GEOLOGICAL AND PETROLOGICAL ASPECTS OF GARNET-PYROXENE-SPINEL SKARNS AND LUDWIGITE-BEARING PERICLASE MARBLE IN THE SOUTH ALTA CONTACT AUREOLE, UTAH

Intrusion of granodiorite magma 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, spinel, and 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-andradite solid solutions, with andradite contents ranging from Adr₇₃-Adr₈₅. Pyroxenes are predominantly diopside-hedenbergite solid solutions and are diopside-rich (Di₆₁-Di₈₇). Pyroxenes coexisting with spinel and clintonite can contain significant Ca- or Fe-Tschermak components (up to 16 and 11 mole %, respectively. At one location analyzed, both spinel and ludwigite exhibit Fe-enrichment (and Mg-depletion) toward the sill. Additional chemical variations in ludwigite were also found: ludwigite with fibrous and needle-like crystal habits can contain measurable amounts of Al (< 0.34 moles) and Ti (< 0.08 moles). The geological 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.