Notes to Consolidated Financial Statements  Years ended December 31, 2012 and 2011 26. Accounting for Financial Instruments (continued)  Prepayment Rights on Notes Due 2016 and 2019 Our 2016 and 2019 notes (Note 17(a)) included prepayment rights that were considered to be embedded derivatives.  These notes were redeemed in 2012 so there is no value for the embedded derivatives at December 31, 2012. At  December 31, 2011, these prepayment rights were recorded as other assets (Note 12) on the balance sheet at a fair  value of $313 million based on then-current market interest rates for similar instruments and our credit spread. In addition to the above, one of our road and port contracts contains a contingent zinc escalation payment that is  considered to be an embedded derivative. The fair value of this embedded derivative was $7 million at December 31,  2012 (2011 - $3 million), and is included in other liabilities and provisions on the consolidated balance sheet. Derivatives not designated as hedging instruments are recorded in trade accounts receivable of $3 million and trade  accounts payable and accrued liabilities of $4 million on the consolidated balance sheet. All free-standing derivative contracts mature in 2013. The fair value of our fixed commodity forward sale and purchase contracts is calculated using a discounted cash flow  method based on forward metal prices. A summary of our free-standing derivative contracts and related fair values as  at December 31, 2012 is as follows: Fair Value  Asset (Liability) Quantity  Average Price  (Cdn$ in millions) Derivatives not designated as hedging instruments Forward sales contracts Zinc 12.5 million lbs  US$0.91/lb  $  (1) Lead  27.4 million lbs  US$1.01/lb  (2) U.S. dollar  US$257 million  Cdn$/US$0.99  (1) U.S. dollar  US$2.3 million  CLP/US$644  1 Forward purchase contracts Zinc 14.2 million lbs  US$0.91/lb  1 Lead  9.3 million lbs  US$1.00/lb  1 $  (1) Derivatives designated as cash flow hedges U.S. dollar forward sales contracts  US$295 million  Cdn$/US$0.99  $  – Teck 2012 Annual Report 128