recovery totaled approximately 73% silver, 78% zinc, 68% lead and 61% gold. The doré is further refined by precious metal refiners and sold to banks,  and the three concentrate products are sold to a number of major smelters worldwide. See  Note 11  of  Notes to Consolidated Financial Statements  for  information on the significant customers for Greens Creek’s products. Concentrates are shipped from the Hawk Inlet marine terminal about nine miles  from the mill. Underground exploration activities at the Greens Creek unit during 2012 focused on continued expansion of the 200 South, Gallagher, NWW and  Southwest Bench ore zones along trend of already-existing reserves and in new areas such as the Northeast and West contacts. Definition drilling of  the 200 South, 9a and NWW ore zones resulted in additions to reserves. Surface exploration drilling was conducted in 2012 on the Killer Creek and  West Gallagher targets near the Greens Creek mine where mineralization was identified. Approximately 25,000 feet of surface drilling is planned at the  Greens Creek unit for 2013. Electricity for the Greens Creek unit is provided through the purchase of surplus hydroelectric power from Alaska Electric Light and Power Company  (“AEL&P”), to the extent it is available after the power needs of Juneau and the surrounding area are met. When weather conditions are not favorable  to maintain lake water levels sufficient for all of the power needs at Greens Creek to be met by available hydroelectric power, the mine relies on power  provided by on-site diesel generators . The employees at Greens Creek are employees of Hecla Greens Creek Mining Company, our wholly-owned subsidiary, and are not represented by a  bargaining agent. There were 386 employees at the Greens Creek unit at December 31, 2012. As of December 31, 2012, we have recorded a $32.9 million asset retirement obligation for reclamation and closure costs. We maintained a $30  million reclamation bond for Greens Creek as of December 31, 2012. The net book value of the Greens Creek unit property and its associated plant,  equipment and mineral interests was approximately $681 million as of December 31, 2012. Based on current ore reserve estimates, the currently known remaining mine life at Greens Creek is 10 years. Information with respect to production,  average costs per ounce of silver produced and proven and probable ore reserves is set forth in the following table. Years Ended December 31,  Production 2012 2011 2010 Ore milled (tons) 789,569 772,069 800,397 Silver (ounces) 6,394,235 6,498,337 7,206,973 Gold (ounces) 55,496 56,818 68,838 Zinc (tons) 64,249 66,050 74,496 Lead (tons) 21,074 21,055 25,336 Average Cost per Ounce of Silver Produced  (1) Total cash costs $ 2.70 $ (1.29) $ (3.90) Total production costs $ 9.68 $ 5.19 $ 3.36 Proven Ore Reserves (2,3,4,5,6,7)  Total tons 12,000 — — Contained silver (ounces) 112,500 — — Contained gold (ounces) 1,100 — — Contained zinc (tons) 930 — — Contained lead (tons) 330 — — Probable Ore Reserves (2,3,4,5,6,7)  Total tons 7,845,600 7,991,000 8,243,100 Silver (ounces per ton) 12.0 12.3 12.1 Gold (ounces per ton) 0.09 0.09 0.09 Zinc (percent) 9.0 9.2 9.3 Lead (percent) 3.4 3.5 3.5 Contained silver (ounces) 94,481,200 98,383,300 99,730,000 Contained gold (ounces) 718,400 742,400 757,000 Contained zinc (tons) 702,300 733,140 766,500 Contained lead (tons) 267,410 281,620 291,300 Total Proven and Probable Ore Reserves  (2,3,4,5,6,7)  Total tons 7,857,600 7,991,000 8,243,100 Silver (ounces per ton) 12.0 12.3 12.1 Gold (ounces per ton) 0.09 0.09 0.09 Zinc (percent) 9.0 9.2 9.3 Lead (percent) 3.4 3.5 3.5 Contained silver (ounces) 94,593,700 98,383,300 99,730,000 Contained gold (ounces) 719,500 742,400 757,000 Contained zinc (tons) 703,230 733,140 766,500 Contained lead (tons) 267,740 281,620 291,300 (1) Includes by-product credits from gold, lead and zinc production. Cash costs per ounce of silver represents a measurement that is not in accordance with GAAP that management uses to monitor and  evaluate the performance of our mining operations. We believe cash costs per ounce of silver provides an indicator of economic performance and efficiency at each location and on a consolidated basis,  14 [ HL:10-K ] 