A Paleoproterozoic Andean-type iron oxide copper-gold environment, the Great Bear magmatic zone, Northwest Canada

Abstract

Iron oxide copper-gold (IOCG) and associated iron-oxide apatite (IOA) styles of metallic mineralization are recognized throughout the Paleoproterozoic Great Bear magmatic zone of the northwest Canadian Shield. The Great Bear magmatic zone was constructed between ca. 1876 and 1855Ma on top of the older Hottah terrane, which preserves continental arc magmatism that began around ca. 2.0 to 1.97Ga and continued between ca. 1.93 and 1.89Ga. The Great Bear represents the final stages of ca. 150 million years of intermittent and pulsed magmatism related to an evolving continental orogenic belt. The preserved geology supports a dramatic geodynamic change in the subduction zone process at ca. 1875Ma, a key driving mechanism for magma and metal mobilization, and was rapidly followed by a large-scale introduction of felsic-intermediate plutons. The overall tectonic setting is partially constrained from new and previously published geochemical data that show that the volcanic and plutonic rocks are high-K calc-alkaline to shoshonitic in nature (e.g., high K2O, Th/Yb, and Ce/P205). They also have suprasubduction-zone geochemical signatures, including primitive mantle normalized positive Th and negative Nb, P, and Ti anomalies. The data support the primary melts were derived from a GLOSS-modified mantle wedge. Three-dimensional rendering of geophysical datasets suggest that two (of four) preserved surfaces within the upper mantle lithosphere, at 70 to 120km depths, represent frozen, subducted oceanic slabs, and likely were the drivers for the bulk of Hottah and Great Bear arc magmatism. The older slab is northwest-striking and dips 12° to 15° northeast, whereas the younger is deeper and north-striking, dipping 13° east. The geometry of the surfaces are comparable with 4D modeling, where a subduction zone is temporarily shut down due to plateau collision, and then steps oceanward and re-initiates; there is no need for polarity reversal of the subduction system. This new geometry and the related inferences about process should be the focus of future research in the region, but for the time-being it can be stated that these subduction and collisional processes were the first order control on lithospheric evolution, and therefore metallic mineralization. Overall, the Great Bear magmatic zone IOCG and related mineralization is not comparable to other Proterozoic IOCG belts, such as those in Australia. However, the complexity of mineralization styles, the spatial-temporal relationship between IOA and IOCG mineralization, the suprasubduction zone environment, and a major change in tectonic regime are features similar to Andean-type IOCG mineralization, as well as Cordilleran alkali porphyry Cu-Au deposits. This further establishes the linkages between subduction zone processes and IOCG formation, as well as relationships in the IOCG-porphyry deposit continuum model.