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