FAYALITE AS A PRODUCT OF SERPENTINIZATION OF PERIDOTITE, DULUTH COMPLEX, MINNESOTA

We have discovered fayalite (Fa53-79) as a product of the serpentinization of two samples of peridotite (olivine Fa38-43) from the Layered Series of the mafic Duluth Complex, Minnesota. Phyllosilicate alteration products extend from Mg-rich minerals (lizardite, chrysotile, and clinochlore) that are accompanied by magnetite, to Fe-rich (ferrosaponite, minnesotaite, chamosite, and magnesian hisingerite). The latter are identified by their chemical formulae in EMPA-WDS and TEM-EDS analyses. Phyllosilicates are physically intermixed with serpentine on submicroscopic to nano scales. Serpentine ranges from X_Fe = 0.07 to 0.31 and Fe is predominantly ferroan. Hisingerite contains up to 1.5 wt.% CaO and ferrosaponite has interlayer Na and K. Fayalite crystals are idiomorphic against phyllosilicate along alteration veinlets in the primary olivine. No brucite, carbonate or quartz was detected. The relative abundance of Fe-rich phyllosilicates (X_Fe = 0.5 to 0.75) and magnetite argue for a major loss of Mg caused by infiltration of acidic aqueous fluids. This feature is ordinarily associated with the weathering of serpentinite. However, the production of fayalite, ferric-iron free minnesotaite, and ferrosaponite shows that this fluid was not fundamentally an oxidizing one associated with weathering, except probably for the formation of later hisingerite. The properties of the fluid may have been derived by alteration of nearby CuNi-sulfide deposits. Stable phase relations suggest alteration temperatures on the order of at least 300°C.