CONTROLS ON MINERALIZATION IN THE BATHTUB INTRUSION, BABBITT DEPOSIT (DULUTH COMPLEX, MINNESOTA, USA): HALOGEN VARIATIONS IN APATITE AND COEXISTING FLUID INCLUSIONS

Accessory apatite from the basal mineralized troctolitic rocks of the Cu-Ni ± PGE Babbitt deposit (Bathtub intrusion, Duluth Complex, NE-Minnesota), is very useful in recording magmatic and hydrothermal processes, due to the preferred incorporation of trace and rare earth elements. Apatite halogen-contents are helpful in determining chemistries of fluids involved in late fluid-rich melts or metasomatic events. In general, the Babbitt mineralization follows the direction of an E-W trending anticline within the footwall (Virginia Formation). The distribution of mineralization is thought to be associated with the N-trending Grano Fault that was reactivated during the emplacement of the Duluth Complex.

Along an E-W-profile away from the Grano Fault analysis of apatites display pronounced trends in halogen-contents with decreasing Cl- (1.6 wt. %–0.2 wt. %) and increasing F-concentrations (2.2 wt. %–3.3 wt. %). Various apatite textures include euhedral inclusions of single crystals or accumulations in olivine and pyroxene (± growth zonation); euhedral elongated and larger fractured grains along olivine rims, and overgrowth textures; subhedral crystals between olivine-plagioclase and pyroxene-plagioclase interfaces; acicular inclusions in cumulus plagioclase and pyroxene. Concentric or oscillatory zoned apatites could result from magmatic differentiation; overgrowth textures could be linked to late hydrothermal processes.

Primary and secondary fluid inclusions and fluid inclusion planes hosted by quartz and apatite display various fluid chemistries: CO₂-and H₂O-rich primary fluid inclusion are found in quartz-rich layers of the Biwabik Iron Formation. Overlying Virginia Formation units host in addition to H₂O-rich fluid inclusions in quartz, CH₄-rich ones with trace amounts of CO₂. Troctolites (Duluth Complex), however, contain trail-bound H₂O-and CO₂-rich fluid inclusions exclusively hosted by apatite. Such a CO₂-rich nature of the fluid, the Cl-enrichment of apatite in ore-bearing troctolites and the presence of platinum-group minerals in hydrothermally altered portions, suggests the involvement of aqueous complexes (Cl- and vapor-rich) in the transportation and final redistribution of platinum-group elements. FWF (P23157-N21) financial support to A. Mogessie is acknowledged.