TEMPERATURE-SENSITIVE BRINE VISCOSITY: KEY TO COPPER DEPOSITION IN THE FINE-GRAINED BASAL NONESUCH FORMATION, WHITE PINE-PRESQUE ISLE DISTRICT, NORTHERN MICHIGAN

Economic stratiform copper mineralization in the fine-grained basal Nonesuch Formation of the ~50-km wide White Pine-Presque Isle mining district, northern Michigan, has long been explained by the infiltration of cupriferous brine from the footwall Copper Harbor Conglomerate aquifer. Given that infiltration model, why do lesser amounts of copper occur in coarser-grained, presumably more permeable basal Nonesuch strata extending ~100 km further east and west of the mining district? The answer may lie in thermal blanketing by early Nonesuch sediments and an increased infiltration rate for brine warmed by latent volcanic heat from a large dormant or recently extinct Porcupine Volcanics caldera buried by the Copper Harbor aquifer and lying directly below the White Pine-Presque Isle area. Not only would warming have lowered the brine density, but more importantly it could have reduced the brine viscosity remarkably. An increase in temperature from 15 to 100^oC could have increased the upward infiltration rate by four times. Even if the permeability of fine-grained Nonesuch sediment over the warm caldera were one-half order of magnitude lower than that of coarser-grained caldera-distal Nonesuch sediment, vertical infiltration into the basal Nonesuch could have been 1.3 times higher over the caldera.

At the rift basin-scale, brine flow would have been driven within the Copper Harbor aquifer by meteoric recharge in hydrogeologically connected rift-margin highlands. As a consequence of the exceptionally rapid local flow of brine upward into the Nonesuch over the warm caldera area, an equivalent exceptional volume of brine must have been drawn laterally within the Copper Harbor Conglomerate aquifer to replace the local flow into the basal Nonesuch. This lateral focus of brine toward the warm caldera area would have created a more efficient basin-scale ore-forming system than that resulting from a linear recharge-driven flow.