brines related to Mississippi Valley Type (MVT) mineralization and formation as silcretes at low temperature (10–30 C).

www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 71 (2007) 3812–3832The homogeneity of d O for quartz overgrowths determined by ion microprobe rules out a systematic regional variation of temperature as predicted for MVT brines and there are no other known heating events in these sediments that were never buried to depths>1 km. The data in this study suggest that quartz overgrowths formed as silcretes in the St. Peter Sandstone from meteoric water with d18O values of 10 ‰ to 5 ‰ at 10–30 C. This interpretation runs counter to con-ventional wisdom based on fibrous or opaline silica cements suggesting that the formation of syntaxial quartz over-growths requires higher temperatures. While metastable silica cements commonly form at high degrees of silica oversaturation following rapid break-down reactions of materials such as of feldspars or glass, the weathering of a clean quartz arenite is slower facilitating chemical equilibrium and precipitation of crystallographically oriented overgrowths of a-quartz. 2007 Elsevier Ltd. All rights reserved.Optically continuous silcrete quartz cements of the St. Peter Sandstone: High precision oxygen isotope analysis by ion microprob