RI-70, Characterization of the Franklin Peridotite and Other Similar Intrusions in East-Central and Southwestern Minnesota

Abstract

Outcrops of peridotite adjacent to the Minnesota River near the town of Franklin in Renville County were sampled and petrographically characterized as part of a small study funded by the Minnesota Department of Natural Resources in 1997. That study obtained mineral separates with the intent of examining them for kimberlite indicator minerals. The results were not formally published, but rather summarized in an unpublished final report to the Minnesota Department of Natural Resources titled "Mineral Investigations of Franklin Kimberlites." In addition to petrographic and geochemical characterizations, ground magnetic traverses were made across the outcrop area in order to quantify the size and shape of the peridotite body. Based on simple ground magnetic surveys, the peridotite body is approximately 1 square kilometer (0.4 square mile) in area. The peridotite in the outcrops is extensively silicified, most likely by low-temperature alteration associated with lateritic weathering beneath Cretaceous sedimentary strata. Peridotite that is not silicified is composed of olivine (serpentinized), orthopyroxene, hornblende, magnetite, and minor spinel, phlogopite, ilmenite, and sulfide minerals; all the silicate phases are Mg-rich. The silicified peridotite contains abundant secondary quartz and chalcedony, but the silicification did not affect the Fe/Mg ratio, as both silicified and unsilicified peridotite have Mg numbers of 87 to 88. The Franklin peridotite is similar to ultramafic peridotite and pyroxenite bodies in east-central Minnesota, as well as the Cottonwood peridotite body intersected by drilling in northern Lyon County, 55 kilometers (34 miles) west–northwest of the Franklin peridotite and south of the Minnesota River valley. The peridotites in east-central Minnesota are between 1,770 and 1,791 Ma in age, whereas the age of the Franklin and Cottonwood peridotites is unknown.