Geology and Structure of a Portion of the Partridge River Intrusion: A Progress Report

Abstract

Studies of the basal contact of the Partridge River intrusion, as deduced by compilation of all drill hole data and relogging of 37 drill holes, has indicated more structμre than previously recognized. Structure contour maps of the footwall rocks have been prepared showing the nature of the basal contact, the top of the Biwabik Iron-Formation, and the thickness of the Virginia Formation beneath the Partridge River intrusion. These data indicate that preexisting folds in the basement rocks at both Minnamax and Dunka Road exerted a strong control over the form of the base of the intrusion. Several northeasttrending normal faults and northwest-trending strike-slip(?) faults were al so delineated in this study which supports the half-graben model proposed by Weiblen and Morey (1980). A northeast~trending pre-Keweenawan fault has also been located in the Wetlegs area. Along this fault an inferred window of Biwabik Iron Formation is in direct contact with the Partridge River intrusion. Three oxidebearing ultramafic bodies {Longnose, Longear, and Section 17) are exposed at the surface along this zone. The spatial location of oxide-bearing ultramafics to areas where the iron formation is in direct contact with the Duluth Complex suggests that they may be genetically related. At least five major units within the basal portion of the Partridge River intrusion have been delineated for the Wetlegs area. They are present in 23 drill holes at Wetlegs and extend northeast into the Dunka Road Cu-Ni deposit and southwest into the Wyman Creek Cu-Ni deposit. From the base up, these units are characterized by: sulfide-bearing augite troctolite {175-1570 ft. thick); troctolite with abundant layers of picrite (melatroctolite), peridotite and dunite (450 ft. thick); a 250 ft. thick, fine-grained, mottled-textured troctolitic anorthosite ("marker bed" for the area); augite troctolite (400 ft. thick); and augite-bearing anorthositic troctol ite (250 ft. thick). To the northeast and southwest of Wetlegs, most of these units are present but the omission of particular units in either direction indicates an irregular stacked pattern. Establishment of an internal stratigraphy has provided an excellent opportunity to: 1) study the nature of any structural discontinuities present within the intrusion, 2) determine the extent and variability of intrusive lithologic units, 3) more fully understand any background geochemical variations that may be present within and between the l i tho logic units, and 4) better understand the origin of the Partridge River intrusion, the various Cu-Ni, CuNi- Ti and Fe-Ti deposits and their relationship to the origin of the Duluth Complex.