Lower and Middle Precambrian Geology of the Denham Area, Northern Pine County, East-Central Minnesota
1985-05-15
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Lower and Middle Precambrian Geology of the Denham Area, Northern Pine County, East-Central Minnesota
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1985-05-15
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Thesis or Dissertation
Abstract
The Precambrian rocks of the McGrath Gneiss, the Denham Formation and the Thomson Formation have been studied in outcrop and thin section within a several mile radius of Denham, Minnesota. This study of the petrology, metamorphism, structural geology, sedimentology and uranium potential of this very diverse geologic setting has aided in the determining of the various relationships between the three formations. The Lower Precambrian McGrath Gneiss is a quartzo-feldspathic gneiss which forms a broad anticlinorium extending from Mille Lacs Lake to southeast of Denham. An increase in cataclastic textures and recrystallization is evident in thin section as one approaches the contact of the McGrath Gneiss with the Middle Precambrian sedimentary rocks of the Denham Formation. The McGrath's near vertical foliation, defined by biotite and hornblende, corresponds linearly to the fold axes of the overlying Middle Precambrian rocks which suggests that cataclasis and folding probably took place during the same period of deformation. Unconformably overlying the McGrath Gneiss is the Middle Precambrian Denham Formation which consists of a sequence of quartz rich sedimentary rocks and minor amounts of volcanic rocks. Lithologies of the Denham Formation include schist, quartzite, metaarkose, volcaniclastics, pillow lavas and marble. At its type locality, in an abandoned Pleistocene river valley, approximately l.6km southeast of the village of Denham, the rocks of the Denham Formation have been extensively deformed and metamorphosed. Mineral assemblages within the Denham Formation represent lower amphibolite facies suggesting culmination of metamorphism at temperatures in the area of 550° to 650°C and pressures greater than 3.5kb. The rocks have been folded into east-west trending anticlines and synclines with fold axes, foliations, lineations and rod structures also striking east-west. Extensive recrystallization has resulted in nearly total obliteration of relict textures. At its type locality, near Thomson, Minnesota, the Thomson Formation consists of graywacke, siltstone and slate with approximately 2500 feet of section exposed. The Thomson Formation, seen near Denham, represents the southern most exposure of the unit. At the Denham locations the Thomson consists of interbedded metagraywackes and schistose rocks which have been metamorphosed to garnet-staurolite grade. Extensive recrystallization and metamorphism has resulted in near obliteration of original sedimentary structures other than bedding. A two fold increase in grain size can be seen in the Thomson Formation from Moose Lake south to Denham (Weiblen, 1964). Along the Soo Line Railroad tracks, approximately 2km east of Denham, the Thomson has been folded into a series of east-west trending anticlines and synclines with generally steeper dipping north limbs. A schistosity parallel to bedding and a crenulation cleavage axial planer to the folds can also be observed. The unconformity-vein type deposits of northern Australia and northern Saskatchewan have inspired interest in similar environments in Minnesota. Several localities in Pine and Carlton counties have yielded scintillometer readings of up to fifteen times the areas background readings. It appears that most of the potential sites for uranium deposits in Minnesota are found at or near the unconformable Precambrian contacts. Ten samples were powdered and analyzed for uranium and thorium. The results showed that the samples analyzed were not of economic importance. As of October, 1981, no deposits of commercial size have been found, however, some exploration is continuing.
Description
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Scott Nelson Neumann in partial fulfillment of the requirements for the degree of Master of Science, May 1985. There is 1 supplementary file also attached to this record, which contains Plate 1 referenced in the thesis.
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