Origin and Occurrence of Platinum Group Elements, Gold and Silver in the South Filson Creek Copper-Nickel Mineral Deposit, Lake County, Minnesota
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Origin and Occurrence of Platinum Group Elements, Gold and Silver in the South Filson Creek Copper-Nickel Mineral Deposit, Lake County, Minnesota
Published Date
1990-03
Publisher
University of Minnesota Duluth
Type
Technical Report
Abstract
The South Filson Creek Cu-Ni-PGE-Au-Ag mineral occurrence is located on the
western margin of the Duluth Complex in Lake County, northeastern Minnesota. The
occurrence of primary magmatic and late-stage, structurally controlled mineralization is
located in the South Kawishiwi intrusion of the Duluth Complex, approximately 2200 feet
above the basal contact. The primary host rock for the mineralization is a medium-grained
augite troctolite. Petrographic studies indicate that there were at least two episodes of
mineralization. Deposition of primary, coarse-grained, interstitial pyrrhotite, pentlandite, and
chalcopyrite occurred in "cloud zones". Primary mineralization was followed by the
introduction of hydrothermal fluids along fracture zones, as evidenced by the formation of
hydrous minerals, sulfide replacement textures and geochemical signatures suggestive of
remobilization. These late-stage fluids deposited secondary sulfides at redox boundaries
created by the primary sulfides. The secondary assemblage includes chalcopyrite, bornite,
chalcocite, digenite, covellite, violarite, sphalerite, mackinawite, valleriite, and the platinum
group minerals, all which occur in extremely fine, discontinuous veinlets that are rarely
recognizable in hand specimen. The veinlets were created by hydrofracturing of silicate
minerals due to a volume increase initiated by serpentinization of olivine. These veinlets are
always proximal to highly serpentinized fractures and are possibly associated with a proposed
NE-trending fault zone along the south branch of Filson Creek.
The copper-nickel ratio for the deposit is about 3:1. Platinum + palladium correlates
with high copper and sulfur. Also, high inter-element correlation between Cu, Ni, Pd, Pt and
Au suggests that secondary enrichment of these elements is local in extent and related to
faulting and redox boundaries. Statistical analysis suggests, given the available data, that infill
drilling could discover a significant quantity of mineralization.
The alteration assemblage associated with the secondary mineralization is serpentine,
biotite, stilpnomelane, iddingsite, chlorite, sericite, and clay minerals. The alteration is very
subtle and is best recognized in thin section. Both alteration and mineralized zones range in
thickness from less than one foot to 90 feet.
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The files attached to this record include the report (GMIN-TR-1989-15.pdf) and the Plates 1-3 mentioned in the report. Appendix C on the disk in the back pocket has yet to be located.
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Natural Resources Research Institute, University of Minnesota, Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811; Dept. Civil and Min. Engineering, University of Minnesota, 500 Pillsbury Drive S.E., Minneapolis, Minnesota 55455; Funded by the Greater Minnesota Corporation
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Kuhns, Mary Jo P; Hauck, Steven A; Barnes, Randal J. (1990). Origin and Occurrence of Platinum Group Elements, Gold and Silver in the South Filson Creek Copper-Nickel Mineral Deposit, Lake County, Minnesota. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/188508.
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