Geology, fluid inclusion and H-O-S-Pb isotope constraint on the genesis of the Changxing gold deposit, Fujian, South China

Abstract

The Changxing gold deposit is located in the Wuyi metallogenic belt of South China. Orebodies are primarily hosted in K-feldspar migmatitic granite along the NE- and NNE-striking ductile–brittle fracture structures. The ore minerals mainly include native gold, pyrite, chalcopyrite, whereas the main gangue minerals are quartz, sericite and chlorite. Fluid inclusion studies were conducted on the pre-ore stage quartz veins and main-ore stage quartz veins. Three types of fluid inclusions, namely, CO2-bearing inclusion with smaller carbonic proportion (<40%) (type I), CO2-rich inclusion with higher carbonic proportion (>60%) (type II), and aqueous fluid inclusions (type III) can be observed in the quartz. Primary type I inclusions in pre-ore quartz veins show consistent CO2 phase volumetric proportions with homogenization temperatures of 362–396 °C and salinities of 5.0–6.4 wt% NaCl equivalent. The primary coexisting type II and type III inclusions are observed in ore stage quartz vein, sharing similar homogenization temperatures in the range of 292–353 °C and 279–335 °C, but contrasting salinity values at 1.4– 5.3 and 5.3–10.5 wt% NaCl equivalent, respectively. Petrographic observations and microthermometric results collectively show that fluid immiscibility occurred concurrently with gold mineralization during the main-ore stage. The δD (−88.9‰ to −71.8‰) and δ18O (5.9‰ to 8.5‰) of ore-forming fluids imply that the ore-forming fluids were mainly of magmatic origin. The δ34S values (2.1‰ to 4.9‰, with average value of 3.1‰) of the main-ore stage pyrite suggest that sulfur was derived from a homogeneous magmatic source. The lead isotopic compositions suggest that the migmatitization of the K-feldspar granitization granite (206Pb/204Pb = 17.953 to 18.147, 207Pb/204Pb = 15.636 to 15.688, 208Pb/204Pb = 37.727 to 38.851) in the Caledonian had no genetic relationship with gold mineralization of Changxing gold deposit (206Pb/204Pb = 17.466 to 18.157, 207Pb/204Pb = 15.551 to 15.619, 208Pb/204Pb = 37.727 to 38.508), and the Mesozoic magmatic activity likely provided ore materials of Changxing gold deposit. In summary, we propose that the Changxing deposit is a magmatic hydrothermal gold deposit.