SEQUENCE OF CRYSTALLIZATION AND THE RELATIONSHIPS BETWEEN FLOW BANDING AND TRANSPORTATION OF IONS IN THE TOPAZ BEARING RHYOLITE OF TOPAZ MOUNTAIN, JUAB COUNTY, UTAH

Topaz Mountain, located in the Thomas Range of Juab County, Utah, contains extensive lava flows, domes, and tuffs. These structures, caused by the explosive eruptions of F-rich rhyolite lava, are approximately 6.5 million years in age, and are characterized throughout the area by complex flow banding. At outcrop scale, the flow banding is one of the most distinct features of Topaz Mountain and appears to have provided pathways for vapor transport, with crystal-lined vugs concentrated along the flow banding. Within the transported vapor, ions such as F, Si, O, Na, K, Fe, Mg, Ti, Ca, P, Al, Mn, Cl, Be, Sr, Li, Cr, Cs, Ba, Pb, Co, Ga, Sc, Y, Zr, Nb, Hf, Ta, Th, and U were able to flow into the vugs, creating not only phenocrysts of feldspar and quartz within the flow, but a plethora of vapor phase minerals within any open space as well. In addition to topaz, the most common vapor phase minerals include quartz, garnet, hematite, pseudobrookite, bixbyite, red beryl, and fluorite; many of which are uncommon in silicic volcanic rocks. We hypothesize that a direct correlation between flow banding and the orientations of crystallized phenocrysts will become apparent within the thin sections. We also hypothesize that the location of vapor phase minerals will also directly correspond to the position of various flow bands. Over the years, several specimens were collected for the purpose of research at UWG. From the samples collected at Topaz Mountain, a series of orthogonal thin sections were created. Pictures taken of each individual slide, with a polarizing microscope at low power, are examined for medium to large elongated crystals as well as evidence of flow banding within the thin section. A straight line is drawn down the longitudinal axis of the crystal, to determine angulation, and used to establish a pattern of consistent orientations with any flow banding present. The pictures are then examined for determination of crystallization sequences used to predict mineral formations and transportation. A timeline of mineralization will be established based on the shape, whether regular or irregular, of vugs housing vapor minerals. These vugs appear as holes within the thin sections and we hypothesize that these occurred after the flow had stopped moving.