The Yellow Dog peridotite and a possible buried igneous complex of lower Keweenawan age in the northern peninsula of Michigan

Partly serpentinized peridotite of early Keweenawan age crops out in two places along a 20-kilometer-long zone of positive aeromagnetic anomalies in northern Marquette County, Michigan. Most of the area is mantled by Pleistocene drift with few bedrock exposures.

Petrographic and electron microprobe studies show that the peridotite was originally a plagioclase lherzolite containing 40 to 50 percent olivine (Fo8₀) and approximately 10 to 15 percent each of enstatite (En₇₈Wo₀₄Fs₁₈) and diopsidic augite (En₄₇Wo₄₂Fs₁₁). The plagioclase varies from 5 to 10 percent, and according to Morris (1977) is labradorite with a composition of An_57-65. Major oxide minerals (4 to 6 percent) are ilmenite and magnetite. Sulfides comprise 1 to 2 percent of the peridotite and are chiefly pyrrhotite, pentlandite, and chalcopyrite.

Ground magnetic, gravity, and very low frequency electro-magnetic (VLF-EM) surveys have refined the location and magnitude of anomalies previously known only from aeromagnetic studies. These surveys together with soil geochemical studies suggest that peridotite, and possibly other mafic rocks forming a differentiated igneous complex, may occur throughout a belt 20 kilometers long (east-west) and 1 to 2 kilometers wide (north-south).

Differentiated igneous complexes in many parts of the world are hosts for copper, nickel, chromium, or precious-metal deposits. The peridotite in the area of this study is anomalously rich in copper and sulfur compared to world-wide averages for peridotite. Positive electro-magnetic anomalies found near the peridotite outcrops may be caused by sulfide-rich zones in the igneous rocks and should be explored further for copper-nickel mineralization.