Cu-Ni-PGE mineralization of felsic dikes along the Grano fault in the Bathtub intrusion (Duluth Complex, Minnesota, USA)

Occurrence of felsic dikes is extremely common along the N trending Grano fault in the mafic, layered Bathtub Intrusion (BTI) of the Duluth Complex. Based on their geochemical characteristics, the felsic dikes with syenitic-monzonitic composition can be derived from partial melting of granitic rocks that are locally the footwall of the BTI. The dikes with granitic compositions are comparable with rhyolites and icelandites that formed contemporaneously with the emplacement of the BTI in the main magmatic phase of the Keweenawan rift event. The felsic veins are no carriers of precious or base metals but the exsolving fluids during the crystallization may have contributed to metal remobilization and precipitation. Both the felsic dikes and the host troctolite are intensively altered. Chemical composition of the rocks indicates that high temperature and saline fluids interacted with rocks in these alteration zones. These fluids can be connected to local hydrothermal cells developed during emplacement of felsic dikes. Late reductive fluids, produced by serpentinization of troctolite, also resulted in in situ modification of base metal mineralogy. We conclude that the Grano fault is not only a feeding channel for the mafic intrusions but a principal pathway for hydrothermal fluids and therefore it forms a zone where the original mineralogy of the magmatic Cu-Ni-PGE ore was modified.