Hydrothermal Alteration, Mineralization, and Structural Geology of the Zijinshan High-Sulfidation Au-Cu Deposit, Fujian Province, Southeast China

Abstract

Zijinshan is a world-class Au-Cu district located in southwest Fujian Province, southeast China. It contains a diverse array of ore deposits hosted by the Zijinshan granite complex and surrounding volcano-sedimentary rocks. Associated deposits include high-sulfidation epithermal gold-copper, intermediate-sulfidation epithermal polymetallic silver-base metal, and porphyry molybdenum-copper deposits. The Zijinshan high-sulfidation Au-Cu deposit is located in the middle of the Zijinshan district, and ore zones are hosted in, below, and adjacent to the Zijinshan lithocap. The host rocks are part of a Jurassic granite complex emplaced between 165 and 157 Ma. 40Ar/39Ar age spectra of hydrothermal alunite indicate that hydrothermal activity at Zijinshan occurred at 102.86 ± 0.61 to 101.19 ± 0.60 Ma and was associated with dacite porphyry dikes that intruded at 104.8 ± 0.9 Ma. Hydrothermal activity produced pervasive silicic and advanced argillic alteration assemblages that are zoned from a central, massive quartz alteration domain, outward to massive quartz-dickite, disseminated quartz-alunite ± dickite, and disseminated quartz-muscovite-dickite zones. Copper sulfides occur in hydrothermally cemented breccias, veins, and minor disseminations in granite; copper minerals include covellite, digenite, and minor enargite. Oxide Au ores are associated with strongly weathered silicic alteration in the upper parts of the deposit. A set of subvertical NE-striking faults were active prior to mineralization at Zijinshan. During mineralization, a NW-striking fault system controlled the emplacement of veins, breccias, and dacite dikes. Kinematic indicators show that most of the NW-striking faults were active as normal oblique faults during mineralization. Postmineralization ENE-striking dextral strike-slip faults are associated with minor conjugate NNW-striking faults. The premineralization structures at Zijinshan likely formed during NW-directed compression. Synmineralization structures formed under subvertical contraction and subhorizontal NNE-plunging extension. Postmineralization structures developed in a strike-slip regime defined by a NW-oriented axis of compression and NE-trending axis of extension. Changes in the styles of pre- and synmineralization structures are related to fluctuations in the regional stress regime. The transition from compression to extension was fundamental to mineralization. Postmineralization faults are related to transient local stress regime variations at the deposit scale.