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

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.