A porphyritic copper (gold) ore-forming model for the Shaxi-Changpushan district, Lower Yangtze metallogenic belt, China: geological and geochemical constraints

Abstract

Ore-forming P-T-X conditions and the nature of gold occurrence in the Shaxi-Changpushan porphyry copper deposit, central Anhui Province, eastern China, were investigated employing a wide range of geological and geochemical methods. Our results document liquid-vapour conditions and show that abundant fluid inclusions occur in quartz veins accompanied by pyrite-chalcopyrite-gold mineralization. Most economic deposits involved coexisting liquid and gas phases, whereas a few formed in equilibrium with a homogeneous aqueous liquid. The ore-forming temperature lies between 230 and 350°C. Isotope studies show that the δ34 S values are between –0.20 and 3.00‰ for most of the sulphides; δ34 S values of chalcopyrite are somewhat more homogenous than those of pyrite. Ore-forming fluids and materials were mainly derived from magmatic sources. Meteoric water played a small role in the ore-forming process, judging by the oxygen and hydrogen isotope data for fluid inclusions measured by the explosion method (δ18 O values ranging from 3.51 to 5.52‰, and δD ranging from −59.8‰ to −82.4‰). In the ore deposit, the gold occurs as micro-inclusions heterogeneously distributed in chalcopyrite and pyrite. Gold mineralization is positively correlated with As in chalcopyrite, pyrite, and some Cu-bearing ores. Igneous rocks and sedimentary rock distributions in the Shaxi-Changpushan ore district were strictly controlled by the regional fault system since the Jurassic period, especially the Tan-Lu fault system in east China. Intrusive bodies comprising porphyritic quartz dioritoid, biotite-quartz dioritoid, and fine-grained dioritoid are ore-bearing, cutting sedimentary rocks of the Upper Jurassic and Middle–Lower Silurian series. Sediments exposed in the ore district consist of Upper Devonian–Middle Silurian clastic rocks, Middle and Lower Jurassic and Upper Cretaceous terrestrial clastic rock series. Petrologic data show that formation of Cu-Au ore bodies was related to adakitic intrusives in the Shaxi-Changpushan area. Based on geochemical exploration and the tectonic background of the southern part of Tan-Lu fault zone, we propose a porphyric copper (gold) ore-forming model for the Lower Yangtze metallogenic belt: ore bodies were controlled by structural shielding in the core of the regional anticline. Combined geological and geochemical evidence suggests that a super-large porphyry (gold) deposit may be present in the region.