HOLOCENE AND PALEOGENE ARKOSES OF THE MASSIF-CENTRAL, FRANCE - MINERALOGY, CHEMISTRY, PROVENANCE, AND HYDROTHERMAL ALTERATION OF THE TYPE ARKOSE

Paleogene arkoses of the Auvergne region of France represent the type arkose originally described by Brongniart (1826). They are alluvial-fa n and fluvial deposits including traction-current sandstones (arenites ) and matrix-rich debris-flow deposits (wackes). Locally, they have be en extensively altered by geothermal waters related to nearby Tertiary -Holocene volcanic activity. The alteration is typified by leaching of detrital grains and precipitation of ubiquitous chert cement. The ave rage Gazzi-Dickinson composition of unaltered arenites is Q40F60L0, wi th K/F (potash feldspar/total feldspar) = 0.63. Altered arenites have an average composition of Q55F43L2, with K/F= 0.73, reflecting loss of plagioclase due to intense alteration. When secondary chert is includ ed with detrital quartz and chert, the altered rocks have an average c omposition of Q66F32L1, emphasizing silica enrichment associated with alteration. Holocene sands derived from crystalline basement of the Ma ssif Central have the same provenance as the Paleogene sandstones. The y have an average Gazzi-Dickinson composition of Q46F49L5 with K/F = 0 .52 and compare favorably with their unaltered ancient counterparts. A rkosic alluvium at Roosevelt Hot Springs, Utah is believed to represen t a modern analog for alteration of the Auvergne sandstones. The alluv ium has been altered and cemented with silica by geothermal waters at near-surface conditions in the epithermal zone. Chemical analyses of 1 30 sand and rock samples demonstrate original sediment compositions an d changes due to hydrothermal alteration. Holocene sands and unaltered Paleogene clastics are compositionally similar and show variation tre nds similar to feldspathic clastics derived from granitic basement in other basins. Silica enrichment in the altered sandstones causes dilut ion of all other elements as an effect of constant summation. Conseque ntly, most element abundances decline proportionately with silica dilu tion. Na and Rb, however, are reduced below the levels predicted by di lution due to plagioclase and biotite destruction. As a result, K/Rb r atios are higher in the altered sandstones. Ba, S, SO4, As, and Sb are enriched in the altered rocks by precipitation from hydrothermal solu tions. Barium is in barite as fracture-filling veins and scattered pat ches in sandstone matrix. Destruction of mafic minerals during alterat ion released Fe, Co, Cr, Ni, Zn, Pb, and Th that later precipitated as oxides and sulfides in fractures and locally in chert matrix in the s andstones.