Two generations of garnets and their relevance for the hydrothermal fluid evolution of the Hongyuntan deposit, NW China

Abstract

The Hongyuntan deposit, which is one of the typical volcanic-hosted Fe deposits in the Aqishan–Yamansu metallogenic belt of the Eastern Tianshan Orogenic Belt, primarily consists of Fe and skarn minerals (especially garnet). Two generations (three types) of garnets were identified in the Hongyuntan deposit based on field and optical characteristics. Hongyuntan garnets are Fe-rich and belong to the grossular–andradite solid solution series (And = 50.99%–100.00%, average: 79.06%). Garnets of the first generation (Grt1) are massive aggregates and are fine-grained with well-developed oscillatory zonings. They have Type I (two-phase liquid and vapor) and Type II (multi-phase daughter mineral-bearing) fluid inclusions and experienced high temperature (540–560 °C) and salinity (9.2–63.2 wt% NaCl eqv) conditions. The Grt1 have high ΣREE, LfREE/HREE, δEu, U, Y, and high field strength element (HFSE: sum of Nb, Ta, Zr, and Hf) contents. Garnets of the second generation (Grt2) are vefin types and are characterized by irregular shapes with irregular zonings. Type A garnets of the second generation (Grt2-A) usually coexist with mafgnetite and have Type I and Type II fluid inclusions with relatively high temperature (480–500 °C) and salinity (9.5–36.6 wt% NaCl eqv) values. Compared to Grt1, Grt2-A have low And, U, and LREE/HREE together with medium ΣREE, δEu, HFSE, and Y contents. Type B garnets of the second generation (Grt2-B) usually coexist with epidote and quartz. They merely have Type I fluid inclusions with medium–low temperature (260–280 °C) and salinity (6.6–12.7 wt% NaCl eqv) values. Grt2-B have high And, δEu, and LREE/HREE contents but very low ΣREE, HFSE, and Y contents.The fluid inclusions and geochemical properties of the garnets indicate that the temperature, salinity, oxygen fugacity (fO2), and pH of the ore-forming fluid fluctuated, indicating Hongyuntan deposit likely experienced superimposed mineralization. Grt1 originated in a magmatic–hydrothermal fluid and formed through diffusive metasomatism under relatively high fO2, neutral pH, and low water/rock (W/R) ratio conditions. The reduced Fe and other siderophile elements in the ore-forming fluid, which formed Grt1, are believed to be the material basis for iron ores in the later main metallogenic stage. Grt2-A demonstrate the characteristics associated with transitional fluid, which most likely originated from the episodic hydrothermal supplies. However, the ore-forming fluid system gradually opened up during the crystallization of Grt2-B, and the decreasing temperatures and depletion of ore-forming materials were ultimately unfavorable for the deposition of magnetite.