By-product credits were lower in 2012 compared to 2011 due to lower zinc and lead prices, lower zinc production due to lower ore grades, and  lower gold production. By-product credits were lower in 2012 compared to 2010 due to lower zinc and lead prices and lower production for both  metals due to lower ore grades. The difference between what we report as “production” and “payable metal quantities sold” is due to the difference between the quantities of  metals contained in the concentrate we produce versus the portion of those metals actually paid for by our smelter customers according to the terms  of the smelter contracts (not all of the metals contained in our concentrate shipments are physically recovered through the smelting process).  Differences can also arise from inventory changes incidental to shipping schedules. While revenue from zinc, lead and gold by-products is significant, we believe that identification of silver as the primary product of the Greens Creek  unit is appropriate because: y silver has historically accounted for a higher proportion of revenue than any other metal and is expected to do so in the future; y we have historically presented Greens Creek as a producer primarily of silver, based on the original analysis that justified putting the project  into production, and believe that consistency in disclosure is important to our investors regardless of the relationships of metals prices and  production from year to year; y metallurgical treatment maximizes silver recovery; y the Greens Creek deposit is a massive sulfide deposit containing an unusually high proportion of silver; and y in most of its working areas, Greens Creek utilizes selective mining methods in which silver is the metal targeted for highest recovery. We periodically review our revenues to ensure that reporting of primary products and by-products is appropriate. Because we consider zinc, lead  and gold to be by-products of our silver production, the values of these metals offset operating costs within our cost per ounce calculations. The Lucky Friday Segment Years Ended December 31,  Dollars are in thousands (except per ounce and per ton amounts) 2012 2011 2010 Sales  $ 248 $ 134,728 $ 105,495 Cost of sales and other direct production costs  —  (52,180)  (47,159) Depreciation, depletion and amortization  —  (6,053)  (8,340) Gross profit $ 248  $ 76,495  $ 49,996  Tons of ore milled — 298,672 351,074 Production: Silver (ounces)  — 2,985,339 3,359,379 Lead (tons) — 18,095 21,619 Zinc (tons) — 7,305 9,286 Payable metal quantities sold:  Silver (ounces)  — 2,805,402 3,136,205 Lead (tons) — 16,983 20,213 Zinc (tons) — 5,465 6,850 Ore grades:  Silver ounces per ton — 10.69 10.25 Lead percent — 6.51 6.60 Zinc percent — 2.82 3.04 Mining cost per ton $ — $ 60.76 $ 54.27 Milling cost per ton $ — $ 16.96 $ 14.74 Total cash cost per silver ounce  (1) $ — $ 6.47 $ 3.76 (1) A reconciliation of this non-GAAP measure to cost of sales and other direct production costs and depreciation, depletion and amortization, the most comparable GAAP measure, can be found below in  Reconciliation of Total Cash Costs (non-GAAP) to Costs of Sales and Other Direct Production Costs and Depreciation, Depletion and Amortization (GAAP). The decrease in gross profit for 2012 compared to 2011 and 2010 resulted from the suspension of production at the Lucky Friday mine during 2012,  as discussed below. Gross profit for 2011 increased compared to 2010 due to higher average prices for all metals produced at the Lucky Friday (further  discussed in  Results of Operations  above), partially offset by higher costs of sales and other direct production costs due primarily to increased  employee profit sharing and taxes resulting from increased profitability.  At the end of 2011, MSHA began a special impact inspection at the Lucky Friday mine which resulted in an order to remove built-up cementitious  material from the Silver Shaft, as further discussed in  Item 2. Property Descriptions, Operating Properties, The Lucky Friday Unit.  The Silver Shaft is an  approximately one-mile deep, 18-foot diameter, concrete-lined shaft from surface. It is the primary access to the Lucky Friday mine’s underground  workings. In response to the MSHA order, we submitted a plan to MSHA and received approval to remove the built-up cementitious material, and that  work commenced in the first quarter of 2012. The plan also included removal of unused utilities, construction of a water ring to prevent ice from  [ HL:10-K ] 25 In the fourth quarter of 2012, we updated our asset retirement obligation (“ARO”) at Greens Creek to reflect a preliminary revised reclamation and  closure plan having estimated undiscounted costs of approximately $73.9 million, an increase from the $53.4 million in the previous plan. See  Note 4  of  Notes to Consolidated Financial Statements  for more information on the ARO update. We expect to again update our ARO and revise our closure  plan in 2013 for a tailings capacity expansion at Greens Creek which is currently under consideration by the government agencies. Adjustments to the  ARO liability are recorded with corresponding changes to the ARO asset balance, which is included in properties, plants, equipment, and mineral  interests, net on our Consolidated Balance Sheets. As a result, we do not anticipate future updates in the ARO for Greens Creek to have a material  impact on our annual results of operations. As part of the revised closure plan, we may be required to increase our current $30 million reclamation  bond for Greens Creek. Although we do not know the amount of such increase, it likely will be a material amount, and there can be no assurance that  this bonding capacity will be available to us at that time. 