Chapter 3 GEOCHEMISTRY and AGE DETERMINATION of VOLCANO-PLUTONIC ROCKS

Chapter 3 GEOCHEMISTRY AND AGE DETERMINATION OF VOLCANO-PLUTONIC ROCKS 3.1 Introduction Volcanic and plutonic rocks are thought to be formed by partial melting of mantle and lower crustal rocks above subduction zones and mantle plumes, during rifting or during other tectonic processes (Campbell et al., 1974). This magmatism tends to produce hydrothermal fluids either through circulation of pre-existing water through heated rock or directly through the expulsion of magmatic fluids during magmatic crystallization. Most metals are soluble in these hot fluids but precipitate to form ore deposit when these fluids undergo major physical or chemical changes, e.g. porphyry copper, and volcanic hosted massive sulphide (Burnham, 1979; Tatsumi and Eggins, 1993). In Chapter 2 it was demonstrated that the occurrences of major gold and iron-gold provinces in Thailand were closely associated with the occurrences of volcano-plutonic rocks indicating a hydrothermal origin for these deposits. In this chapter the link amongst the magmatism, the tectonics and the ore deposit will be explored by undertaking a brief study of the geochemistry and age of mineralisation. The occurrences of major gold and iron-gold deposits in northern and northeastern Thailand are restricted to two orogenic belts (Jungyusuk and Khositanont, 1992); the Sukhothai located to the west of the Nan-Uttaradit suture and the Loei- Phetchabun Fold Belt, to the east (Fig. 3.1). With notable exceptions, previous tectonic reconstructions of Thailand and mainland Southeast Asia were based mostly on sedimentary basin analysis and paleobiogeographic reconstruction but tended to ignore the geochemistry of volcanic and plutonic rocks. This chapter, therefore, examines the major elements, trace elements and age of the igneous and volcaniclastic rocks within the Sukhothai and 22 Figure 3.1 Geological map of Thailand showing approximate distribution of Sukhothai and Leoi – Phetchabun Fold Belts, and distribution of major rock units (modified from 1:2,500,000 Geological Map of Thailand, Department of Mineral Resources, 1999) 23 Loei – Phetchabun Fold Belts (Fig. 3.2). The geochemistry is then combined with the field observations in order to characterise the tectonic setting and evolution of the Sukhothai and Loei-Phetchabun orogenic belts. 3.2 Methodology 3.2.1 Major, Trace Elements and REE Analysis Igneous rocks erupted in different tectonic settings tend to have different major, and trace element characteristics (Rollinson, 1993). By comparing the geochemistry of the rocks from the Sukhothai and Loei Phetchabun Fold Belts to those from modern tectonic settings using well characterised geochemical diagrams, this study will test the tectonic models for these areas determined by previous studies. 3.2.1.1 Sample preparation Selected samples were crushed into 10 cm size using a rock splitter, cleaned and crushed into 1 cm pieces using Jaw Crusher and then finally crushed to fine powder using both W carbide and Cr-steel ring mills. Fusion discs and pellets were prepared for XRF major elements analysis, whereas sample solutions were prepared for ICP- MS REE analysis. 3.2.1.2 Analytical Techniques Major and trace elements were analysed by a PANalytical (Philips) PW 1480 X- Ray Fluorescence (XRF) spectrometer installed at the CODES ARC Center of Excellent in Ore Deposit, University of Tasmania, Australia. Major elements were measured from fusion discs, which were prepared at 1,100 °C in 5%Au/95%Pt crucibles using 0.500g of sample, 4.500g of 12-22 Flux (lithium tetraborate- metaborate mix) and 0.0606g of LiNO3, following the technique described in Robinson (2003). Loss on ignition (LOI) for each sample was determined by heating 24 Figure 3.2 Map of Thailand showing sample locations (open stars), the Nan- Uttaradit suture and the Sukhothai and Loei-Phetchabun Fold Belts 25 1-2 g of sample at 1,000 ºC for 12 hours and reweighing. Pressed powder pellets for trace element analysis were prepared using 10 g of sample and PVPMC (Polyvinyl- pyrrolidone-Methylcellulose) as a binder. The samples were pressed to 3.5 tonnes cm-2 within a mold with a diameter of 32 mm. A 3kW (maximum) ScMo anode X-Ray tube and 3kW (maximum) Au anode X-Ray tube were used in order to measure trace elements. For the low-abundance trace elements and rare earth elements (REE), 16 volcanic and plutonic rocks were selected and analysed using a HP4500 Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Sample solutions were prepared using PicoTrace® high pressure acid (HF/H2SO4) digestion. Aliquots (100 mg) of powdered sample were weighed into 30 ml PTFE digestion containers. A few drops of ultra-pure water were added into samples after weighting and adding 0.1 ml of µg g -1 indium solution to each digestion container, 3 ml HF and 3 ml H2SO4 were slowly added. After shaking a few times for thorough mixing, the PTFE containers were left in the digestion block at 180 °C for 16 hours. The digestion mixture was then left over in the evaporation block for four days at 180 ºC. HClO4 (1 ml) was then added to the residue and dried before adding the final 2 ml HNO3 and 1 ml HCl. The residue was then dissolved by warming the solution in the digestion block at 60-70 ºC for ~ 1 hour. Finally, it was transferred into a polypropylene bottle and diluted to 100 ml after the solution became clear (Yu and Robinson, 2003), and then analysed. The results obtained from XRF analysis and from solution ICP-MS analysis show that the concentrations of individual elements, which were analysed by both techniques, are less than 5% differences. 3.2.2 U-Pb zircon age determination Zircon is an accessory mineral that generally forms simultaneously with crystallisation of intermediate to acid igneous rocks. Since, the blocking temperature of the zircon is as high as 800 ºC. It is, therefore, resistant to physical and chemical changes. In other words, the isotopic values are not easily changed and can be used to determine the age of the igneous host rocks. The age of rock is determined by determining the amount of U, which has decayed to Pb after crystallization of the zircon. 26 Zircons generally separate non-radiogenic Pb from their crystal lattice during crystallisation and therefore the age of rock can be determined directly from the U-Pb decay constants. 3.2.2.1 Sample preparation . Zircons were separated from 100 to 200g of rock by crushing to < 400 microns in a mortar and pestle or a Cr-steel ring mill, depending on sample hardness. Heavy minerals were then separated using a combination of mechanised panning device (superpan) and a hand pan. The heavy mineral residue was then dried. Magnetic and paramagnetic minerals were removed using a hand magnet and a Franz magnetic separator. Approximately 20 to 30 zircons were picked from the non-magnetic heavy mineral separate using a single hair from a fine artist’s paint brush, and mounted on double sided sticky tape. Epoxy glue was then poured into a 2.5 cm diameter mould on top of zircon grains. The mount was dried for 12 hours and polished using clean sandpaper and a clean polishing lap. The samples were then washed in distilled water in an ultrasonic bath prior to analysis. 3.2.2.2 Analytical techniques Zircons were ablated in a He atmosphere in a custom-made chamber with the laser pulsing at 5 Hz and a 30 micron diameter beam delivering ~ 12 J/cm2 and drilling at approximately 1 micron/s. A total of 11 masses were counted (96Zr, 146Nd, 178Hf, 202Hg, 204Pb, 206Pb, 207Pb, 208Pb, 232Th , 238U), with longer counting times on Pb isotopes giving a total quadrupole cycling rate of 0.2 second. Each analysis began with a 30 seconds analysis of background gas followed by 30 seconds with the laser switched on. Four primary (Temora zircons of Black et al., 2004) and 2 secondary standards 91500 were analysed both before and after every 12 zircon analyses to correct for mass bias, machine drift and down hole fractionation. Repeated monitoring of U/Pb mass fractionation during drilling showed an average fractionation of U/Pb varying from 0.050 at the start of a 30 second analysis to 0.053 at the deepest level of laser ablation. Monitoring of international standards showed a reproducible error of approximately ± 2 Ma at 200 Ma. 27 3.3 Volcano-plutonic rock in the Lampang – Phrae volcanic belt The formation of Sukhothai orogenic belt was especially closely associated with N-S arcuate trending volcano-plutonic rocks, which have been known as Lampang- Phrae volcanic belt (Jungyusuk and Khositanont, 1992). The Lampang – Phrae volcanic rocks can be geographically subdivided into three major belts. Doi Ton Belt is located in the western flank of the Sukhothai Fold Belt. The majority of Lampang – Phrae volcanic belt is occupied by Doi Luang Belt, which forms a N-S trending arcuate S-shape (Fig. 3.3) in the central Sukhothail Fold Belt, whereas Den Chai Belt formed complex structure next to the eastern part of Doi Luang Belt in the eastern Sukhothai Fold Belt. Previous study showed that Doi Luang Belt was formed at 240 ± 1 Ma (Barr et al., 2000). Since volcano-plutonic rocks in the Doi Luang Belt are closely associated with gold and iron-gold mineralisation (e.g. Huai Kham On, Mae Mok, and Mae Bo Thong Deposits), this project extends more detailed studies on geochemical characteristics and U-Pb zircon age determination of the Doi Luang Central Belt of the Lampang-Phrae volcano-plutonic rocks in order to obtain better understanding of gold and iron-gold mineralisation in relation to tectonic setting and evolution of the Sukhothai Fold Belt.

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