Geochemical characteristics of pyrite in the Dabaozhuang deposit in the Middle-Lower Yangtze River Metallogenic Belt, Eastern China

Dabaozhuang is a pyrite deposit in the Luzong Volcanic Basin, Eastern China, and has similar exhalation-sedimentary features to volcanic massive sulfide deposits (VMS), which commonly form in seawater. To better understand the ore-forming process of this special type of deposit, we employed SHRIMP-SI and LA-ICP MS to measure sulfur isotope and trace element compositions of anhydrite and four types of pyrite from the Dabaozhuang deposit. SHRIMP in situ sulfur isotope analysis indicates a narrow range in δ³⁴S of anhydrite, 21.0–21.7‰, and variable ranges for pyrite, 9.9–10.7‰, for pyrite vein in the deep (Py I); −9.2 to 2.0‰, for cyclic annular and oval-shaped pyrite in massive ore (Py II); 3.1–5.3‰, for massive or disseminated pyrite in the footwall tuff and hematite (Py III); and −29.7 to 30.4‰, for euhedral and coarse pyrite in the carbonate cement (Py IV). δ³³S and δ³⁶S in four types of pyrite have no abnormal values, indicating that sulfur isotope fractionation is mainly controlled by thermochemical sulfate reduction (TSR). The higher δ³⁴S values in type I pyrite and anhydrite indicate a mixing process of magmatic fluid and evaporates at depth. Py II and IV have negative δ³⁴S data, indicating the formation temperature of pyrite decreases because of the mixing of lake water. δ³⁴S values in Py III are close to magmatic sulfur, which shows the igneous sulfur (tuff) is an important sulfur source for the disseminated mineralization. Cobalt and Ni content also show that the formation temperature of Py I is highest, and Py IV is lowest. This research indicates that the “VMS-like” deposits could be formed in continental volcanic lakes and are genetically related to SO₄²⁻-rich IOA (iron oxide-apatite) deposits.