The Paleozoic Aktogai Group in Kazakhstan ranks among the thirty 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 remain unclear. 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 deposit’s 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 microthermometry and mineral thermometers reveal that VQA1, 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 of meteoric 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, with temperatures of ~275-370 °C and brittle conditions. The Paleozoic Aktogai deposit exhibits formation depths and fluid evolution processes similar to Mesozoic and Cenozoic PCDs 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 during early high-temperature conditions in porphyry Cu deposits.

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