A paleohydrologic model for mineralization of the White Pine copper deposit, northern Michigan

Abstract

Pertinent physical properties of the upper Keweenawan rocks can be measured or in{erred within a sufficiently narrow range to make the quantitative evaluation of various paleohydrologic models for the origin of the White Pine copper deposit feasible. The approach is illustrated -here by calculations for models that involve lateral migration of fluids through the subjacent Copper Harbor Conglomerate to the site of the deposit and stripping of copper from these solutions where they percolated upward through the Nonesuch Shale. The calculations reveal limitations to theories of origin that would not be evident from purely qualitative consideration; some of these limitations could be useful to exploration. For example, if the water was yielded by compaction of the Copper Harbor Conglomerate and contained 50 ppm Cu, there must have been significant convergence-of solution paths toward White Pine. Surface water entering the Copper Harbor Conglomerate on the north limb of the Lake Superior syncline is an adequate source if it could be shown that the point of entry was once significantly higher in altitude than the water table at White Pine; this model implies a major copper deposit at great depth north of the axis of the syncline. Ground water entering the Nonesuch Shale up dip from White Pine is not a possible source of mineralizing solutions. Symbols Used be = thickness of Copper Harbor Conglomerate (cm). b• = thickness of Nonesuch Shale (cm). h' = excess head (cm) at base of Nonesuch Shale. z•h '= difference ooe excess head at base of Nonesuch Shale between two specified localities (cm). z•h • divided by the distance (cm) between the two localities is the average hydraulic gradient between the localities. K• = hydraulic conductivity of Copper Harbor Conglomerate (cm/sec). K• = hydraulic conductivity of Nonesuch Shale (cm/ sec). qh = rate of lateral movement of fluid in the Copper Harbor Conglomerate (cm/sec). {/, = rate of movement of fluid upward across the base of the Nonesuch Shale (cm/sec). x Publication authorized by the Director, U.S. Geological Survey. the volume of water flowing past a vertical strip 1 cm wide and extending from top to bottom of the Copper Harbor Conglomerate in 1 second •(cc/sec). Where that formation is essentially horizontal, Vh = q• X the volume of water flowing upward past a strip 1 cm wide parallel to the base of the Nonesuch Shale between two specified localities in 1 second (cc/sec). It is the product of the average rate of upward movement of fluid (q0 and the distance between the localities.

Constraints on Conceptual and Quantitative Modeling of Early Diagenetic Sediment-Hosted Stratiform Copper Mineralization

“…Disseminated copper sulfides were deposited by replacement of disseminated syndiagenetic pyrite [1][2][3][4][5]. Early semi-quantitative modeling followed, both at the deposit-scale [6,7] and basin-scale [8].…”

“…Although only recognized much later to be misleading, the implication was that the ferric oxide of the redbeds signaled oxidizing conditions and therefore the hematite of footwall redbeds assured that conditions were suitably oxidizing for transporting copper. At the time, our understanding of the solubility of copper seemed chemically complete and sufficient for designing basin-scale models (e.g., [8,[15][16][17][18][19][20]). …”

“…Two prominent general models emerged to explain the transport of eSSC-stage copper at the basin scale: (a) compaction-driven cupriferous pore (connate) waters [8,17,18]; and (b) topography-driven (i.e., highland recharge-driven) cupriferous meteoric water (e.g., [8,21]). Both models were found to be plausible according to White [8] in his classic paleohydrologic modeling of the White Pine deposit, northern Michigan, assuming that the brine carried 50 ppm copper (considered reasonable, given that Figure 2 shows that concentrations of 6 to 64 ppm copper are feasible and that lower copper concentrations would require unreasonably large volumes of ore solution to form an ore deposit).…”

“…Both models were found to be plausible according to White [8] in his classic paleohydrologic modeling of the White Pine deposit, northern Michigan, assuming that the brine carried 50 ppm copper (considered reasonable, given that Figure 2 shows that concentrations of 6 to 64 ppm copper are feasible and that lower copper concentrations would require unreasonably large volumes of ore solution to form an ore deposit).…”

Early Mesozoic basins workshop Fieldtrip No. 2; Base- and pecious-metal occurrences in the Culpeper basin, northern Virginia

“…Most of the known copper deposits of this type are relatively small, although some larger and more significant deposits occur, such as the White Pine district, Michigan (White, 1971), and Corocoro, Bolivia (Ljunggren and Meyer, 1964). Some of the Zambian copper belt deposits may belong in this category as well (particularly Chibuluma).…”