CU ISOTOPE VARIATIONS BETWEEN NI-CU MINERALIZED INTRUSIONS OF PICRITIC AND HIGH-AL OLIVINE THOLEIITE DESCENT IN THE MIDCONTINENT RIFT SYSTEM, NORTH AMERICA

Disseminated sulfides in igneous rocks from the Midcontinent Rift have significantly different δ⁶⁵Cu signatures. Sheet-like intrusions of the Duluth Complex were derived from high-Al olivine tholeiites (HAOT) and have disseminated sulfides with δ⁶⁵Cu values from -0.85 to 0.45 ‰. The Eagle and Tamarack deposits were generated by fractionation of picrites, and disseminated sulfides have δ⁶⁵Cu values from 0.69 to 1.32 ‰ and 0.99 to 1.84 ‰, respectively. δ⁶⁵Cu values of mantle peridotites and basalts are in the range of -0.1 to 0.18 ‰. Based on Cu isotope values of sedimentary country rocks, assimilation of sedimentary Cu could have contributed significantly to mineralization at Duluth only if the mantle δ⁶⁵Cu value was less than -0.25 ‰.

Geochemical studies of high-MgO magmas suggest picrites of the Midcontinent Rift were formed from the central, hotter portions of the mantle plume and subjected to higher degrees of partial melting. Conversely, HAOTs may have been produced in the margins of the plume, at lower temperatures and lower degrees of partial melting. This suggests that the mantle sources of the two magmas may have had different δ⁶⁵Cu signatures. Luck et al. (2005) proposed two reservoirs of Cu based on their studies of iron meteorites and carbonaceous chondrites, but the cause of such fractionation is unclear. The potentially lower degrees of partial melting required to produce the HAOTs may have led to incomplete dissolution of mantle sulfides, and possibly, isotopic fractionation of Cu. It is also possible that HAOTs were produced by fractional crystallization of picrites. Although this process may explain some characteristics of HAOTs, fractionation in sulfide-undersaturated magmas would not have changed the Cu isotope ratios, due to Cu incompatibility.

Cu isotope fractionation in low temperature environments is thought to be caused by redox changes, liquid-mineral fractionation, and fractionation between dissolved species, but very little is known about Cu isotope fractionation in the mantle and magmatic processes. Experimental studies of high temperature Cu isotope fractionation and regional scale studies of δ⁶⁵Cu variations are being developed to assess mechanisms of Cu isotope disparity between magmatic sulfides associated with picritic and HAOT parental magmas in the Midcontinent Rift.