Graduation Year

Spring 2014

Document Type

Open Access Senior Thesis



Reader 1

Jade Star Lackey

Reader 2

Robert Gaines

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© 2014 Alexa R. Zilberfarb


Underground coal fires commonly metamorphose or melt surrounding rocks at temperatures exceeding 1000°C. Numerous “baked” sandstone clinker deposits occur in the Cretaceous sedimentary rocks exposed in the San Rafael Swell, UT. This study examines clinker in three main localities: 1) East Carbon, UT, 2) Helper, UT, and 3) Emery, UT. The extent of pyrometamorphism in these areas is variably developed, but reached high enough temperature in Helper, UT to initiate melting and the production of paralavas. These paralavas were examined compositionally and mineralogically to determine melting conditions, peak temperatures, and mobility of different metals as a result of pyrometamorphism. X-ray diffraction and petrographic analysis showed that paralavas in the Helper locality contain the high temperature SiO2 polymorphs tridymite and cristobalite which alone indicate temperatures exceeding 875°C in several samples. Paralavas containing diopside+tridymite and cordierite+mullite+cristobalite provide more restrictive estimates of temperature as they form cotectic and eutectic assemblages in the SiO2-Mgo-CaO and SiO2-MgO-Al2O3, respectively. The assemblages indicate minimum temperatures of melting and metamorphism of 1330–1465ºC. The high temperatures of the paralavas generate increased metal mobility, potentially signifying a hazard if leached out into the environment

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