Rare Earth Element Enriched Minerals in Hydrothermal Copper Deposits from the Keweenaw Peninsula, Michigan, USA

Student Author(s)

Connor Mulcahy

Faculty Mentor(s)

Dr. Edward Hansen; Dr. Dieter Rhede (Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum); Dr. Theodore Bornhorst (Geological and Mining Engineering and Sciences, Michigan Technological University)

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Hydrothermal metamorphism associated with copper mineralization in Michigan’s Keweenaw Peninsula created concentrated masses of calc-silicate minerals in both basalts and arkosic conglomerates. These masses consisted of epidote, pumpellyite, prehnite, and titanite in the basalts and epidote, titanite, and sporadic andradite in the conglomerates. The calc-silicate masses were examined by Scanning Electron Microscope, and electron microprobe analyses were completed on the calc-silicate minerals from three localities of mineralized basalts and two localities of mineralized conglomerates. Epidotes enriched in rare earth elements (REE) were found in samples from both conglomerate localities and can be classified into three textural types. Type I enrichment is a growth zoning characterized by narrow zones with up to 6 wt.% REE outlining euhedral crystal zones enclosed within REE-poor epidote grains. Type II enrichment is a dissolution-reprecipitation zoning and consists of irregular REE-enriched zones around fractures or as incomplete rims/embayments at the margins of REE poor crystals. Type III enrichment consists of masses of small, acicular crystals with an allanite component of up to 47 mole% projecting from the margins of REE poor epidote crystals or intergrown with titanite/REE poor epidote. The REE-fluorocarbonate synchysite occurs with calcite in some conglomerate samples that contain little or no epidote. Only one basalt sample was found to contain REE-enriched minerals where it occurs in the form of clusters of irregularly shaped patches with up to 4.3 wt.% REE within REE-poor epidote. Either a change in the hydrothermal fluid composition or a sudden decrease in temperature during the last stages of epidote growth led to super-saturation of REE elements that in turn led to the development of Type II and Type III enrichment. The most probable source for the REE are the rhyolite clasts in the conglomerates.

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