Osisko Intersects 611 g/t Au Over 2.0 Metres At Windfall

Additional High Grade Results from Caribou, Wolf, Zone 27 and Underdog

(Toronto. September 20, 2017)  Osisko Mining Inc. (OSK:TSX. “Osisko” or the “Corporation”) is pleased to provide new results from the ongoing drill program at its 100% owned Windfall Lake gold project located in Urban Township, Abitibi, Québec. The recently expanded 800,000 metre drill program combines definition, expansion and exploration drilling in and around the main Windfall gold deposit and in the adjacent Lynx deposit (located immediately NE of Windfall). Significant new analytical results from 42 intercepts in 28 drill holes and wedges focused on infill and expansion drilling in the Underdog, Caribou, Zone 27 and Mallard corridors of the Windfall deposit are presented below.
 
Highlights from the new results include: 611 g/t Au over 2.0 metres (15.4 g/t Au over 2.0 metres cut) in OSK-W-17-913; 297 g/t Au over 2.1 metres (14.5 g/t Au over 2.1 metres cut) in OSK-W-17-862-W1; 120 g/t Au over 2.0 metres (28.7 g/t Au over 2.0 metres cut) in OSK-W-17-871-W1; 60.5 g/t Au over 2.4 metres in OSK-W-17-973; 65.5 g/t Au over 2.0 metres in OSK-W-17-978; and 47.0 g/t Au over 2.3 metres in OSK-W-17-820-W1. Maps showing hole locations and full analytical results are available at www.osiskomining.com.

Notes:

1. True widths are estimated at 65 – 80% of the reported core length interval.  See “Quality Control” below.
2. Definitions: FW = foot wall; HW = hanging wall; VNCR = crustiform vein.

Underdog Corridor

OSK-OBM-16-601 intersected 16.2 g/t Au over 2.0 metres. Mineralization is composed of 5% pyrite stringers and ptygmatic tourmaline veins and disseminated pyrite hosted in a strongly sericitized andesite. The interval extends the FW3 upper 40 metres north-east of OSK-OBM-16-609 (63.2 g/t Au over 7.7 metres previously reported August 1, 2017).

OSK-W-17-820-W1 returned two intersections in the FW0 Footwall Zone: 5.78 g/t Au over 2.0 metres (including 11.3 g/t Au over 1.0 metre) and 9.02 g/t Au over 2.0 metres (including 12.0 g/t Au over 1.0 metre). Mineralization is composed of 1% disseminated pyrite or pyrite stringers within a fragmental felsic intrusion. The two intersections are 50 metres up-plunge and south-west of OSK-W-17-820 (5.79 g/t Au over 6.3 metres previously reported July 19, 2017). OSK-W-17-820-W1 also intersected the FW3 Zone returning 31.7 g/t Au over 2.2 meters, (including 115 g/t Au over 0.6 metres) (27.7 g/t Au over 2.2 metres cut) and the Z14 Zone, 47.0 g/t Au over 2.3 metres (including 266 g/t Au over 0.4 metres) (18.1 g/t Au over 2.3 metres cut). Both intersections are similar and associated with felsic porphyritic dikes with up to 2% pyrite stringers and local quartz veins. The interval in FW3 is 40 metres south-west of OSK-W-17-821 (9.27 g/t Au over 3.3 metres previously reported May 30, 2017).

OSK-W-17-847 intersected two intervals: 6.87 g/t Au over 2.0 metres and 8.85 g/t Au over 3.9 metres in the Underdog corridor. In the first interval, mineralization is within the fragmental felsic dike and composed of 1% pyrite stringers, and correlates with the FW0 hanging wall. In the second interval mineralization is composed of up to 20% pyrite stringers and pyrite-tourmaline stringers hosted in a strongly silicified felsic porphyritic intrusion. The interval correlates to the FW2 Zone 50 metres north-east of OBM-15-560 (11.0 g/t Au over 6.1 metres previously reported January 28, 2016).

OSK-W-17-855-W1 intersected 21.0 metres over 2.0 metres (including 41.2 g/t Au over 1.0 metre). Mineralization is composed of up to 30% pyrite in a fragmental felsic dike. The interval correlates to the FW1 Zone.

OSK-W-17-855-W2 intersected 26.1 g/t Au over 3.5 metres (including 66.2 g/t Au over 0.4 metres, 33.5 g/t over 0.7 metres and 31.5 g/t Au over 1.3 metres) in the FW3 Zone. Mineralization is composed of up to 7% pyrite stringers, traces of tourmaline-pyrite stringers, up to 5% disseminated pyrite, and local decimeter scale pyrite massive bands. It is hosted in a porphyritic felsic dike with quartz-eyes and strong silica alteration. This intersection is 50 metres north-east of OSK-W-17-807-W2 (32. 9 g/t Au over 2.0 metres previously reported August 14, 2017). OSK-W-17-855-W2 also intersected 9.2 g/t over 2.2 metres (including 18.6 g/t Au over 1.0 metre) in the Z14 Zone. Mineralization is composed of 1% disseminated pyrite associated to large quartz-eyes porphyritic dike. It is 70 metres north-east of OSK-W-17-807-W2 (4.29 g/t Au over 2.0 metres previously reported June 20, 2017).

OSK-W-17-864 intersected 14.0 g/t Au over 2.7 metres (including 28.6 g/t Au over 1.0 metre) in the FW3 Upper Zone. Mineralization is composed of up to 3% disseminated pyrite and up to 1% pyrite stringers hosted in the andesite. It is 25 metres south-west of OSK-W-17-854-W2 (18.6 /t Au over 3.5 metres previously reported August 1, 2017) and 15 metres north-east of OSK-W-17-871-W1 (120 g/t Au over 2.0 metres, this press release).

OSK-W-17-871-W1 intersected 120 g/t Au over 2.0 metres, (including 557 g/t Au over 0.4 metres) (28.7 g/t Au over 2.0 metres cut). Mineralization is composed of local visible gold, up to 10% disseminated pyrite, up to 10% pyrite tourmaline stringers and 2% pyrite stringers in a sericitized and chloritized rhyolite. The intersection is related to the FW3 Upper Zone and is 40 metres south-west of OSK-W-17-854-W2 (18.6 /t Au over 3.5 metres previously reported August 1, 2017).

OSK-W-17-885 returned two intersections: 6.69 g/t Au over 8.0 metres (including 22.3 g/t Au over 1.0 metre) and 6.55 g/t Au over 2.6 metres (including 18.2 g/t au over 0.8 metres). The first interval consists of a strongly sericitized and moderately silicified felsic intrusion with 2-5% disseminated pyrite and 1-2 % pyrite stringers in the FW0 Footwall Zone and is 100 metres north-east of OSK-17-780-W1 (5.97 g/t Au over 17.0 metres previously reported April 11, 2017). The second interval is a silica-rich zone with 3-5% tourmaline-pyrite stringers and 2% disseminated pyrite. This intersection is in the FW3 Zone and is 50 metres south-west of OSK-W-17-780 (9.38 g/t Au over 4.0 metres previously reported April 6, 2017).

OSK-W-17-889 intersected 4.35 g/t Au over 3.5 metres in the Wolf 2 Zone. Mineralization contains up to 20% pyrite stringers within a bleached andesite. It is 40 metres west-south-west of OSK-W-17-844 (8.14 g/t Au over 2.5 metres previously reported August 1, 2017).

OSK-W-17-1051 intersected 5.36 g/t Au over 2.7 metres. The strongly sericitized felsic dike hosts 3% pyrite stringers and trace tourmaline stringers. The interval correlates to Z14 Zone, a division of the FW3 Zone, 30 metres north-east of OSK-W-16-708-W1 (3.74 g/t Au over 3.0 metres previously reported August 31, 2016).

Caribou Corridor

OSK-EAG-13-494 intersected two intervals in Wolf: 6.52 g/t Au over 2.1 metres (including 19.6 g/t Au over 0.6 metres) and 4.56 g/t Au over 2.0 metres. Both intersectiions follow a contact between the andesite and a large quartz-eye porphyritic intrusion, with mineralization comprising up to 2% disseminated pyrite and pyrite stringers and associated with strong sericite alteration. The intersections infill 35 meters north-east of OSK-W-16-708-W1, and 25 meters south-west of OSK-16-708-W2 (12.5 g/t Au over 3.8 metres and 4.47 g/t Au over 2.8 metres, previously reported August 31 and September 12, 2016).

OSK-W-17-842 intersected the CS1 Zone within the Caribou Corridor and returned 14.5 g/t Au over 5.5 metres (including 182 g/t Au over 0.4 metres) (8.90 g/t Au over 5.5 metres cut). Mineralization is composed of 10% pyrite stringers and tourmaline-pyrite veins hosted in a fragmental andesite strongly sericitized, and is 50 metres north-east of OSK-W-16-749 (12.7 g/t Au over 5.4 metres previously reported January 23, 2017).

OSK-W-17-858-W2 intersected three intervals: 8.51 g/t Au over 2.3 metres; 32.6 g/t Au over 2.4 metres (including 76.6 g/t Au over 0.9 metres) and 9.50 g/t Au over 2.0 metres (including 34.5 g/t Au over 0.5 metre). The first interval correlates to the Wolf Zone and is composed of 2% pyrite and carbonate veins in a strongly sericitized felsic dike. The second and third intervals consists of a crustiform quartz carbonate vein in the Red Dog intrusion and in the andesite. Local visible gold, 5% pyrite stringers and tourmaline-pyrite stringers are also observed. The deepest vein is within the FW3 Upper Zone, 25 metres north-east of OSK-OBM-16-697 (29.1 g/t Au over 2.6 metres previously reported May 30, 2017).

OSK-W-17-862 intersected 17.4 g/t Au over 2.9 metres (including 30.8 g/t Au over 1.5 metres). Mineralization is at a contact between a gabbro and rhyolite with up to 4% disseminated pyrite in the silica altered zone. The interval is 30 metres above OSK-W-17-889 (16.2 g/t Au over 2.0 metres in this press release).

OSK-W-17-871 was previously reported on August 1,2017 with a cut value of 13.4 g/t Au over 4.8 metres. Additional analysis was done on the samples greater than 100 g/t and the interval has been recalculated to 17.9 g/t Au over 4.8 metres (including 143 g/t Au over 0.5 metres). The cut value remains unchanged. The mineralized zone is in a bleached to strongly sericitized large quartz-eye felsic intrusion containing 7% pyrite stringers. The intersection is 25 metres north-east of OSK-W-17-901 (16.2 g/t Au over 2.1 metres previously reported August 1, 2017) within the Caribou Corridor.

OSK-W-17-889 intersected 16.2 g/t Au over 2.0 metres (including 58.0 g/t Au over 0.5 metres) in the main Caribou Zone. Mineralization is at a contact between the gabbro and the rhyolite. The 1% pyrite stringers are associated with strong sericite alteration and a quartz-tourmaline vein. The interval is 30 metres below OSK-W-17-862 (17.4 g/t over 2.9 metres in this press release).

OSK-W-17-901 intersected two intervals: 13.8 g/t Au over 2.0 metres and 7.12 g/t Au over 2.0 metres (including 13.3 g/t Au over 1.0 metre). In both intervals, mineralization is composed of up to 1% disseminated pyrite and 1% pyrite stringers. The host rhyolite is strongly alterated in silica, sericite and carbonates. The first interval correlates to Caribou Main Zone and the second interval to Caribou footwall, 65 metres below OBM-16-660 (7.25 g/t Au over 6.9 metres previously published July 11, 2016).

OSK-W-17-913 intersected 611 g/t Au over 2.0 metres (including 4070 g/t Au over 0.3 metres) (15.4 g/t Au over 2.0 metres cut). The high-grade zone contains local visible gold and 1% pyrite stringers associated with pervasive silica flooding within a felsic dike. The host rock is strongly sericitized. The intersection correlates to the Wolf 2 Zone, 50 metres east-north-east of OSK W-16-718 (8.5 g/t Au over 4.7 metres previously reported September 19, 2016). The hole also intersected 13.6 g/t Au over 2.0 metres (including 26.5 g/t Au over 1.0 metre) in the Wolf footwall. Mineralization is associated with a large quartz-eyes porphyritic dike and composed of disseminated pyrite and trace of pyrite stringers. The interval is 40 metres north-east of OSK-W-16-718 (4.78 g/t Au over 2.6 metres previously reported September 19, 2016).

OSK-W-17-989-W1 intersected two proximate intervals in Caribou South 1 Zone: 3.59 g/t Au over 7.0 metres and 7.74 g/t Au over 3.0 metres (including 20.5 g/t Au over 1.0 metre). Mineralization is associated to a porphyritic felsic intrusion crosscutting a strongly sericitized and silicified andesite. Up to 20% pyrite stringers are observed. The interval is 65 metres south-west of OSK-W-16-709 (4.90 g/t Au over 3.7 metres previously reported August 23, 2016).

Zone 27

OSK-W-17-862-W2 intersected 297 g/t Au over 2.1 metres (including 2080 g/t Au over 0.3 metres) (14.5 g/t Au over 2.1 metres cut). The mineralization correlates to Zone 27 along the Red Dog intrusion contact, with visible gold associated with disseminated pyrite and pervasive silica alteration in a strongly silicified andesite.

OSK W-17-903 intersected 10.7 g/t Au over 2.0 metres (including 37.7 g/t Au over 0.5 metres). Mineralization consists of 2% pyrite stringers, 3% pyrite-tourmaline stringers and 5% disseminated pyrite hosted in silica and sericite altered andesite. The intersection correlates to Zone 27, 45 metres above OSK-W-16-746 (16.5 g/t Au over 3.7 metres previously reported January 5, 2017).

OSK-W-17-968 intersected 5.77 g/t Au over 2.4 meters in Zone 27. Mineralization consists of 3% quartz-tourmaline veins and 5% pyrite stringers in a strongly sericitized felsic dike. This intersection is 15 meters above OBM-16-600 (12.9 g/t Au over 4.2 metres previously reported March 16, 2016).

OSK-W-17-973 intersected two intervals in Zone 27: 60.5 g/t Au over 2.4 metres (including 122 g/t Au over 1.0 metre) (51.6 g/t Au over 2.4 metres cut) and 5.12 g/t Au over 2.1 metres (including 25.2 g/t Au over 0.4 metres). The first interval is composed of semi-massive pyrite in a silica altered porpyritic felsic dike. The second interval is at the contact between two felsic dikes and is composed of 1% pyrite in clusters or stringers.

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