High-temperature magmatic fluid exsolved from magma at the Duobuza porphyry copper-gold deposit, Northern Tibet

Abstract

The Duobuza porphyry copper–gold deposit (proven Cu resources of 2.7 Mt, 0.94% Cu and 13 t gold, 0.21 g t−1 Au) is located at the northern margin of the Bangong-Nujiang suture zone separating the Qiangtang and Lhasa Terranes. The major ore-bearing porphyry consists of granodiorite. The alteration zone extends from silicification and potassic alteration close to the porphyry stock to moderate argillic alteration and propylitization further out. Phyllic alteration is not well developed. Sericite-quartz veins only occur locally. High-temperature, high-salinity fluid inclusions were observed in quartz phenocrysts and various quartz veins. These fluid inclusions are characterized by sylvite dissolution between 180 and 360°C and halite dissolution between 240 and 540°C, followed by homogenization through vapor disappearance between 620 and 960°C. Daughter minerals were identified by SEM as chalcopyrite, halite, sylvite, rutile, K–feldspar, and Fe–Mn-chloride. They indicate that the fluid is rich in ore-forming elements and of high oxidation state. The fluid belongs to a complex hydrothermal system containing H2O – NaCl – KCl ± FeCl2 ± CaCl2 ± MnCl2. With decreasing homogenization temperature, the fluid salinity tends to increase from 34 to 82 wt% NaCl equiv., possibly suggesting a pressure or Cl/H2O increase in the original magma. No coexisting vapor-rich fluid inclusions with similar homogenization temperatures were found, so the brines are interpreted to have formed by direct exsolution from magma rather than trough boiling off of a low-salinity vapor. Estimated minimum pressure of 160 MPa imply approximately 7-km depth. This indicates that the deposit represents an orthomagmatic end member of the porphyry copper deposit continuum. Two key factors are proposed for the fluid evolution responsible for the large size of the gold-rich porphyry copper deposit of Duobuza: (i) ore-forming fluids separated early from the magma, and (ii) the hydrothermal fluid system was of magmatic origin and highly oxidized.