natural History Museum, London, SW7 5BD, United Kingdom
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
College of Earth Sciences, ChangAn University, Xian, 710054, China
Laboratory of Geological Formations, K. Satpaev Institute of Geological Sciences, Almaty, 050010, Kazakhstan
Key Laboratory of Mineral Resources
Institutions of Earth Science
University of Chinese Academy of Sciences
Center for Russian and Central EurAsian Mineral Studies
State Key Laboratory of Lithospheric Evolution
College of Earth Sciences
Laboratory of Geological Formations
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