Mineralogical constraints on magmatic and hydrothermal Sn-W-Ta-Nb mineralization at the Nong Sua aplite-pegmatite, Thailand
Linnen, Robert L.; Williams-Jones, Anthony E.
European Journal of Mineralogy Volume 5 Number 4 (1993), p. 721 - 736
published: Jul 22, 1993
manuscript accepted: Feb 19, 1993
manuscript received: Feb 14, 1992
ArtNo. ESP147050504004, Price: 29.00 €
Abstract The chemistry of silicate minerals provides some constraints on the origins of disseminated cassiterite and Ta-Nb-Ti oxide minerals, and vein-hosted cassiterite-wolframite mineralization at the Nong Sua aplite-pegmatite complex. Zoned tourmaline crystals in aplite and pegmatite have preserved their primary compositions. Similar compositions are observed in hydrothermal tourmaline, suggesting an overlap of P, T, ƒHF and cation activities between the magmatic and hydrothermal stages. All tourmaline types contain excess (Y-site) aluminum, which is typical of the pegmatite environment. A temperature dependence of Al substitution in tourmaline, proposed for pegmatites elsewhere, is shown to be implausible at Nong Sua, indicating that the alkali:aluminum ratio of the melt and fluid have a greater importance in controlling tourmaline composition than has previously been suggested. Muscovite compositions record a history of post-magmatic re-equilibration down to approximately 400°C. By comparing the fluorine contents of muscovite and tourmaline, which have a contrasting tendency to re-equilibrate, it is apparent that the ƒHF was relatively high at the magmatic and early hydrothermal stages, and was lower during late hydrothermal activity. Enrichment of Ca and Mg in the rims of magmatic and hydrothermal tourmaline indicates that there was infiltration or diffusion of these elements at the magmatic and hydrothermal stages. Such infiltration-diffusion may have also triggered cassiterite crystallization by increasing ƒO2. Textural and compositional data for garnet suggest that it crystallized from melt. Some garnet cores are defined by primary fluid inclusions. The garnet in these cores contains up to 1500 ppm SnO2 and provides a link between the late magmatic and the early hydrothermal evolution of the pegmatite system. Columbite, ixiolite and niobian-tantalianrutile are restricted to being aplite-pegmatite-hosted and commonly occur as inclusions in disseminated cassiterite, or in the cores of Sn-rich garnet. These oxide minerals are interpreted to have crystallized from melt.