Evidence for sulfide and Fe-Ti-P-rich liquid immiscibility in the Duluth Complex, Minnesota

Abstract

Apatite- and oxide (ilmenite + magnetite)-rich dunite, pyroxenite, peridotite, melatroctolite, and gabbro occur as transgressive bodies in the Boulder Lake area of the southern Duluth Complex. Nelsonite (apatite + ilmenite) is locally found as vein or dikelike bodies whose geometric form resembles veinlike massive Cu-Ni sulfide mineralization in the Babbitt deposit. Massive sulfide mineralization at Babbitt has been attributed to the separation of an immiscible sulfide magma from a tholeiitic parent. Nelsonite contains assessory minerals such as Zn-rich hercynitic spinel, biotite, zircon, baddeleyite, and sulfide minerals. Massive sulfide mineralization at Babbitt is characterized by the presence of minor (1-10 vol %) amounts of euhedral apatite, as well as Fe-Ti oxides and the same assemblages of accessory minerals that is found in the nelsonite. Textures developed in the nelsonite, together with the unusual composition, are supportive of an origin involving Fe-Ti-P and S liquid immiscibility. Apatite from the nelsonites and related rocks contain very low REE and Cl contents relative to that in the massive sulfide. These data are thought to reflect the evolution of a Cl- and REE-bearing fluid from the magma that gave rise to the nelsonite and a later separation of an Fe-Ti-P-rich immiscible liquid in comparison to the early separation of an immiscible sulfide-rich liquid from a basaltic magma in the Babbitt area.Sulfur, oxygen, and hydrogen isotope data are not supportive of an origin for the apatite-oxide-rich rocks that is linked to assimilation of metasedimentary country rock. However, elevated delta 18 O values (up to 13.2 in plagioclase) in many of the apatite-oxide-rich rocks, together with anomalously high An content of plagioclase, suggest that subsolidus interaction with a Ca-rich fluid derived from dehydration reactions in the pelitic rocks of the Virginia Formation has been important.Although an origin involving immiscibility of an Fe-Ti-P-rich liquid is strongly indicated for the nelsonites, the origin of spatially associated apatite-oxide-rich rock types is problematic. The SiO 2 content of most of the crosscutting bodies is less than 30 wt percent, and derivation by extreme closed-system fractional crystallization of a tholeiitic parent is considered unlikely due to predicted early saturation in Fe-Ti oxides. The oxideapatite-rich rocks may themselves be products of liquid immiscibility, reflecting a continuum in compositions with nelsonite as an end member.