Abstract The Middle Devonian Geneseo Formation and its lateral equivalents in the Northern Appalachian Basin are regarded as crucial secondary targets to the extensively explored Marcellus subgroup. High-resolution sedimentology, stratigraphy, and petrography have yielded differentiation of genetically related packages, comprised of distinct lithofacies with characteristic physical, biological, and chemical attributes. In addition, argon ion milling and nanoscale scanning electron microscopy of shale sections has shown that the pore structure of the Geneseo derives from pores defined by phyllosilicate frameworks, carbonate dissolution, and within organic matter. Intervals of silt-rich mudstones and muddy siltstones occur in multiple facies types and “interrupt” facies, reflecting background sedimentation. These deposits and their sedimentary features are interpreted as products of high-density fluvial discharge events. Pore morphology and distribution correlates with distinct mudstone lithofacies as a result of small-scale compositional and textural characteristics. Phyllosilicate framework pores are small triangular openings (100-1500 nm wide) and are the dominant pore type observed in hyperpycnites. Organic matter porosity is common (10-500 nm pore size) and dominates the organic-rich facies that represents “background” sedimentation with high organic content. Carbonate dissolution pores (50-500 nm wide) are observed in calcareous intervals and reflect partial dissolution of carbonate grains during catagenetic formation of carboxylic/phenolic acids.

Gold and uranium mineralization within the Archean Witwatersrand Basin exhibits a close association with carbonaceous matter. In order to understand the gold and uranium mineralization associations, it is necessary to understand something about the carbonaceous matter itself as well as the fluids involved in remobilizing both hydrocarbons and mineralization within the basin. Diagenetic maturation of primitive bacterial material released hydrocarbons that were able to migrate through the sediments of the economically important Witwatersrand Basin. Migrating hydrocarbons underwent polymerization and condensation due to the effect of ionizing radiation in the proximity of detrital uraninite during catagenesis. Isotopically heavy hydrocarbons precipitated out as bitumen seams where uraninite concentrations were high or as “fly-speck” bitumen around isolated uraninite grains, giving rise to sediment-hosted bitumen types. Lighter hydrocarbons, liberated during catagenic radiolysis, were incorporated into circulating basinal fluids. During late-catagenic/early-metagenic processes, hydrocarbon-bearing fluids migrating along aquifers (faults, bedding planes, and unconformities) precipitated insoluble carbonaceous matter, or distal/vein-related bitumen, in suitable sites. Light hydrocarbons precipitated as crosscutting veins where the vein intersected uraniferous seam bitumen, as bitumen nodules within late-stage quartz veins and as nodules and/or inclusion linings within fluid inclusions in late-stage quartz veins. Where fault zones permeated the Archean granitic basement, hydrocarbons were precipitated by radiolytic processes around high-U phases as nodular, granitoid hosted, or distal bitumen. Distal- and vein-related bitumen exhibits a higher degree of structural ordering than sediment-hosted bitumen as a result of elevated pressure-temperature conditions experienced during metagenic and fault-related processes. Liberated light hydrocarbons in basinal fluids formed organo-urano-gold-, thio-gold-, and/or organo-urano-gold-sulphide complexes. Circulation of these hydrocarbon-rich fluids and subsequent precipitation of bitumen and mineralization provide an explanation for the apparent hydrothermal component of Witwatersrand mineralization.