The possible bearing of the granite of the UPH Deep Drill Holes, northern Illinois, on the origin of Mississippi Valley ore deposits

Abstract

The Pb‐Pb, Th‐Pb, and U‐Pb systems of whole‐rock drill‐core samples of the granite of UPH record at least one postcrystallization event. Treatment of the data on a concordia plot indicate the timing of this disturbance was probably 260 ±35 m.y. ago and probably no older than 400 m.y. ago. The upper intercept age on concordia is 1416±20 m.y and the Pb‐Pb isochron age for the four least disturbed rocks plus the four potassium feldspars is 1451±19 m.y. These ages are in close agreement with Rb‐Sr whole rock and U‐Pb zircon ages determined by others and should be the age of granite formation. Considerable Pb loss is indicated for many of the shallower samples that have a more red rather than pink color, probably to a factor of two to three or more as a result of the disturbance, although the granite remains Pb‐rich today (40–80 ppm). In some other studies of disturbed U‐Th‐Pb systems, the radiogenic lead has been taken up by potassium feldspars; however, the Pb for most samples seems to have been lost from the system for the granite of UPH. One sample with an unusually high Pb content of about 195 ppm has the most radiogenic Pb of any sample but ‘normal’ values of Th‐U. This sample has obviously gained radiogenic Pb (by more than a factor of four) and confirms that Pb was indeed mobilized in parts of the system. Within the limits of the uncertainties, equal amounts of Pb were lost or gained in both the U‐Pb and Th‐Pb systems, which is compatible with mobilization being restricted to lead. There is some tendency, however, for the more affected samples to have lower Th‐U values and higher U contents. A complex model in which U is gained, Pb lost, and 208Pb lost preferentially to 207Pb and 206Pb cannot be ruled out. U and Th have somewhat different crystallographic locations and decay energies so that 208Pb might be in more accessible sites than the other Pb isotopes. Fission track mapping of the samples indicates most of the U today is located in submicroscopic sites along edges and cleavages of biotite and iron oxides so U is readily available for remobilization. Lead extraction with only minor U or Th removal may have been accomplished by chloride‐rich brines. Lead isotope data on the granite of UPH indicates that some of the Pb in the ore in the nearby Upper Mississippi Valley lead‐zinc district may have been derived from these granites. An even better match is made with the ores of southeastern Missouri, although a granite like that of UPH would only be one end member (most likely the radiogenic end member) of a mixed source. Curiously, similar aged and heavy metal rich granites from southeastern Missouri are not a good match for theckn nearby ores. Basement rocks such as the granite of UPH must be reconsidered as an important contributor to the ores of the Mississippi Valley, but it is unlikely that they constitute the sole source.