Geochemical Characteristics of Mineral Assemblages from the Yamansu Iron Deposit, NW China, and Their Metallogenic Implications

Abstract

The Yamansu deposit, which is hosted in the volcanic-sedimentary sequence of the Carboniferous Yamansu Formation in Eastern Tianshan, NW China, contains many skarns, and the orebodies occur in the ore district in stratoidal, banded or lenticular forms. Four alteration stages, namely, albite–tourmaline–apatite–Grt1 (Stage I), K-feldspar–Grt2 (Stage II), magnetite–chlorite–epidote (Stage III), and quartz–calcite–axinite–Grt3 (Stage IV), are distinguished in the Yamansu deposit. The mineral geochemistry associated with each different stage is presented to provide a better understanding of the corresponding metallogenic processes. The ore-forming fluid in Stage I was derived from a magmatic–hydrothermal source and formed at high temperatures with many volatiles. This ore-forming fluid, which contained considerable metallogenic materials during the early stage, likely experienced diffusive metasomatism in a closed system with low water/rock (W/R) ratios. Mineral geochemical analyses show that the Fe content gradually increases from Stage I to Stage II, indicating that accumulated ore-forming materials were available during changes in the physicochemical conditions from a reducing environment with neutral pH to oxidizing conditions with mildly acidic pH. During the main metallogenic stage (Stage III), mineral assemblages reflect moderate- to high-temperature conditions, and the ore-forming fluid was created and destroyed periodically; the magnetite ores were deposited in a fluctuating fluid system. The multilayered orebodies, multigenerational garnets, and minerals with oscillatory zoning indicate that the ore-forming fluid may have developed periodic fluctuations, and this special multistage fluctuation of the hydrothermal fluid in the Yamansu deposit was the key factor controlling the multiple extraction, enrichment and precipitation of metallogenic materials.