Textural and geochemical characteristics of garnet from the Luoyang Fe skarn deposit, eastern China: implications for ore-forming fluid evolution and mineralization conditions

Abstract

The late Palaeozoic Yong’an–Meizhou depression belt is an important iron (Fe) and polymetallic metallogenic belt in southern China. It has undergone a transformation from Tethys to the circum-Pacific tectonic domain. The Luoyang deposit is one of the typical Fe skarn deposits in the Yong’an–Meizhou depression belt of eastern China. Garnet is a characteristic mineral in the deposit. Two generations of garnets are detected in the deposit based on their textural characteristics and trace-element contents, and are represented by Fe-enriched andradite. The first generation of garnets (Grt1) have two types of garnets (Grt1-A and Grt1-B). Type A garnets of the first generation (Grt1-A) (Adr80-88) replaced by massive diopside-magnetite assemblage exhibit distinct oscillatory zonings and display patterns of enriched light rare earth elements (LREE) to weak heavy rare earth elements (HREE), with weak negative to positive Eu anomalies, and highest U, ΣREE and Sn contents. Type B garnets of the first generation (Grt1-B) are irregular zones (Adr94-96) coexisting with magnetite, in which Grt1-A is generally dissolved, and have obviously LREE-enriched and HREE-depleted patterns, with weak negative to positive Eu anomalies, and moderate U, ΣREE and Zn contents. Garnets of the second generation (Grt2) (Adr96-99) that replaced massive magnetite together with sphalerite show unzoned patterns, with a flat REE pattern and pronounced negative Eu anomalies as well as contents of lowest U and ΣREE, and highest W. The substitution of REEs in garnets occurs as [X2+]VIII –1[REE3+]VIII +1[Si4+]IV –1[Z3+]IV +1in an Al-enriched environment. Luoyang hydrothermal fluids shifted from reducing conditions with relatively high-U and -ΣREE characteristics to oxidizing conditions with relatively low-U and -ΣREE characteristics. The reduced siderophile elements and increased fO2 in fluid during Grt1-B formation caused magnetite mineralization and reduced Zn contents during Grt2 formation, causing the deposition of sphalerite. All garnets formed from magmatic fluid and were controlled by infiltrative metasomatism in an opened system.

Geochemical Characteristics of Garnet from Zinc–Copper Ore Bodies in the Changpo–Tongkeng Deposit and Its Geological Significance

“…Y and ΣREEs have similar geochemical behaviors, and the linear correlation between the Y and ΣREE contents suggested that the garnets formed in a relatively closed system without fluid mixing. Conversely, deviations from the linear trend may have indicated changes in the fluid composition during garnet growth [5,[40][41][42][43][44][45]. As shown in Figure 11, there was a linear correlation between the Y and ΣREE contents in both garnet generations, indicating that the compositions of the ore-forming fluids were stable.…”

“…Due to the smaller ionic radius of Fe 2+ compared to that of Ca 2+ , it is difficult for Fe 2+ to substitute for Ca 2+ at the X-site in garnets through isomorphic substitution. As a result, under relatively reducing conditions, grossular is mainly formed, while most Fe 2+ is transported in the hydrothermal fluids for long periods of time and over long distances in the form of Na(FeCl) 4 and other complexes [41,50]. In contrast, under relatively oxidizing conditions, the Fe 2+ in hydrothermal fluids oxidizes to Fe 3+ , substituting for Al 3+ in garnet to form andradite, so the formation of andradite requires higher oxygen fugacity than grossular formation [30,[51][52][53].…”

Genesis of the Lamasu large skarn Cu deposit, northern Xinjiang: Constraints from garnet U–Pb dating, micro‐textural and geochemical analyses

“…This is because more Ca 2+ and Mg 2+ can be leached from the wall rocks under lower pH conditions, which reduces Fe concentration in the fluids. Conversely, andradite forms readily under alkaline and oxidizing conditions (Ai & Jin, 2017;Wang et al, 2016;Zhao et al, 2018). The garnets from Lamasu are of the andradite-grossularite series, which belong to typical contact metasomatism genesis (Chang et al, 2019).…”

“…For Grt-I, the andradite content (57%-86%) is higher than that of grossularite T A B L E 3 Electron probe microanalysis results of garnets from the Lamasu deposit. discontinuously with the host strata, changing the composition of magmatic fluids and volatiles (Bai, 2019;Wang et al, 2016). The fluid inclusions study by Liu et al (2018) Similar to Grt-I, Grt-II has also andradite (54%-74%) and minor grossularite (25%-43%), suggesting an alkaline and oxidizing formation condition.…”