ILLITE-SMECTITE GEOTHERMOMETRY OF THE PROTEROZOIC ORONTO GROUP, MIDCONTINENT RIFT SYSTEM

Characterization of the Nonesuch Formation, middle unit of the Protero zoic Oronto Group, as a potential hydrocarbon source for the Lake Supe rior basin portion of the Midcontinent Rift system requires an underst anding of the thermal maturity of the region and its relationship to t he thermal history. Illite/smectite (I/S) expandability data were coll ected from the Nonesuch Formation and the overlying Freda Sandstone an d compared with organic thermal maturity data; both data sets coupled with a thermal and burial history for the White Pine area of Michigan allow regional interpretation of maximum formation temperatures of the Nonesuch Formation and the Freda Sandstone with respect to time. Samp les collected from drill holes in northeastern Wisconsin display nearl y pure smectite within the lower Freda Sandstone trending abruptly to ordered I/S within the Nonesuch Formation. Regular trends of decreasin g expandability with depth occur in four other drill holes to the nort heast. Comparison of I/S expandability between similar stratigraphic i ntervals reveals a significant trend of increasing thermal maturity to the northeast, with the lowest thermal maturities observed in the Iro n River Syncline area just west of White Pine, Michigan. I/S geothermo metry suggests maximum temperatures in the Nonesuch Formation of 140-d egrees-C in Wisconsin, 115-degrees-C in the Iron River Syncline area, 160-degrees-C at White Pine, and 190-degrees-C near the southern porti ons of the Keweenaw Copper District. The geographic pattern of tempera tures determined from I/S geothermometry is identical to that determin ed from organic thermal maturity indicators in the Nonesuch Formation (Imbus et al., 1988, 1990; Hieshima and Pratt, 199 1; Pratt et al, 199 1; Mauk and Hieshima, 1992). Regular variations in I/S expandability with depth occur in the Freda Sandstone and the Nonesuch Formation nea r the southern limits of the Keweenaw Copper District. These variation s suggest a fossil geothermal gradient of 55-degrees-C/km and limit th e thickness of sediment above the Nonesuch Formation to approximately 3 km. In comparison, 3.6 km of Freda Sandstone are presently exposed n ear the Wisconsin border, and numerical modeling suggests a range of 4 -6 km of sediment overlying the Nonesuch Formation. None of the data i ndicate the presence of the Bayfield Group sediments above the Nonesuc h Formation at the time of clay diagenesis. Samples from White Pine su ggest a two-stage burial history: 1) clay reaction, possible hydrocarb on maturation, and copper-sulfide mineralization at maximum temperatur es above 100-degrees-C during the main riffing and burial event, follo wed by 2) fracturing, reverse faulting, and fluid circulation during a rift-terminating compressional event that may have allowed petroleum migration and native copper mineralization at temperatures below 100-d egrees-C. Abrupt changes in I/S expandability with depth and the prese nce of poorly crystalline I/S (greater than 80% expandable) and kaolin ite in the Freda Sandstone in Wisconsin appear to represent later over printing of the diagenetic assemblage by fluids that were probably coo ler and of differing composition than earlier diagenetic fluids. Howev er, the authigenic assemblage from the vicinity of White Pine, Michiga n, which includes up to 25% expandable I/S, appears to represent a dia genetic profile formed during the main rifting and burial event. There fore, these expandable I/S-type clays are essentially 1.0 billion year s old.