Abstract

Tin granites that host greisens have been extensively studied, but surprisingly little work has been carried out on the properties of the fluids during greisen formation. In the Triberg Granite Complex, southwestern Germany, it has been possible to reconstruct part of the chemical and physical evolution of these greisen-forming fluids.This granite complex contains an area of about 30 km 2 with cassiterite-bearing greisen assemblages and beryl-bearing pegmatites that have been locally affected by the greisen-forming fluids. The cassiterite occurs in narrow veins and in miarolitic cavities and is disseminated in parts of the granite. In places, beryl-bearing pegmatites, which are hosted by the same granite unit and texturally predate the greisen, interacted with the greisen-forming fluids. Production of secondary beryl, albite, phenakite, bertrandite, and kaolinite is associated with the replacement of the primary beryl in the pegmatites.The systematic variation of mineral assemblages in different parts of the leucogranitic part of the Triberg Granite Complex indicated temperature and chemical gradients in the fluids. Calculations of mineral stabilities and fluid inclusion studies on cassiterite and beryl support the existence of the gradients that were inferred from the field observations. Estimated pressure and salinity were nearly constant: pressure was estimated to be 1,500 bars and the measured salinity was 4 to 5 wt percent NaCl equiv. From the interior to the margin of the study area, the temperature decreased from 550 degrees to about 250 degrees C and the concentration of hydrofluoric acid (HF) in the fluid decreased from 0.06 to 0.002 moles/l.The best explanation of the greisen formation is a model that postulates a source of the mineralizing fluids in the center of the mineralized area. These fluids may have been released from a fluid trap in a cupolalike structure and may have penetrated radially outward into the cooled granite where they formed the greisen assemblages, affected beryllium pegmatites, and caused thermal and chemical gradients.

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