Elsevier

Ore Geology Reviews

Volume 126, November 2020, 103725
Ore Geology Reviews

In situ LA-ICP-MS study of garnets in the Makeng Fe skarn deposit, eastern China: Fluctuating fluid flow, ore-forming conditions and implication for mineral exploration

https://doi.org/10.1016/j.oregeorev.2020.103725Get rights and content

Highlights

  • •
    Various textures in the Makeng garnet show four-stage garnet growth.
  • •
    The four-stage garnet growth records fluctuating fluid flow.
  • •
    The W-Mo-rich garnet can be used as an indicator mineral for W-Mo exploration in this area.

Abstract

Most garnets in Fe-Cu skarn deposits have simple petrographic textures, LREE-enriched geochemical patterns, are enriched in Fe, and are distinguished from garnets of typical W or W-Mo skarn deposits by their extremely low W, Sn, and Mo contents. However, little is known about the garnets in Fe skarn systems that have more complex textures and high concentrations of W, Sn, and Mo. Here we describe diverse textures and LA-ICP-MS data for garnets in the largest Fe skarn deposit (Makeng) in eastern China, and we discuss the dynamics of the mineralizing fluids and the significance of garnet for W-Mo exploration. Garnets from the Makeng deposit fall into four main types, corresponding to four-stage garnet growth based on their textural characteristics and trace element contents: Stage 1, Al-rich relatively homogeneous garnets (Adr51-55) with a portion (Adr17-48) and HREE-enriched patterns and weak negative Eu anomalies; Stage 2, Fe-rich garnet with LREE-enriched or flat patterns and weak negative or positive Eu anomalies, represented by oscillatory zoned rims (Adr50-90) with dodecahedral faces and oscillatory zones (cores Adr54-68, rims Adr65-80) with composite dodecahedral-trapezohedral faces; Stage 3, nearly pure andradite garnets with significantly LREE-enriched patterns and strong positive Eu anomalies, as represented by irregular zones (Adr70-99) in oscillatory zoned rims and relatively homogeneous garnets (Adr95-100) with extremely weak zoning; and Stage 4, Al-rich garnet with HREE-enriched patterns and weak negative or weak positive Eu anomalies, as represented by garnet veinlets (Adr45-77) with weak zoning. These four stages of garnet were formed by periodic fluctuations in the fluid flux which can be explained by a kinetic dispersion model: a low flux responsible for the Al-rich garnet, a high flux responsible for the Fe-rich garnets, the highest flux responsible for the nearly pure andradite garnet, and a low flux responsible for the veins of Al-rich garnet. Relative to garnet in Fe-Cu skarn deposits, all garnet types in the Makeng Fe skarn deposit are rich in trace elements such as W, Sn, and Mo, especially the pure andradite garnet in stage 3, which has the highest contents of W, Sn, and Mo. These elemental enrichments are closely correlated with Fe, LREE/HREE, and fO2 signatures. The W and Mo signatures are identical to those of garnets in typical W or W-Mo skarn deposits, and they are useful geochemical indicators during exploration for W-Mo ore deposits.

Introduction

Garnet has a wide range of compositions as solid solutions, and in most skarn deposits garnet is marked by zonal patterns, which has attracted much attention (Jamtveit, 1991; Yardley et al., 1991, Jamtveit and Andersen, 1992, Jamtveit et al., 1993, Jamtveit et al., 1995, Jamtveit and Hervig, 1994, Smith et al., 2004, Gaspar et al., 2008, Peng et al., 2015, Park et al., 2017b, Tian et al., 2019). Zonation patterns in garnet have been interpreted as being related mainly to repeated hydrofracturing (Park et al., 2017b), mineral reactions (Dziggel et al., 2009), or periodic changes in fluid flux (Jamtveit et al., 1995). While the irregular superposition of zones in oscillatory zoned garnet is not common, it commonly reflects the dissolution of early zones and records a high fluid flux in mineralized hydrothermal systems (Dziggel et al., 2009) or a significant change in the composition (e.g., salinity) of the fluid (Smith et al., 2004). The combination of the REE signature of garnet with its texture can effectively provide a record of the history of hydrothermal fluid evolution, and the kinetics of mineral growth (Rakovan and Reeder, 1996, Smith et al., 2004), the physical and chemical properties of the ore-forming fluids (Zhai et al., 2014, Zhang et al., 2017a, Zhang et al., 2017b, Ding et al., 2018, Fu et al., 2018), and metasomatism dynamics, i.e., diffusive metasomatism or advective metasomatism (Gaspar et al., 2008).
Early studies proposed a series of discriminate diagrams for skarn deposit classification and mineral exploration using garnet major element compositions (Meinert, 1992, Zhang et al., 2017a). Moreover, garnet also incorporates trace elements related to skarn ore deposits, and some metal elements (e.g., W, Sn and Mo) have been found to be enriched in andradite garnets (Xu et al., 2016, Park et al., 2017a, Zhou et al., 2017). Their enrichment signature is controlled by intrinsic mechanisms (e.g., the substitution of these elements for Fe3+ in garnet) and extrinsic mechanisms (e.g., the physicochemical parameters of the hydrothermal fluids) (Xu et al., 2016, Zhou et al., 2017, Park et al., 2017a). The W-Sn-Mo-rich garnets in skarns can be used as an indicator mineral for W-Sn-Mo ore exploration (Xu et al., 2016, Park et al., 2017a, Zhou et al., 2017, Tian et al., 2019).
The Makeng Fe skarn deposit is located in the Yong’an-Meizhou depression belt and is the largest iron ore deposit in Southeast China. It contains 430 Mt of resources with average grade of 41.6% Fe. Most studies have been carried out on petrogenic and metallogenic geochronology (Zhao et al., 1980, Ge et al., 1981, Zhang et al., 2012, Zhang et al., 2015b, Yang et al., 2017b), petrogenic and metallogenic setting (Yang et al., 2017b), and ore-forming mechanism (Zhao et al., 1980, Ge et al., 1981, Wang et al., 1981, Li and Chen, 1982, Zhang et al., 2013, Yang et al., 2017a). Despite garnet is a widespread alteration mineral in the Makeng deposit, only a few studies have examined its major element characteristics (Zhang and Zhang, 2014). Moreover, no attention has been paid to the compositional evolution of garnets with different textures and to revealing enrichment features of some metal elements (e.g., W, Sn and Mo) in garnets that can be used in ore exploration. Therefore, in this paper we present the results of our detailed study of the textures of garnet varieties in the Makeng Fe deposit and their in situ major and trace element signatures. Our study has revealed high levels of W and Mo in the garnets and four stages of fluid flux recorded by garnet. We outline the implications of these results for W and W-Mo exploration, and hope that our results provide useful new reference data when examining W or W-Mo skarn deposits in this region.

Section snippets

Regional geology

The South China Block is composed of the Yangtze Block in the northwest which is separated by the Jiangshan-Shaoxing Fault from the Cathaysia Block in the southeast (Fig. 1a). Fujian Province, which lies in the Cathaysia Block, contains a magmatic arc along the east coast, the Precambrian Wuyishan Terrane in the north, and the Yong’an-Meizhou depression belt in the west (Fig. 1b). Our study area is in the southwestern part of the Hercynian Yong’an-Meizhou depression belt (Bian and Gao, 1982),

Descriptions of collected samples

We collected four skarn samples representing three different types (283 m, 248 m, and 330 m levels) of iron ores from the Makeng main iron ore body in order to undertake studies of the petrography and mineral chemistry: (1) massive iron ores (samples MK15 and MK21) from 283 m level in 81 prospecting line, (2) massive iron ore (sample MK26) from 248 m level in 81 prospecting line, and (3) massive iron ore (sample MK67) from 330 m level 82 prospecting line. In detail, garnets are brown in hand

Garnet major element geochemistry

The LA-ICP-MS analytical results for the four stages of garnet are presented in Appendix 1 and its major and trace elements are summarized in Table 1. The data indicate generally that garnet types from the Makeng deposit belong to the grossular-andradite solid solution with compositions ranging from Adr20Grs80 to almost pure andradite [Adr100] (Fig. 6). The spessartine, pyrope, and almandine contents are collectively less than 13%, except for one analysis from sample MK015 (23%) (Appendix 1).

Significance of REE patterns of the garnets

The REE patterns of garnet in hydrothermal systems have been investigated by many workers (e.g., Jamtveit and Hervig, 1994, Smith et al., 2004, Gaspar et al., 2008) who pointed out that physico-chemical fluid conditions (e.g., temperature, pressure, oxygen fugacity, pH, and REE complex speciation) adds an important external factor to the role of crystal chemistry (e.g., substitution mechanisms, surface sorption, lattice relaxation energy, and growth rate). Recently, some workers (Dziggel et

Conclusions

  • (1)
    Garnet from the Makeng skarn deposits has diverse textural patterns including core-rim textures with obvious oscillatory zoning, crystals that lack zoning, dissolution texture marked by irregular zones in the oscillatory zoned rims, and thin veins with zoning cutting garnet crystals. These various textures reflect four-stage of garnet growth: unzoned garnet and core portions (Stage 1), rim portions and zoned garnet (Stage 2), irregular zones and slightly zoned garnet (Stage 3), and garnet

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are greatly indebted to Senior Engineer Jin-Xiang Wang (Fujian Makeng Mine CO., LTD) for the assistance during fieldwork. This research was supported by the following funding agencies: the National Key Research and Development Program of China (Grant No. 2016YFC0600205), the opening funding of State Key Laboratory for Mineral Deposits Research, Nanjing University (Grant No. 2019-LAMD-K08), Hunan Key Laboratory of Land Resources Evaluation and Utilization (Grant No. SYS-ZX-201901), the

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