RARE EARTH ELEMENT ENRICHED MINERALS IN HYDROTHERMAL COPPER DEPOSITS FROM THE KEWEENAW PENINSULA, MICHIGAN, USA

Low grade hydrothermal metamorphism associated with copper mineralization in Michigan’s Keweenaw Peninsula created concentrated masses of calc-silicate minerals in intralayered tholeiitic basalts and rhyolite-pebble conglomerates within the fill of the 1.1 Ga Midcontinent rift. These masses consist of epidote, pumpellyite, prehnite, and titanite in the basalts and epidote, titanite, and sporadic andradite in the conglomerates and were examined by SEM, and electron microprobe analyses from five localities (three in basalts; two in conglomerates). Epidote grains enriched in REE were found in samples from both conglomerate localities and can be classified into: Type I characterized by narrow growth zones with up to 6 wt.% Ce₂O₃ + La₂O₃ + Nd₂O₃ ; Type II characterized by dissolution-reprecipitation zoning consisting of irregular REE-enriched zones around fractures or as incomplete rims/embayments at the margins of REE poor crystals; and Type III characterized by 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. Synchysite, REE-fluorocarbonate, occurs with calcite in some conglomerate samples that contain little or no epidote. Only one basalt sample was found to contain REE-enriched minerals as clusters of irregularly shaped patches with up to 4.3 wt.% Ce₂O₃ + La₂O₃ + Nd₂O₃ within REE-poor epidote. Type I REE-enrichment represents a brief increase in the activity of REE-elements during epidote growth. 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 hydrothermal fluids may have acquired REE from leaching of rhyolite clasts in conglomerates at depth in the source area for the fluids