METASOMATIC FORMATION OF CALC-SILICATE MINERALS DURING HYDROTHERMAL ALTERATION OF ARKOSIC CONGLOMERATES AND SANDSTONES, FROM THE KEWEENAW PENINSULA, MICHIGAN

Arkosic interflow conglomerates containing sandstone layers and lenses are a major host for native copper deposits in the Portage Lake Volcanic series, Keweenaw Peninsula, Michigan. We used catholuminescence, SEM, and electron microprobe to examine the mineralogy and mineral chemistry of samples of the C&H Conglomerate from the Centennial Mine. Samples from the upper mine level contain small (<100 microns) epidote grains with high iron (0.29 < Fe/(Fe+AL) < 0.4). Epidote abundance in these samples varies with some areas containing only a few scattered grains and other areas containing clusters of hundreds of epidote grains along with pumpellyite and titanite. There is an inverse correlation between the abundances of small dispersed hematite grains and epidote. Samples from the lower levels of the mine have up to approximately 50 modal percent calc-silicate minerals, and typically contain markedly less alkali feldspar than samples from the upper level. Epidote grains display a patchy Fe-Al zoning (0.16 < Fe/(Fe+Al) < 0.43). Epidote with up to 36 mole% allanite component occurs as veins and small patches within REE-poor epidote grains. Pumpellyite frequently occurs in embayments within epidote grains, contains epidote inclusions, and displays a patchy zoning in both Fe/(Fe+Mg) ratios (0.32 – 0.72) and (Fe+Mg)/(Fe+Mg+Al) ratios (0.24 – 0.45). Titanite contains up to 2.6 wt% F with a positive correlation between F and Al (which ranges up to 7.6 wt %). Andradite-grossular hydrogarnet with up to 2.5 wt % F occurs as small relatively rare grains and ranges in composition from 99% to 41% andradite. Calcite veins occur at all levels at the mine, and in the upper levels are associated with REE-fluorocarbonates. Textures, mineral abundances, and mineral compositions suggest that epidote formed by a reaction between feldspars, iron oxides, and a hydrothermal solution which brought in Ca and removed K and Na. Pumpellyite began forming after epidote and partially at its expense. Titanite formed by reactions between the hydrothermal fluid and primary iron-titanium oxides. Titanite and hydrogarnet chemistries both indicate that the hydrothermal fluid contained F which may have played an important role in the transport of REE.