A DOMINANT EVOLVED-METEORIC WATER COMPONENT IN THE GENESIS OF NATIVE COPPER LODES, KEWEENAW PENINSULA, NORTHERN MICHIGAN

The Keweenaw Peninsula of northern Michigan is well known for sediment-hosted stratiform copper mineralization hosted mainly by basal sulfide-rich siltstone-shale units of the Nonesuch Formation in the White Pine district, and for native copper lodes hosted mainly by flow-top breccias and amygdaloidal portions of flood basalts and interbedded conglomerates of the stratigraphically lower Portage Lake Volcanics (PLV) some 100 kms to the northeast. The native copper mineralization is generally attributed to a metamorphogenic fluid generated during down-dip burial metamorphism of its host strata. A complementary influx of water, similar to water responsible for the White Pine mineralization, would provide the salinity and appropriately oxidizing Eh-pH conditions necessary for copper transport in the native copper lode-forming fluid.

The timing of the native copper mineralization may be tied to the Grenvillian compressional event which closed the Midcontinental Rift, produced reverse thrust movement on the Keweenaw rift-margin fault, and tilted Keweenaw strata toward the rift axis. The PLV would have been subjected to increased burial metamorphism and metamorphogenic water should have been remobilized toward surface along porous flow tops and coarse-grained sediments. In parallel, a highland-driven evolved-meteoric brine could continue to circulate and redden deep Oronto Group sediments as well as still deeper, permeable amygdaloidal and fragmented flow tops and interbedded conglomerates of the PLV. The brine, warmed with mantle heat, would have aggressively leached copper from all strata encountered, including those portions of the PLV releasing metamorphogenic water. As hematitization continued, the combined evolved-meteoric/metamorphogenic fluid would have become still more depleted in oxygen and would have evolved toward the highly reduced conditions typical of deep groundwater. Under such conditions, metallic copper is stable and hematite unstable, and the cupriferous hybrid fluid should then have simultaneously bleached (de-hematitized) the host rock and deposited native copper, aided by cooler up-dip temperatures and retrograde dehydration of the ore-forming fluid.