Eruptive fountains of silicic magma and their possible effects on the tin content of fountain-fed lavas, Taylor Creek Rhyolite, New Mexico

Abstract

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 Duffield, W. A., 1990, Eruptive fountains of silicic magma and their possible effects on the tin content of fountain-fed lavas, Taylor Creek Rhyolite, New Mexico, in Stein, H. J., and Hannah, J. L., eds., Ore-bearing granite systems; Pedogenesis and mineralizing processes: Geological Society of America Speci...

Eruption and emplacement of the Yamakogawa Rhyolite in central Kyushu, Japan: A model for emplacement of rhyolitic spatter

“…In microscopic observation, devitrification has strongly overprinted primary textures. These lithofacies and microscopic features resemble the welded-agglutinate deposit of Taylor Creek Rhyolite ( Duffield, 1990). Therefore, these deformed clasts are interpreted to be fluidal spatter from fallout not pyroclastic flow.…”

“…This shows that this deposit is not underwent significant rheomorphism such as fountain-fed lava. From these characteristics, we interpreted that this deposit is welded agglutinate from stromborian fire-fountaining (e.g., Duffield, 1990;Turbeville 1992). …”

Physical volcanology, stratigraphy, and lithogeochemistry of an archean volcanic arc :

“…Lobate felsic lavas are more consistent with synvolcanic inflation of lava (Duffield, 1990) into unconsolidated material (Kano et al, 1991;. Bedded tuffs are deformed in contact with the intruded lava (Figure 114), however, the absence of felsic hyaloclastites imply that the sediments, while unconsolidated, were dry.…”

“…Nevertheless, these tuffs were deposited contemporaneous to the intrusion of the felsic lavas based on synsedimentary deformation of bedding in contact with the lobate felsic faciès (Figure 114), which can occur only if the sediments were unconsolidated at the time of the felsic intrusion (Duffield, 1990).…”

“…Fissural-fed lavas are typically high-volume, high-temperature events (Duffield, 1990) that form sheet flows (Bonatti and Harrison, 1988), which favor the lateral propagation of (felsic) lava. Bonnichsen and Kauffmann (1987) suggested that subaerial felsic lava flows originate from high-discharge, fissurai eruptions at temperatures upwards to 1000°C.…”