Economic stratiform copper mineralization in the fine-grained, carbonaceous basal Nonesuch Formation of the ~50-km-wide White Pine-Presque Isle mining district, northern Michigan, has long been explained by the essentially vertical infiltration of cupriferous brine across the full width of basal Nonesuch strata from the underlying Copper Harbor Conglomerate aquifer, followed by copper precipitation mainly by reactions with in situ iron sulfide. Based on that model, why do lesser amounts of total copper, measured in vertical sections, occur in coarser grained (presumably more permeable) basal Nonesuch strata situated laterally to the east and west of the mining district? The answer may lie in increased infiltration rates for Copper Harbor-hosted ore-forming brine warmed locally by latent volcanic heat from a resurgent caldera (or calderas) located within the large Porcupine Volcanics dome underlying the mine district. Not only would caldera heat have warmed the brine and lowered its density, but more importantly it would have produced a remarkable decrease in the brine viscosity. An increase in temperature from 15° to 100°C could have increased the brine infiltration rate by more than four times in very early diagenetic time when the sediments were poorly compacted and highly porous. Because progressive compaction would have opposed the temperature effect, a very early diagenetic timing is suggested for main-stage copper mineralization over the Porcupine Volcanics caldera. Textural evidence has also long supported an early mineralization event.
At the rift-basin scale, the exceptionally rapid upward flux of warmed brine into fine-grained Nonesuch over the caldera would have required equivalent volumes of replacement brine, which would in turn have required a lateral convergence of brine within the footwall Copper Harbor Conglomerate aquifer toward the White Pine-Presque Isle district. 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 hydrogeologic flow. Most concepts described here deserve further evaluation through quantitative hydrogeologic modeling.
Thermal effects on ore brine viscosities and densities, developed here for the sediment-hosted stratiform copper mineralization of the White Pine-Presque Isle district, could be considered in genetic models for other deposits and deposit types. The effects are especially strong for solutions warmed from low temperatures (e.g., 20°C or lower) to 100°C, and could also be important to ore fluid flow for ores involving much higher hydrothermal temperatures if solutions with initially low temperatures played a role in their genesis.