Pliocene water-laid tuffs are widespread in western Utah and provide the host rock for beryllium-fluorite mineralization at Spor Mountain. Systematic study of the mineral and chemical composition of the tuff in the southern part of the Thomas Range was undertaken to examine regional effects of mineralization at Spor Mountain and to differentiate these from the effects of other petrologic processes. The water-laid tuff is zoned inward from the periphery of the Thomas Range; successive mineral zones are outlined by the vitric, zeolitic, and feldspathic facies. Study of thin sections indicates that the inward succession of facies reflects the alteration of glass to clinoptilolite and fin ally to potassium feldspar. The zonation of mineral facies and paragenesis is similar to that in other zeolitized tuffs where alkaline waters derived from leaching of glass are believed to have altered vitric tuff to zeolites and potassium feldspar. R-mode factor analysis was used to test and further develop a petrogenetic model for the water-laid tuff. Important factors in tuff petrogenesis are beryllium-fluorite mineralization, three stages of diagenetic alteration of glass, concentration of calcite versus silica, and concentration of detrital heavy minerals. Mineralization effects occur mainly as a halo around the Spor Mountain beryllium-fluorspar district. Diagenetic effects are most intense in the tuff of the interior of the Thomas Range. The highest concentration of calcite occurs mainly in tuff beneath Topaz Mountain; heavy mineral elements are concentrated locally. The concentration of some elements, notably chlorine, is affected by additional factors. Although the tuff in the southern part of the Thomas Range is mineralized only weakly or not at all, the beryllium-fluorite mineralization at Spor Mountain has left a strong imprint on the trace element content of the tuff. The elements F, Cs, Li, Be, Ga, Nb, Y, and to a lesser extent MgO and Rb were concentrated by mineralization and constitute a distinct geochemical signature which should be useful in searching for other beryllium-fluorspar mineral districts. Most of the mineralization-related elements form primary dispersion halos extending 1-2 miles (1.6-3.2 km) outward from Spor Mountain into unaltered tuff. Such large halos nearly triple the exploration target presented by a mineralized district like Spor Mountain. Diagenesis has been of major importance in changing the chemical composition of tuff. Soda, K,O, F, Rb, U, Mn, and Ph are depleted during zeolitization and MgO, Ba, Cr, and Sr are concentrated. Potassium feldspar diagenesis did not affect the chemical composition of the tuff. Possible release ofF, U, and Mn during zeolitization suggests a potential diagenetic contribution of these elements to ore deposits, notably the sedimentary uranium deposits found nearby.

for hydrothennal deposits must be sought around rhyolite vents and in a hypothesized pluton of alkali rhyolite composition beneath Spor Mountain; for groundwater deposits, reducing environments may occur in basin fill such as that of the Dugway Valley cauldron.

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Introduction 1 Purpose and Scope 3 Overview of Methods 3 Acknowledgements 3

Chemical classification of sandstone is worth attempting because modern analytical methods are producing extensive data sets on rock composition. Previous compilations and statistical studies have demonstrated the application of chemical data to problems in sandstone petrology, but classification has been largely ignored. Chemical data has value for classification, but the limitations of using chemical data for classification must be identified. Consideration of the processes of sandstone formation permits formulation of classification guidelines and systems for classification. Alternative systems for chemical classification can be then evaluated for their power to identify major types of sandstones and to facilitate genetic interpretation.The relative concentrations of three major oxide groups silica and alumina, alkali oxides, and iron oxide plus magnesia have been used to classify sandstones. The enrichment of SiOi by weathering and transport produces quartz arenites (orthoquartzites). Silica enrichment can be represented by SiO^A^Oa. Abundant alkalis (Na2O and K^O) characterize immature sandstones such as graywackes and arkoses; the ratio K2O/Na2O is determined by the net effects of provenance and diagenesis. The concentration of iron oxide and MgO is the net effect of provenance and the hydraulic concentration and diagenesis of detrital ferromagnesian and iron minerals. The relative concentration of iron oxide plus magnesia can be represented by (FeiO3+MgO)/(K2O+Na2O). These ratios form the basis for chemical classification, and may be represented by two 2-dimensional diagrams or one 3-dimensional (triangular) diagram. Variants of these ratios have been used in previous classification systems.With varying success, the classes quartz arenite, graywacke, and arkose can be defined by chemical composition. Guidelines for chemical classification are defined by plotting major oxide ratios for major classes of sandstone, and by examining compositional variation within and among the major classes. Thus, quartz arenites are here defined by log (SiO2/Al2O3) > 1.5 and graywackes are defined by log (SiO^AhOs) < 1 and log (K20/Na2O) < 0. Arkoses are defined by log (SiO2/Al2O3) < 1.5, log (K2O/Na2O) > 0 and log ((FeTO3+MgO)/(K2O+Na2O)) < 0. However, graywackes from passive continental margins have log (K2O/Na2O) > 0 and some arkoses have log (K20/Na2O) < 0. A fourth sandstone class, lithic arenite, is compositionally diverse; various classifications were tested with modest success, but lithic arenites cannot be identified with great confidence by chemical composition alone.Alternative classification systems proposed previously rely on various oxide combinations, ratios, plotting methods, definitions of petrographic terms, and class boundaries. Comparison of these systems with the guidelines offered here reveals strengths and weaknesses of each. No system is ideal, and classification is facilitated by using chemical data in conjunction with petrographic data, and by exploring relationships among chemical variables that mig...