Multistages of original sapphire formation related to basaltic magmatism in the Bo Phloi basaltic gem field, Kanchanaburi, Western Thailand: Evidence from trace elements and ages of zircons
Graphical abstract
Introduction
Sapphires deposited within basaltic terranes commonly contain zircon inclusion which they normally assume to be cogenetic assemblage (Guo et al., 1996a, Guo et al., 1996b, Khamloet et al., 2014) which is supported by unusual sapphire + zircon bearing gravels discovered in those basaltic gem fields (e.g., Coenraads et al., 1995, Pisutha-Arnond et al., 1998). Hence the trace element geochemistry of zircon may provide the fingerprint and important information for the genetic model. In addition, a zircon is often used for U-Pb geochronology by which the measured date could represent the crystallization age of its original formation. Zircon inclusion in sapphire as well as composite zircon within sapphire-bearing gravels, all from alluvial gem deposits associated with Cenozoic basalt in Bo Phloi District, Kanchanaburi Province, Western Thailand (Fig. 1), were collected for this study.
Many genetic models of sapphire occurred in basaltic terrane have been obtained from the REE geochemistry and age of zircon inclusion (e.g., Sutherland and Fanning, 2001, Sutherland et al., 2002, Sutherland et al., 2008, Graham et al., 2008). From previous data, many U-Pb isotopic dating techniques including Sensitive High Resolution Ion Microprobe (SHRIMP), Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), and Secondary Ion Mass Spectrometry (SIMS) have been used for measuring the zircon ages (e.g., Compston, 1999, Zhao et al., 2002, Black et al., 2003, Kusiak et al., 2008). Among these techniques, LA-ICP-MS is the one widely used for age determination as well as trace element analysis due to its high sensitivity (e.g., Fryer et al., 1993, Norman et al., 1998, Kosler and Sylvester, 2003, Black et al., 2004, Jackson et al., 2004, Xia et al., 2004, Chang et al., 2006); it is also a robust and cost effective technique.
Bo Phloi gem field in Kanchanaburi province, western Thailand is a well-known basaltic gem deposit that has supplied large quantities of a good quality sapphire to the world market. Geologically, this area is situated along a Cenozoic rift basin caused by crustal extension after collision of Indo-Australian plate and Eurasian plate (McCabe et al., 1988). Thick Quaternary sedimentary sequences have accumulated and coincided with multiple basaltic eruptions, particularly in the upper sequence in which the main gem mining activities had been operated at about 15–20 m depth. However, only small remnant of basalt is still present in the middle of the basin surrounded by Paleozoic sedimentary rocks. Precambrian gneiss overthrusted onto Mesozoic sandstones expose in the northern part whereas Mesozoic granites distribute along the western flank (Department of Mineral Resources, 1977, Fig. 1). These Mesozoic (Triassic) granites belong to the Central Granite Belt (Nantasin et al., 2012).
In-depth studies of sapphires and their mineral inclusions from the Bo Phloi gem field have been reported by a few researchers (Saminpanya, 2000, Guo et al., 1994, Khamloet et al., 2014). More varieties of these inclusions were categorized, based on mineral chemistry, into two groups related to alkaline felsic magma and contact metamorphism by Khamloet et al. (2014). Zircon is a typical inclusion in alkaline felsic suite which also contains alkali feldspar, nepheline, manganiferous ilmenite, monazite and calcite. On the other hand, Si-rich enstatite, almandine–pyrope garnet, sapphirine, staurolite, biotite–phlogopite mica, and hercynitic spinel are grouped as contact metamorphic suite. Khamloet et al. (2014) also slightly touched REE analyses of zircon. Mineral inclusions belonging to the alkaline felsic suite have been observed more often than those of the contact metamorphic suite. Consequently, they proposed the bimodal genetic model of original sapphire formation which appear to have significantly related to a highly evolved felsic melt and been involved partly by contact-metamorphic reaction of basaltic magma and crustal rocks. More detail will be present in the discussion part.
An unusual gravel, named as ‘Corsilzirspite’, containing corundum (sapphire), sillimanite, tiny zircon, and black spinel, by Pisutha-Arnond et al. (1998). They also described petrographic feature and reported mineral chemical compositions of the Corsilzirspite gravel; consequently, they concluded that contact metamorphism/metasomatism/gabbroic melt contamination appears to have involved partly in the original formations of sapphire in the Bo Phloi gem field.
For this manuscript, we report detailed trace analyses and U-Pb ages of zircon inclusions in sapphire and zircons in sapphire-bearing gravels which were mentioned above. All of the data are then used to improve genetic model of original sapphire formations in association with Cenozoic basaltic magmatism which were suggested by Khamloet et al. (2014).
Section snippets
Sample collection and methodology
Rough sapphire samples (Fig. 2a) from alluvial deposit were firstly examined under a binocular microscope to sort out suitable grains that contain zircon inclusions (Fig. 2b) before mounting them in epoxy resin. Subsequently, they were carefully polished until the inclusions were exposed on the surface prior to trace element analyses and age determinations. It should be notified again that zircon inclusions in the Bo Phloi sapphire have been grouped as alkaline felsic magmatic suite by Khamloet
Trace element geochemistry of zircons
Trace element contents of the zircon inclusion in sapphire (1B3ib2, same sample as previously reported in Fig. 7 of Khamloet et al., 2014), as well as the zircons in the metamorphic sapphire-bearing gravels (GRA1 and GRA2 equivalent to samples BXL02-2 and BLX1-1, previously reported in Fig. 7 of Khamloet et al., 2014) are depicted in Table 1. For comparison, the average trace element contents of zircons from different rocks, i.e., syenite pegmatite, nepheline-syenite pegmatite and carbonatite
Sequences of sapphire/zircon crystallization and basaltic eruption
In this study, the Bo Phloi zircon inclusion gave the U-Pb age of 24 ± 0.9 Ma using LA-ICP-MS technique. Based on the same analytical technique, the U-Pb age of zircon in ‘Corsilzirspite’ gravel (GRA2) gave 10.86 ± 0.14 Ma and the U-Pb ages of zircon in GRA1 sapphire-bearing gravel at 4.4 ± 0.4 Ma. These ages were compared with the Cenozoic basaltic activities in Thailand as shown in Table 3 which both K/Ar and 40Ar/39Ar ages of the Bo Phloi basalt previously reported by Barr and Macdonald, 1981
Conclusions
The trace element geochemistry of zircon inclusion in sapphire and zircons in the sapphire-bearing gravels indicate that the Bo Phloi sapphire has its origin related to highly evolved alkaline felsic (syenitic) melts. These data in combination with the geochemical affinities of the mineral inclusions found in sapphire host (see Khamloet et al., 2014), a bimodal origin of the Bo Phloi sapphire, namely syenitic melt-related origin and contact metamorphic-related origin has been proposed to occur
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
This research project was partially supported by Thailand Research Fund (TRF). The research was completed at the Department of Geology, Faculty of Science, Chulalongkorn University. The trace-element analysis and U–Pb dating of the zircons were carried out by Dr. Elena Belousova and Ms. Tin Tin Win (Department of Earth and Planetary Sciences, Macquarie University, Sydney), for which the authors are sincerely grateful. Finally, we would like to thank Prof. Khin Zaw for handling the manuscript
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