MONTREAL, QUEBEC--(Marketwire - March 31, 2011) - Osisko Mining Corporation (News - Market indicators)(FRANKFURT:EWX) is pleased to provide an updated reserve and resource estimate for its 100%-owned Canadian Malartic project. This new estimate, calculated at US$1000 gold, combines the previously-reported US$825 reserves and resources of the Canadian Malartic (CM) and South Barnat deposits (see February 10 2010 press release) with new resources defined from the ongoing drill program, including the Barnat Extension and Gouldie Zones.

Belzile Solutions Inc. of Rouyn-Noranda, Québec ("BSI") and G Mining Services of Montreal, the independent resource/reserve estimate consultants for Osisko, have authorized the release of the following estimates. Highlights include:

• The open pit Proven & Probable reserve has increased to 10.71 million (M) ounces gold at a fully diluted average grade of 0.97 g/t, a 1.74 million ounce or 19.3 percent increase relative to the previously-published estimate of 8.97 million ounces (see February 10, 2010 press release);
  
  
• Global Measured and Indicated (M&I) resources above a cut-off grade of 0.30 g/t Au have increased to 11.80 M ounces gold, a 600,000 ounce or 5.4 percent increase relative to the previously-published estimate of 11.20 M ounces (see December 14, 2009 press release);
  
  
• Global Inferred resources above a cut-off grade of 0.30 g/t Au have increased to 850,000 ounces gold, a 510,000 ounce or 150% percent increase relative to the previously-published estimate of 340,000 ounces (see December 14, 2009 press release);
  
  
• In-pit Inferred resources (not included in reserves) consist of an additional 197,000 oz gold (9.0 M tonnes at 0.68 g/t Au), which was considered as waste and could potentially be converted to reserves with additional drilling.
  
  
• Recoverable gold has increased by 1.47 M ounces to 9.19 M ounces (at 85.8% recovery) from 7.72 M ounces (at 86.1% recovery; see February 10, 2010 press release).
  
  
• 7.21 M ounces of gold in the in-pit M&I resource category, representing 67.3 percent of the reserve, has an average grade of 1.80 g/t using a very conservative cut-off grade of 1 g/t gold.
  
  
• At a gold price of US$1400 per ounce, the in-pit diluted M&I resource increases to 11.80 M ounces of gold at an average grade of 0.84 g/t Au.
  
  
• These reserve/resource estimates were calculated from a robust database comprising 636,200 metres of drilling completed by Osisko since March 2005, as well as 228,500 metres of historical drilling that was tested and validated.

Reserve and resource estimates using base case US$1000 engineered pit design with 0.30 g/t Au (South Barnat) to 0.32 g/t Au (CM) lower cut-off grade

Sean Roosen, President and CEO, noted: "We are very proud to be able to present our shareholders with these updated reserve and resource estimates. We have increased our reserves by over 19 percent with respect to last year and by over 70 percent with respect to the 2008 Feasibility Study. This is a very significant increase in reserves which will positively impact the mine life and economics of the project. Ongoing drilling continues to define new mineralization in areas such as the Jeffrey and Western Porphyry Zones, which has not been included in this reserve/resource estimate".

"I would also like to add that using a very conservative cut-off grade of 1 g/t gold, the in-pit M&I resource is 7.21 million ounces gold, that is 67 percent of the reserve, at an average undiluted grade of 1.80 g/t Au. Furthermore, at US$1400 per ounce gold, the in-pit M&I resource increases to 11.80 million ounces at an average diluted grade of 0.84 g/t gold, virtually all of which could potentially be converted to reserves, if one assumes a similar conversion factor as in the case of the present estimates."

For the purpose of the reserve estimate, the optimal Whittle pit shell was used as a guideline for the manual design of the engineered pit, and only the in-pit measured and indicated resources were considered. Optimization parameters included a gold price of US$1000 per ounce, processing costs of US$5.66 per tonne, total ore-based costs of US$7.18 per tonne, average metallurgical recovery of 85.8% and inter-ramp pit slope angles between 46 and 55 degrees. The strip ratio is estimated at 2.28 with average diluted grade of 0.97 g/t Au based on a calculated dilution of 8.9%. The quoted mineral reserve is exclusive of estimated historical stope crown pillar losses. The proven & probable reserves lost in crown pillars are estimated at 2.8 M tonnes at an average grade of 1.98 g/t Au for 178,260 ounces gold.

Details on updated costs, mine plan, production schedules and mine life will be provided in Q2 2011. Projected production for 2011 and 2012 may be less than previously projected (see February 10, 2010 press release and subsequent disclosure), due to the increased overall reserve and its effect on the mine plan. The new mine plan will be modified to include new ounces defined in lower grade zones that require incorporation into the 2011 – 2012 mining schedule, to accommodate required waste dumps over these newly identified mineralized deposits. For example, modifications to the mine plan now include earlier mining of the Gouldie zone (reserves of approximately 185,652 ounces at a diluted grade of 0.91 g/t Au) which requires early extraction because of its location. Furthermore, noise levels imposed on operations currently restrain ramping up to full production capacity of 55,000 tpd. Osisko continues to implement noise reduction measures and is working to resolve noise level issues with the Quebec government to allow production ramp-up to full capacity for Q3 2011.

Updated Resource Estimates

BSI, in collaboration with G Mining Services Inc. of Montreal, estimated an in-pit M&I resource within a single Whittle-optimized pit shell using a base case gold price of US$1000 per ounce. The total in-pit M&I resource for the Canadian Malartic Project is 10.63 million ounces of gold at an average undiluted grade of 1.08 g/t Au, with an additional 0.20 million ounces gold at an average grade of 0.68 g/t Au in the inferred category, based on a derived lower cut-off grade of 0.32 g/t Au for the Canadian Malartic portion of the pit and a derived lower cut-off grade of 0.30 g/t Au for the South Barnat portion of the pit. The tables below summarize the in-pit estimates using variable lower cut-off grades:

Canadian Malartic Undiluted Resource Estimates within US$1000 Whittle Pit Shell

Sensitivity of the in-pit diluted M&I resource to gold price is as follows (inferred resources excluded):

The global Measured and Indicated ("M&I") resource is 11.80 million ounces of gold at an average undiluted grade of 1.04 g/t Au, with an additional 0.85 million ounces gold at an average grade of 0.78 g/t Au in the Inferred category, based on a lower cut-off grade of 0.30 g/t Au at US$1000 per ounce gold. The tables below summarize the BSI estimates using variable lower cut-off grades:

Canadian Malartic Updated Global Resource Estimates

Details on the parameters of the resource estimates are as follows:

• The database comprised a total of 228,500 metres of drilling validated from a larger database of historical drilling (Canadian Malartic Mines and Lac Minerals) and from approximately 636,200 metres of drilling completed and assayed by Osisko as of the end of January 2011 on a maximum 25 m x 25 metre grid. (All NQ or HQ core assays reported by Osisko were obtained by standard 50 g fire assaying-AA finish or gravimetric finish at ALS Chemex laboratories in Val d'Or, Quebec.)
  
  
• The database also comprised a total of 572,280 assays with an average of 1.39 metre per sample for a total of 793,939 assayed metres.
  
  
• The estimates were done using Ordinary Kriging (OK) as the geostatistical interpolation method based on 5.0 metre analytical composites. Resources were also estimated using Inverse Distance Squared (ID2) interpolation for testing and comparative purposes, which produced similar results, i.e. less than 2 percent difference in total ounces at all cut-offs.
  
  
• Calculations were based on original samples cut to a maximum of 10 g/t to 70 g/t Au depending on the domain. The apparent reduction in the gold content varies from 0 to 14.5% depending of the domains. All five metre composites were calculated based on cut original data.
  
  
• All estimates were based on a Parent Cell dimension of 20 metres E, 10 metres N and 10 metres height with estimation parameters determined by variography.
  
  
• Geological interpretation identified thirty-seven different domains (sixteen in Canadian Malartic, thirteen in South-Barnat and eight in Gouldie).
  
  
• Underground voids were modeled from historical mine plans and adjusted according to positions of drill intersections in stopes and drifts. Void volumes of stopes were increased by one metre along all edges to compensate for uncertainties. The void models were then used to deplete the resource estimate. All samples located within the voids were also removed from the estimate.
  
  
• Tonnage estimates were based on rock densities of 2.69 tonnes/cubic metre for the porphyry, 2.75 for altered sediments and 2.83 for altered ultramafic rocks.
  
  
• The resource estimates using the lower cut-offs of 0.32 g/t Au (CM) to 0.30 g/t Au (Barnat) are emphasized for reporting purposes as this is the in-pit cut-off estimated for the US$1000 Whittle shell.
  
  
• The US$1000 engineered pit has approximate maximum dimensions of 3,485 metres in length, 1,265 metres in width and a vertical depth of 400 metres. 

Mr. Elzéar Belzile, P. Eng. of BSI, Mr. Louis-Pierre Gignac, P. Eng. of G Mining Services Inc. and Mr. Robert Wares, P. Geo. and Executive Vice-President of Osisko, are the Qualified Persons who have reviewed this news release and are responsible for the technical information reported herein, including verification of the data disclosed. A NI 43-101 compliant report on these updated resource and reserves estimates will be filed on SEDAR within 45 days of the date of this press release.

Cautionary Notes Concerning Estimates of Mineral Resources

This news release uses the terms measured, indicated and inferred resources as a relative measure of the level of confidence in the resource estimate. Readers are cautioned that mineral resources are not economic mineral reserves and that the economic viability of resources that are not mineral reserves has not been demonstrated. In addition, inferred resources are considered too geologically speculative to have any economic considerations applied to them. It cannot be assumed that all or any part of an inferred mineral resource will ever be upgraded to a higher category. Under Canadian rules, estimates of inferred mineral resources may not form the basis of feasibility or pre-feasibility studies or economic studies except for Preliminary Assessment as defined under NI 43-101. Readers are cautioned not to assume that that further work on the stated resources will lead to mineral reserves that can be mined economically.

Forward-Looking Statements

Certain statements contained in this press release may be deemed "forward-looking statements". All statements in this release, other than statements of historical fact, that address events or developments that Osisko expects to occur, are forward looking statements. Forward looking statements are statements that are not historical facts and are generally, but not always, identified by the words "expects", "plans", "anticipates", "believes", "intends", "estimates", "projects", "potential", "scheduled" and similar expressions, or that events or conditions "will", "would", "may", "could" or "should" occur including, without limitation, the commencement of commercial production. Although Osisko believes the expectations expressed in such forward-looking statements are based on reasonable assumptions, including, without limitation, that all technical, economical and financial conditions will be met in order to warrant further development of the project, such statements are not guarantees of future performance and actual results may differ materially from those in forward looking statements. Factors that could cause the actual results to differ materially from those in forward-looking statements include gold prices, access to skilled consultants, mining development personnel, results of exploration and development activities, Osisko's limited experience with production and development stage mining operations, uninsured risks, regulatory changes, defects in title, availability of personnel, materials and equipment, timeliness of government approvals, actual performance of facilities, equipment and processes relative to specifications and expectations, unanticipated environmental impacts on operations market prices, continued availability of capital and financing and general economic, market or business conditions. These factors are discussed in greater detail in Osisko's most recent Annual Information Form and in the most recent Management Discussion and Analysis filed on SEDAR, which also provide additional general assumptions in connection with these statements. Osisko cautions that the foregoing list of important factors is not exhaustive. Investors and others who base themselves on forward-looking statements should carefully consider the above factors as well as the uncertainties they represent and the risk they entail. Osisko believes that the expectations reflected in those forward-looking statements are reasonable, but no assurance can be given that these expectations will prove to be correct and such forward-looking statements included in this press release should not be unduly relied upon. These statements speak only as of the date of this press release.