ABSTRACT

Paleogene arkoses of the Auvergne region of France represent the type arkose originally described by Brongniart (1826). They are alluvial-fan and fluvial deposits including traction-current sandstones (arenites) and matrix-rich debris-flow deposits (wackes). Locally, they have been 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 average Gazzi-Dickinson composition of unaltered arenites is Q₄₀F₆₀L₀, with 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 a teration. When secondary chert is included with detrital quartz and chert, the altered rocks have an average composition of Q₆₆F₃₂L₁, emphasizing silica enrichment associated with alteration. Holocene sands derived from crystalline basement of the Massif Central have the same provenance as the Paleogene sandstones. They have an average Gazzi-Dickinson composition of Q₄₆F₄₉L₅ with K/F = 0.52 and compare favorably with their unaltered ancient counterparts. Arkosic alluvium at Roosevelt Hot Springs, Utah is believed to represent a modern analog for alteration of the Auvergne sandstones. The alluvium has been altered and cemented with silica by geothermal waters at near-surface conditions in the epithermal zone.

Chemical analyses of 130 sand and rock samples demonstrate original sediment compositions and changes due to hydrothermal alteration. Holocene sands and unaltered Paleogene clastics are compositionally similar and show variation trends similar to feldspathic clastics derived from granitic basement in other basins. Silica enrichment in the altered sandstones causes dilution of all other elements as an effect of constant summation. Consequently, most element abundances decline proportionately with silica dilution. Na and Rb, however, are reduced below the levels predicted by dilution due to plagioclase and biotite destruction. As a result, K/Rb ratios are higher in the altered sandstones. Ba, S, SO₄, As, and Sb are enriched in the altered rocks by precipitation from hydrother al solutions. Barium is in barite as fracture-filling veins and scattered patches in sandstone matrix. Destruction of mafic minerals during alteration 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 sandstones.