Browsing by Subject "Interstitial pyrrhotite"

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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
    (University of Minnesota Duluth, 1990-03) Kuhns, Mary Jo P; Hauck, Steven A; Barnes, Randal J
    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.