Serpentinization of ultramafic rocks can produce alteration minerals such as brucite (Mg (OH) 2), which has the potential to sequester carbon dioxide, and awaruite (Ni3Fe), a potential source of nickel. The Trembleur ultramafite is part of a dismembered ophiolite in central British Columbia. Field and petrographic data indicate that it is heterogeneous both in protolith and alteration. The protolith consists mainly of harzburgite, lherzolite, dunite, and lesser pyroxenite. Dunite, more abundant than previously recognized, could be a replacement of harzburgite (±lherzolite). All ultramafic rocks in the Decar area, north of Trembleur Lake, are altered to some extent (mainly partially to pervasively serpentinized) and locally contain higher temperature metamorphic assemblages. Carbonate alteration post-dated serpentinization, degrading minerals such as brucite and awaruite to locally produce ophicarbonates, soapstone, and listwanite. The primary olivine-pyroxene ratio of the protolith may have controlled fluid pathways and thus the extent of serpentinization and the abundance, distribution, and grain size of brucite and awaruite, which has implications for carbon sequestration and nickel potential in the Decar area.