The Water Hen intrusion is an oxide-bearing (ilmenite + magnetite) ultramafic
intrusion (OUI) that is emplaced along a pre-basement fault into the troctolitic series rocks
of the Duluth Complex. The intrusion consists of medium-grained dunite and peridotite
and local pegmatitic pyroxenite approximately 1,600 ft. x 500 ft. x 700 ft. in size. Oxide
(>90% ilmenite) composes from 5-50% of the various lithologies. Sulfides are minor,
about 2-5%, and are predominantly pyrrhotite with minor cubanite, chalcopyrite and
pentlandite. Concentrations of 5-80% graphite also occur within the intrusion.
Surrounding the Water Hen intrusion is a zone of mixed lithologies (Mixed Zone)
consisting of the host rock troctolites, apophyses of OUI and local inclusions of footwall
rocks. The Mixed Zone (M) is dominated by >60% troctolitic rocks with OUI composing the
remainder. The OUI apophyses vary from 1-50 ft. thick and have sharp contacts with the
The troctolitic host rocks for the Water Hen intrusion consist of medium- to coarsegrained
troctolite to anorthositic troctolite (TA unit) and a troctolitic cyclically layered series
(TL unit). The cyclically layered series is similar to troctolitic layered rocks at Bardon Peak.
The individual cyclic layers are 6 in. to 10 ft. in thickness and the entire unit is over 300 ft.
thick. The An content decreases from An80 at the bottom of the unit to An60 near the top of
the unit. The individual cyclic layers are composed of ilmenite-dunite at the base and
grade upward to anorthositic troctolite. The bottom contacts are sharp and each
successive layer within the individual unit is identified by the occurrence of biotite or
clinopyroxene. In the bottom olivine-rich layer, the oxides (<5%) are ilmenite >>
magnetite. The sulfides in this same layer (3-5%) are coarse-grained with cubanite >
chalcopyrite > pentlandite >> pyrrhotite. In the more feldspathic layers, the sulfides (1-3%)
are fine-grained with chalcopyrite >> pentlandite = cubanite + pyrrhotite. The oxides (1-
5%) are also fine-grained with ilmenite >> magnetite.
The footwall rocks in the Water Hen area consist of very fine-grained
metamorphosed Virginia Formation and fine-grained hornfelsed basalt and/or troctolite.
There are >100 ft. of basalt or chilled margin rocks within the footwall. This mafic hornfels
commonly occurs between the Virginia Formation and the TA unit. Orthopyroxenite dikes
and dikelets also occur in the mafic hornfels. These dikes contain anomalous PGEs and
secondary sulfide mineralization.
The copper-nickel sulfides are primary igneous sulfides associated with the
troctolitic rocks. Violarite, pyrite and secondary magnetite in cross-cutting veinlets and
other secondary sulfides indicate that the primary sulfides were altered and remobilized by
a later event. Cu:Ni ratios have a bimodal distribution that is not followed by the PGEs.
However, Cu, Ni, Ag, Au, Pt, Pd are all highly correlated with each other. This high interelement
correlation suggests that the late-stage (secondary) remobilization locally
redistributed and reconcentrated these elements.
The files attached to this record include the report (GMIN-TR-1989-17.pdf) and the Plates 1-12 mentioned in the report. Appendices A-C on the disk in the back pocket have yet to be located.
Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442, Dept. Civil and Mineral Eng., University of Minnesota, 500 Pillsbury Drive S.E., Minneapolis, Minnesota 55455; Dept. of Geology, University fo Minnesota, Duluth, Duluth, Minnesota, 55812; Funded by the Greater Minnesota Corporation
Strommer, James; Morton, Penelope; Hauck, Steven A; Barnes, Randal J.
Geology and Mineralization of a Cyclic Layered Series, Water Hen Intrusion, St. Louis County, Minnesota.
University of Minnesota Duluth.
Retrieved from the University of Minnesota Digital Conservancy,
Content distributed via the University of Minnesota's Digital Conservancy may be subject to additional license and use restrictions applied by the depositor.