Possible crustal contamination of Midcontinent Rift igneous rocks: examples from the Mineral Lake intrusions, Wisconsin

Abstract

Interlayered mafic–telsic intrusions from the Mineral Lake intrusive complex in northwest Wisconsin reflect the typical bimodal basalt–rhyolite compositional pattern of the Midcontinent Rift flood basalt province in the Lake Superior region. The later felsic intrusions were emplaced between the mafic intrusions and overlying basalt flows, and postemplacement fractional crystallization produced gradational mineralogical and geochemical variations. Isotopic and trace-element data for the Mineral Lake intrusions are consistent with mantle sources for both mafic and felsic intrusions, with compositional differences explained by the extent of fractional crystallization and crustal contamination or mantle source characteristics.εNd–εSr plots of analyzed Midcontinent Rift igneous rocks define three largely separate isotopic fields that suggest separate sources. However, the spread in isotopic data and a spider diagram plot of mafic samples from the εNd = εSr = 0 field suggest a crustal component and derivation from depleted rather than chondritic mantle. Evolved felsic rocks plotting in two negative εNd – positive εSr fields can be explained by derivation from separate enriched mantle sources or crustal contamination or both.

The Eagle and East Eagle sulfide ore‐bearing mafic‐ultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution

“…The recent discovery of the Eagle Ni‐Cu‐PGE deposit in Upper Michigan represents a major breakthrough in the exploration for magmatic sulfide deposits in the Midcontinent Rift System (MRS) (Figure 1a). Prior to the Eagle discovery, sulfide mineralization in the MRS was known to occur mainly in the basal zones of large layered mafic intrusions within the Duluth and Mellen complexes [ Klewin , 1990; Seifert et al , 1992; Miller and Ripley , 1996; Ripley et al , 2007]. The sulfide deposits in these large sheet‐like mafic intrusions generally have low grades of Ni (<0.2 wt %) and Cu (<0.6 wt %) and were considered economically unattractive until recent technological advancements.…”