Potassium Distribution and Metasomatism In Pelites and Schists: How and When, Relation To Postdepositional Events

>Abstract: A large global database for over 9000 Archean to Holocene pelites in numerous basins has been developed to evaluate diagenetically unaltered and altered compositions. The mineralogy and geochemistry of Quaternary muds and ancient mudstones provide first-order controls on diagenetic potassium metasomatism in pelites. Clay mineralogy of Quaternary muds is dominated by smectite and kaolinite with minor detrital mica and illite. Thus, K2O is < 4%, K/Al is < 0.25, and K/Rb is < 220 in most Quaternary muds. With diagenetic alteration of clays to illite–smectite and illite, K2O content increases to 4–8% and K/Al rises to 0.2–0.5 or more. In extreme potassium metasomatism, authigenic K-feldspar develops. Sources of potassium are internal to shale–sandstone units wherein detrital K-feldspar is dissolved during diagenesis, providing K for clay illitization in closed-system (isochemical) alteration as in some Gulf Coast and North Sea Basin cases. Potassium in amounts greater than that available from closed-system alteration is provided from extraformational circulating brines in open-system diagenesis. K/Rb, with diagenesis, varies according to whether the metasomatizing fluid was relatively poor in Rb (seawater derived brines) or richer in Rb as in many hydrothermal waters. Diagenetic fluids may originate from seawater, or meteoric water enriched in cations by water–rock interactions along their subsurface migration paths and/or by dissolution of evaporites. Potassium metasomatizing fluids penetrate low-permeability pelites through faults, fractures, and microfractures and by diffusion in the mineral matrix. Typically, K-metasomatism, possibly with Pb, Zn, Cu, Ba, and F mineralization, occurs long after sediment deposition and burial at temperatures of 50° to 150°C+, related to tectonic activity and fluid movement. Different modes of metasomatism are recognized and correlated with geological setting. In the passive-margin Gulf Coast Tertiary basin, open-system K-metasomatism, where present, is limited to areas adjacent to steeply dipping growth faults which act as conduits for episodic pulses of altering fluids migrated upward from deeper basin sources. In contrast, widespread pervasive open-system K-metasomatism produced by continuous long-term gravity, advective, and diffusive flow of brine from the late Paleozoic Appalachian–Ouachita–Wichita Mountains highland to altered earlier Paleozoic shales plus sandstones, bentonites, and carbonates in the Illinois and Anadarko–Ardmore foreland basins. This mode of K-alteration following major mountain-building events is characteristic of many Precambrian and Paleozoic collision orogens and associated foreland basins. Another mode is in rift basins with no external drainage in which indigenous waters are evaporated to brines. These brines cause albitization of K-feldspar to yield potassium for illitization of detrital clays.