Abstract: Paragenesis of Authigenic Minerals in St. Peter Sandstone

I. Edgar Odom, T. M. Willand, R. J. Lassin

Owing to its initial high degree of mineralogic maturity (usually 99% quartz), the St. Peter Sandstone (Ordovician) is ideal for the study of the geochemical factors involved in diagenesis and the paragenesis of authigenic clay and other minerals. Quartz and perhaps some dolomite are the only primary minerals whose solution has directly influenced the pore-fluid chemistry of the St. Peter.

The St. Peter contains an impressive array of clay and other authigenic minerals, including illite, kaolinite, smectite, chlorite, K-feldspar, quartz, pyrite, dolomite, jarosite, alunite, and gypsum. Kaolinite is the dominant clay mineral in the main body of the St. Peter in most outcrop sections, whereas illite is dominant in subsurface section, except that in some areas where the St. Peter contains potable water there is abundant kaolinite. Authigenic K-feldspar is abundant in both outcrop and subsurface sections, especially at the base of the St. Peter, locally. Jarosite, alunite, and gypsum are found only in near-surface samples and appear to be related to "recent" weathering. Although outcrop and subsurface samples differ in gross mineralogy, the localization and abundance of aut igenic clay minerals, feldspar, quartz, and pyrite in various parts of a single section may be very complex, which indicates that past and perhaps even present pore-fluid chemistry bears a relation to permeability.

The stages of authigenic mineral formation in the St. Peter are as follows: (1) K-feldspar and illite formed from potassium-rich pore fluids, (2) smectite and chlorite formed as the potassium to magnesium ratio decreased, (3) kaolinite and pyrite formed when pore fluids became depleted in alkali and alkaline earth ions (acidic), (4) a second stage of illite formation occurred locally where acidic pore fluids dissolved some K-feldspar, and (5) quartz overgrowths formed last and during the preceding stages from silica derived mainly from pressure solution.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado