Gold mineralization related to rifting of the Rodinia surpercontinent: As gauged from the Dongyuan Au deposit, Jiangnan orogen, China

Abstract

Abundant orogenic gold (Au) deposits, widely distributed in medium to low-grade metamorphic rocks, formed episodically since the Middle Archean. Orogenic Au mineralization waned during the Neoproterozoic era, although Neoproterozoic orogenies were prevalent. The Jiangnan Orogen, South China, as part of the sutures uniting the Rodinia (Meso- and Neoproterozoic) supercontinent, is situated between the Cathaysia and Yangtze blocks. Many orogenic Au deposits, such as the Wan’gu, Huangjindong, Dongyuan, and Jinshan Au deposits, are located in the northeastern section of the Jiangnan Orogen. The orebodies of the recently discovered Dongyuan deposit display a horizontal zonation, as follows: a peripheral quartz–chlorite–sulfide–calcite (ankerite, manganocalcite, and calcite) veinlets/veins and a central sulfide (mainly arsenopyrite) veinlets/veins. The former veinlets/veins also contain a small amount of albite, scheelite, TiO2 minerals, xenotime, and monazite. Compared with the other orogenic Au deposits, arsenopyrite from the Dongyuan Au deposit is characterized by enrichment of Sb–Co–Cd–Ag element assemblages. Based on the electron microprobe and Raman analyses, TiO2 mineral grains, including anatase, rutile, and anatase–rutile intergrowths, are present in the Au veins. In contrast with previous studies, TiO2 minerals (anatase and rutile) have a nearly consistent concentration range of each element detected, which taken together can be used for constraining fluid sources of ores. TiO2 minerals with enriched V–Fe assemblages, combined with initial 187Os/188Os ratio (0.64 ± 0.11) of arsenopyrite, reflect that the Dongyuan Au veins were probably precipitated from a metamorphic fluid. The Liyangdou biotite granodiorite and Donggang granite, located about one kilometer north and six kilometers south of the deposit, have LA-ICP-MS U–Pb zircon ages of 803 ± 3.4 Ma and 814 ± 3.4 Ma, respectively. The analysis of five arsenopyrite samples selected from the Dongyuan Au veins yields an isochron age of 744 ± 36 Ma (with an initial 187Os/188Os ratio of 0.64 ± 0.11 (2σ) and MSWD = 30). We found that the Dongyuan Au mineralization was later than the peripheral Neoproterozoic magmatic activities and metamorphic events of country rocks. Combined with previous studies, the Dongyuan deposit was formed in the tectonic setting of Rodinia supercontinent rifting, after the Neoproterozoic collision event in the Jiangnan Orogen, which likely triggered orogenic Au mineralization.