The silidor deposit, Rouyn-Noranda District, Abitibi Belt: Geology, structural evolution, and paleostress modeling of an Au quartz vein-type deposit in an Archean trondhjemite

Late orogenic gold-bearing quartz vein deposits within the Rouyn-Noranda mi ning district occur mainly within Archean tonalitic plutons of the Blake Ri ver Group (southern Abitibi belt). The Silidor deposit is a representative example of a pluton-hosted lode gold deposit. The Silidor mine contained 2. 95 million tons (Mt; mined and estimated reserves), grading 5.1 g/t Au (sim ilar to 15 t Au). The mineralized zone is 900 m in length and has a vertica l extent of 900 m, an average thickness of 3.5 m, and trends northwest-sout heast with a dip of 50 degrees to 70 degrees NE. Its alteration envelope co nsists of a red hematite-altered trondhjemite. Measured delta(18)O values f or quartz veins (7.7-10.9 parts per thousand) and delta(34)S values for pyr ite (-6.1 to -9.4 parts per thousand) imply oxidizing conditions during gol d de position. The mineralized zone comprises: vein quartz (white, gray, an d smoky varieties), beige mineralized trondhjemite, and green carbonate-ser icite-fuchsite breccia, which resulted from shearing and metasomatism of an early northwest-southeast dioritic dike. A quantitative microtectonic study (>400 measurements) was carried out on t he Powell tonalitic sill in the Silidor mine area to reconstruct the deform ation before, during, and after the mineralizing events. The reduced stress tensors display large variations in sigma 1 orientation trending successiv ely northwest-southeast. northeast-southwest,;md finally north-south. The S ilidor deposit is the result of several vein-filling events, which occurred during evolution from strike-slip faulting to reverse faulting regimes, wi th al remaining northeast-southwest but with exchange of orientation of sig ma 2 and sigma 3. Such variations may reflect an oblique collision and proc esses of stress permutation. Paleostress mapping of Archean terranes can be used as a targeting tool for mesothermal lode gold deposits. Reduced stress tensors were used for tilt: computation of paleostress maps, using a distinct element model. The Silid or Au quartz mineralization appears on these paleostress maps in an area ch aracterized by low mean stress throughout the deformation history of the ar ea near the Horne Creek fault. This study emphasizes the role of second-ord er faulting in the location of low- and high-pressure zones in the Archean crust and the possible role of a tectonic indentor in the location of Au mi neralization.