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

Abstract

The Changpo–Tongkeng tin polymetallic deposit in Dachang, Guangxi, is a world-class, superlarge, polymetallic tin deposit consisting of lower skarn zinc–copper ore bodies and upper tin polymetallic ore bodies. Garnet is the main gangue mineral in the skarn zinc–copper ore bodies and has a granular texture. Based on hand specimens and microscopic observations, the existing garnet can be divided into two generations: an early generation (Grt I) and a late generation (Grt II). The results of electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in situ microanalysis show that the contents of SiO2 and CaO in the garnets from the two generations present limited variations, while the FeOT and Al2O3 contents vary significantly, indicating the grossular–andradite solid solution series (Gro29–82And12–69). Compared with Grt I (Gro72And25), Grt II (Gro39And59) is Fe-enriched and oscillatory zoning is developed. The total rare earth element (REE) contents in the two generations of garnet are relatively low, showing light rare earth element (LREE) depletion and heavy rare earth element (HREE) enrichment patterns. Grt II has higher REE content than Grt I and exhibits significant negative Eu anomalies (δEu = 0.18–0.44). The contents and variation characteristics of the major and trace elements in the two generations of garnet suggest that there were variable redox conditions and water/rock ratios in the hydrothermal system during the crystallization process of garnet. In the early stage, skarnization was in a relatively closed and low-oxygen fugacity system, with hydrothermal diffusion metasomatism being dominant, forming homogeneous Grt I lacking well-developed zoning. In the late stage of skarnization, the oxygen fugacity of the ore-forming fluids increased, with infiltration metasomatism being dominant, forming Grt II with well-developed oscillatory zoning. The contents of Sn, As, W, In, and Ge in the garnets are relatively high and increase with the proportion of andradite. Sn in zinc–copper ore bodies mainly exists in the form of isomorphic substitution in garnet, which may be the main reason for the lack of tin ore bodies during the skarn stage. This paper compares the trace element contents in garnets from domestic skarn deposits. The results indicate that the Sn content and δEu in garnet can be used to evaluate the tin-forming potential of skarn deposits.

Skarn Formation and Zn–Cu Mineralization in the Dachang Sn Polymetallic Ore Field, Guangxi: Insights from Skarn Rock Assemblage and Geochemistry

“…The EPMA results for two generations of garnets were published by He et al [24]. The SiO 2 and CaO contents in the garnets show limited variation, ranging from 36.04 to 39.31 wt% and 33.62 to 35.84 wt%, respectively.…”

“…The trace element analysis results for garnet were published by He et al [24]. The results showed that both generations of garnets are depleted in large ion lithophile elements (LILEs) such as Rb, Cs, Ba, and K compared to the primitive mantle [35].…”

“…The earliest mineral formed during the skarn stage of the Dachang ore field was grossular, indicating that the early ore-forming fluid during the skarn stage was relatively reduced [24]; with the evolution of the ore-forming fluid, considerable andradite, pyroxene, and wollastonite were formed, indicating an increase in the oxygen fugacity of the oreforming fluid in the later stage. In the final skarn stage, garnet and pyroxene were replaced by Fe-actinolite, chlorite, and quartz, indicating that the ore-forming conditions changed from relatively oxidized to reduced (Figure 9).…”

“…The earliest mineral formed during the skarn stage of the Dachang ore field was gro sular, indicating that the early ore-forming fluid during the skarn stage was relatively r duced [24]; with the evolution of the ore-forming fluid, considerable andradite, pyroxen and wollastonite were formed, indicating an increase in the oxygen fugacity of the or…”

“…Previous researchers have carried out extensive studies here, but the focus of previous work has mainly been on the genesis of deposits, emplacement ages of granitic pluton, metallogenic age and isotopic studies, etc. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. However, mineralogical research, especially systematic studies of skarn rock petrography and geochemistry, is still relatively weak, which restricts the improvement of deposit models and in-depth understanding of skarn mineralization.…”