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The Taylor Creek Rhyolite, a group of Tertiary high-silica-rhyolite lava domes and flows in southwestern New Mexico, contains cassiterite-bearing veins whose tin was derived from the host rhyolite as it degassed, cooled, and devitrified immediately after emplacement. Theoretical considerations and studies of fumarolic deposits at many volcanoes worldwide indicate that tin is highly mobile in a vapor phase, probably as halogen complexes, thus favoring the occurrence of such auto-mineralization in a cooling-lava environment. Mass-balance calculations for the New Mexico situation indicate that much of the tin evolved during devitrification of the rhyolite cannot be accounted for in the mineralized deposits. Some of this “missing” tin almost certainly was dispersed into alluvium during erosion of mineralized parts of the lavas, and some may have been transferred to the atmosphere around fumaroles rooted in the cooling lavas. In addition, tin may have been lost to the atmosphere from Taylor Creek Rhyolite magma that was erupted in fountains. The recent recognition of fountain-fed fallback in the New Mexico rhyolite field suggests that this third means of moving tin out of erupting magma may indeed have contributed to the overall tin history in the Taylor Creek Rhyolite magma system.

Fountain-fed flows of silicic lavas are not well known, whereas mafic counterparts are known to be common as a result of observations of many eruptions of basaltic magma. Characteristic properties of silicic magmas that collectively tend to result in relatively high viscosity inhibit the occurrence of eruption columns whose fallback is hot enough to thoroughly weld and perhaps totally rehomogenize into a melt that subsequently feeds lava flows. However, high volumetric rates of eruption, high magmatic temperature (relative to solidus temperature), and any other conditions that help to reduce viscosity, in concert with factors that result in relatively brief periods of trajectory for lava clots, favor the formation of fountain-fed silicic lava flows. Fluorine-rich magma of about 830°C produced such silicic lavas in the Taylor Creek Rhyolite, and rocks with a similar mode of emplacement in other volcanic areas are herein hypothesized to be far more abundant than presently recognized. Possible examples elsewhere include large-volume sheets of silicic lavas in southwestern Idaho (eruption temperature of 950° to 1,100°C) and in Trans-Pecos Texas, where lava-flow and welded pyroclastic textures intermixed within individual eruptive units have led to confusion and difficulty in interpreting the mechanism of emplacement. Documentation of fountain-fed silicic lavas is rare; eruptions of silicic magmas are infrequent relative to the average human lifespan, and very few have occurred during historic time. Moreover, evidence of a lava-fountain origin may be only weakly preserved in the rocks so formed; the evidence also may be entirely lacking, as is commonly the case for the closely observed mafic examples.

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