Behavior of Cu, Zn, and Ni During Prehnite-Pumpellyite Rank Metamorphism of the Keweenawan Basalts, Northern Michigan

Abstract

Both copper and zinc were mobilized during prehnite-pumpellyite facies metamorphism in the Keweenawan basalts of northern Michigan. Cu, averaging 70 ppm in basalts, was leached from epidotized or more highly metamorphosed rocks in the zone of dehydration; it was introduced into hydrated rocks where pumpellyite, prehnite, laumontite, and chlorite are the principal hydrous secondary phases. Native copper is a common associate of the metamorphic assemblage in such rocks. The chemical data suggest that fluids evolved by dehydration at depth may well have contained from 1,500 to 2,000 ppm Cu. These fluids percolated upward toward lower temperature levels where hydration was dominant and where the residual concentration of Cu-ions in the fluids could have reached high levels if precipitation had not occurred. The model supplies nearly five orders of magnitude more copper than has been produced by a century of mining in the Keweenaw Peninsula. The data suggest that enrichment or depletion of native copper should be spatially related to the distribution of certain hydrous minerals in the Keweenawan lava pile. Zn, which averages 100 ppm in unaltered olivine tholeiites, was leached from all highly altered rocks in permeable zones and removed by metamorphic fluids. Abundance of Ni was not appreciably affected by the metamorphic process except where there was dilution by void-filling secondary minerals. Introduction TI-XE Keweenawan tholeiitic lavas of northern Michi-gan, called collectively the Portage Lake Volcanics, underwent a postdepositional metamorphic event which produced secondary mineral assemblages of the zeolite and prehnite-pumpellyite facies (Stoiber and Davidson, 1959; Jolly and Smith, 1972). In the upper part of the five-kilometer-thick section exposed in the Keweenaw Peninsula, albite, prehnite, laumontite, chlorite, minor analcime, and sphene, along with rare native copper, are present in metamorphosed rocks. In the middle part of the section, albite, pumpellyite, prehnite, native copper, quartz, chlorite, sphene, and minor epidote are characteristic of altered lavas. Finally, in the lowest exposed rocks, albite, epidote, quartz, chlorite, sphene, and minor pumpellyite form metamorphic assemblages. These three parts of the lava pile will here be called the chlorite, pumpellyite, and epidote zones respectively after Jolly and Smith (1972). The purpose of this report is to suggest relationships between the metamorphic process and behavior of three common trace metals, Cu, Zn, and Ni. Metamorphic PetrologyBecause the trace element redistribution is intimately related to the metamorphic process, the' major characteristics of this alteration must be briefly summarized. More complete information on this topic may be found in Stoiber and Davidson (1959), White (1968), and Jolly and Smith (1972). The metamorphic alterations of the presently exposed Keweenawan lavas were localized in permeable parts of the lava pile only; extensive readjustment took place primarily in amygdular zones, breccias, conglomerate units, o...

A theoretical basis for exploration for native copper in northern Wisconsin

“…All but one of the major deposits lie within the zone of overlap, and only one lies below the lower limit of prehnite, in rocks characterized by the amygdule-mineral assemblage, quartz-epidote without prehnite. Jolly (1974) and Jolly and Smith (1972) studied the metamorphic alterations of the basaltic rock itself, rather than the amygdules, and were able to refine considerably the concept of regional metamorphic zoning. In the region occupied by the major copper deposits, they distinguish an upper laumontite (chlorite) zone, an intermediate pumpellyite zone, and a lower epidote zone ( fig.…”

“…If one believes, following Stoiber and Davidson (1959) and Jolly (1974), that there is a genetic connection between copper deposition and metamorphic grade, then a logical early step in exploration would involve an attempt to find areas in which the rocks belong to Jolly's pumpellyite zone, thus locating his epidotepumpellyite boundary. In Michigan, metamorphic grade decreases as stratigraphic distance increases above the base of the Portage Lake Volcanics, so areas of lower metamorphic grades should be sought where the rocks are stratigraphically higher than, or along strike from, areas where copper is associated with epidote metadomains.…”

“…The water would leak toward the surface along any available channelways. This water, if saline like the deep formation waters common throughout the world, would inevitably be much enriched in copper by the metamorphic reactions (Stoiber and Davidson, 1959;Jolly, 1974), thus fulfilling the basic requirements for the ascending ore-forming fluid that deposited the native-copper deposits.…”

“…The area of Portage Lake Volcanics closest to the deepest part of the basin is an area in which the rocks have been metamorphosed to the grade of Jolly's (1974) epidote zone, and are thus of higher grade than the empirical optimum for major copper deposits. Within this area of higher metamorphic grade, there are actually a number of native copper deposits (see the two areas shown as containing minor deposits in fig.…”

“…Hubbard (1975) reported that all feldspars contain, or are altered to, epidote. Pumpellyite, however, does occur, at least locally, in epidote metadomains in the uppermost part of the Portage Lake Volcanics, so the top of the formation may lie fairly close to Jolly's (1974) boundary between epidote and pumpellyite zones.…”