Mineralogical Evidence for Oxidizing, Rare-Earth Transporting Fluids in the Metamorphism of Conglomerates from the Keweenaw Peninsula, Michigan

The Allouez conglomerate is a copper bearing sedimentary unit within the Portage Lake Lava Series in the Keweenaw Peninsula, Michigan. Scanning electron microscopy and energy dispersive X-ray analysis of the Allouez conglomerates reveal several features suggesting interaction with oxidizing, calcium and light-rare-earth element bearing fluids relatively late in the metamorphic history. Evidence for oxidation includes the formation of ilmenite lamellae in titaniferous magnetites and the replacement of those lamellae by titanite and rutile suggesting the reactions:

1. 4Fe2TiO4 + O2 = 4FeTiO3 + 2Fe2O3

2. 4 FeTiO3 + 4Ca++ + 4H4SiO4 + O2 = 4CaTiSiO5 + 2Fe2O3 + 8H+ + 4H2O

3. 4FeTiO3 + O2 = 4TiO2 +2 Fe2O3

Copper sulfide minerals in calcite veins were oxidized forming copper oxide rims.

Light-rare-earth element transport is suggested by:

1. rims and patches of light-rare-earth element enriched epidote (La2O3 + Ce2O3 + Nd2O3 up to 13 weight percent) on and in epidote grains with no detectable rare-earth elements.

2. needles of a calcium, light-rare-earth bearing carbonate (possibly parasite, rontgenite or sychysite) in calcite veins and in vugs.