Regional and local controls on Archean rock-hosted cobalt mineralization at the McAra deposit, southern Superior Province, Ontario, Canada

The McAra deposit is in eastern Ontario, Canada, and is hosted in an Archean inlier to the Paleoproterozoic Huronian basin. It is currently estimated to contain ∼2.4 million pounds of cobalt at an average grade of 1.25%. New drill data show the mineralized zone comprises glaucodot–cobaltite veins and breccias that transect a mafic–siliciclastic volcanogenic massive sulfide (VMS) deposit. The high cobalt grade and host stratigraphy at the McAra deposit contrast with five-element (Ag–Co–Ni–Bi–As) deposits at the Cobalt and Gowganda camps in the region that produced high-grade silver and by-product cobalt from veins spatially associated with Nipissing Gabbro intrusions. However, geochemical data from recent core samples alongside fluid inclusion and mineralogical data suggest the cobalt zone at McAra and the five-element veins share a similar metal assemblage and were deposited from similar fluids. The mafic–siliciclastic VMS deposit at McAra contains anomalous amounts of cobalt, suggesting the Archean host stratigraphy was the source for the high-grade cobalt zone. Basin brines in the Paleoproterozoic are interpreted to have leached cobalt from Archean rocks and then redeposited it through oxidation–reduction reactions along synvolcanic faults that controlled earlier VMS deposit formation. High-resolution aeromagnetic data show that McAra is immediately adjacent to a mafic dike that transects the Huronian basin along a northwest-striking, crustal-scale fault system. These data, alongside observations from field mapping, also suggest the deposit is on the margin of a sub-basin that contains an 80 km2 Nipissing sill that may have originally overlain the deposit area and been a hydrologic seal during mineralization. The new deposit- and regional-scale data and interpretations are used to create a model for the McAra deposit and provide evidence for why it is cobalt-rich relative to other five-element veins. The model and data can be used to guide exploration for additional cobalt-rich deposits in the region and similar settings globally.

Dating the Sedimentary Protolith of the Daldyn Group Quartzite, Anabar Shield, Russia: New Detrital Zircon Constraints

Quartzites and paragneisses of the Archean granulite series of the Anabar Shield (Siberian Craton, Russia) are described geochemically. The Sm-Nd isotope systematics of the rocks and the U-Pb age (SHRIMP II) and geochemistry of zircons from quartzites and paragneisses are studied. Newly formed zircons from quartzites display geochemical characteristics of the magmatic type and were produced by rock anatexis upon granulite-facies metamorphism. The Paleoproterozoic age of the latest detrital zircons, 2250 ± 24 Ma, constrains the maximum age of sedimentary rock deposition. The anatectic rims around detrital zircons were formed ca. 2000 ± 9 Ma ago. The time of deposition of the sedimentary protolith of gneisses and quartzites falls within the age interval of the above-mentioned dates and is tentatively accepted as 2.1 Ga. The presence of Paleoproterozoic metasedimentary rocks in the Daldyn Group implies the tectonic heterogeneity of the series and the existence of Paleoproterozoic rock bodies among the predominant Archean rock sequences.

Precambrian history of the eastern Ferris Mountains and Bear Mountain, south-central Wyoming Province

The eastern Ferris Mountains and Bear Mountain area of south-central Wyoming contain a complex assemblage of Archean and Proterozoic rock units, including a metasedimentary and metavolcanic supracrustal sequence named the Spanish Mine metamorphic suite, three granitic plutons (Turkey Creek, Ferris Mountains, Bear Mountain), and at least three sets of mafic dikes. The Spanish Mine metamorphic suite was deposited, intruded by mafic sills and (or) dikes, and underwent amphibolite-grade metamorphism and folding just prior to, or synkinematic with, the intrusion of the Turkey Creek metaplutonic suite, U–Pb dated at 2733.5 ± 2 Ma. A second set of mafic dikes intruded the Turkey Creek metaplutonic suite prior to mylonitic shearing and late-stage folding along the Miners Canyon shear zone. These events were followed by intrusion of the Ferris Mountains plutonic suite at ~2717 Ma. The emplacement of the yet undated granite of Bear Mountain records the last phase of Neoarchean magmatism. The timing of magmatism and deformation support a model that the Ferris Mountains basement rocks formed in an arc terrane distinct from the older Wyoming craton and were accreted to the province in the Neoarchean. Intrusion of a third mafic dike set is dated at 2113 ± 15 Ma and may date the silicified, epidote-rich zones that crosscut all of the Archean rock units. Lead isotopic compositions of galena within the Spanish Mine metamorphic suite indicate additional Proterozoic activity and mineralization.

Paleomagnetism of the Tulemalu dykes, Northwest Territories, Canada

Easterly trending Tulemalu diabase dykes that cut an area of predominantly Archean rock in southern Keewatin were magnetized more than 2200 Ma ago. Thirteen of the sampled sites (dykes) exhibit a southeast-down remanent magnetization and five sites give a more poorly grouped direction that is reversed to this. Their combined pole is at 122.4°E, 0.5°S, δm = 10.4°, δp = 6.0°. Six northeasterly trending Kazan dykes, which are probably correlatives of the Kaminak dykes, give a metamorphic pole at 94.2°W, 30.7°N, δm = 35°, δp = 31°. The age of their magnetization is thought to be between 1900 and 2000 Ma.The pole for the Tulemalu dykes lies within a group of poles of similar age derived from basic rocks of the Slave Province. It is about 30° away from the pole for the MacKay dykes, which occur north of Great Slave Lake and which have an easterly trend similar to that of the Tulemalu.Poles whose ages range between 1900 and 2200 Ma and that are derived from the Slave, western Churchill, and Superior provinces all suggest that during this period these three continental plates occupied approximately the same relative positions as they do today.