Paper No. 4
Presentation Time: 2:15 PM

A GEOCHEMICAL AND PETROLOGICAL STUDY OF MAFIC DIKES AND INTRUSIONS ASSOCIATED WITH THE EAGLE NI-CU-PGE DEPOSIT IN THE BARAGA BASIN, UPPER MICHIGAN


DUNLOP III, Matthew, Department of Geological Sciences, Indiana University, 1001 E 10th Street, Bloomington, IN 47405, RIPLEY, Edward M., Department of Geological Sciences, Indiana Univ, 1001 East 10th St, Bloomington, IN 47405 and LI, Chusi, Department of Geological Sciences, Indiana University, Bloomington, IN 47405, mdunlop@umail.iu.edu

The Baraga Basin in northern Michigan is comprised of Paleoproterozoic meta-sedimentary rocks that host a number of mafic intrusions and dikes, including the Eagle sulfide ore-bearing intrusion (1107.3 ± 3.7 Ma). The Eagle intrusion hosts a high grade Ni-Cu-PGE ore deposit, and is part of a belt of mafic intrusive bodies extending from northwest to southeast across the Baraga Basin. This belt is bracketed to the north and south by similar-trending Cu-depleted mafic dikes. A large number of Cu-enriched mafic dikes (of the Baraga Dike Swarm) are present within the basin, and are oriented northeast-southwest. Also present is a series of Cr- and Ni-enriched mafic dikes which trend northwest-southeast.

Samples from various types of mafic dikes and intrusions were analyzed for PGE. The data shows that the Cu-depleted mafic dikes are also depleted in all PGE+Au, suggesting that depletion of chalcophile elements may have resulted from magmatic rather than hydrothermal processes. Furthermore, mafic intrusive bodies located along the Eagle trend and between the Cu-depleted dikes, specifically the Boulderdash and Roland Lake intrusions, show extremely low Pd/Ir ratios and high Pt/Pd ratios relative to the more abundant Cu-enriched dikes located throughout the Baraga Basin.

A comparison of Boulderdash and Roland Lake shows that the two intrusions have similar Pt/Pd and Pd/Ir ratios, but with Boulderdash being more depleted in all PGE. Additionally, Roland Lake appears to be related to a more primitive parental magma, crystalizing olivines ranging from Fo 75 to Fo 85, while olivines in the Boulderdash intrusion range from Fo 61 to Fo 69. Boulderdash also shows a small population of Fo 55 olivine crystals, suggesting a possible mixing of two different magmas. Roland Lake olivines show a much steeper positive correlation on a Ni vs Fo plot than olivines in the Boulderdash intrusion. However, both intrusions show a nearly identical slope on a Mn vs Fo plot.

The parental magma to the Cu-depleted dikes likely experienced large scale sulfide saturation via crustal S contamination, while the Cu-enriched dikes’ parental magma likely did not. The Roland Lake and Boulderdash parental magmas are more primitive than the Cu-enriched and -depleted dikes, and do not appear to have undergone extensive sulfide saturation.