The mineralogical, chemical, and isotopic compositions were determined for a white tripoli from the footwall of the Jurassic U??rku??t Mn-oxide ore deposit in the Bakony Mountains, Hungary. The tripoli consists of quartz and chalcedony, with SiO2 contents up to 100 wt.%; consequently, trace-element contents are very low. Oxygen isotopes and quartz crystallinity indicate a low-temperature diagenetic origin for this deposit. The tripoli was formed by dissolution of the carbonate portion of the siliceous (sponge spicules) Isztime??r Limestone. Dissolution of the carbonate was promoted by inorganic and organic acids generated during diagensis and left a framework composed of diagenetic silica that preserved the original volume of the limestone layer. The relative enrichment of silica and high porosity is the result of that carbonate dissolution. The silty texture of this highly friable rock is due to the structurally weak silica framework. ?? 2005 Elsevier B.V. All rights reserved.
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A porous silica rock ("tripoli") in the footwall of the Jurassic U??rku??t manganese deposit, Hungary: Composition, and origin through carbonate dissolution