Garnets in the Yinachang Fe-Cu deposit, Kangdian region, China: Mineralogical and geochemical insights into fluid-rock dominated iron oxide-copper–gold (IOCG) mineralization

Abstract

Garnet is an important metasomatic mineral in iron oxide-copper–gold (IOCG) ore systems, but its crystallization mechanisms and relationship to fluid evolution in IOCG are poorly understood. In this study, major and trace element analyses were conducted on garnets from the Yinachang IOCG deposit in the Kangdian region, southwest China, in order to reveal their characteristics and forming process.Garnet commonly occurs in the K-Fe ± REE mineralization stage and Cu ± Au mineralization stage of the Yinachang deposit. Three types of garnets are identified from this deposit, which generally have high almandine but low grossular and spessartine contents (Alm65-78Gro5-17Sp5-17). The type 1 garnet (Grt 1) in the ore bodies and the type 2 garnet (Grt 2) in the proximal silicate alteration zones are pink to purple, euhedral to subhedral grains. Grt 1 exhibit zoning pattern, and Grt 2 were altered into chlorite and was mainly crosscut by ore-bearing veins. They are both extremely depleted in LILEs and enriched in HFSEs, with low total rare earth element ∑REE contents but variable LREE vs HREE ratios. The type 3 garnets (Grt 3) in the intermediate garnet-biotite-quartz zones are purple-brown, anhedral grains with high LILE, HFSE, and ∑REE contents, and exhibit LREE-depleted and HREE-enriched patterns, with marked positive Eu anomalies. All three types garnets that formed during alteration and mineralization are hydrothermal, but their composition is controlled by different mechanisms due to different environments. Compositions of Grt 1 and Grt 2 are mainly related to substitution mechanism controlled by crystal chemistry, physicochemical conditions of the closed hydrothermal system, and buffering effects of carbonate wall rocks during slow infiltration metasomatism, whereas the composition of Grt 3 is mainly controlled by surface adsorption and related to the composition of external fluid during later rapid garnet growth. Decreases in temperature, pressure, and fO2 triggered by mixing of fluids could be responsible for the distinct mineralogical and geochemical characteristics of the three types of garnets.Garnets from IOCG deposits show various textual and geochemical characteristics worldwide. Generally, almandine and andradite are dominating end members for IOCGs. Andradite is typical for metasomatic origin. Although almandine garnet is generally considered to form during metamorphism, the occurrence of hydrothermal almandine in the alteration zones of IOCG deposits reveal a metasomatism between Fe-bearing fluid and Al-enriched pelite protolith in a sufficiently reduced environment. Also, it is possible that the metasomatism during mineralization has enriched the protolith with Fe, and the post-ore metamorphism formed almandine by providing high temperature and pressure.