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

摘要

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 (Grtl) 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 degrees C) and salinity (9.2-63.2 wt% NaCl eqv) conditions. The Grt1 have high Sigma REE, LfREE/HREE, delta Eu, 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 degrees C) and salinity (9.5-36.6 wt% NaCl eqv) values. Compared to Grtl, Grt2-A have low And, U, and LREE/HREE together with medium Sigma REE, delta Eu, 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 degrees C) and salinity (6.6-12.7 wt% NaCl eqv) values. Grt2-B have high And, delta Eu, and LREE/HREE contents but very low Sigma REE, HFSE, and Y contents.