THE ECONOMIC GEOLOGY OF THE WESTERN SYNCLINE COPPER DEPOSIT, UPPER PENINSULA, MICHIGAN

The Western Syncline, located in the Upper Peninsula, Michigan, USA, is a blind, Mesoproterozoic, Kupferschiefer-type, sedimentary rock-hosted stratiform copper deposit hosted within the thick clastic sedimentary rock sequence that comprises the upper fill of the Mesoproterozoic (1.1 to 1.0 Ga) Midcontinent rift. It was discovered in 1956, but activity ceased in 1959. Exploration and development work was revived in 2008 and in 2013 Copperwood, the highest-grade portion of one of four distinct potential orebodies in the Western Syncline, was fully permitted.

Chalcocite is concentrated in gray to black shales and siltstones of the lowermost Nonesuch Formation forming gently-dipping, tabular bodies within a shallow-plunging open fold. The Western Syncline is structurally simple and has no hydrothermal or metamorphic overprint.

Formation of potential orebodies is a function of the depositional and diagenetic conditions in the Western Syncline. Anoxic conditions in a low-energy, clastic environment facilitated decomposition and preservation of organic material that enabled seawater sulfate reduction. These processes resulted in development of diagenetic pyrite in the sediments and upon the passing of copper in neutral, saline, oxidized fluids through the unlithified sediments, chalcocite replaced pyrite. The best potential orebodies are where sedimentary conditions and processes deposited thick packages of favorable host rocks: gray to black shales and siltstones.