Re-Os Systematics of Cr-Pge Mineralized Zones in the Partridge River and South Kawishiwi Intrusions and the Layered Series at Duluth, Duluth Complex, Northeastern Minnesota

Abstract

This report is a portion of a project on the relationship between chromium and platinum-group elements (PGEs) in the Duluth Complex (Fig. 1). The rhenium-osmium and oxygen isotope data in this report were produced from the magnetic fraction of samples in Table 1, where possible. These data are used to evaluate origin of chromium and related PGEs from various parts of the basal zone of the Duluth Complex (Fig. 1). Chromium mineralization associated with PGEs in the Duluth Complex was first recognized by Sabelin and Iwasaki (1985, 1986) and Sabelin (1985, 1987) in drill hole Du-15 from the Birch Lake area (BLA). The presence of chromium spinels was noted earlier by Weiblen and Morey (1976) at the Spruce Road deposit. Severson (1995) identified chromium and platinum mineralization in drill hole SL-19A northeast of the Water Hen deposit. Also, Severson (1991) identified Cr-rich spinels in the Local Boy ore zone of the Babbitt deposit. Hauck et al. (in prep.; Hauck unpubl. data; Hauck et al., 1998) have identified a variety of Cr-rich spinels, e.g., Cr-magnetite, Cr-hercynite, Cr-Timagnetite, in the BLA, not all of which are directly associated with PGE mineralization. Cr-Timagnetite + Cr-pleonaste was identified by Sassani (1992) in the Fish Lake area of the Layered Series at Duluth. In addition, Heine et al. (1998) reported 1.64 % Cr2O3 in saprolite in drill hole 265 3/1 P2 in a Keweenawan-age (?) ultramafic body in Stearns County in central Minnesota. Samples for this study were collected from oxide-bearing, semi-massive, and massive oxide zones or layers to answer some fundamental questions about the origin of the Cr-PGE mineralization within the Duluth Complex. Results of the sample from Stearns County are not discussed in this report.