BH250-129
Title
Subject
Description
Minor Minerals: pyrite, biotite, iron-titanium oxide
Alteration: Chlorite, sericite,
BH250-129 is a sample from the Duluth Gabbro near Nickel Lake, Minnesota, part of the Duluth Complex—one of the largest mafic layered intrusions in the world, formed during the Midcontinent Rift. These rocks provide critical insights into magmatic differentiation, crystal settling, and the large-scale evolution of rift-related magmatism.
The rock is mineralogically dominated by plagioclase, clinopyroxene, and orthopyroxene, consistent with a typical gabbroic composition. However, what makes this sample particularly significant is the extensive alteration observed. The plagioclase feldspars show clear evidence of fluid interaction, indicating that hydrothermal fluids passed through the rock after crystallization. These fluids altered the primary minerals and introduced new chemical components into the system.
This fluid activity is directly linked to the presence of sulfide mineralization, represented by pyrite in the sample. The sulfides likely formed as metals were mobilized and concentrated by these fluids, either during late-stage magmatic processes or subsequent hydrothermal circulation. Texturally, this may be expressed as alteration halos, replacement textures, or sulfide grains associated with fractures and grain boundaries.
Although the Duluth Complex is globally significant for its nickel, copper, and platinum-group element (PGE) deposits, the Nickel Lake area itself is considered relatively weakly mineralized. This makes BH250-129 especially useful as a teaching example: it captures the processes of fluid-driven alteration and early-stage sulfide formation, even in areas where economic concentrations were not fully developed.
Overall, this sample highlights the important link between magmatic systems and hydrothermal processes. It demonstrates how primary igneous minerals can be modified by later fluids and how these interactions can lead to the redistribution and concentration of metals, key steps in the formation of ore deposits within large igneous complexes.
Coverage
GPS Coordinates: 93°27'43.79"W, 47°36'27.23"N