Mercury is often coincident with gold and silver in precious metals ores, and is commonly produced as a byproduct during gold and silver recovery.   The amount of mercury in ore can vary greatly, from less than 0.1 mg/kg to over 100 mg/kg.    A portion of this mercury can be released to the atmosphere from a variety of thermal and non-thermal processes used in precious metals mining.   Because of concerns regarding atmospheric release, implementation of control technology has resulted in a decrease of mercury emissions to the atmosphere, from over 13000 lbs in 1999 to 4718 lbs in 2004 (Toxics Release Inventory Data).   Mercury reacts with cyanide to form very soluble mercury cyanide complexes that follow gold and silver during recovery, and any thermal process during recovery can potentially release mercury, including carbon stripping, carbon regeneration, and precious metals refining.  A recent survey of precious metals mines in Nevada from the Nevada Division of Environmental Protection has revealed that in 2004, 110.7 metric tons of mercury were produced as byproduct in Nevada, mostly as elemental mercury.   The three largest sources of byproduct mercury in Nevada were the Goldstrike Mine (Barrick; 173,538 lbs), the Rochester Mine (Coeur; 31,875 lbs), and the Twin Creeks Mine (Newmont; 26,746 lbs.)  The mercury is sold to recyclers and then into the open market.