EXPLANATIONS FOR UNIQUE ASPECTS OF SEDIMENT-HOSTED STRATIFORM COPPER MINERALIZATION AT WHITE PINE, MICHIGAN: THE EFFECTS OF LATENT VOLCANIC HEAT

The conventional redbed-to-graybed infiltration model for main-stage sediment-hosted stratiform copper (SSC) mineralization in the White Pine district has been up-graded recently: the rate of infiltration of ore brine into the basal Nonesuch could be approx. 4.2 times greater due to warming by latent volcanic heat from the underlying Porcupine Volcanics shield volcano. That unique warming could also explain other aspects of the main-stage mineralization at White Pine. For example, the transition from cupriferous mineralization to overlying pyritic black "shales" typically occurs within centimeters at White Pine, whereas that transition extends across the complete cupriferous zone in most other SSCs. Also, the cupriferous zone at White Pine is uniquely chalcocitic, whereas most SSCs exhibit a basal chalcocite-dominant zone overlain gradually by equally important bornite- and chalcopyrite-dominant zones. And even though main-stage copper deposition at White Pine is attributed mainly to replacement of in-situ fine-grained pyrite euhedra and framboidal pyrite, no pseudomorphs after pyrite occur within the cupriferous zone (other than chalcocitic nodules which replaced former pyritic nodules), whereas most SSCs worldwide have pseudomorphs and minor fine-grained pyrite within their cupriferous zones. Moreover, the grain-size of disseminated main-stage chalcocite at White Pine is distinctly coarser compared to the grain-size of the pyrite replaced by chalcocite.

Obvious explanations for these unusual aspects of main-stage copper at White Pine have been lacking for decades. Now, with higher reaction rates and greater recrystallization at elevated temperatures, the same latent volcanic heat called upon to have accelerated the rate of ore brine infiltration into the basal Nonesuch beds may explain: the narrow sulfide reaction zone across the top of the cupriferous zone, the complete reaction to stable chalcocite throughout the cupriferous zone, the complete replacement of pyrite within the cupriferous zone, and at least initial recrystallization of chalcocite within the cupriferous zone to coarser-grained chalcocite. A single answer (latent volcanic heat) for such diverse questions tends to confirm the validity of the latent volcanic heat model.