Mineralogy, geochronology, and genesis of the Andrew Lake uranium deposit, Thelon Basin, Nunavut, Canada

The Thelon Basin located in Nunavut, Canada, shares many similarities with the U-producing Athabasca Basin in Saskatchewan. The Kiggavik project area, located near the northeastern edge of the Thelon Basin, contains U deposits and showings along the ∼30 km long NE–SW Kiggavik – Andrew Lake structural trend. The Andrew Lake deposit is near the southern end of this trend. Pre-mineralization is characterized by quartz ± carbonate veins that occupy fault systems later reactivated as conduits for U-mineralizing fluids. A four-phase genetic model is proposed for the Andrew Lake deposit. Phase 1 comprises vein-style uraninite (U1; 1031 ± 23 Ma) that is associated with illite and hematite, and contains variable PbO contents (0.2–9.5 wt.%). Phase 2 is characterized by altered uraninite (U2; ∼530 Ma) that is associated with coffinite. Altered uraninite (U3; <1 Ma) characterizes phase 3 and occurs as centimetre-scale “roll-fronts”. In phase 4, all three uraninite stages, and coffinite, are altered to boltwoodite. Although the oldest uraninite U–Pb age is ∼1030 Ma, illite associated with the U mineralization gives 40Ar/39Ar ages of 941 ± 31 and 1330 ± 36 Ma. The younger age is similar to the age for U1, suggesting that there was a fluid event that either precipitated U1 or reset the U–Pb isotopic system at ∼1000 Ma. While the older age for illite (1330 Ma) does not correlate with Andrew Lake U–Pb uraninite ages, it does correlate with ages previously reported for uraninite and clay alteration minerals in the Kiggavik area.

A 2169 Ma U–Pb baddeleyite age for the Otish Gabbro, Quebec: implications for correlation of Proterozoic magmatic events and sedimentary sequences in the eastern Superior Province

Otish Gabbro sills intrude sedimentary rocks in the Otish Basin of the southeastern Superior Province. Here, deposition of Otish Supergroup sediments had previously been thought to be older than K–Ar and Sm–Nd ages of ca. 1750–1710 Ma for Otish Gabbro sills, and younger than ca. 2515–2500 Ma U–Pb ages of underlying Mistassini dykes. However, a much older U–Pb baddeleyite age of 2169.0 ± 1.4 Ma is presented here for an Otish sill, indicating that they are coeval with, and likely genetically related to, the giant 2172–2167 Ma Biscotasing dyke swarm to the southwest and (or) the Cramolet sills and Payne River dykes to the north. The new date also indicates that the age of the Otish Supergroup falls between ca. 2515 Ma and ca. 2169 Ma, only a little different from the ca. 2450–2217 Ma bracket for the Huronian Supergroup of the Southern Province, and is consistent with both supergroups spanning the oxy-atmo inversion. The Otish Supergroup could also be coeval with the Sakami Formation to the north, but is likely older than the Richmond Gulf Group on the east coast of Hudson Bay. Early paleomagnetic study of Otish sills yielded a remanence ∼20° from that expected for Biscotasing-aged intrusions. This may indicate that too few distinct sills were studied to average out paleosecular variation, that demagnetization techniques failed to fully remove unstable magnetization components, or that the remanence is a stable secondary overprint, perhaps acquired during a fluid event related to uranium mineralization at ca. 1720 Ma.

THE A-TYPE KERKINI GRANITIC COMPLEX IN NORTH GREECE: GEOCHRONOLOGY AND GEODYNAMIC IMPLICATIONS

The Kerkini granitic complex (KGC) intrudes the Serbomacedonian massif KGC comprises the Mûries granite (MUR), the Miriofito granite (MIR), and the Kastanusa (KAS) granodiorite. The main rock-type is two-mica granite. Feldspars are represented by albite andperthitic microcline, biotite is iron-rich and white mica is phengite. Fluorite is also present. The rocks are peraluminous, enriched in total alkalis, depleted in MgO and CaO and have high FeOt/MgO ratios. They are enriched in Zr, Nb, Y, Ga and REE, and have strong negative Eu anomaly. They plot in the Atype granite fields of various discriminant diagrams and their chemistry suggests a WPG tectonic environment. Sr initial ratio ranges from 0.7107 to 0.7182. The most probable genetic model is fluid-absent melting of a biotite-rich tonalitic crustal source at 950 -975 C and at considerable depths. Rb-Sr white mica ages and SHRIMP U-Pb zircon ages yielded 246±3 Ma and 247±2Ma, respectively, interpreted as the crystallization age of the KGC. K-Ar ages of 130±3 and 131 ±3 Ma (biotite) and 133±3 Ma (white mica) can be interpreted by a metamorphic/fluid event at about 133 Ma. Rb-Sr white mica dates at 152±2 Ma probably resulted by incomplete resetting of the Rb-Sr isotopie system and yielded "mixing ages" between crystallization (ca. 247 Ma) possibly related to a Permian - Triassic rift event and metamorphic/fluid event (ca. 133 Ma).