Origin and evolution of ore-forming fluids in the Yamansu deposit, Eastern Tianshan, NW China: Constraints from geology, fluid inclusions and H O C S isotopes

Abstract The Yamansu deposit is located in the Aqishan–Yamansu belt of the Eastern Tianshan and is hosted by submarine volcanic rocks (spilite-keratophyre series), which are characterized by high Na2O contents (2.85 – 7.59 wt. %) and Na2O/K2O ratios (1.12 – 68.20). The Yamansu orebodies feature stratoidal, banded, or lenticular shapes in the spilite-keratophyre series. Four principal stages can be recognized in the Yamansu deposit and the main iron mineralization occurred in Stage III. Microthermometry studies on two types of fluid inclusions indicate that the temperature of ore-forming fluid fluctuated and an additional heat caused by multiple periods of volcanic activity may have incorporated into the ore-forming system. The δD and δ18Ofluid values of the ore-forming fluids in the Yamansu deposit vary broadly from -117.1‰ to -80.0‰ and from -17.4‰ to +14.6‰, respectively, indicating a magmatic fluid in the early stage and a mixture of magmatic and meteoric water or seawater in the late stage. The δ13CV-PDB (ranging from -2.5‰ to +1.1‰) and δ18OV-SMOW (ranging from -7.7‰ to +14.8‰) values of calcite reveal that the ore-forming fluids experienced water-rock interactions, which changed the physicochemical conditions of the ore-forming fluids and facilitated the deposition of variable amounts of magnetite and skarn minerals (mainly chlorite and epidote) in Stage III. The wide range of δ34S values (-2.9‰ to +8.5‰) of pyrite from the Yamansu deposit indicates that the ore-forming materials mainly involved deeply sourced magmatic sulfur but were affected by mixing with seawater. The geochemical signatures suggest that submarine spilite-keratophyre series favored the formation of the Yamansu iron deposit and these signatures can be applied to the prospecting on the periphery of the Yamansu ore district or in the Aqishan–Yamansu metallogenic belt.