Asynchronous formation of the adjacent epithermal Au-Cu and porphyry Cu-Mo deposits in the Zijinshan orefield, southeast China

Abstract

Adjacent high-sulfidation epithermal (HSE) and porphyry deposits often have similar Cu-Au metal association, suggesting they are cogenetic and formed contemporaneously. The Zijinshan orefield (ZOF) is the largest Au producer in China and contains a series of HSE Au-Cu and porphyry Cu-Mo deposits. This study presents new (1) muscovite 40Ar-39Ar ages for the HSE deposit in this area, new (2) zircon laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb ages for felsic igneous rocks associated with the epithermal and porphyry mineralization, and (3) new zircon Lu-Hf isotopic data for these intrusions, all of which provides insights into why these adjacent epithermal Au-Cu and porphyry Cu-Mo deposits have different metal associations.The age data indicate that the alunite-dickite alteration related to the HSE Au-Cu mineralization formed before ca. 110 Ma and the main HSE Au-Cu mineralizing event occurred at ca. 113 Ma. In comparison, the adjacent porphyry Cu-Mo mineralization formed at ca. 104 Ma, ca. 9 Myr after the HSE event. The volcanic rocks associated with the HSE Au-Cu mineralization yield zircon εHf (t) values from −4.1 to 0.3, whereas the Cu-Mo mineralized porphyry has lower zircon εHf (t) values (−4.5 to −1.2), suggesting that the magmas that formed the latter intrusion were derived from a source containing more crustal material than the magmas associated with the HSE mineralization. The differences in the timing of mineralization and the sources of the magmas indicate that the adjacent HSE and porphyry deposits in the ZOF formed from different magmatic-hydrothermal systems that controlled the different metal associations of the mineralizing systems. This means that future mineral exploration within the ZOF should target porphyry Au-Cu deposit at depths that is genetically related to the HSE Au-Cu deposit. The results also show that not all adjacent porphyry and epithermal deposits, especially those with different metal associations, are cogenetic.