Abstract

The Selbaie mine is a large tonnage, low-grade Cu-Zn-Ag-Au deposit with both stratiform and epithermal characteristics. Mineralization comprises lower grade, disseminated Zn-Ag ore and pyritic siliceous sinter of the A1 zone at the surface; and higher grade Cu-Zn-Ag-Au quartz sulfide lenses of the A2 zone below and transitional with the A1 zone, and of the B zone approximately 1 km to the west. The copper-rich quartz sulfide lenses comprise networks of subvertical quartz sulfide veins, vein arrays, and breccia. They have irregular, locally gradational boundaries and variable attitudes, with dips from 25 degrees to 90 degrees . The mineralization is within a volcanic stratigraphic succession of heterolithic breccias and felsic and intermediate tuffs and flows, which are cut by mafic and felsic intrusions and felsite dikes. The felsic volcanic rocks and a tonalitic marginal phase from the adjacent Brouillan pluton are calc-alkalic and have geochemical signatures that support a cogenetic relationship. The mineralization and mine stratigraphic succession are truncated by the Brouillan faults, a series of subparallel, west-directed, predominantly brittle thrusts with the Brouillan pluton on the upper plate above the mine stratigraphic succession.Galenas from stratiform and vein ore have uniform lead isotope compositions of 206 Pb/ 204 Pb = 13.184 to 13.211, 207 Pb/ 204 Pb = 14.351 to 14.369, and 208 Pb/ 204 Pb = 33.039 to 33.100, with model mu values of 8.1-8.2, and model K values of 4.1 to 4.2 (n = 5); among the most primitive of Abitibi volcanic-associated massive sulfide deposits. They have model ages that range from 2.71 to 2.68 Ga.A U-Pb zircon age of 2729 (super +3) (sub -2) Ma for a sericite-altered, calc-alkalic quartz porphyry flow 150 m up section from stratiform mineralization closely approximates the age for this mineralization type. A U-Pb zircon age for a felsite dike that cuts postore east-west foliation is 2726 + or - 3 Ma, slightly younger but overlapping at the 2Sigma level with the age for the quartz feldspar porphyry flow, indicating that the copper-rich quartz sulfide lenses are essentially coeval with the lower grade stratiform mineralization. A sample of sheared, chloritic granodiorite taken from one of the Brouillan faults near the margin of the Brouillan pluton has a complex zircon population. Six concordant analyses of abraded, multigrain fractions, individual grains, or grain tips have 207 Pb/ 206 Pb minimum ages between 2729 and 2694 Ma (2Sigma errors <4 Ma). Decreasing 207 Pb/ 206 Pb ages are accompanied by decreasing Th, U, and Pb concentrations and decreasing Th/U, suggesting that the formation of low Th and U zircon rains occurred in the presence of fault-related fluids. These U-Pb data are interpreted as representing a 207 Pb/ 206 Pb minimum age for the Brouillan pluton at 2729 + or - 4 Ma, coeval with the quartz porphyry flow and both mineralization types; and an age of approximately 2.69 Ga for fluid migration during thrust fault movement.The geology, geochemistry, and geochronology at Selbaie describe (1) a transgressive, stratiform to epithermal mineralizing system that formed within a calc-alkalic, volcanic-plutonic suite at 2725 to 2730 Ma, at the peak of volcanic activity in the northern Abitibi subprovince; followed by (2) west-directed thrusting over the mineralization, resulting in fluid movement and partial to complete regrowth of zircon and resetting of the U-Th-Pb systematics within the fault at approximately 2.69 Ga.

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