Abstract

The Seminoe Mountains expose a Precambrian-cored Laramide uplift hear the southern margin of the Wyoming Province. The core of the range consists of Archean-age crystalline supracrustal (>2.7 Ga) rock folded into a vertically plunging open fold intruded by granodiorite (>2.6 Ga).

The amphibolite grade supracrustal rocks are subdivided into three mappable units. The lowermost unit (Sunday Morning Creek Metavolcanics) consists of approximately 11,000 feet of mafic schists with minor ultramafic schist and metasedimentary rock. This unit is overlain by the Bradley Peak Ultramafics which consist of approximately 1,000 feet of ultramafic and mafic metavolcanic rocks. These rocks include several mappable ultramafic flows that grade upward from cumulate-textured serpentinite flow bottoms to tremolite-talc-chlorite schist spinifex flow tops. Compositions and textures indicate they are peridotitic and basaltic komatiites. The uppermost unit (Seminoe Formation), consists of approximately 4,000 feet of metasedimentary rock including metagreywacke, banded iron formation, and pelitic schist with minor amounts of mafic and ultramafic schist.

Modern exploration of the district has identified more than 100 million tons of low-grade banded iron formation, thin high-grade gold veins, localized copper-silver-gold veins, gold placers, serpentine, jade, leopard rock, asbestos, and anomalous zones of zinc and lead.