Serpentinized ultramafic rocks of Churchill age occur along a northeasterly trending zone of complexly faulted gneissic rocks in the Setting Lake-Moak Lake region of central Manitoba. The ultramafic rocks are classified as serpentinites, tremolite serpentinites, tremolite phlogopite serpentinites, phlogopite serpentinites, serpentinized peridotites, tremolite olivine orthopyroxenites and amphibole orthopyroxenites. They originated from dunites, peridotites and orthopyroxenites. The predominant serpentine is a fibrous variety resembling chrysotile, which can only be satisfactorily indexed on the basis of a 3-layer structure. The derived 3-layer ortho-hexagonal cell has dimensions of ao = 5.346Ao, bo = 9.205Ao and co = 21.93Ao. Minor occurrences of antigorite and lizardite are identified by optical, X-ray diffraction and differential thermal analysis methods. Alteration of serpentinite adjacent to siliceous country rocks results in the formation of chlorite, anthophyllite and tremolite zones with gain of Si, Al, Ca and Mn and loss of Mg, Fe, OH, Cr and Ni... The average composition of nickel belt serpentinites is typical of alpine-type peridotites. They contain higher MgO and lower SiO2 and CaO than rocks believed representative of the upper mantle. MnO and TiO2 show an inverse relationship to the MgO/FeO ratios. The intrusion of ultrabasic magma as a crystal mush up faults from a source in the mantle, quick marginal cooling and subsequent serpentinization are considered the most likely mode of origin for the ultramafic rocks.