SYNTAXIAL SILCRETE CEMENTS OF THE ST. PETER SANDSTONE: A COMPARISON OF OXYGEN ISOTOPE ANALYSIS BY ION MICROPROBE AND LASER FLUORINATION

A detailed oxygen isotope study of detrital quartz (DQ) and authigenic quartz overgrowths from shallowly buried (<1 km) quartz arenites of the St. Peter Sandstone (in SW Wisconsin) constrains temperature and fluid sources during diagenesis.  Quartz overgrowths are syntaxial (optically continuous) and show multiple growth generations by cathodoluminescence.  Cores of DQ grains were separated from 53 rocks and analyzed by laser fluorination, resulting in an average δ¹⁸O of 10.0±0.2‰ (1SD, n=108).  Twelve thin sections were analyzed by CAMECA IMS-1280 ion microprobe (10 micron spot, analytical precision ±0.1-0.2‰, 1SD) and have an identical average δ¹⁸O(DQ) of 10.0±1.4‰ (1SD, n=90), but reveal true variability.  Silicate whole rock (WR) samples (DQ plus quartz overgrowth) from the same 53 rocks were analyzed by laser fluorination, giving δ¹⁸O between 9.8 and 16.7‰ (n=110).  Quartz overgrowths in thin sections from ten rocks were analyzed by ion microprobe and have an average δ¹⁸O of 29.3±1.0‰ (1SD, n=161). 

Given the similarity, on average, of δ¹⁸O for all DQ and for all quartz overgrowths, samples with higher δ¹⁸O(WR) values have more cement.  The volume percent of quartz cement in 53 rocks was calculated by mass balance.  There is from <1 to 21 vol. % cement, with one outlier at 35%.  Eolian samples have an average of 11% cement compared to marine samples, which average 4% cement. 

Two models for quartz cementation have been investigated.  Model 1 hypothesizes that quartz overgrowths formed from hydrothermal (50º to 110ºC) precipitation by ore-forming brines related to Mississippi Valley Type (MVT) mineralization.  Model 2 proposes that quartz overgrowths formed at low temperature (10º to 40ºC) as silcretes.  The homogeneity of the oxygen isotopic composition for quartz overgrowths rules out a systematic regional variation of temperature as predicted for MVT brines and indicates that quartz overgrowths from the St. Peter Sandstone formed as silcretes from meteoric water with δ¹⁸O values between -2 and -9‰ at temperatures of 10º to 40ºC.  The increased cement in eolian samples also supports this conclusion, as this increase may have resulted from dissolution of abraded grain boundaries and fine-grained dust in desert, eolian sand-forming environments, as well as from increased groundwater pH in arid environments.