Strong tin enrichment in a pegmatite-forming melt |
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| Authors: | R Thomas J D Webster |
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| Institution: | (1) GeoForschungsZentrum Potsdam, Albert-Einstein-Strasse, Telegrafenberg, B120, 14473 Potsdam, Germany e-mail: thomas@gfz-potsdam.de, DE;(2) Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th St, New York, NY 10024-5192, USA, US |
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| Abstract: | To investigate processes of magmatic tin enrichment and cassiterite deposition, we studied the abundances of major, trace,
and volatile elements in a large number of rehomogenized silicate melt inclusions in quartz and topaz from a pegmatite body
at the Ehrenfriedersdorf Sn–W deposit. This deposit is associated with evolved Variscan granites of the central Erzgebirge,
southeast Germany. The melt inclusions are peraluminous; the molar aluminum saturation index (ASI) ranges from 1.15 to 2.0,
and many inclusions are characterized by a very high content of fluxing components and volatiles. Some inclusions contain
more than 20 wt% of H2O, F, Cl, and P2O5, plus Li as well as very high levels of Sn. Some rare, highly evolved fractions of late-stage pegmatite-forming liquid at
Ehrenfriedersdorf contained up to 7000 ppm Sn. The presence of hydrogen and methane in addition to water and carbon dioxide
in the vapor phase of the melt inclusions suggests a very low oxygen fugacity for some fractions of magma. The extreme levels
of tin, volatiles, and fluxing components in this magma had an important influence on processes of melt movement and cassiterite
precipitation. Melts, like these, that are high in volatiles and alkalis (sum of Li2O, Na2O, K2O, Rb2O, and Cs2O is >8 wt%) have low densities (≤1.8 g/cm3), low viscosities (<10 Pa.s at 700 °C), facilitate relatively rapid diffusion of ions through melts, and hence are excellent
solvents for extracting and transporting ore-forming elements.
Received: 1 February 1999 / Accepted: 19 January 2000 |
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