EVAPORATED SEAWATER FORMED SEDIMENT-HOSTED STRATABOUND COPPER ORE IN THE MIDCONTINENT RIFT

The Mesoproterozoic North American Midcontinent Rift System (MRS) contains sediment hosted stratiform Cu deposits at White Pine and Copperwood, MI, which together held at least 4 Mt Cu and 75 Moz Ag. Main stage Cu mineralization formed at both deposits during diagenesis at ≤100°C, and White Pine also contains structurally controlled second-stage mineralization that was likely synchronous with Keweenaw Peninsula native Cu mineralization. We report chemical data from fluid inclusions from (1) synsedimentary calcite (~1080 Ma), (2) diagenetic calcite, (3) main-stage chalcocite, and (4) second-stage calcite (~1020 Ma) from White Pine. Fluid inclusions were extracted from 100-600 mg of calcite and chalcocite, and analyzed for Na⁺¹, NH₄⁺¹, Ca⁺², Mg⁺², K⁺¹, Rb⁺¹, Sr⁺², Ba⁺², Cl^-1, Br^-1, F^-1, S₂O₃^-2, SO₄^-2, and acetate.

The Cl-Br-Na data from all four groups of minerals from White Pine plot in a relatively small compositional field, with Cl/Br and Na/Br molar ratios that are less than 250. The data plot close to or along the seawater evaporation curve with no evidence for the dissolution of salt and no input from non-marine brines. The Na/Br molar ratios are much less than those of most basinal fluids worldwide, indicating that there was no significant salt dissolution to form the MRS brines. Some of the analyzed MRS brines evolved beyond halite precipitation to approach Mg- and K-salt saturation. Main- and second-stage brines have similar to overlapping compositions, suggesting that even though the two stages of mineralization were separated by ~60 m.y., they formed from similar brines or the same brine. If so, a large volume of porous sedimentary rocks and/or access to a surficial brine pool was required to store the large volume of brine necessary to form both stages of Cu minerals. Some earlier workers suggested that the fine-grained clastic sedimentary rocks of the Nonesuch Formation, which host the sediment-hosted Cu deposits, were deposited in a lacustrine environment, but our data require seawater that evaporated beyond the point of halite precipitation, and therefore a marine depositional environment. Integrating these results with the current understanding of basin architecture and location of deposits may provide new insights into why some areas of the MRS produced world-class deposits and other segments are barren.