Formation of late-stage hydrothermal mineralization in the Mesoarchean Boula-Nuasahi ultramafic complex, Odisha, India: Constraints from arsenopyrite geothermometry and trace element data

https://doi.org/10.1016/j.oregeorev.2021.104482Get rights and content

Highlights

  • Purest forms of arsenopyrite are reported from Boula-Nuasahi ultramafic complex, India.

  • As and S show a clear negative correlation according to the substitution FeAs1-xS1+x..

  • Temperatures were estimated to be 335 to 425 °C using arsenopyrite geothermometry.

  • Arsenopyrite geothermometry can be applied for low-temperature hydrothermal mineralizations.

Abstract

The Mesoarchean Boula-Nuasahi ultramafic complex is situated in the south eastern flank of the Singhbhum Craton, India and consists mainly of gabbro-anorthosite, peridotite and pyroxenite litho-units that host a series of chromitite bodies. A contemporaneous but slightly younger magmatic event has structurally disturbed the area with intrusion of a coarse-grained gabbro unit known as the Bangur gabbro (∼3.1 Ga). This has caused extensive hydrothermal alteration of the original host rocks and is associated with hydrothermal sulphide- and Pt-Pd mineralizations. Petrographic study shows that well-crystallized and euhedral arsenopyrite crystals are associated with quartz and carbonates in veins aligned along the foliation planes/fractures in the host rocks. This vein assemblage (arsenopyrite + quartz + calcite) has formed during the late stage of hydrothermal mineralization. Electron probe micro-analysis and LA-ICP-MS analysis of arsenopyrite demonstrates that its chemical composition varies within narrow limits and that the concentrations of trace elements are below 900 ppm. The only trace element present in significant concentrations is Sb with an average value of 490 ppm. The As/S atomic ratio varies between 0.93 and 1.01 and As and S show a clear negative correlation according to the substitution FeAs1-xS1+x. The average composition is very close to the stoichiometric value. Because the arsenopyrite crystals are vein minerals, have very low concentrations of any impurities and do not show any compositional zoning, they were used for geothermometric calculations. Matching the measured As atom% with one of the experimentally calibrated isopleths in the established Fe-As-S phase diagram, calculated temperatures are in the range of 335–425 °C. This temperature range is in very good agreement with temperature estimates of 300–500 °C (based on cobaltite–gersdorffite compositions) for late-stage hydrothermal mineralization in the Boula-Nuasahi ultramafic complex available in the literature.

Introduction

There are only very few common sulphide minerals such as sphalerite and arsenopyrite that are suitable as geothermometers or geobarometers. This is because these phases exhibit appreciable solid-solution, the chemical variations are sensitive to pressure or temperature and they have a refractory nature and are not easily modified by post-depositional processes. Experiments on the stability fields of arsenopyrite in the system Fe-As-S have been conducted by several researchers (Clark, 1960a, Clark, 1960b, Barton, 1970, Scott, 1976). Because of the refractory nature of arsenopyrite, these experiments were limited by extremely slow rates of reaction. Sharp et al. (1985) reviewed and re-evaluated the arsenopyrite geothermometer by analysing natural assemblages whose P-T conditions were independently measured or approximated by fluid-inclusion trapping temperatures, silicate phase-equilibria and stable isotope fractionation data. They concluded that application of the arsenopyrite geothermometer may be valid for deposits metamorphosed to greenschist and lower amphibolite facies, but yields too low temperatures for deposits metamorphosed to upper amphibolite and granulite facies and inconsistent temperatures for low-temperature hydrothermal deposits. The As/S atomic ratio of the arsenopyrite that may coexist with other phases buffering the S activity in the system As-S-Fe has been experimentally verified and calibrated as a geothermometer (Kretschmar and Scott, 1976, Sharp et al., 1985). However, application of the arsenopyrite geothermometer has not been straightforward because of the widespread presence of other minor and trace elements in arsenopyrite and because in many instances, arsenopyrite is found to be zoned with respect to the arsenic content (Vesselinov and Kerestedjian, 1995, Kerestedjian, 1997, Choi and Youm, 2000, Mikulski, 2005). Additional complications may arise from post-depositional modification of the arsenopyrite.

In this paper, we report the textural features, paragenesis and mineral chemistry of arsenopyrites that occur as late-stage vein minerals in the mafic–ultramafic rocks of the Bangur area in Boula-Nuasahi ultramafic complex (BNUC) situated in the south eastern flank of the Singhbhum Craton, India. Although a detailed account of various sulphide minerals in the BNUC has been previously reported (Mondal et al., 2001; Mohanty et al., 2002; Augé et al., 2002, Jena et al., 2016), the arsenopyrite mineralization has not been investigated. These arsenopyrites are found to be a surprisingly pure variety with almost close to stoichiometric compositions. We have therefore used the composition of the arsenopyrite for geothermometric applications, yielding inferred formation temperatures of the last phase of hydrothermal sulphide mineralization in the area.

Section snippets

Geological setting

The Boula-Nuasahi ultramafic complex is situated on the southeastern flanks of the Singhbhum Craton and has been emplaced into the regionally metamorphosed greenschist facies metasedimentary rocks of the Precambrian Iron-Ore Group of India (>3.1–3.3 Ga; Mukhopadhyay, 2001, Augé et al., 2003). It forms an elongate body (∼3 km long and 0.5 km wide) trending NW-SE in its northern part and N-S in the southern part (Fig. 1).

The BNUC consists primarily of four major lithostructural units: 1) an

Methods of investigation

Polished thin-sections (2 cm by 3 cm) of the samples were prepared by conventional techniques by mounting on glass-slides in araldite, followed by grinding and polishing with diamond paste. The samples were petrographically studied using a Leica DM 4000 P transmitted- and reflected-light microscope. On the same polished sections, electron probe micro-analysis (EPMA), QEMSCAN mineral mappings and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses were carried out at

Petrography and textures of arsenopyrite

Well-crystallized and euhedral arsenopyrite crystals, which are not associated with any other sulphide minerals, have been found as vein minerals in a partially altered fine-grained anorthositic rock of the hanging-wall side gabbro-anorthosite suite in the Bangur area. The arsenopyrite crystals vary in size from <1 mm to ∼1 cm and occur along the foliation planes/fractures (Fig. 5a and b). Most of the crystals are fractured. Under plane polarized light, arsenopyrite is pleochroic and shows

Mineral chemistry of arsenpoyrite

Electron probe micro-analysis of 45 points in different arsenopyrite grains reveals that their chemical composition varies only within narrow limits (Table 2). The concentrations of As are 44.62–46.84 wt% (average: 45.41 wt%), of Fe are 34.17–34.97 wt% (average: 34.47 wt%) and of S are 19.78–20.72 wt% (average: 20.22 wt%). From the atomic percentages, it can be inferred that these crystals are among the purest forms of arsenopyrite found in natural occurrences with very uniform and

Conclusion

Despite many potential issues related to the application of arsenopyrite geothermometry for low-temperature hydrothermal mineralizations, the chemically pure and unzoned arsenopyrite crystals in the BNUC provide a good opportunity to use them for estimating their temperature of formation. The calculated temperatures of 335–425 °C thus constrain the temperature of formation of the last stage of hydrothermal mineralization in the area that is likely related to the late-stage cross faulting.

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.

Acknowledgements

The CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, India is thanked for supporting the initial work on this project. Dr. M.S. Jena had helped during the sample collection. The visit of B. Nayak to RWTH was financially supported by the Alexander von Humboldt Foundation, Bonn, Germany. Roman Klinghardt and Lars Gronen assisted in operating the analytical equipment. Thomas Derichs prepared the polished thin sections. Prof. S. Basu, Director, CSIR-IMMT is thanked for encouraging

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