ARTICLE R. L. Linnen Depth of emplacement, ¯uid provenance and metallogeny in granitic terranes: a comparison of western Thailand with other tin belts Received: 8 September 1997 / Accepted: 28 October 1997 Abstract The most important tin mineralization in Thailand is associated with the Late Cretaceous to Middle Tertiary western Thai granite belt. A variety of deposit types are present, in particular pegmatite, vein and greisen styles of mineralization. A feature common to most of the deposits is that they are associated with granites that were emplaced into the Khang Krachan Group, which consists of poorly sorted, carbonaceous, pelitic metasediments. Most of the deposits contain low to moderately saline aqueous ¯uid inclusions and aqueous-carbonic inclusions with variable CH 4 /CO 2 ratios. Low salinity aqueous inclusions represent trap- ped magmatic ¯uid in at least one case, the Nong Sua pegmatite, based on their occurrence as primary inclu- sions in magmatic garnet. Aqueous-carbonic inclusions are commonly secondary and neither the CO 2 nor NaCl contents of these inclusions decrease in progressively younger inclusions, implying that they are not magmatic in origin. Reduced carbon is depleted in the metasedi- ments adjacent to granites and the dD values greisen muscovites are variable, but are as low as )134 per mil, indicative of ¯uid interaction with organic (graphitic) material. This suggests that the aqueous-carbonic ¯uid inclusions represent ¯uids that were produced, at least in part, during contact metamorphism-metasomatism. By comparing the western Thai belt with other Sn-W provinces it is evident that there is a strong correlation between ¯uid composition and pressure in general. Low to moderately saline aqueous inclusions and aqueous- carbonic inclusions are characteristic of mineralization associated with relatively deep plutonic belts. Mineral- ized pegmatites are also typically of deeper plutonic belts, and pegmatite-hosted deposits may contain cas- siterite that is magmatic (crystallized from granitic melt) or is orthomagmatic-hydrothermal (crystallized from aqueous or aqueous-carbonic ¯uids) in origin. The magmatic aqueous ¯uids (those that were exsolved from granitic melts) are interpreted to have had low salinities. As a consequence of the low salinities, tin is partitioned in favour of the melt on vapour saturation. Thus with a high enough degree of fractionation, the crystallization of a magmatic cassiterite (or dierent Sn phase such as wodginite) is inevitable. Because tin is not partitioned in favour of the vapour phase upon water saturation of the granitic melts, it is proposed that relatively deep vein and greisen systems tend to form by remobilization processes. In addition, many deeper greisen systems are hosted, in part, by carbonaceous pelitic metasediments and the reduced nature of the metasediments may play a key role in remobilizing tin. Sub-volcanic systems by contrast are characterized by high temperature-high sa- linity ¯uids. Owing to the high chlorinity, tin is strongly partitioned in favour of the vapour and cassiterite mineralization can form by of orthomagmatic-hydro- thermal processes. Similar relationships between the depth of emplacement and ¯uid composition also appear to apply to other types of granite-hosted deposits, such as dierent types of molybdenum deposits. Introduction Evolved granites are interpreted to be the source of mineralization for most tin deposits. These granites typically contain one or more of muscovite or Li-micas, garnet, ¯uorite, topaz and tourmaline and their genesis is commonly explained by fractional crystallization fol- lowed by high temperature metasomatism, e.g., Charoy (1986). Three dierent mechanisms have been proposed for cassiterite crystallization: Mineralium Deposita (1998) 33: 461±476 Ó Springer-Verlag 1998 Editorial handling: B. Lehmann R.L. Linnen Bayerisches Geoinstitut, UniversitaÈt Bayreuth, 95440 Bayreuth, Germany Present address: Department of Earth Science, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Fax: 519-746-7484; email: rlinnen@uwaterloo.ca