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Abstract

Detailed study of the Late Cambrian Mt. Simon Sandstone, in the Illinois and southern part of the Michigan Basins as well as on the intervening Kankakee and Wisconsin Arches in Illinois and Indiana, reveals that the unit has undergone a complex diagenetic history with widespread alterations. Numerous events of authigenic mineral precipitation that occur throughout the study area and at paragenetically similar times include precipitation of early grain-coating illitic illite/smectite, subsequent potassium feldspar (as overgrowths and rhombic cement) and quartz and a late void-filling illitic illite/smectite. Dolomite (both ferroan and nonferroan), pyrite, barite, anhydrite and fluorite occur locally as minor cements. Dissolution of at least some authigenic cements, such as potassium feldspar and dolomite, also occurred throughout the study area. Mechanical compaction occurred throughout the alteration history of the Mt. Simon.

Homogenization temperatures for fluid inclusions in quartz overgrowths range from 100° to 130°C, and freezing point depression data indicate that the inclusions formed from solutions that were very saline (>20 wt % equivalent NaCl). Petrographic observations reveal that the quartz formed at about the same time as the potassium feldspar; previous radiometric dating of authigenic potassium feldspar indicates that it formed at about 400 Ma, during the Late Silurian to Early Devonian. Burial history reconstruction of the Mt. Simon Sandstone reveals that burial temperatures expected in the Mt. Simon Sandstone during Late Silurian to Early Devonian time were significantly less than the temperature of fluids from which quartz precipitated. Thus, it is likely that the fluids from which quartz precipitated migrated from other, probably deeper parts of the protoIllinois basin or other deep sources where fluid temperatures and salinities could be expected to be higher than within the shallower Mt. Simon Sandstone.

The widespread nature of at least some of the alterations in the Mt. Simon Sandstone taken together with the similar timing of alterations throughout the region and the fluid inclusion data suggest that diagenesis in the Mt. Simon Sandstone resulted from regional migration of fluids that were al times hotter than expected temperatures due to burial alone. Thus, the Mt. Simon Sandstone appears to have been a paleofluid-flow conduit through which thermally anomalous pore waters of varying composition migrated over time. Because it has served as a regional paleoflow conduit, it seems likely that solutions responsible for formation of economic deposits of base metals in the region may also have flowed through the unit. In fact, the Mt. Simon Sandstone may well have been a principal conduit through which ore-forming fluids migrated regionally from their source to sites where mineralizing processes resulted in ore deposits.

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