| OCR Text |
Show COLLEGE OF MINES & EARTH SCIENCES UNDERGRADUATE RESEARCH ABSTRACTS Jonathan Casey Root 50 GEOLOGICAL AND PETROLOGICAL ASPECTS OF GARNET-PYROXENE-SPINEL SKARNS AND LUDWIGITE-BEARING PERICLASE MARBLE IN THE SOUTH ALTA CONTACT AUREOLE, UTAH Jonathan Casey Root (John R. Bowman) Department of Geology and Geophysics University of Utah Intrusion of granodiorite m a g m a at Alta, Utah has produced extensive contact metamorphism and skarn mineralization in siliceous dolostone wallrocks. Both the contact metamorphism, particularly the development of the inner periclase zone, and skarn formation result from infiltration of water-rich fluids from the granodiorite Alta stock (Cook and Bowman, 2000). A series of sills (0.5-2m thick) of granodiorite composition, and often with pegmatite texture, intrude these carbonate wallrocks and extend up to 150m from the pluton contact. Detailed mapping shows that mineralogically-zoned skarns, ranging from <0.1 to 2m in thickness, are developed at the contact between these sills and periclase (now replaced by brucite) marbles. The mineralogical zoning sequence within these skarns is: garnet (adjacent to the sill), pyroxene, pyroxene + spinel, spinel + olivine, and the borate mineral, ludwigite (within periclase marble). Garnet is not always present. Clintonite sometimes occurs with pyroxene and spinel. Thin (<0.02m) bands of ludwigite extend into periclase marbles, largely concordant to bedding, for distances up to 10's of meters beyond the tips of sills. Electron microprobe analyses show garnets are grossular (Grs)-andradite (Adr) solid solutions, with compositions ranging from Grs29-Grs62 and Adr32-Adr69. Pyroxenes are predominantly diopside (Di)-es-seneite (Fe-tschermaks) solid solutions and are diopside-rich (Di63-Di100). Pyroxenes within the pyroxene zone contain less than 10 mole percent hedenbergite (Hd), and are enriched in Di (and depleted in tscher-maks components) as distance from the intrusion contact increases; however, pyroxenes in contact with garnet can be Hd-rich (Hd14-Hd35). In one sample (12-2d), individual pyroxene grains are systematically zoned from Di-rich cores to tschermaks-rich rims, suggesting a change in metasomatic fluid chemistry during mineral growth. Spinels exhibit Fe-depletion (and Mg-enrichment) with increasing distance from sills. Olivines are predominantly forsterite (Fo)-rich (Fo92-Fo95). Additional chemical variations in ludwigite were also found: ludwigite grains with euhedral and blocky habits are typically Al-poor and exhibit depletion in Fe (and enrichment in M g ) as distance from the intrusion contact increases. Samples with a fibrous habit are relatively Fe-deficient and contain significant amounts of aluminum. The geological and penological relationships indicate development of silicate skarn and subsequent development of periclase + ludwigite marble through progressive interaction between the marble wallrocks and infiltrating water-rich (and B-bearing) fluids initially equilibrated (quartz-saturated) with the Alta granodiorite. Cook, S. J. and Bowman, J. R., 2000, Contact metamorphism surrounding the Alta stock-Fluid rock interaction accompanying metamorphism of siliceous dolomites: Journal of Petrology, v. 41, p. 739-757. |
