Abstract

In the Portage Lake lava series of the Keweenaw Peninsula, Michigan, 3 overlapping amygdule mineral zones - prehnite, epidote, and quartz zones, respectively - can be distinguished. Higher stratigraphic horizons typically lie within the prehnite zone and lower horizons within the epidote and quartz zones. Zone boundaries cross stratigraphic horizons, indicating that the amygdule minerals were deposited after many successive flows were extruded. Temperature control of the over-all distribution is likely. Mineralogy of flow tops of the Keweenawan series throughout the Lake Superior basin is like that of Keweenaw Peninsula, indicating that the mineral zones have regional extent. Cu concentrations are commonly near the boundary of the quartz zone, but may lie well within the quartz zone where quartz is not abundant. Most barren areas and unsuccessful mines lie either well outside the quartz zone, or where quartz or prehnite or both are plentiful. Detailed mineral mapping of mines on the Kearsarge amygdaloid shows that quartz is a prominent mineral only in the deeper portions of the mined area and that major concentrations of Cu lie near the boundary of the quartz zone. In the Kearsarge amygdaloid deposition temperatures of quartz and calcite, determined by inclusion thermometry, reach a maximum of about 360 degrees C.; this indicates a temperature gradient of greater than 2 degrees C. per 100 ft. at the time of deposition, in contrast to a gradient of 0.6 degrees C. per 100 ft. at the present time - indicating that fluids of hydrothermal character, rising along the flow top, deposited the amygdule minerals, one of which is native Cu. The chemical constituents of epidote, prehnite, and quartz, and the wide distribution and post-extrusion age of these minerals is compatible with formation by regional hydrothermal rearrangement and alteration of materials of the lavas as shown by chemical analyses of the altered rocks of the Kearsarge flow top. Pumpellyitization of parts of the flow top, caused by calcium oxide- and carbon dioxide-bearing waters, provided material to alter other parts of the top to albite basalt. Both alteration processes freed chemicals that formed amygdule minerals. A slightly greater volume of pumpellyitized rock than albitized rock is needed to provide the chemicals required for region-wide formation of amygdule minerals. The suggestion is made that pumpellyitization at depth is more extensive than in the ground now opened by mining and that the chemicals freed were transported regionally upward by hydrothermal solutions. Cu distribution seems related to the zonal amygdule mineral pattern resulting from this alteration and may itself have been freed from the lavas at depth during alteration to pumpellyite.

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