Hydrothermal Alteration and Mineralization of the Middle to Late Jurassic Dabaoshan Porphyry Cu Deposit in South China

Abstract

The Dabaoshan porphyry Cu deposit (420 kilotons (kt) of Cu @ 0.36%) is located in South China. The newly discovered Cu orebodies are hosted in the dacite porphyry adjacent to a granodiorite porphyry. The alteration and mineralization timing and stages of the porphyry Cu deposit were not well-constrained. In this study, we combine field mapping, petrography, whole-rock geochemistry, hydrothermal rutile U-Pb dating and Cu isotopes to synthesize an ore model at Dabaoshan. In situ hydrothermal rutile U-Pb dating yields an age of 159 ± 13 Ma, which brackets the timing of porphyry Cu mineralization. From top to bottom, the alteration zones in Dabaoshan are divided into quartz-sericite, biotite, chlorite-epidote, and chlorite-sericite subzones. Veins are classified into four stages (Stage 1 to 4) with Stage 4 quartz-sericite-chalcopyrite veins being the main Cu ore-bearing veins. The mineralized dacite porphyry has high SiO₂, but low MgO, CaO, and Na₂O contents. The chalcopyrite hosted in veins exhibits δ⁶⁵Cu = values ranging from –1.29‰ to 0.51‰. Such copper isotope fractionation is attributed to vapor-brine phase separation, and mixing of fluids from different geochemical reservoirs. The timing of Cu mineralization and hydrothermal alteration support that the Jurassic granodiorite porphyry is an ore-forming intrusion at Dabaoshan.