ESTIMATING CONCEALED LITHOLOGY UNDER GLACIAL AND PALEOZOIC COVER IN THE WISCONSIN MAGMATIC TERRANES WITH GROUND-BASED MAGNETIC PROFILE DATA

The Wisconsin magmatic terranes (WMT) are comprised of two terranes that were accreted to the Archean Superior craton in the Early Proterozoic Penokean Orogeny. To the north lies the Pembine-Wausau terrane (PWT), a volcanic island arc assemblage with abundant tholeiitic to calc-alkaline metavolcanic rocks deposited from 1889 to 1860 Ma. These bimodal volcanic rocks, almost entirely concealed by glacial drift, host a number of significant volcanogenic massive sulphide (VMS) deposits that are associated with felsic volcanic centers. To the south and separated from the PWT by the Eau Pleine shear zone (EPSZ) is the Marshfield terrane (MT). This terrane consists of an interlayered sequence of calc-alkaline volcanic rocks and sediments deposited at about 1870-1860 Ma on an Archean gneissic basement. Small, possibly co-magmatic, intrusions suggest it was also part of an evolving arc system. While glacial drift is thin to nonexistent south of the EPSZ, much of this terrane is concealed by a relatively thin veneer (10-50 m) of Cambrian sandstone.

In order to better understand the distribution of volcanic rocks in the WMT and their relation to ore deposits; over 5000 km of Earth’s total magnetic field data has been acquired in the WMT using a truck-mounted magnetometer. These data are acquired at a height of 3.5 m at horizontal intervals of 1 – 3 m and have much higher information content than aeromagnetic data. Profile plots of these data display anomalies with various textures that, in some cases, are unique to a lithology over which the data was acquired. In addition, when acquired over a lithology concealed by thin (<100 m) relatively nonmagnetic glacial drift, sediments, or sedimentary rock, magnetic data textures due to the concealed lithology may be observable and identifiable. In the PWT, felsic centers associated with known VMS deposits as well as potential felsic centers can be defined in these data in a number of cases. The Flambeau VMS deposit displays a small uniform positive magnetic anomaly and similar anomalies can be seen in other locations associated with felsic centers. In the MT, geologic mapping along the Eau Claire and Black rivers can be extended for some distance beneath Cambrian sandstone. Several examples are presented from the Flambeau VMS deposit region and near the aforementioned rivers.