Evolution of quartz cementation and burial history of the Eau Claire Formation based on in situ oxygen isotope analysis of quartz overgrowths

Individual quartz overgrowths in siltstone of the late Cambrian Eau Claire Formation (Fm.) are systematically zoned in oxygen isotope ratio (delta O-18). In situ analysis of delta O-18 was performed with 3 and 15 mu m beam spots by secondary ion mass spectrometer (SIMS) on detrital quartz grains and quartz overgrowths. These results from thin lenses within impermeable mudstones reflect samples that were sealed from basin-wide fluid flow and compliment previous studies of more permeable sandstones. Individual grains of detrital quartz (DQ) are homogeneous in delta O-18. The average delta O-18 values in fine-grained detrital quartz in mudstones and siltstones and in coarser-grained quartz in the Eau Claire Fm., Mt. Simon and St. Peter Sandstones (Ss.) are essentially identical at delta O-18 - 10 parts per thousand VSMOW, suggesting that detrital quartz is dominantly igneous in origin. The delta O-18 values of overgrowth quartz (OQ) of buried samples from the Illinois Basin are higher and quartz overgrowths are systematically zoned outward from the detrital cores. These gradients are similar to those from the underlying Mt. Simon Ss., and are best explained by increasing temperatures during burial. Pressure solution is evident in thin section and may have supplied significant silica for overgrowths. In contrast to the deeply buried samples from the Illinois Basin, quartz overgrowths in samples from the Wisconsin Arch are homogeneous and higher in delta O-18. Those overgrowths are interpreted as quartz cements formed in a near-surface environment (<40 degrees C), which is consistent with geological evidence that these rocks were only shallowly buried (<500 m). Based on these delta O-18(OQ) results and the modeled thermal history during burial of the basin, the earliest-formed quartz overgrowths were produced at low temperature from low delta O-18(water) around 450 Ma. The delta O-18 values in traverses of single overgrowths decrease by up to 9.1 parts per thousand, showing continued cementation with increased burial, pressure solution, and heating until similar to 250 Ma. In traverses of the outermost zone of some overgrowths, oxygen isotope values become constant or increase slightly, possibly due to clay mineral dehydration reactions or later fluid infiltration. We present a new cementation and basin evolution model, in which the delta O-18 of cement correlates to the age of formation and the late overgrowths formed between 270 and 250 Ma, during and/or after the migration of brines that formed the Pb-Zn deposits of the Upper Mississippi Valley District (270 Ma). Cementation around 270 Ma would have reduced permeability, possibly ending the flow of ore forming brines. (C) 2014 Elsevier B.V. All rights reserved.