Geological features and ore-forming mechanisms of the Chating Cu–Au deposit: A rare case of porphyry deposit in the Middle–Lower Yangtze River metallogenic belt

Abstract

The Chating Cu–Au deposit is an important porphyry deposit located in the Middle–Lower Yangtze River Metallogenic Belt (MLYMB) of eastern China. Drill core logging and ore petrographic observations were systematically employed to recognize the geological features of the deposit. Several important and peculiar geological characteristics are revealed as fellows: (1) the ore-bearing quartz diorite porphyry emplaced within carbonate-dominated strata rather than clastic strata and volcanic rocks, (2) the ore-hosting location confined in the whole cryptoexplosive breccia pipe inside the porphyry stock, and (3) large-scale and barren marbles, marbled limestones, and hornfels almost encircling the porphyry stock and sporadic and barren skarns scattered within the porphyry stock. The fluid-inclusion study at the deposit reports a wide range of homogenization temperatures (161.4 °C–454.2 °C) and salinities (0.2–54.7 wt% NaCl eq) for the ore-forming fluids, and two fluid boiling events as suggested by the coexistence of halite-bearing liquid-rich inclusions and vapor-rich inclusions. In contrast with classical porphyry deposits, the Chating deposit has similar characteristics in the close relationships between the mineralization and the porphyry stock, the hydrothermal alteration zonation, and the fluid evolution process, while the wall-rock strata and ore-hosting position mark outstanding differences of the Chating deposit. The comprehensive geological and geochemical research in this study has been integrated to explore the ore-forming mechanisms of the deposit. The carbonate wall-rock strata were baked by early magma to form low-permeability thermal metamorphic shield at the contact zone, which prevents the migration and loss of the ore-forming fluids and avoid hydrothermal metasomatism for skarn ores. After that, the cryptoexplosions open up the porphyry system and promote the magmatic hydrothermal fluids mixing with the meteoric water, which successively induce the ore-forming fluids boiling and further cause ore-forming materials unloading and precipitation in the cryptoexplosive breccia pipe.