Skarn Hosted Sn-W Mineralization, Samoeng Mine, Northern Thailand: Petrology, Geochemistry and Geochronology

The Samoeng (Bo Kaeo) mine, the largest primary Sn-W deposit in northern Thailand, is part of the Southeast Asian Tin Belt, which extends 2,800 km from Myanmar and Thailand to Peninsular Malaysia and Sumatra Island. In Thailand, Sn-W deposits are found in the western and central granitoid belts, but their origins are not well constrained as there are only few studies about their petrology, geochemistry and dating of the mineralization. The Sn-W ore bodies at Samoeng are associated with skarn formation between a granite and the calcareous host rock. The magmatic intrusion is a medium to coarse-grained biotite granite with pegmatite and aplite veins intruding the country rock. The biotite granite is enriched in alkalis, Rb, Th and Pb. They display S-type peraluminous characteristics (A/CNK =1.07-1.14), low FeO_T/MgO ratio (0.24-0.44), and belong to the high-K calc-alkaline to shoshonitic series. The granites have elevated LREEs, a negative Eu anomaly, and a flat HREE profile. Tectonic discrimination plots (Pearce et al., 1984) classify most samples as syn-collisional granites (syn-COLG), while the Batchelor and Bowden (1985) diagram categorizes them as syn-, late, and post-collisional. Zircon U-Pb geochronology dates the biotite granite to 214±2 Ma. Monazite U-Pb geochronology yields besides the Late Triassic intrusion age (~215 Ma) a younger Cretaceous age of ~80 Ma.

The calcareous rocks include calc-silicate rocks and marble. The calc-silicates are fine to medium-grained with interlocking crystals, primarily composed of diopside and subordinate wollastonite, phlogopite, quartz, plagioclase, clinohumite, grossular, calcite, Sn-bearing rutile or titanite, spinel, apatite, and minor cassiterite. The calcite marble is medium grained and contains quartz and diopside as accessory minerals.

The Sn-W mineralization in this area is primarily found in breccia dykes, which manifest as narrow, tabular structures intersecting the biotite granite and calc-silicate rock. These dykes, generally ranging from 2 to 5 meters in thickness, are highly weathered (kaolinitized). The breccia fragments are composed of biotite, quartz, K-feldspar and tourmaline, with lesser quantities of magnetite, spinel, cassiterite, and scheelite. The heavy mineral fraction from the Samoeng mine primarily include cassiterite and scheelite, with smaller amounts of magnetite, ilmenite, spinel, zircon, apatite, rutile, titanite, xenotime, allanite, thorianite, and monazite. Monazite U-Pb geochronology indicates several age populations of ~230, ~205 and ~75 Ma, suggesting recrystallization and re-precipitation during multiple thermal events.

Based on our data we conclude that the Sn-W mineralization occurred in the Late Triassic during the emplacement of the granitoid body which led to skarn formation and an associated contact aureole. This event is related to the Sukhothai-Sibumasu collision (closure of the Paleo-Tethys, Indosinian Orogeny). During Cratacous times a thermal overprint is recorded in monazite from the granite body but also from the mineralized skarn zone. This event in the Late Cretaceous is associated with the collision between the Sibumasu and West Burma blocks.

Batchelor, R.A. and Bowden, P. (1985) Petrogenetic Interpretation of Granitoid Rock Series Using Multicationic Parameters. Chemical Geology, 48, 43-55.

Julian A Pearce, Nigel BW Harris, Andrew G Tindle (1984). Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25, 956-983.