Depth of emplacement, fluid provenance and metallogeny in granitic terranes: a comparison of western Thailand with other tin belts |
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| Authors: | R L Linnen |
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| Institution: | Bayerisches Geoinstitut, Universit?t Bayreuth, 95440 Bayreuth, Germany, DE
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| 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 fluid inclusions and aqueous-carbonic inclusions with variable CH4/CO2 ratios. Low salinity aqueous inclusions represent trapped magmatic fluid in at least one case, the Nong Sua pegmatite, based
on their occurrence as primary inclusions in magmatic garnet. Aqueous-carbonic inclusions are commonly secondary and neither
the CO2 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 metasediments adjacent to granites and the δD values greisen muscovites are variable,
but are as low as −134 per mil, indicative of fluid interaction with organic (graphitic) material. This suggests that the
aqueous-carbonic fluid inclusions represent fluids 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 fluid
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. Mineralized pegmatites are also typically of deeper plutonic
belts, and pegmatite-hosted deposits may contain cassiterite that is magmatic (crystallized from granitic melt) or is orthomagmatic-hydrothermal
(crystallized from aqueous or aqueous-carbonic fluids) in origin. The magmatic aqueous fluids (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 different 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 salinity fluids. Owing to the high chlorinity, tin is strongly partitioned in favour
of the vapour and cassiterite mineralization can form by of orthomagmatic-hydrothermal processes. Similar relationships between
the depth of emplacement and fluid composition also appear to apply to other types of granite-hosted deposits, such as different
types of molybdenum deposits.
Received: 8 September 1997 / Accepted: 28 October 1997 |
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