SUBSURFACE GEOLOGY AND MCR INTRUSIONS OF CHIPPEWA COUNTY, WI

The Precambrian geology of Chippewa County, WI is incredibly diverse but exposure is limited to weathered outcrops along streams. To better understand the subsurface geology, ~600 meters of exploration drill core, possessed by the Wisconsin Geologic and Natural History Survey at the Mt. Horeb Research Collections and Education Center (MHRCEC), were examined. Of the of cores logged, CH-317, which extends to 289 m, contains the contact with the Cambrian Mt. Simon Formation at 48 m with a thick saprolite horizon penetrating to 73 m into the Precambrian rock. Most commonly, the saprolite is developed in amphibolite, but here most of the horizon is granitic. Similar to field observations of this horizon, the plagioclase and orthoclase are weathered to green clays. Below the saprolite, biotite-feldspar and amphibole-feldspar gneiss extends to the end of the core with foliation alternating from 45° to 85°. Within the gneiss are several granitic intrusions. A 44 m thick brecciated sulfide-containing zone, beginning at 159 m, where quartz veins containing pyrite are abundant. While this core is a good representation of the diversity of Precambrian lithologies in Chippewa County, the other cores logged also contain granitic gneisses and mafic intrusions.

Mafic intrusions can also be observed in several surface outcrops in Chippewa and adjacent counties. A previous study (Myers, 1980) proposed these mafic intrusions are related to the ~1.1 Ga Mid-Continent Rift (MCR) and Chan (1991) reported paleomagnetic dipoles suggest the exposed dikes represent two pulses of intrusion, one at 1.1Ga and the second at 1.0 Ga. Samples of mafic intrusions were collected from previously mapped locations and the MHRCEC cores. These samples were analyzed using an X-Ray Fluorescence Mass Spectrometer (XRF) and found to be an average of 48 wt% SiO2 and similar major element compositions to those published from the volcanics of the MCR.