Reconnaissance geologic mapping during 1984 revealed previously unreported ultramafic rocks of probable ophiolitic origin in the northern

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part of the Iditarod quadrangle. The ultramafic rocks are poorly exposed on rounded tops of the low rolling hills in the area, and they form a northeast-southwest belt at least 8 mi long and as much as 3 mi wide. Although these rocks are associated with Mississippian to Jurassic(?) chert, tuff, argillite, and basalt of the Innoko terrane, the contact relations are uncertain.

The ultramafic rocks consist of harzburgite, lherzolite, pyroxenite, and pods of altered dunite; disseminated chrome spinel is a common but minor constituent. Serpentinization is ubiquitous, all samples showing at least some effects and many being entirely serpentinized. Discontinuous pods of former serpentinized dunite(?) within lherzolite are entirely altered to a magnesite + talc + chrome spinel assemblage, indicative of low-grade metamorphism in the presence of water and carbon dioxide.

The ultramafic rocks of Iditarod quadrangle are similar to and on trend with those of the Mount Hurst area, 25 mi to the northeast in the Ophir quadrangle. The Mount Hurst rocks represent part of a dismembered ophiolite, probably related to the Yukon-Koyukuk ophiolite belt. Although several of the classic ophiolite components are missing from the ultramafic sequence in Iditarod quadrangle, an ophiolite origin is strongly suggested by correlation with the Mount Hurst rocks and is supported by the presence of a structural slice of hypersthene gabbro in close proximity to the ultramafic sequence.

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