CORRELATION OF THE WHITE PINE SEDIMENT-HOSTED STRATIFORM COPPER DEPOSIT WITH AN UNDERLYING RESURGENT CALDERA?

Why are several geological features stacked one above the other in the area of the White Pine stratiform copper deposit: in the footwall, 1) a large felsic dome and 2) a rapid thinning of the Copper Harbor Conglomerate, and at the ore deposit level, 3) a large marine(?) delta and 4) the best-mineralized basal Nonesuch Formation? We propose that, at the scale of the Mid-Continent Rift, the felsic dome (and its presumed subjacent intrusive equivalents) on the southern margin of the Keweenaw rift had a large central resurgent caldera in which a major portion of the deltaic Nonesuch "shale" accumulated as anoxic pyritic sediments, similar to those forming now in the Black Sea. The felsic volcanism/plutonism would have been effectively extinct in Nonesuch time but would have represented a source of latent heat directly beneath the current position of the best-mineralized Nonesuch strata; the ore solution rose in that area by moderate heat-induced convection. The pertinent geological context of the mineralized Nonesuch extends back to the end of Portage Lake Volcanics time when a Valles-type caldera with a felsic volcanic dome formed in the Porcupine Mountains area. A shallow structural dome was intruded during the late stage of caldera resurgence. In post-rift time, extensive Copper Harbor Conglomerate continental redbeds were deposited throughout the rift basin and over the felsic dome. With subsequent subsidence, a large marine incursion occupied the rift basin and led to the deposition of the Nonesuch siltstone-shale unit, a carbonaceous pyritic sediment suggestive of stagnant anoxic conditions prevalent in basins located at low latitudes, as the Lake Superior region was (10 to 15oN) approximately 1.1 Ga ago. Main-phase copper mineralization occurred along the base of the Nonesuch, largely by replacement of the original pyrite when low-temperature cupriferous brines circulated from the Copper Harbor Conglomerate into the basal Nonesuch. Supporting observations for our concept include: (1) the volcanic facies, and (2) the emplacement of rhyolitic lava domes with structural control by caldera ring fractures. Further testing of the concept may include (1) sedimentologic analysis; (2) structural analysis; (3) quantitative modeling.