The Mt. Simon Sandstone (Cambrian) in core from Hole UPH-3 in northwestern Illinois contains three distinct diagenetic zones. A zone with K-feldspar overgrowths characterizes the upper 51 m of Mt. Simon Sandstone. An intermediate, underlying sericite zone is about 126 m thick, and a lowermost feldspar zone is about 87 m thick. Authigenic K-feldspar in the Mt. Simon Sandstone occurs in several modes: overgrowths, rhombs, penetration-twinned crystals, chert-like aggregates surrounded by coarser K-feldspar, mosaics of anhedral to euhedral crystals, and as replacements of microcline. Sericite (I/S) occurs as pore-filling aggregates of minute flakes. Quartz overgrowths are common throughout these three diagenetic zones. K-feldspar was the first to form, closely followed by quartz, then by sericite, and lastly, possibly by a second addition of quartz overgrowth. The upper 6.1 m of Precambrian granite gneiss basement exhibits similar potassic alteration to the overlying Mt. Simon Sandstone. The uppermost 2.5 m of the basement rock is a probable weathering profile in which pedogenic clays were replaced by sericite (I/S) and chlorite. Sericite in the basement rock was likely formed during the same episode as in the Mt. Simon Sandstone, based on delta 18 O values. Authigenic K-feldspar occurs as pore-fillings, overgrowths, replacements of microcline, and intergrown with phyllosilicate. Below the profile it partially replaces plagioclase. Similar potassic alteration of the Mt. Simon Sandstone of UPH-3 occurs in cores of both UPH-1 and 2, indicating a lateral extent of about 7 km. A K/Ar date of 394.6 + or - 6 Ma was obtained from authigenic K-feldspar of the basal Mt. Simon Sandstone. This Early Devonian age is within the range of ages from authigenic K-feldspar in Ordovician tuffs of the Upper Mississippi Valley, suggesting that the K-feldspar is the product of a single, regional diagenetic event. K/Ar dates of 271.3 + or - 4.6 Ma, 254.4 + or - 4.6 Ma, and 214.1 + or - 4.6 Ma were obtained from authigenic sericite of the Mt. Simon Sandstone. The Devonian age for the K-feldspar and Permian ages for the sericite are nearly coeval with times of uplift of the Findlay and Pascola Arches respectively. These diagenetic episodes are attributed to regional migration of basinal brines caused by groundwater recharge on these uplifted arches. Potassium for both the K-feldspar and sericite possibly came from the Michigan and Illinois basins, respectively. Devonian and Permian dates for potassic diagenesis possibly coincide with two episodes of Pb-leaching of basement rocks at 400 Ma and 260 Ma. Thus the potassic fluids may have leached Pb and Zn from Precambrian basement, particulary paleosols, and then formed Upper Mississippi Valley Pb-Zn deposits.